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Bit error rate

In digital transmission, the number of bit errors is the number of received bits of a data stream over a communication channel that have been altered due to noise, interference, distortion or bit synchronization errors.

The bit error rate (BER) is the number of bit errors per unit time. The bit error ratio (also BER) is the number of bit errors divided by the total number of transferred bits during a studied time interval. Bit error ratio is a unitless performance measure, often expressed as a percentage.

The bit error probability pe is the expected value of the bit error ratio. The bit error ratio can be considered as an approximate estimate of the bit error probability. This estimate is accurate for a long time interval and a high number of bit errors.

Example
As an example, assume this transmitted bit sequence:

1 1 0 0 0 1 0 1 1

and the following received bit sequence:

0 1 0 1 0 1 0 0 1,

The number of bit errors (the underlined bits) is, in this case, 3. The BER is 3 incorrect bits divided by 9 transferred bits, resulting in a BER of 0.333 or 33.3%.

Packet error ratio
The packet error ratio (PER) is the number of incorrectly received data packets divided by the total number of received packets. A packet is declared incorrect if at least one bit is erroneous. The expectation value of the PER is denoted packet error probability pp, which for a data packet length of N bits can be expressed as

 * $$p_p = 1 - (1 - p_e)^N = 1 - e^{N \ln(1 - p_e)}$$,

assuming that the bit errors are independent of each other. For small bit error probabilities and large data packets, this is approximately

 * $$p_p \approx p_eN.$$

Similar measurements can be carried out for the transmission of frames, blocks, or symbols.

The above expression can be rearranged to express the corresponding BER (pe) as a function of the PER (pp) and the data packet length N in bits:

 * $$p_e = 1 - \sqrt[N]{(1 - p_p)}$$

Factors affecting the BER
In a communication system, the receiver side BER may be affected by transmission channel noise, interference, distortion, bit synchronization problems, attenuation, wireless multipath fading, etc.

The BER may be improved by choosing a strong signal strength (unless this causes cross-talk and more bit errors), by choosing a slow and robust modulation scheme or line coding scheme, and by applying channel coding schemes such as redundant forward error correction codes.

The transmission BER is the number of detected bits that are incorrect before error correction, divided by the total number of transferred bits (including redundant error codes). The information BER, approximately equal to the decoding error probability, is the number of decoded bits that remain incorrect after the error correction, divided by the total number of decoded bits (the useful information). Normally the transmission BER is larger than the information BER. The information BER is affected by the strength of the forward error correction code.

Analysis of the BER
The BER may be evaluated using stochastic (Monte Carlo) computer simulations. If a simple transmission channel model and data source model is assumed, the BER may also be calculated analytically. An example of such a data source model is the Bernoulli source.

Examples of simple channel models used in information theory are:
 * Binary symmetric channel (used in analysis of decoding error probability in case of non-bursty bit errors on the transmission channel)
 * Additive white Gaussian noise (AWGN) channel without fading.

A worst-case scenario is a completely random channel, where noise totally dominates over the useful signal. This results in a transmission BER of 50% (provided that a Bernoulli binary data source and a binary symmetrical channel are assumed, see below).

In a noisy channel, the BER is often expressed as a function of the normalized carrier-to-noise ratio measure denoted Eb/N0, (energy per bit to noise power spectral density ratio), or Es/N0 (energy per modulation symbol to noise spectral density).

For example, in the case of BPSK modulation and AWGN channel, the BER as function of the Eb/N0 is given by:

$$\operatorname{BER}=Q(\sqrt{2E_b/N_0})$$,

where $$Q(x) := \frac{1}{\sqrt{2\pi}} \int_{x}^{ \infty } e^{-x^2/2}dx$$.

People usually plot the BER curves to describe the performance of a digital communication system. In optical communication, BER(dB) vs. Received Power(dBm) is usually used; while in wireless communication, BER(dB) vs. SNR(dB) is used.

Measuring the bit error ratio helps people choose the appropriate forward error correction codes. Since most such codes correct only bit-flips, but not bit-insertions or bit-deletions, the Hamming distance metric is the appropriate way to measure the number of bit errors. Many FEC coders also continuously measure the current BER.

A more general way of measuring the number of bit errors is the Levenshtein distance. The Levenshtein distance measurement is more appropriate for measuring raw channel performance before frame synchronization, and when using error correction codes designed to correct bit-insertions and bit-deletions, such as Marker Codes and Watermark Codes.

Mathematical draft
The BER is the likelihood of a bit misinterpretation due to electrical noise $$w(t)$$. Considering a bipolar NRZ transmission, we have

$$x_1(t) = A + w(t)$$ for a "1" and $$x_0(t) = -A + w(t)$$ for a "0". Each of $$x_1(t)$$ and $$x_0(t)$$ has a period of $$T$$.

Knowing that the noise has a bilateral spectral density $$\frac{N_0}{2} $$,

$$x_1(t)$$ is $$\mathcal{N}\left(A,\frac{N_0}{2T}\right)$$

and $$x_0(t)$$ is $$\mathcal{N}\left(-A,\frac{N_0}{2T}\right)$$.

Returning to BER, we have the likelihood of a bit misinterpretation $$p_e = p(0|1) p_1 + p(1|0) p_0$$.

$$ p(1|0) = 0.5\, \operatorname{erfc}\left(\frac{A+\lambda}{\sqrt{N_o/T}}\right)$$ and $$ p(0|1) = 0.5\, \operatorname{erfc}\left(\frac{A-\lambda}{\sqrt{N_o/T}}\right)$$

where $$ \lambda$$ is the threshold of decision, set to 0 when $$p_1 = p_0 = 0.5$$.

We can use the average energy of the signal $$E = A^2 T$$ to find the final expression :

$$p_e = 0.5\, \operatorname{erfc}\left(\sqrt{\frac{E}{N_o}}\right).$$ ±§

Bit error rate test
BERT or bit error rate test is a testing method for digital communication circuits that uses predetermined stress patterns consisting of a sequence of logical ones and zeros generated by a test pattern generator.

A BERT typically consists of a test pattern generator and a receiver that can be set to the same pattern. They can be used in pairs, with one at either end of a transmission link, or singularly at one end with a loopback at the remote end. BERTs are typically stand-alone specialised instruments, but can be personal computer–based. In use, the number of errors, if any, are counted and presented as a ratio such as 1 in 1,000,000, or 1 in 1e06.

Common types of BERT stress patterns

 * PRBS (pseudorandom binary sequence) – A pseudorandom binary sequencer of N Bits. These pattern sequences are used to measure jitter and eye mask of TX-Data in electrical and optical data links.
 * QRSS (quasi random signal source) – A pseudorandom binary sequencer which generates every combination of a 20-bit word, repeats every 1,048,575 words, and suppresses consecutive zeros to no more than 14. It contains high-density sequences, low-density sequences, and sequences that change from low to high and vice versa. This pattern is also the standard pattern used to measure jitter.
 * 3 in 24 – Pattern contains the longest string of consecutive zeros (15) with the lowest ones density (12.5%). This pattern simultaneously stresses minimum ones density and the maximum number of consecutive zeros. The D4 frame format of 3 in 24 may cause a D4 yellow alarm for frame circuits depending on the alignment of one bits to a frame.
 * 1:7 – Also referred to as 1 in 8.  It has only a single one in an eight-bit repeating sequence.  This pattern stresses the minimum ones density of 12.5% and should be used when testing facilities set for B8ZS coding as the 3 in 24 pattern increases to 29.5% when converted to B8ZS.
 * Min/max – Pattern rapid sequence changes from low density to high density. Most useful when stressing the repeater's ALBO feature.
 * All ones (or mark) – A pattern composed of ones only. This pattern causes the repeater to consume the maximum amount of power. If DC to the repeater is regulated properly, the repeater will have no trouble transmitting the long ones sequence. This pattern should be used when measuring span power regulation.  An unframed all ones pattern is used to indicate an AIS (also known as a blue alarm).
 * All zeros – A pattern composed of zeros only. It is effective in finding equipment misoptioned for AMI, such as fiber/radio multiplex low-speed inputs.
 * Alternating 0s and 1s - A pattern composed of alternating ones and zeroes.
 * 2 in 8 – Pattern contains a maximum of four consecutive zeros. It will not invoke a B8ZS sequence because eight consecutive zeros are required to cause a B8ZS substitution. The pattern is effective in finding equipment misoptioned for B8ZS.
 * Bridgetap - Bridge taps within a span can be detected by employing a number of test patterns with a variety of ones and zeros densities. This test generates 21 test patterns and runs for 15 minutes. If a signal error occurs, the span may have one or more bridge taps. This pattern is only effective for T1 spans that transmit the signal raw. Modulation used in HDSL spans negates the bridgetap patterns' ability to uncover bridge taps.
 * Multipat - This test generates five commonly used test patterns to allow DS1 span testing without having to select each test pattern individually. Patterns are: all ones, 1:7, 2 in 8, 3 in 24, and QRSS.
 * T1-DALY and 55 OCTET - Each of these patterns contain fifty-five (55), eight bit octets of data in a sequence that changes rapidly between low and high density. These patterns are used primarily to stress the ALBO and equalizer circuitry but they will also stress timing recovery. 55 OCTET has fifteen (15) consecutive zeroes and can only be used unframed without violating one's density requirements. For framed signals, the T1-DALY pattern should be used. Both patterns will force a B8ZS code in circuits optioned for B8ZS.

Bit error rate tester
A bit error rate tester (BERT), also known as a "bit error ratio tester" or bit error rate test solution (BERTs) is electronic test equipment used to test the quality of signal transmission of single components or complete systems.

The main building blocks of a BERT are:
 * Pattern generator, which transmits a defined test pattern to the DUT or test system
 * Error detector connected to the DUT or test system, to count the errors generated by the DUT or test system
 * Clock signal generator to synchronize the pattern generator and the error detector
 * Digital communication analyser is optional to display the transmitted or received signal
 * Electrical-optical converter and optical-electrical converter for testing optical communication signals

title: Bit error rate


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Bit inversion

In computing and telecommunications, bit inversion means the changing of the state of a bit or binary number to the opposite state, i.e. the changing of a 0 bit to 1 or of a 1 bit to 0. It is often represented with a tilde (~). It also refers to the changing of a state representing a given bit to the opposite state.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Bit inversion


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Bit pairing

In telecommunication, bit pairing is the practice of establishing, within a code set, a number of subsets that have an identical bit representation except for the state of a specified bit.

