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Digital subscriber line

Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric digital subscriber line (ADSL), the most commonly installed DSL technology, for Internet access.

In ADSL, the data throughput in the upstream direction (the direction to the service provider) is lower, hence the designation of asymmetric service. In symmetric digital subscriber line (SDSL) services, the downstream and upstream data rates are equal.

DSL service can be delivered simultaneously with wired telephone service on the same telephone line since DSL uses higher frequency bands for data transmission. On the customer premises, a DSL filter is installed on each telephone to prevent undesirable interaction between DSL and telephone service.

The bit rate of consumer ADSL services typically ranges from 256 kbit/s up to 25 Mbit/s, while the later VDSL+ technology delivers between 16 Mbit/s and 250 Mbit/s in the direction to the customer (downstream), with up to 40 Mbit/s upstream. The exact performance is depending on technology, line conditions, and service-level implementation. Researchers at Bell Labs have reached SDSL speeds over 1 Gbit/s using traditional copper telephone lines, though such speeds have not been made available for the end customers yet.

History
It was originally thought that it was not possible to operate a conventional phone line beyond low-speed limits (typically under 9600 bit/s). In the 1950s, ordinary twisted-pair telephone cable often carried four megahertz (MHz) television signals between studios, suggesting that such lines would allow transmitting many megabits per second. One such circuit in the United Kingdom ran some 10 mi between the BBC studios in Newcastle-upon-Tyne and the Pontop Pike transmitting station. However, these cables had other impairments besides Gaussian noise, preventing such rates from becoming practical in the field. The 1980s saw the development of techniques for broadband communications that allowed the limit to be greatly extended. A patent was filed in 1979 for the use of existing telephone wires for both telephones and data terminals that were connected to a remote computer via a digital data carrier system.

The motivation for digital subscriber line technology was the Integrated Services Digital Network (ISDN) specification proposed in 1984 by the CCITT (now ITU-T) as part of Recommendation I.120, later reused as ISDN digital subscriber line (IDSL). Employees at Bellcore (now Telcordia Technologies) developed asymmetric digital subscriber line (ADSL) by placing wide-band digital signals at frequencies above the existing baseband analog voice signal carried on conventional twisted pair cabling between telephone exchanges and customers. A patent was filed by AT&T Bell Labs on the basic DSL concept in 1988.

Joseph W. Lechleider's contribution to DSL was his insight that an asymmetric arrangement offered more than double the bandwidth capacity of symmetric DSL. This allowed Internet service providers to offer efficient service to consumers, who benefited greatly from the ability to download large amounts of data but rarely needed to upload comparable amounts. ADSL supports two modes of transport: fast channel and interleaved channel. Fast channel is preferred for streaming multimedia, where an occasional dropped bit is acceptable, but lags are less so. Interleaved channel works better for file transfers, where the delivered data must be error-free but latency (time delay) incurred by the retransmission of error-containing packets is acceptable.

Consumer-oriented ADSL was designed to operate on existing lines already conditioned for Basic Rate Interface ISDN services. Engineers developed high speed DSL facilities such as high bit rate digital subscriber line (HDSL) and symmetric digital subscriber line (SDSL) to provision traditional Digital Signal 1 (DS1) services over standard copper pair facilities.

Older ADSL standards delivered 8 Mbit/s to the customer over about 2 km of unshielded twisted-pair copper wire. Newer variants improved these rates. Distances greater than 2 km significantly reduce the bandwidth usable on the wires, thus reducing the data rate. But ADSL loop extenders increase these distances by repeating the signal, allowing the local exchange carrier (LEC) to deliver DSL speeds to any distance.

Until the late 1990s, the cost of digital signal processors for DSL was prohibitive. All types of DSL employ highly complex digital signal processing algorithms to overcome the inherent limitations of the existing twisted pair wires. Due to the advancements of very-large-scale integration (VLSI) technology, the cost of the equipment associated with a DSL deployment lowered significantly. The two main pieces of equipment are a digital subscriber line access multiplexer (DSLAM) at one end and a DSL modem at the other end.

A DSL connection can be deployed over existing cable. Such deployment, even including equipment, is much cheaper than installing a new, high-bandwidth fiber-optic cable over the same route and distance. This is true both for ADSL and SDSL variations. The commercial success of DSL and similar technologies largely reflects the advances made in electronics over the decades that have increased performance and reduced costs even while digging trenches in the ground for new cables (copper or fiber optic) remains expensive.

These advantages made ADSL a better proposition for customers requiring Internet access than metered dial up, while also allowing voice calls to be received at the same time as a data connection. Telephone companies were also under pressure to move to ADSL owing to competition from cable companies, which use DOCSIS cable modem technology to achieve similar speeds. Demand for high bandwidth applications, such as video and file sharing, also contributed to the popularity of ADSL technology. Some of the first field trials for DSL were carried out in 1996.

Early DSL service required a dedicated dry loop, but when the U.S. Federal Communications Commission (FCC) required incumbent local exchange carriers (ILECs) to lease their lines to competing DSL service providers, shared-line DSL became available. Also known as DSL over unbundled network element, this unbundling of services allows a single subscriber to receive two separate services from two separate providers on one cable pair. The DSL service provider's equipment is co-located in the same telephone exchange as that of the ILEC supplying the customer's pre-existing voice service. The subscriber's circuit is rewired to interface with hardware supplied by the ILEC which combines a DSL frequency and POTS signals on a single copper pair.

Since 1999, some ISPs started to offer microfilters, which were installed indoors and performed the same function as DSL splitters which were installed outdoors, which was to separate ADSL frequencies from POTS frequencies used for phone calls. These filters originated out of a desire to make self-installation of DSL service possible, and eliminate early outdoor DSL splitters which were installed at or near the demarcation point between the customer and the ISP.

By 2012, some carriers in the United States reported that DSL remote terminals with fiber backhaul were replacing older ADSL systems.

Operation
Telephones are connected to the telephone exchange via a local loop, which is a physical pair of wires. The local loop was