Note: An example of bit pairing occurs in the International Alphabet No. 5 and the American Standard Code for Information Interchange (ASCII), where the upper case letters are related to their respective lower case letters by the state of bit six.

title: Bit pairing


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Bit-sequence independence

In telecommunication, bit-sequence independence is a characteristic of some digital data transmission systems that impose no restrictions on, or modification of, the transmitted bit sequence.

Bit-sequence-independent protocols are in contrast to protocols that reserve certain bit sequences for special meanings, such as the flag sequence, 01111110, for HDLC, SDLC, and ADCCP protocols.

Bit-sequence-independence allows only line codes that have the same number of transitions per bit, otherwise, the line code is dependent on the bit sequence and, therefore, bit-sequence dependent.

title: Bit-sequence independence


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Bit slip

In digital transmission, bit slip is the loss or gain of a bit or bits, caused by clock drift – variations in the respective clock rates of the transmitting and receiving devices.

One cause of bit slip is overflow of a receive buffer that occurs when the transmitter's clock rate exceeds that of the receiver. This causes one or more bits to be dropped for lack of storage capacity.

One way to maintain timing between transmitting and receiving devices is to employ an asynchronous protocol such as start-stop. Alternatively, bit slip can be prevented by using a self-clocking signal (such as a signal modulated using OQPSK) or using a line coding such as Manchester encoding.

Another cause is "losing count", as on a hard drive: if a hard drive encounters a long string of 0s, without any 1s (or a string of 1s without 0s), it may lose track of the frame between fields, and suffer bit slip. When a pulse of N consecutive zero bits are sent, clock drift may cause the hardware to apparently detect N-1 zero bits or N+1 zero bits – both kinds of errors are called bit slip. Thus one prevents long strings without change via such devices as run length limited codes.

Many communication systems use linear-feedback shift register scrambling to prevent long strings of 0s (or other symbol), including VSAT, 1000BASE-T,, etc. While a scrambler makes the "losing count" type of bit slip error occur far less often, when bit slip errors do occur (perhaps for other reasons), scramblers have the property of expanding small errors that add or lose a single bit into a much longer burst of errors.

The optimized cipher feedback mode (OCFB), the statistical self-synchronization mode, and the "one-bit CFB mode" also expand small bit-slip errors into a longer burst of errors, but eventually recover and produce the correct decrypted plaintext. A bit-slip error when using any other block cipher mode of operation generally results in complete corruption of the rest of the message.

title: Bit slip


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Bit-stream transmission

In telecommunication, the term bit-stream transmission has the following meanings:

1. In bit-oriented systems, the transmission of bit strings.

2. In character-oriented systems, the transmission of bit streams that represent characters.

In bit-stream transmission, the bits usually occur at fixed time intervals, start and stop signals are not used, and the bit patterns follow each other in sequence without interruption.

title: Bit-stream transmission


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Bit stuffing

In data transmission and telecommunication, bit stuffing (also known—uncommonly—as positive justification) is the insertion of non-information bits into data. Stuffed bits should not be confused with overhead bits.

Bit stuffing is used for various purposes, such as for bringing bit streams that do not necessarily have the same or rationally related bit rates up to a common rate, or to fill buffers or frames. The location of the stuffing bits is communicated to the receiving end of the data link, where these extra bits are removed to return the bit streams to their original bit rates or form. Bit stuffing may be used to synchronize several channels before multiplexing or to rate-match two single channels to each other.

Another use of bit stuffing is for run length limited coding: to limit the number of consecutive bits of the same value in the data to be transmitted. A bit of the opposite value is inserted after the maximum allowed number of consecutive bits. Since this is a general rule the receiver doesn't need extra information about the location of the stuffing bits in order to do the de-stuffing. This is done to create additional signal transitions to ensure reliable reception or to escape special reserved code words such as frame sync sequences when the data happens to contain them.

Bit stuffing does not ensure that the payload is intact (i.e. not corrupted by transmission errors); it is merely a way of attempting to ensure that the transmission starts and ends at the correct places. Error detection and correction techniques are used to check the frame for corruption after its delivery and, if necessary, the frame will be re-sent.

Zero-bit insertion
The NRZI coding scheme transmits a 0 bit as a signal transition, and a 1 bit as no change. In this case, bit stuffing is most easily described as the insertion of a 0 bit after a long run of 1 bits.

It was popularized by IBM's SDLC (later renamed HDLC), and is also used in Low- and full-speed USB.

After a long sequence of 1 bits there would be no transitions in the transmitted data, and it would be possible for the transmitter and receiver clocks to lose synchronisation. By inserting a 0 after five (SDLC) or six (USB) consecutive 1 bits the transmitter guarantees a maximum of six (SDLC) or seven (USB) bit times between transitions. The receiver can synchronise its clock against the transitions to ensure proper data recovery.

In SDLC the transmitted bit sequence "01111110" containing six adjacent 1 bits is the Flag byte. Bit stuffing ensures that this pattern can never occur in normal data, so it can be used as a marker for the beginning and end of the frame without any possibility of being confused with normal data.

The main disadvantage of bit-stuffing is that the code rate is unpredictable; it depends on the data being transmitted.

Source: from Federal Standard 1037C in support of MIL-STD-188

title: Bit stuffing


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Bit-synchronous operation

Bit-synchronous operation is a type of digital communication in which the data circuit-terminating equipment (DCE), data terminal equipment (DTE), and transmitting circuits are all operated in bit synchronism with a clock signal.

In bit-synchronous operation, clock timing is usually delivered at twice the modulation rate, and one bit is transmitted or received during each clock cycle.

Bit-synchronous operation is sometimes erroneously referred to as digital synchronization.

title: Bit-synchronous operation


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Blind transmission

A blind transmission, in telecommunications, is a transmission made without obtaining a receipt, or acknowledgment of reception, from the intended receiving station. Blind transmissions may occur or be necessary when security constraints, such as radio silence, are imposed, when technical difficulties with a sender's receiver or receiver's transmitter occur, or when lack of time precludes the delay caused by waiting for receipts.

Examples

 * In aviation it is common to "transmit in the blind" when an aircraft is approaching a non-towered airport. The pilot will broadcast his position and intentions over a common frequency. If no other aircraft are in the traffic pattern, the pilot continues to transmit his position in the blind in case another aircraft is in the area but unable to respond.
 * If air traffic control is attempting to establish communication with a presumed NORDO aircraft, or aircraft on the incorrect frequency, it would be appropriate phraseology to issue a transmission that includes the blind preamble, e.g., "New York Center in the blind, N1234, if you copy this transmission, squawk ident."
 * The less common reason would be a ground control station not being able to transmit (due to transmitter failure) or not wanting to transmit any acknowledgment (to avoid giving away further information and putting a military mission at risk).

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Block

Block or blocked may refer to:

Broadcasting

 * Block programming, the result of a programming strategy in broadcasting
 * W242BX, a radio station licensed to Greenville, South Carolina, United States known as 96.3 the Block
 * WFNZ-FM, a radio station licensed to Harrisburg, North Carolina, United States, branded as 92.7 The Block
 * Blocked (The Flash), an episode of the television series The Flash

Music

 * Block Entertainment, a record label
 * Blocks Recording Club, a record label
 * Woodblock (instrument), a small piece of slit drum made from one piece of wood and used as a percussion instrument
 * "Blocks", by C418 from Minecraft - Volume Beta, 2013

Toys

 * Toy block, one of a set of wooden or plastic pieces, of various shapes
 * Unit block, a type of standardized wooden toy block for children

Video game

 * Blocked (video game), a puzzle game for the iPhone and iPod Touch

Building and construction

 * Breeze block, cinder block or cement block, a concrete masonry unit for building
 * Compressed earth block, a building block or unit for construction
 * Tower block, a high-rise building

Land subdivisions

 * Block (district subdivision), administrative region in some South Asian countries
 * Block (rural Australia), a small agricultural landholding
 * City block, the smallest area that is surrounded by streets

United States

 * Block, Illinois, an unincorporated community
 * Block, Kansas, an unincorporated community
 * Block, Tennessee, an unincorporated community
 * Block Island, an island in the state of Rhode Island

Science and technology

 * Block (periodic table), a set of adjacent groups in the periodic table
 * Block (meteorology), large-scale patterns in the atmospheric pressure field
 * Fault block, a geologic zone or geologic province
 * Block, Inc., an American technology company formerly known as Square

Computing

 * Block (blockchain), a segment of an open list of data records
 * Block (data storage), the practice of storing electronic data in equally sized units
 * Block (Internet), technical measures to restrict users' access to certain internet resources
 * Blocking access to certain websites is one form of internet censorship
 * IP range or block of the Internet, a subset of the internet in a set of contiguous IP addresses
 * Block (programming), a group of declarations and statements treated as a unit
 * Block (telecommunications), a unit of data transmission
 * Block artifact, a type of distortion in a compressed image
 * Block-level element, in the HTML markup language
 * Blocks (C language extension), an extension to the C programming language designed to support parallel programming
 * Unicode block, a named range of codepoints in Unicode
 * Block Elements, a Unicode block of block-shaped characters

Engineering

 * Engine block and cylinder block, the main part of an internal combustion engine
 * Block (sailing), a single or multiple pulley used on sailboats

Medicine

 * Nerve block, or regional nerve blockade, any deliberate interruption of signals traveling along a nerve, often for the purpose of pain relief
 * Local anesthetic nerve block (sometimes referred to as simply "nerve block"), a short-term block
 * Nerve block, the deliberate temporary degeneration of nerve fibers to produce a block that may persist for weeks, months, or indefinitely
 * Neurectomy, the cutting through or removal of a nerve or a section of a nerve, which usually produces a permanent block
 * Third-degree atrioventricular block (AV block), a medical condition

Mathematics

 * Block (permutation group theory)
 * Block, in modular representation theory
 * Block, in graph theory, is a biconnected component, a maximal biconnected subgraph of a graph
 * Aschbacher block of a finite group
 * Block design, a kind of set system in combinatorial mathematics
 * Block matrix
 * Block of a ring, a centrally primitive idempotent or the ideal it generates

Sports

 * Block, a defensive shot in pickleball
 * Block (basketball), when a defensive player legally deflects a shot
 * Block (cricket), when a batsman deflects the ball to avoid getting out
 * Blocking (American football)
 * Blocking (martial arts)
 * Starting blocks, devices used by sprinters to assist in preventing their feet from slipping as they break into a run
 * The Block (basketball), a defensive play in game 7 of the 2016 NBA Finals

Transportation

 * Block number, a system to differentiate between groups of aircraft of the same type with minor variants
 * Signalling block system, a way of controlling train movement

People

 * Block (musician) (Jamie Block), an anti-folk musician based in New York, US

Other uses

 * Block letters (also known as printscript, manuscript, print writing, or ball and stick in academics), a sans-serif (or "Gothic") style of writing
 * Block party (also known as street party), a community social event
 * Block scheduling, a type of academic scheduling
 * Hat block, or block shaper, a wooden block carved into the shape of a hat by a craftsman
 * Postage stamp block, an attached group of postage stamps

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Block check character

In telecommunications, a block check character (BCC) is a character added to a transmission block to facilitate error detection.

In longitudinal redundancy checking and cyclic redundancy checking, block check characters are computed for, and added to, each message block transmitted. This block check character is compared with a second block check character computed by the receiver to determine whether the transmission is error free.

title: Block check character


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Blocking

Blocking may refer to:

Computing and telecommunications

 * Blacklist (computing)
 * Blocking (computing), holding up a task until an event occurs
 * Blocking (radio), interference by an off-frequency signal
 * Blocking probability, for calls in a telecommunications system
 * Head-of-line blocking, in some network switching fabrics
 * Internet blocking; see Block (Internet)

Psychology

 * Thought blocking, a type of thought disorder
 * Blocking effect, in psychology
 * Mental block, a type of suppression or repression

Other uses in science

 * Blocking (linguistics), where the existence of a competing form blocks the application of a morphological process
 * Blocking (statistics), in the design of experiments, the arranging of experimental units in groups (blocks) which are similar to one another
 * Atmospheric blocking, a phenomenon in meteorology of large scale stationary pressure cells
 * Blocking, in the western blot technique, a process to prevent unwanted binding of antibodies to a membrane

Other technologies

 * Blocking (construction), short boards used as reinforcement or to provide attachment points in wood-framed and other forms of construction

Sport

 * Blocking (American football), the legal interference with another player's motion
 * Blocking (martial arts), a defensive technique in martial arts

Theatre and film

 * Blocking (stage), the movement and positioning of actors on a stage, or within a frame in film
 * Blocking (animation), a technique in which key poses establish timing and placement of items in a scene

Other uses

 * Blocking (textile arts), the process of stretching a knitted garment into shape while wet or using steam
 * Blocking (transport), the process of dividing a transit schedule into parts that can be operated by a single vehicle
 * Blocking troops - military barrier troops positioned to forestall unauthorised retreats

title: Blocking


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Bonding

Bonding may refer to:

Arts and entertainment

 * Bonding (TV series), a 2019 Netflix Original TV series
 * "Bonding" (Kim Possible), a 2004 episode of the television series Kim Possible
 * "The Bonding", a 1989 episode of the television series Star Trek: The Next Generation
 * The Bonding (album), a 2013 album by Austrian symphonic metal band Edenbridge

Other uses

 * Bonding (dental), a dental procedure in which a dentist applies a resin material to the tooth

title: Bonding


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Branch

A branch, sometimes called a ramus in botany, is a stem that grows off from another stem, or when structures like veins in leaves are divided into smaller veins.

History and etymology
In Old English, there are numerous words for branch, including seten, stofn, telgor, and hrīs. There are also numerous descriptive words, such as blēd (that is, something that has bled, or "bloomed", out), bōgincel (literally "little bough"), ōwæstm (literally "on growth"), and tūdornes (literally "offspringing"). Numerous other words for twigs and boughs abound, including tān, which still survives as the "-toe" in mistletoe.

Latin words for branch are ramus or cladus. The latter term is an affix found in other modern words such as cladodonts (prehistoric sharks with branched teeth) or cladogram.

Woody branches
Large branches are known as boughs and small branches are known as twigs. The term twig usually refers to a terminus, while bough refers only to branches coming directly from the trunk.

Due to a broad range of species of trees, branches and twigs can be found in many different shapes and sizes. While branches can be nearly horizontal, vertical, or diagonal, the majority of trees have upwardly diagonal branches. A number of mathematical properties are associated with tree branchings; they are natural examples of fractal patterns in nature, and, as observed by Leonardo da Vinci, their cross-sectional areas closely follow the da Vinci branching rule.

Specific terms
A bough can also be called a limb or arm, and though these are arguably metaphors, both are widely accepted synonyms for bough. A crotch or fork is an area where a trunk splits into two or more boughs. A twig is frequently referred to as a sprig as well, especially when it has been plucked. Other words for twig include branchlet, spray, and surcle, as well as the technical terms surculus and ramulus. Branches found under larger branches can be called underbranches.

Some branches from specific trees have their own names, such as osiers and withes or withies, which come from willows. Often trees have certain words which, in English, are naturally collocated, such as holly and mistletoe, which usually employ the phrase "sprig of" (as in, a "sprig of mistletoe"). Similarly, the branch of a cherry tree is generally referred to as a "cherry branch", while other such formations (i.e., "acacia branch" or "orange branch") carry no such alliance. A good example of this versatility is oak, which could be referred to as variously an "oak branch", an "oaken branch", a "branch of oak", or the "branch of an oak tree".

Once a branch has been cut or in any other way removed from its source, it is most commonly referred to as a stick, and a stick employed for some purpose (such as walking, spanking, or beating) is often called a rod. Thin, flexible sticks are called switches, wands, shrags, or vimina (singular vimen).

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Brewster's angle

Brewster's angle (also known as the polarization angle) is an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with no reflection. When unpolarized light is incident at this angle, the light that is reflected from the surface is therefore perfectly polarized. The angle is named after the Scottish physicist Sir David Brewster (1781–1868).

Explanation
When light encounters a boundary between two media with different refractive indices, some of it is usually reflected as shown in the figure above. The fraction that is reflected is described by the Fresnel equations, and depends on the incoming light's polarization and angle of incidence.

The Fresnel equations predict that light with the p polarization (electric field polarized in the same plane as the incident ray and the surface normal at the point of incidence) will not be reflected if the angle of incidence is
 * $$\theta_\mathrm{B} = \arctan\!\left(\frac{n_2}{n_1}\right)\!,$$

where n1 is the refractive index of the initial medium through which the light propagates (the "incident medium"), and n2 is the index of the other medium. This equation is known as Brewster's law, and the angle defined by it is Brewster's angle.

The physical mechanism for this can be qualitatively understood from the manner in which electric dipoles in the media respond to p-polarized light. One can imagine that light incident on the surface is absorbed, and then re-radiated by oscillating electric dipoles at the interface between the two media. The polarization of freely propagating light is always perpendicular to the direction in which the light is travelling. The dipoles that produce the transmitted (refracted) light oscillate in the polarization direction of that light. These same oscillating dipoles also generate the reflected light. However, dipoles do not radiate any energy in the direction of the dipole moment. If the refracted light is p-polarized and propagates exactly perpendicular to the direction in which the light is predicted to be specularly reflected, the dipoles point along the specular reflection direction and therefore no light can be reflected. (See diagram, above)

With simple geometry this condition can be expressed as
 * $$\theta_1 + \theta_2 = 90^\circ,$$

where θ1 is the angle of reflection (or incidence) and θ2 is the angle of refraction.

Using Snell's law,
 * $$n_1 \sin \theta_1 = n_2 \sin \theta_2,$$

one can calculate the incident angle θ1 = θB at which no light is reflected:
 * $$n_1 \sin \theta_\mathrm{B} = n_2 \sin(90^\circ - \theta_\mathrm{B}) = n_2 \cos \theta_\mathrm{B}.$$

Solving for θB gives
 * $$\theta_\mathrm{B} = \arctan\!\left(\frac{n_2}{n_1}\right)\!.$$

For a glass medium (n2 ≈ 1.5) in air (n1 ≈ 1), Brewster's angle for visible light is approximately 56°, while for an air-water interface (n2 ≈ 1.33), it is approximately 53°. Since the refractive index for a given medium changes depending on the wavelength of light, Brewster's angle will also vary with wavelength.

The phenomenon of light being polarized by reflection from a surface at a particular angle was first observed by Étienne-Louis Malus in 1808. He attempted to relate the polarizing angle to the refractive index of the material, but was frustrated by the inconsistent quality of glasses available at that time. In 1815, Brewster experimented with higher-quality materials and showed that this angle was a function of the refractive index, defining Brewster's law.

Brewster's angle is often referred to as the "polarizing angle", because light that reflects from a surface at this angle is entirely polarized perpendicular to the plane of incidence ("s-polarized"). A glass plate or a stack of plates placed at Brewster's angle in a light beam can, thus, be used as a polarizer. The concept of a polarizing angle can be extended to the concept of a Brewster wavenumber to cover planar interfaces between two linear bianisotropic materials. In the case of reflection at Brewster's angle, the reflected and refracted rays are mutually perpendicular.

For magnetic materials, Brewster's angle can exist for only one of the incident wave polarizations, as determined by the relative strengths of the dielectric permittivity and magnetic permeability. This has implications for the existence of generalized Brewster angles for dielectric metasurfaces.

Applications
While at the Brewster angle there is no reflection of the p polarization, at yet greater angles the reflection coefficient of the p polarization is always less than that of the s polarization, almost up to 90° incidence where the reflectivity of each rises towards unity. Thus reflected light from horizontal surfaces (such as the surface of a road) at a distance much greater than one's height (so that the incidence angle of specularly reflected light is near, or usually well beyond the Brewster angle) is strongly s-polarized. Polarized sunglasses use a sheet of polarizing material to block horizontally-polarized light and thus reduce glare in such situations. These are most effective with smooth surfaces where specular reflection (thus from light whose angle of incidence is the same as the angle of reflection defined by the angle observed from) is dominant, but even diffuse reflections from roads for instance, are also significantly reduced.

Photographers also use polarizing filters to remove reflections from water so that they can photograph objects beneath the surface. Using a polarizing camera attachment which can be rotated, such a filter can be adjusted to reduce reflections from objects other than horizontal surfaces, such as seen in the accompanying photograph (right) where the s polarization (approximately vertical) has been eliminated using such a filter.

When recording a classical hologram, the bright reference beam is typically arranged to strike the film in the p polarization at Brewster's angle. By thus eliminating reflection of the reference beam at the transparent back surface of the holographic film, unwanted interference effects in the resulting hologram are avoided.

Entrance windows or prisms with their surfaces at the Brewster angle are commonly used in optics and laser physics in particular. The polarized laser light enters the prism at Brewster's angle without any reflective losses.

In surface science, Brewster angle microscopes are used to image layers of particles or molecules at air-liquid interfaces. Using illumination by a laser at Brewster's angle to the interface and observation at the angle of reflection, the uniform liquid does not reflect, appearing black in the image. However any molecular layers or artifacts at the surface, whose refractive index or physical structure contrasts with the liquid, allows for some reflection against that black background which is captured by a camera.

Brewster windows
Gas lasers using an external cavity (reflection by one or both mirrors outside the gain medium) generally seal the tube using windows tilted at Brewster's angle. This prevents light in the intended polarization from being lost through reflection (and reducing the round-trip gain of the laser) which is critical in lasers having a low round-trip gain. On the other hand, it does remove s polarized light, increasing the round trip loss for that polarization, and ensuring the laser only oscillates in one linear polarization, as is usually desired. And many sealed-tube lasers (which do not even need windows) have a glass plate inserted within the tube at the Brewster angle, simply for the purpose of allowing lasing in only one polarization.

Pseudo-Brewster's angle
When the reflecting surface is absorbing, reflectivity at parallel polarization (p) goes through a non-zero minimum at the so-called pseudo-Brewster's angle.

title: Brewster's angle


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Bridging loss

Bridging loss is the loss, at a given frequency, that results when an impedance is connected across a transmission line. It is expressed as the ratio, in decibels, of the signal power delivered to a given point in a system downstream from the bridging point prior to bridging, to the signal power delivered to the given point after bridging. The term is introduced because return loss is not applicable to the high-impedance input conditions. The term is also used in telephone practice and synonymous with the insertion loss that result from bridging an impedance across a circuit.

title: Bridging loss


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Brightness

Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. The perception is not linear to luminance, and relies on the context of the viewing environment (for example, see White's illusion).

Brightness is a subjective sensation of an object being observed and one of the color appearance parameters of many color appearance models, typically denoted as $$Q$$. Brightness refers to how much light appears to shine from something. This is a different perception than lightness, which is how light something appears compared to a similarly lit white object.

The adjective bright derives from an Old English beorht with the same meaning via metathesis giving Middle English briht. The word is from a Common Germanic berhtaz, ultimately from a PIE root with a closely related meaning, * "white, bright". "Brightness" was formerly used as a synonym for the photometric term luminance and (incorrectly) for the radiometric term radiance. As defined by the US Federal Glossary of Telecommunication Terms (FS-1037C), "brightness" should now be used only for non-quantitative references to physiological sensations and perceptions of light. Brightness is an antonym of "dimness" or "dullness".

With regard to stars, brightness is quantified as apparent magnitude and absolute magnitude.

Two pictograms resembling the Sun with rays are used to represent the settings of luminance in display devices. They have been encoded in Unicode since version 6.0 (October 2010) in the Miscellaneous Symbols and Pictographs block under U+1505 as "low brightness symbol" (🔅) and U+1F506 as "high brightness symbol" (🔆).

The United States Federal Trade Commission (FTC) has assigned an unconventional meaning to brightness when applied to lamps. When appearing on light bulb packages, brightness means luminous flux, while in other contexts it means luminance. Luminous flux is the total amount of light coming from a source, such as a lighting device. Luminance, the original meaning of brightness, is the amount of light per solid angle coming from an area, such as the sky. The table below shows the standard ways of indicating the amount of light.

title: Brightness


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Buffer

Buffer may refer to:

Science

 * Buffer gas, an inert or nonflammable gas
 * Buffer solution, a solution used to prevent changes in pH
 * Lysis buffer, in cell biology
 * Metal ion buffer
 * Mineral redox buffer, in geology

Technology and engineering

 * Buffer (GIS), a HASS zone around a map feature
 * Buffer (optical fiber), a GG component of a fiber optic cable
 * Buffer (rail transport), a device that cushions impacts between vehicles
 * Buffer amplifier, an isolating circuit used in electronics or telecommunications
 * Buffer stop, a device that keeps rail vehicles on tracks
 * Buffer wheel, a device used to smooth a workpiece's surface
 * Digital buffer, an electronic circuit used to isolate the input from the output
 * Floor buffer, an appliance used to polish hard floors
 * Optical buffer, a device that stores optically transmitted data
 * Recoil buffer, a firearm component
 * Seismic buffers, protect structures against the effects of earthquakes

Computing

 * Buffer (application), a software application for managing social network accounts
 * Data buffer, memory used temporarily to store output or input data while it is transferred.
 * Framebuffer, a type of data buffer for use in graphical display
 * Memory buffer register, the connection between processor and memory

People

 * Bruce Buffer (born 1957), American sports announcer for UFC events
 * Michael Buffer (born 1944), American ring announcer for boxing and wrestling events

Other uses

 * Buffer (navy), a colloquial title
 * Buffer state, a country separating two rival or hostile powers, thought to prevent conflict between them
 * Buffer zone, a region separating two areas, possibly to segregate or conjoin them
 * Buffering (horse), an Australian thoroughbred
 * Buffering (TV series), a British TV series

title: Buffer


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Burst switching

In a packet switched network, burst switching is a capability in which each network switch extracts routing instructions from an incoming packet header to establish and maintain the appropriate switch connection for the duration of the packet, following which the connection is automatically released.

In concept, burst switching is similar to connectionless mode transmission, but differs in that burst switching implies an intent to establish the switch connection in near real time, so that only minimum buffering is required at the node switch.

A variant of burst switching used in optical networks is optical burst switching.

Applications
Examples of technology using burst switching include:
 * EtherLoop, used after the late-1990s to facilitate telephone company offerings to compete with the high-speed local data access that was then beginning to be offered by cable TV providers as an internet service offering.
 * Intra-vehicle communication: Tesla Cybertruck in 2023 began using a gigabit Ethernet loop with burst switching for near real-time, redundant control of all motor vehicle subsystems, including steer-by-wire, motor controls, all vehicle controllers, plus vehicle audio with active road-noise cancellation.

title: Burst switching


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Burst transmission

In telecommunication, a burst transmission or data burst is the broadcast of a relatively high-bandwidth transmission over a short period.

Burst transmission can be intentional, broadcasting a compressed message at a very high data signaling rate within a very short transmission time.

In the 1980s, the term "data burst" (and "info burst") was used for a technique used by some United Kingdom and South African TV programmes to transmit large amounts of primarily textual information. They would display multiple pages of text in rapid succession, usually at the end of the programme; viewers would videotape it and then read it later by playing it back using the pause button after each page.

Data bursts can occur naturally, such as when the download of data from the internet briefly experiences higher speeds. It can also occur in a computer network where data transmission is interrupted at intervals. Burst transmission enables communications between data terminal equipment (DTEs) and a data network operating at dissimilar data signaling rates.

title: Burst transmission


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Busy signal (disambiguation)

A busy signal is information communicated to a user or apparatus attempting a connection, indicating the requested connection cannot be completed.

 * Busy Signal (born 1982), or Reanno Gordon, a dancehall singer from Jamaica
 * "Busy Signal", a song by Prefuse 73 from the 2003 album One Word Extinguisher

title: Busy signal (disambiguation)


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Bypass

Bypass may refer to:

 * Bypass (road), a road that avoids a built-up area (not to be confused with passing lane)
 * Flood bypass of a river

Medicine

 * Bypass surgery, a class of surgeries including for example:
 * Heart bypass
 * Gastric bypass
 * Cardiopulmonary bypass

Other

 * Bypass capacitor, used to bypass a power supply or other high impedance component
 * High bypass, a turbofan aircraft gas turbine engine
 * Bypass duct
 * Bypass ratio
 * Bleach bypass, an optical effect
 * Bypass switch, access port for an in-line active security appliance
 * Bypass (telecommunications)
 * Bypass valve
 * A blowoff valve on a compressor
 * A manual gas addition valve on a rebreather

Other uses

 * The Bypass, a 2003 short silent Bollywood film
 * Railroad bypass, a new railroad line built to replace or supplement an existing route
 * By-pass Variegated, an architectural style

title: Bypass


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Cable television relay service station

In telecommunication, a cable television relay service station (CARS) is a fixed or mobile station used for the transmission of television and related audio signals, signals of standard and FM broadcast stations, signals of instructional television fixed stations, and cablecasting from the point of reception to a terminal point from which the signals are distributed to the public.

Source: from Federal Standard 1037C and from the Code of Federal Regulations, Telecommunications Parts 0-199

title: Cable television relay service station


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Call

Call or Calls may refer to:

Games

 * Call (poker), a bet matching an opponent's
 * Call, in the game of contract bridge, a bid, pass, double, or redouble in the bidding stage

Music and dance

 * Call (band), from Lahore, Pakistan
 * Call, a command in square dancing, delivered by a caller
 * "Call / I4U", a 2011 single by Japanese music group AAA
 * "Call", a 2002 song by Ashanti from her album Ashanti
 * "Call" (Stray Kids song), 2021

Film

 * Call (film), or The Call, 2020 South Korean film
 * Calls (film), 2021 Indian Tamil-language crime thriller film

Television

 * Calls (TV series), a mystery thriller TV series on Apple TV+

Finance

 * Call on shares, a request for a further payment on partly paid share capital
 * Call option, a term in stock trading

Places

 * Call, North Carolina, United States
 * Call, Texas, United States

Computing

 * Call, a shell command in DOS, OS/2 and Microsoft Windows command-line interpreters
 * Call, a method of starting a subroutine
 * Computer-assisted language learning, a concept in language education
 * System call, in computer science, the mechanism used by an application program to request service from the operating system or another application program

Nature

 * Animal communication, a song or noise made by an animal such as:
 * Bird call, a type of bird vocalization
 * Mating call, animal communication to attract a sexual partner
 * Game call, a device that is used to animal communication noises to attract or drive animals to a hunter

Telecommunications

 * Call, an attempt to set up a telecommunication circuit
 * Call, in teletraffic engineering, a unit of traffic measurement
 * Call origination, in telephony
 * Call sign, in broadcasting and radio communications, a unique designation for a transmitting station
 * Telephone call

Other uses

 * Call (surname)
 * Religious calling
 * CALL, U.S. Army Center for Army Lessons Learned
 * CALL, Center for Anglican Learning & Leadership at the Church Divinity School of the Pacific

title: Call


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Call collision

In the field of telecommunications, a call collision, commonly referred to as a glare, can occur in two situations:

 * 1) Terminal and Data Circuit-Terminating Equipment (DCE) Contention: This type of call collision arises when both a terminal and a data circuit-terminating equipment (DCE) simultaneously designate the same communication channel to initiate a call request and handle an incoming call. In such cases, the DCE prioritizes the call request and proceeds with it, resulting in the cancellation of the incoming call.
 * 2) Trunk or Channel Seizure at Both Ends: The second scenario involves a call collision when a trunk or channel is seized simultaneously at both ends. This situation can lead to conflicts and disruptions in the communication flow.

Glare can sometimes be experienced when attempting to make an outgoing call on a private branch exchange (PBX) system but getting connected to an incoming call instead. This occurrence can also happen in residential settings if an outgoing call is initiated at the precise moment when an incoming call is about to start ringing.

To mitigate the risk of glare, Multi-line hunting techniques are employed. These techniques involve selecting circuits in the opposite preference order, ensuring that the highest numbered line, which is typically the last choice for incoming calls, becomes the first choice for outgoing calls. The following example illustrates the sequence: incoming -->1 2           3            4            5            6            7            8<-- outgoing

With PRI circuits, the channel selection sequence is specified when the circuit is provisioned. Common practice is to have the PBX use descending channel selection, and the carrier to use ascending. Glare is not common on PRI circuits because the signaling is so fast, however it is not impossible (especially if there are subtle differences in the timers at either end, and the circuit is being used at near-capacity). The users will not experience a connection to an unexpected call (as would be the case with analog circuits), because glare causes protocol errors that generally prevent any sort of successful connection. Instead, one or both of the call attempts might fail, and ideally an error would appear in the logs (this depends on the logging capabilities of the systems at either end of the circuit). Glare is quite rare on PRI circuits, and can be difficult to troubleshoot.

For old, analog PBX trunks, glare can be reduced by using ground start signaling, which offers a better answer and disconnect supervision. IE: Nortel BSP discouraged using loop start trunks for this and other reasons. Long Distance exchanges in the 1950s and 60s incorporated Glare Detectors to alleviate the problem.

title: Call collision


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Call duration

In telecommunications, the term call duration has the following meanings:

 * 1) In telephone systems, the time between (a) the instant a circuit, i.e., off-hook condition at each end, is established between the call originator and the call receiver and (b) the instant the call originator or the call receiver terminates the call.
 * 2) In data transmission, the duration of the information transfer phase of an information transfer transaction.

Source: from Federal Standard 1037C

title: Call duration


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Called-party camp-on

In telecommunication, a called-party camp-on is a communication system service feature that enables the system to complete an access attempt in spite of issuance of a user blocking signal. This is most often found in a switchboard system at a company. Instead of going to voicemail or simply sitting on hold until the line is free, this feature places you in a queue whereby the call will be put through as soon as the line clears.

Systems that provide this feature monitor the busy user until the user blocking signal ends, and then proceed to complete the requested access. This feature permits holding an incoming telephone call until the called party is free.

title: Called-party camp-on


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Call processing

In telecommunication, the term call processing has the following meanings:

 * 1) The sequence of operations performed by a switching system from the acceptance of an incoming call through the final disposition of the call. See call control for a more complete description.
 * 2) The series of steps and processes by which an organization automates the handling of telephone calls (usually incoming calls).  Call processing in this sense may include the initial greeting of the call (perhaps time-of-day or other factor dependent) to routing the call based on dialed digits or lack thereof.  The automated treatment may include routing the call to an Interactive Voice Response System (IVR), sending the call to a voice mail system, queuing the call, etc. or a combination of steps and real-time decisions.  See also Automated attendant.
 * 3) The end-to-end sequence of operations performed by a network from the instant a call attempt is initiated until the instant the call release is completed.
 * 4) In data transmission, the operations required to complete all three phases of an information transfer transaction.

"Volume Call Processing" is the handling of calls when there are far more incoming calls than can be answered by an individual or group of attendants.

title: Call processing


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Call-second

In telecommunication, a call-second is a unit used to measure communications traffic density, equivalent to one call with a duration of one second.

Traffic is measured independent of users. For example, one user making two 75-second calls is equivalent to two users each making one 75-second call, as each case produces 150 call-seconds of traffic.

A CCS (centacall-second) is often used to describe 100 call-seconds, so 3600 call-seconds = 36 CCS = 1 call-hour.

In a communication network, a trunk (link) can carry numerous concurrent calls by means of multiplexing. Hence a particular number of call-seconds can be carried in infinitely many ways as calls are established and cleared over time. For example, one call-hour could be one call for an hour or two (possibly concurrent) calls for half an hour each. Call-seconds give a measure of the average number of concurrent calls.

Offered load is defined as the traffic density per unit time, measured in erlangs. An erlang is defined as one call-hour per hour, or 3,600 call-seconds per hour.

Hence, if one CCS is measured over a one-hour period, the offered load is 1/36 erlangs.

title: Call-second


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Call-sign allocation plan

In telecommunication, call-sign allocation plan is the table of allocation of international call sign series contained in the current edition of the International Telecommunication Union (ITU) Radio Regulations.

Note: In the table of allocation, the first two characters of each call sign (whether two letters or one number and one letter, in that order) identify the nationality of the station. In certain instances where the complete alphabetical block is allocated to a single nation, the first letter is sufficient for national identity. Individual assignments are made by appropriate national assignment authorities from the national allocation.

title: Call-sign allocation plan


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Call tracing

In telecommunication, call tracing is a procedure that permits an entitled user to be informed about the routing of data for an established connection, identifying the entire route from the origin to the destination.

There are two types of call tracing. Permanent call tracing permits tracing of all calls. On-demand call tracing permits tracing, upon request, of a specific call, provided that the called party dials a designated code immediately after the call to be traced is disconnected.

title: Call tracing


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Camp-on busy signal

In telecommunication, the term camp-on busy signal has the following meanings:

 * A signal that informs a busy telephone user that another call originator is waiting for a connection. Synonym: call waiting
 * A teleprinter exchange facility signal that automatically causes a calling station to retry the call-receiver number after a given interval when the call-receiver teleprinter is occupied or the circuits are busy. Synonym: speed-up tone

title: Camp-on busy signal


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Cancel character

In telecommunication and character encoding, the term cancel character refers to a control character which may be either of:

 * 1) "CAN", "Cancel", U+0018, or  used to indicate that the data with which it is associated are in error or are to be disregarded. Exact meaning can depend on protocol. For example:
 * 2) * In some journalistic text transmission formats, it signifies that the preceding word should be deleted; it is sometimes called "Kill Word" ("KW") in this context.
 * 3) * In some Videotex formats, it stops any running macros. In others, it clears the current line after the cursor position (compare ).
 * 4) "CCH", "Cancel Character", U+0094, or  used to erase the previous character. This character was created as an unambiguous alternative to the much more common backspace character ("BS", U+0008), which has a now mostly obsolete alternative function of causing the following character to be superimposed on the preceding one.

title: Cancel character


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Capacitive coupling

Capacitive coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) nodes, induced by the electric field. This coupling can have an intentional or accidental effect.

In its simplest implementation, capacitive coupling is achieved by placing a capacitor between two nodes. Where analysis of many points in a circuit is carried out, the capacitance at each point and between points can be described in a matrix form.

Use in analog circuits
In analog circuits, a coupling capacitor is used to connect two circuits such that only the AC signal from the first circuit can pass through to the next while DC is blocked. This technique helps to isolate the DC bias settings of the two coupled circuits. Capacitive coupling is also known as AC coupling and the capacitor used for the purpose is also known as a DC-blocking capacitor.

A coupling capacitor's ability to prevent a DC load from interfering with an AC source is particularly useful in Class A amplifier circuits by preventing a 0 volt input being passed to a transistor with additional resistor biasing; creating continuous amplification.

Capacitive coupling decreases the low frequency gain of a system containing capacitively coupled units. Each coupling capacitor along with the input electrical impedance of the next stage forms a high-pass filter and the sequence of filters results in a cumulative filter with a cutoff frequency that may be higher than those of each individual filter.

Coupling capacitors can also introduce nonlinear distortion at low frequencies. This is not an issue at high frequencies because the voltage across the capacitor stays very close to zero. However, if a signal passing through the coupling capacitance has a frequency that is low relative to the RC cutoff frequency, voltages can develop across the capacitor, which for some capacitor types results in changes of capacitance, leading to distortion. This is avoided by choosing capacitor types that have low voltage coefficient, and by using large values that put the cutoff frequency far lower than the frequencies of the signal.

Use in digital circuits
AC coupling is also widely used in digital circuits to transmit digital signals with a zero DC component, known as DC-balanced signals. DC-balanced waveforms are useful in communications systems, since they can be used over AC-coupled electrical connections to avoid voltage imbalance problems and charge accumulation between connected systems or components.

For this reason, most modern line codes are designed to produce DC-balanced waveforms. The most common classes of DC-balanced line codes are constant-weight codes and paired-disparity codes.

Gimmick loop
A gimmick loop is a simple type of capacitive coupler: two closely spaced strands of wire. It provides capacitive coupling of a few picofarads between two nodes. Usually the wires are twisted together.

Parasitic capacitive coupling
Capacitive coupling is often unintended, such as the capacitance between two wires or PCB traces that are next to each other. One signal may capacitively couple with another and cause what appears to be noise. To reduce coupling, wires or traces are often separated as much as possible, or ground lines or ground planes are run in between signals that might affect each other, so that the lines capacitively couple to ground rather than each other. Prototypes of high-frequency (tens of megahertz) or high-gain analog circuits, often use circuits that are built over a ground plane to control unwanted coupling. If a high-gain amplifier's output capacitively couples to its input it may become an electronic oscillator.

title: Capacitive coupling


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Capture effect

In a radio receiver, the capture effect, or FM capture effect, is a phenomenon associated with FM reception in which only the stronger of two signals at, or near, the same frequency or channel will be demodulated.

FM phenomenon
The capture effect is defined as the complete suppression of the weaker signal at the receiver's limiter (if present) where the weaker signal is not amplified, but attenuated. When both signals are nearly equal in strength or are fading independently, the receiver may rapidly switch from one to another and exhibit flutter.

The capture effect can occur at the signal limiter, or in the demodulation stage for circuits that do not require a signal limiter. Some types of radio receiver circuits have a stronger capture effect than others. The measurement of how well a receiver rejects a second signal on the same frequency is called its capture ratio. It is measured as the lowest ratio of the power of two signals that will result in the suppression of the weaker signal.

The capture effect phenomenon was first documented in 1938 by General Electric engineers conducting test transmissions. Two experimental FM stations, located 15 miles (24 km) apart in Albany and Schenectady, New York, were configured to transmit on the same frequency, in order to study how this would affect reception. It was determined that, for most of the path between the two stations, only one of the signals could be heard, with the complete elimination of the other. It was concluded that this effect occurred whenever the stronger signal was about twice as strong as the weaker one. This was significantly different than the case with amplitude modulation signals, where the general standard for broadcasting stations was that to avoid objectionable interference the stronger signal had to be about twenty times that of the weaker one. The capture effect thus allowed co-channel FM broadcasting stations to be located somewhat closer to each other than AM ones, without causing mutual interference.

AM immunity
Amplitude modulation, or AM radio, transmission does not exhibit this effect. For AM reception, the receiver tracks the signal strength of the AM signal as the basis for demodulation. This allows signals to be tracked as just another change in amplitude, so it is possible for an AM receiver to demodulate several carriers at the same time, resulting in an audio mix.

The ability to receive multiple signals simultaneously is in some cases considered beneficial and is one reason that the aviation industry, and others, have chosen to use AM rather than FM for communications.

Phenomena similar to the capture effect are described in AM when offset carriers of different strengths are present in the passband of a receiver. For example, the aviation glideslope vertical guidance clearance beam is sometimes described as a "capture effect" system, even though it operates using AM signals.

Digital modulation
For digital modulation schemes it has been shown that for properly implemented on-off keying/amplitude-shift keying systems, co-channel rejection can be better than for frequency-shift keying systems.

title: Capture effect


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Carrier

Carrier may refer to:

Entertainment

 * Carrier (album), a 2013 album by The Dodos
 * Carrier (board game), a South Pacific World War II board game
 * Carrier (TV series), a ten-part documentary miniseries that aired on PBS in April 2008
 * Carrier (video game), a 2000 video game for the Sega Dreamcast
 * Carriers (film), a 2009 post-apocalyptic horror film
 * The Carrier (band), an American melodic hardcore band
 * The Carrier, spaceship home to the Authority, a team of comic superheroes
 * Carrier (podcast), a podcast produced by QCode and starring Cynthia Erivo

Science

 * Carrier is a diluent used to simplify radioanalytical separations.
 * Carrier protein, a protein that facilitates the transport of another molecule
 * Disease carrier in medicine

Technology

 * Aircraft carrier, a warship primarily hosting fixed-wing aircraft
 * Carrier recovery in telecommunications
 * Carrier signal, a waveform suitable for modulation by an information-bearing signal
 * Helicopter carrier, a warship primarily hosting helicopters
 * Universal Carrier, a tracked vehicle
 * Wireless carrier, an organization that operates a wireless network for mobile phones
 * Information carrier or substrate, the image in a photographic layer

Other

 * Carrier, Oklahoma, a town in Garfield County, Oklahoma
 * Carrier (surname), a surname and list of people with the name
 * Carrier Global, air conditioning and commercial refrigeration manufacturing company
 * Carrier language, the Athabaskan language of the Dakelh people
 * Carrier pigeon, a dove trained to transport messages
 * Common carrier, an organization that transports a product or service
 * JMA Wireless Dome, formerly known as the Carrier Dome, in Syracuse, New York
 * Mail carrier, a postal worker

title: Carrier


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Carrier-sense multiple access with collision avoidance

Carrier-sense multiple access with collision avoidance (CSMA/CA) in computer networking, is a network multiple access method in which carrier sensing is used, but nodes attempt to avoid collisions by beginning transmission only after the channel is sensed to be "idle". When they do transmit, nodes transmit their packet data in its entirety.

It is particularly important for wireless networks, where the alternative with collision detection CSMA/CD, is not possible due to wireless transmitters desensing (turning off) their receivers during packet transmission.

CSMA/CA is unreliable due to the hidden node problem.

CSMA/CA is a protocol that operates in the data link layer.

Details
Collision avoidance is used to improve the performance of the CSMA method by attempting to divide the channel somewhat equally among all transmitting nodes within the collision domain.

 * 1) Carrier Sense: prior to transmitting, a node first listens to the shared medium (such as listening for wireless signals in a wireless network) to determine whether another node is transmitting or not. Note that the hidden node problem means another node may be transmitting which goes undetected at this stage.
 * 2) Collision Avoidance: if another node was heard, we wait for a period of time (usually random) for the node to stop transmitting before listening again for a free communications channel.
 * Request to Send/Clear to Send (RTS/CTS) may optionally be used at this point to mediate access to the shared medium. This goes some way to alleviating the problem of hidden nodes because, for instance, in a wireless network, the Access Point only issues a Clear to Send to one node at a time. However, wireless 802.11 implementations do not typically implement RTS/CTS for all transmissions; they may turn it off completely, or at least not use it for small packets (the overhead of RTS, CTS and transmission is too great for small data transfers).
 * Transmission: if the medium was identified as being clear or the node received a CTS to explicitly indicate it can send, it sends the frame in its entirety. Unlike CSMA/CD, it is very challenging for a wireless node to listen at the same time as it transmits (its transmission will dwarf any attempt to listen). Continuing the wireless example, the node awaits receipt of an acknowledgement packet from the Access Point to indicate the packet was received and checksummed correctly. If such acknowledgement does not arrive in a timely manner, it assumes the packet collided with some other transmission, causing the node to enter a period of binary exponential backoff prior to attempting to re-transmit.

Although CSMA/CA has been used in a variety of wired communication systems, it is particularly beneficial in a wireless LAN due to a common problem of multiple stations being able to see the Access Point, but not each other. This is due to differences in transmit power, and receive sensitivity, as well as distance, and location with respect to the AP. This will cause a station to not be able to 'hear' another station's broadcast. This is the so-called 'hidden node', or 'hidden station' problem. Devices utilizing 802.11 based standards can enjoy the benefits of collision avoidance (RTS / CTS handshake, also Point coordination function), although they do not do so by default. By default they use a Carrier sensing mechanism called exponential backoff (or Distributed coordination function), that relies upon a station attempting to 'listen' for another station's broadcast before sending. CA, or PCF relies upon the AP (or the 'receiver' for Ad hoc networks) granting a station the exclusive right to transmit for a given period of time after requesting it (Request to Send / Clear to Send).

CSMA-CA requires a determination of whether a channel is 'idle', even when incompatible standards and overlapping transmission frequencies are used. Per the standards, for 802.11/Wi-Fi transmitters on the same channel, transmitters must take turns to transmit if they can detect each other even 3 dB above the noise floor (the thermal noise floor is around -101 dBm for 20 MHz channels). On the other hand, transmitters will ignore transmitters with incompatible standards or on overlapping channels if the received signal strength from them is below a threshold Pth which, for non Wi-Fi 6 systems, is between -76 and -80 dBm.

IEEE 802.11 RTS/CTS Exchange
CSMA/CA can optionally be supplemented by the exchange of a Request to Send (RTS) packet sent by the sender S, and a Clear to Send (CTS) packet sent by the intended receiver R. Thus alerting all nodes within range of the sender, receiver or both, to not transmit for the duration of the main transmission. This is known as the IEEE 802.11 RTS/CTS exchange. Implementation of RTS/CTS helps to partially solve the hidden node problem that is often found in wireless networking.

Performance
CSMA/CA performance is based largely upon the modulation technique used to transmit the data between nodes. Studies show that under ideal propagation conditions (simulations), direct-sequence spread spectrum (DSSS) provides the highest throughput for all nodes on a network when used in conjunction with CSMA/CA and the IEEE 802.11 RTS/CTS exchange under light network load conditions. Frequency hopping spread spectrum (FHSS) follows distantly behind DSSS with regard to throughput with a greater throughput once network load becomes substantially heavy. However, the throughput is generally the same under real world conditions due to radio propagation factors.

Usage

 * GNET – an early proprietary LAN protocol
 * Apple's LocalTalk implemented CSMA/CA on an electrical bus using a three-byte jamming signal.
 * 802.11 RTS/CTS implements virtual carrier sensing using short request to send and clear to send messages for WLANs (802.11 mainly relies on physical carrier sensing though).
 * IEEE 802.15.4 (Wireless PAN) uses CSMA/CA
 * NCR WaveLAN – an early proprietary wireless network protocol
 * HomePNA
 * The ITU-T G.hn standard, which provides a way to create a high-speed (up to 1 Gigabit/s) local area network using existing home wiring (power lines, phone lines and coaxial cables), uses CSMA/CA as a channel access method for flows that do not require guaranteed quality of service, specifically the CSMA/CARP variant.


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Carrier-sense multiple access with collision detection

Carrier-sense multiple access with collision detection (CSMA/CD) is a medium access control (MAC) method used most notably in early Ethernet technology for local area networking. It uses carrier-sensing to defer transmissions until no other stations are transmitting. This is used in combination with collision detection in which a transmitting station detects collisions by sensing transmissions from other stations while it is transmitting a frame. When this collision condition is detected, the station stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to resend the frame.

CSMA/CD is a modification of pure carrier-sense multiple access (CSMA). CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus shortening the time required before a retry can be attempted.

With the growing popularity of Ethernet switches in the 1990s, IEEE 802.3 deprecated Ethernet repeaters in 2011, making CSMA/CD and half-duplex operation less common and less important.

Procedure

The following procedure is used to initiate a transmission. The procedure is complete when the frame is transmitted successfully or a collision is detected during transmission.
 * 1) Is a frame ready for transmission? If not, wait for a frame.
 * 2) Is medium idle? If not, wait until it becomes ready.
 * 3) Start transmitting and monitor for collision during transmission.
 * 4) Did a collision occur? If so, go to collision detected procedure.
 * 5) Reset retransmission counters and complete frame transmission.

The following procedure is used to resolve a detected collision. The procedure is complete when retransmission is initiated or the retransmission is aborted due to numerous collisions.
 * 1) Continue transmission (with a jam signal instead of frame header/data/CRC) until minimum packet time is reached to ensure that all receivers detect the collision.
 * 2) Increment retransmission counter.
 * 3) Was the maximum number of transmission attempts reached? If so, abort transmission.
 * 4) Calculate and wait the random backoff period based on number of collisions.
 * 5) Re-enter main procedure at stage 1.

Methods for collision detection are media dependent. On a shared, electrical bus such as 10BASE5 or 10BASE2, collisions can be detected by comparing transmitted data with received data or by recognizing a higher than normal signal amplitude on the bus. On all other media, a carrier sensed on the receive channel while transmitting triggers a collision event. Repeaters or hubs detect collisions on their own and propagate jam signals.

The collision recovery procedure can be likened to what happens at a dinner party, where all the guests talk to each other through a common medium (the air). Before speaking, each guest politely waits for the current speaker to finish. If two guests start speaking at the same time, both stop and wait for short, random periods of time (in Ethernet, this time is measured in microseconds). The hope is that by each choosing a random period of time, both guests will not choose the same time to try to speak again, thus avoiding another collision.

Jam signal
The jam signal or jamming signal is a signal that carries a 32-bit binary pattern sent by a data station to inform the other transmitting stations of the collision and that they must not transmit.

The maximum jam-time is calculated as follows: The maximum allowed diameter of an Ethernet installation is limited to 232 bits. This makes a round-trip-time of 464 bits. As the slot time in Ethernet is 512 bits, the difference between slot time and round-trip-time is 48 bits (6 bytes), which is the maximum jam-time.

This in turn means: A station noting a collision has occurred is sending a 4 to 6 byte long pattern composed of 16 1-0 bit combinations.

The purpose of this is to ensure that any other node which may currently be receiving a frame will receive the jam signal in place of the correct 32-bit MAC CRC; this causes the other receivers to discard the frame due to a CRC error.

Late collision
A late collision is a type of collision that happens further into the packet than is allowed for by the protocol standard in question. In 10-megabit-per-second shared-medium Ethernet, if a collision error occurs after the first 512 bits of data are transmitted by the transmitting station, a late collision is said to have occurred. Importantly, late collisions are not re-sent by the NIC, unlike collisions occurring before the first 64 octets; it is left for the upper layers of the protocol stack to determine that there was loss of data.

As a correctly set up CSMA/CD network link should not have late collisions, the usual possible causes are full-duplex/half-duplex mismatch, exceeded Ethernet cable length limits, or defective hardware such as incorrect cabling, non-compliant number of hubs in the network, or a bad NIC.

Local collision
A local collision is a collision that occurs at the NIC, as opposed to on the wire. A NIC cannot detect local collisions without attempting to send information.

On UTP cable, a local collision is detected on the local segment only when a station detects a signal on the RX pair at the same time it is sending on the TX pair. Since the two signals are on different pairs, there is no characteristic change in the signal. Collisions are only recognized on UTP when the station is operating in half-duplex. The only functional difference between half and full-duplex operation in this regard is whether or not the transmit and receive pairs are permitted to be used simultaneously.

Remote collision
A remote collision, in CSMA/CD computer networks over half-duplex media (10BASE5 or 10BASE2), is a collision that occurs when a frame shorter than the minimum length is transmitted. This frame may cause a collision at the remote end which cannot be detected by the transmitter, so the frame is not resent on the physical layer. Due to interference on the medium, its data is corrupted and frame check sequence fails, requiring recovery at a higher layer, if possible.

Channel capture effect
The channel capture effect is a phenomenon where one user of a shared medium "captures" the medium for a significant time. During this period (usually 16 frames), other users are denied use of the medium. This effect was first seen in networks using CSMA/CD on Ethernet. Because of this effect, the most data-intense connection dominates the multiple-access wireless channel. This happens in Ethernet links because of the way nodes "back off" from the link and attempt to re-access it. In the Ethernet protocol, when a communication collision happens (when two users of the medium try to send at the same time), each user waits for a random period of time before re-accessing the link. However, a user will wait ("back off") for a random amount of time proportional to the number of times it has successively tried to access the link. The channel capture effect happens when one user continues to "win" the link.

For example, user A and user B both try to access a quiet link at the same time. Since they detect a collision, user A waits for a random time between 0 and 1 time units and so does user B. Let's say user A chooses a lower back-off time. User A then begins to use the link and B allows it to finish sending its frame. If user A still has more to send, then user A and user B will cause another data collision. A will once again choose a random back-off time between 0 and 1, but user B will choose a back-off time between 0 and 3 – because this is B's second time colliding in a row. Chances are A will "win" this one again. If this continues, A will most likely win all the collision battles, and after 16 collisions (the number of tries before a user backs down for an extended period of time), user A will have "captured" the channel.

The ability of one node to capture the entire medium is decreased as the number of nodes increases. This is because as the number of nodes increases, there is a higher probability that one of the "other" nodes will have a lower back-off time than the capturing node.

The channel capture effect creates a situation where one station is able to transmit while others are continually backing off, thus leading to a situation of short-term unfairness. Yet, the situation is long-term fair because every station has the opportunity to "capture" the medium once one station is done transmitting. The efficiency of the channel is increased when one node has captured the channel.

A negative side effect of the capture effect would be the idle time created due to stations backing off. Once one station is finished transmitting on the medium, large idle times are present because all other stations were continually backing off. In some instances, back-off can occur for so long that some stations actually discard packets because maximum attempt limits have been reached.

Applications
CSMA/CD was used in now-obsolete shared-medium Ethernet variants (10BASE5, 10BASE2), and in the early versions of twisted-pair Ethernet, which used repeater hubs. Modern Ethernet networks, built with switches and full-duplex connections, no longer need to use CSMA/CD, because each Ethernet segment, or collision domain, is now isolated. CSMA/CD is still supported for backwards compatibility and for half-duplex connections. The IEEE 802.3 standard, which defines all Ethernet variants, for historical reasons still bore the title "Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications" until 802.3-2008, which uses new name "IEEE Standard for Ethernet".


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Carrier shift

In telecommunication, the term carrier shift has the following meanings:

 * 1) In the transmission of binary or teletypewriter signals, keying in which the frequency of the carrier signal is shifted in one direction for marking signals and in the opposite direction for spacing signals.
 * 2) In amplitude modulation, a condition that results from imperfect modulation in which the positive and negative excursions of the modulating envelope are unequal in amplitude.

Note 1: The carrier shift results in a change in carrier power.

Note 2: The carrier shift may be a shift to a higher or to a lower frequency.

title: Carrier shift


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Carrier system

A carrier system is a telecommunications system that transmits information, such as the voice signals of a telephone call and the video signals of television, by modulation of one or multiple carrier signals above the principal voice frequency or data rate.

Carrier systems typically transmit multiple channels of communication simultaneously over the shared medium using various forms of multiplexing. Prominent multiplexing methods of the carrier signal are time-division multiplexing (TDM) and frequency-division multiplexing (FDM). A cable television system is an example of frequency-division multiplexing. Many television programs are carried simultaneously on the same coaxial cable by sending each at a different frequency. Multiple layers of multiplexing may ultimately be performed upon a given input signal. For example, in the public switched telephone network, many telephone calls are sent over shared trunk lines by time-division multiplexing. For long-distance calls several of these channels may be sent over a communications satellite link by frequency-division multiplexing. At a given receiving node, specific channels may be demultiplexed individually.

History
The purpose of carrier systems is to save money by carrying more traffic on less infrastructure. 19th century telephone systems, operating at baseband, could only carry one telephone call on each wire, hence routes with heavy traffic needed many wires.

In the 1920s, frequency-division multiplexing could carry several circuits on the same balanced wires, and by the 1930s L-carrier and similar systems carried hundreds of calls at a time on coaxial cables.

Capacity of these systems increased in the middle of the century, while in the 1950s researchers began to take seriously the possibility of saving money on the terminal equipment by using time-division multiplexing. This work led to T-carrier and similar digital systems for local use.

Due to the shorter repeater spacings required by digital systems, long-distance still used FDM until the late 1970s when optical fiber was improved to the point that digital connections became the cheapest ones for all distances, short and long. By the end of the century, analog connections between and within telephone exchanges became rare.

title: Carrier system


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Carson bandwidth rule

In telecommunication, Carson's bandwidth rule defines the approximate bandwidth requirements of communications system components for a carrier signal that is frequency modulated by a continuous or broad spectrum of frequencies rather than a single frequency. Carson's rule does not apply well when the modulating signal contains discontinuities, such as a square wave. Carson's rule originates from John Renshaw Carson's 1922 paper.

Carson's bandwidth rule is expressed by the relation:
 * $$CBR = 2 (\Delta f + f_m)$$
 * where:
 * $$CBR$$ is the bandwidth requirement;
 * $$\Delta f$$ is the peak frequency deviation;
 * $$f_m$$ is the highest frequency in the modulating signal.

For example, a typical VHF/UHF two-way radio signal using FM mode, with 5 kHz peak deviation, and a maximum audio frequency of 3 kHz, would require an approximate bandwidth of 2 × (5 kHz + 3 kHz)= 16 kHz.

Standard broadcast stereo FM, with a peak deviation of 75 kHz, has a highest modulating frequency (which combines L + R and L − R) of 53 kHz. Most of the energy therefore falls within an approximate bandwidth of 2 × (75 + 53)= 256 kHz. (Geographically close FM broadcast transmitters are almost always assigned nominal center frequencies at least 400 kHz apart).

Carson's bandwidth rule is often applied to transmitters, antennas, optical sources, receivers, photodetectors, and other communications system components.

Any frequency modulated signal will have an infinite number of sidebands and hence an infinite bandwidth but, in practice, all significant sideband energy (98% or more) is concentrated within the bandwidth defined by Carson's rule. It is a useful approximation, but setting the arbitrary definition of occupied bandwidth at 98% of the power still means that the power outside the band is about $$10\log\left(\frac{0.98}{0.02}\right) \approx 17\;\mathrm{dB}$$ less than the carrier inside, therefore Carson's Rule is of little use in spectrum planning.

title: Carson bandwidth rule


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Cassegrain antenna

In telecommunications and radar, a Cassegrain antenna is a parabolic antenna in which the feed antenna is mounted at or behind the surface of the concave main parabolic reflector dish and is aimed at a smaller convex secondary reflector suspended in front of the primary reflector. The beam of radio waves from the feed illuminates the secondary reflector, which reflects it back to the main reflector dish, which reflects it forward again to form the desired beam. The Cassegrain design is widely used in parabolic antennas, particularly in large antennas such as those in satellite ground stations, radio telescopes, and communication satellites.

Geometry
The primary reflector is a paraboloid, while the shape of the convex secondary reflector is a hyperboloid. The geometrical condition for radiating a collimated, plane wave beam is that the feed antenna is located at the far focus of the hyperboloid, while the focus of the primary reflector coincides with the near focus of the hyperboloid. Usually the secondary reflector and the feed antenna are located on the central axis of the dish. However, in offset Cassegrain configurations, the primary dish reflector is asymmetric, and its focus, and the secondary reflector, are located to one side of the dish, so that the secondary reflector does not partially obstruct the beam.

Advantages
This design is an alternative to the most common parabolic antenna design, called "front feed" or "prime focus", in which the feed antenna itself is mounted suspended in front of the dish at the focus, pointed back toward the dish. The Cassegrain is a more complex design, but in certain applications it has advantages over front feed that can justify its increased complexity:
 * The feed antennas and associated waveguides and "front end" electronics can be located on or behind the dish, rather than suspended in front where they block part of the outgoing beam.  Therefore, this design is used for antennas with bulky or complicated feeds, such as satellite communication ground antennas, radio telescopes, and the antennas on some communication satellites.
 * Another advantage, important in satellite ground antennas and radio telescopes, is that because the feed antenna is directed forward, rather than backward toward the dish as in a front-fed antenna, the spillover sidelobes caused by portions of the beam that miss the secondary reflector are directed upwards toward the cold sky rather than downwards towards the warm earth. In receiving antennas this reduces reception of ground noise, resulting in a lower antenna noise temperature.
 * Dual reflector shaping: The presence of a second reflecting surface in the signal path allows additional opportunities for tailoring the radiation pattern for maximum performance. For example, the gain of ordinary parabolic antennas is reduced because the radiation of the feed antenna falls off toward the outer parts of the dish, resulting in lower "illumination" of those parts. In "dual reflector shaping" the shape of the secondary reflector is altered to direct more signal power to outer areas of the dish, resulting in more uniform illumination of the primary, to maximize the gain. However, this results in a secondary that is no longer precisely hyperbolic (though it is still very close), so the constant phase property is lost. This phase error, however, can be compensated for by slightly tweaking the shape of the primary mirror.   The result is a higher gain, or gain/spillover ratio, at the cost of surfaces that are trickier to fabricate and test.   Other dish illumination patterns can also be synthesized, such as patterns with high taper at the dish edge for ultra-low spillover sidelobes, and patterns with a central "hole" to reduce feed shadowing.
 * Another reason for using the Cassegrain design is to increase the focal length of the antenna, to reduce sidelobes, among other advantages. Parabolic reflectors used in dish antennas have a large curvature and short focal length; the focal point is located near the mouth of the dish, to reduce the length of the supports required to hold the feed structure or secondary reflector. The focal ratio (f-number, the ratio of the focal length to the dish diameter) of typical parabolic antennas is 0.25–0.8, compared to 3–8 for parabolic mirrors used in optical systems such as telescopes. In a front-fed antenna, a "flatter" parabolic dish with a long focal length would require an impractically elaborate support structure to hold the feed rigid with respect to the dish. However, the drawback of this small focal ratio is that the antenna is sensitive to small deviations from the focal point: the angular width that it can effectively focus is small. Modern parabolic antennas in radio telescopes and communications satellites often use arrays of feedhorns clustered around the focal point, to create a particular beam pattern. These require the good off-axis focusing characteristics of a large focal ratio, and because the convex secondary reflector of the Cassegrain antenna increases it significantly, these antennas typically use a Cassegrain design.
 * The longer focal length also improves crosspolarization discrimination of off-axis feeds, important in satellite antennas that use the two orthogonal polarization modes to transmit separate channels of information.

A disadvantage of the Cassegrain is that the feed horn(s) must have a narrower beamwidth (higher gain) to focus its radiation on the smaller secondary reflector, instead of the wider primary reflector as in front-fed dishes. The angular width the secondary reflector subtends at the feed horn is typically 10–15°, as opposed to 120–180° the main reflector subtends in a front-fed dish. Therefore, the feed horn must be longer for a given wavelength.

Beam waveguide antenna
A beam waveguide antenna is a type of complicated Cassegrain antenna with a long radio wave path to allow the feed electronics to be located at ground level. It is used in very large steerable radio telescopes and satellite ground antennas, where the feed electronics are too complicated and bulky, or requires too much maintenance and alterations, to locate on the dish; for example those using cryogenically cooled amplifiers. The beam of incoming radio waves from the secondary reflector is reflected by additional mirrors in a long twisting path through the axes of the altazimuth mount, so the antenna can be steered without interrupting the beam, and then down through the antenna tower to a feed building at ground level.

History
The Cassegrain antenna design was adapted from the Cassegrain telescope, a type of reflecting telescope developed around 1672 and attributed to French Province England priest Laurent Cassegrain. The first Cassegrain antenna was invented and patented by Cochrane and Whitehead at Elliot Bros in Borehamwood, England, in 1952. The patent, British Patent Number 700868, was subsequently challenged in court, but prevailed.

title: Cassegrain antenna


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Cell relay

In computer networking, cell relay refers to a method of statistically multiplexing small fixed-length packets, called "cells", to transport data between computers or kinds of network equipment. It is a reliable, connection-oriented packet switched data communications protocol.

Transmission Rates
Cell relay transmission rates usually are between 56 kbit/s and several gigabits per second. ATM, a particularly popular form of cell relay, is most commonly used for home DSL connections, which often runs between 128 kbit/s and 1.544 Mbit/s (DS1), and for high-speed backbone connections (OC-3 and faster).

Cell relay protocols have neither flow control nor error correction capability, are information-content independent, and correspond only to layers one and two of the OSI Reference Model.

Cell relay can be used for delay- and jitter-sensitive traffic such as voice and video.

How Cell Relay Works
Cell relay systems break variable-length user packets into groups of fixed-length cells, that add addressing and verification information. Frame length is fixed in networking hardware, based on time delay and user packet-length considerations. One user data message may be segmented over many cells.

Cell relay systems may also carry bitstream-based data such as PDH traffic, by breaking it into streams of cells, with a lightweight synchronization and clock recovery shim. Thus cell relay systems may potentially carry any combination of stream-based and packet-based data. This is a form of statistical time division multiplexing.

Cell relay is an implementation of fast packet-switching technology that is used in connection-oriented broadband integrated services digital networks (B-ISDN, and its better-known supporting technology ATM) and connectionless IEEE 802.6 switched multi-megabit data service (SMDS).

At any time there is information to be transmitted; the switch basically sends the data units. Connections don't have to be negotiated like circuit switching. Channels don't have to be allocated because channels do not exist in ATM, and on condition that there is an adequate amount of bandwidth to maintain it, there can be indefinite transmissions over the same facility.

Cell relay utilizes data cells of a persistent size. Frames are comparable to data packets; however they contrast from cells in that they may fluctuate in size based on circumstances. This type of technology is not secure for the reason that its procedures do not support error handling or data recovery. Per se, all delicate and significant transmissions may perhaps be transported faster via fixed-sized cells, which are simpler to transmit compared to variable-sized frames or packets.

Reliability
Cell relay is extremely reliable for transporting vital data. Switching devices give the precise method to cells as each endpoint address embedded in a cell. An example of cell relay is ATM, a prevalent form utilized to transfer a cell with a fixed size of 53 bytes.

title: Cell relay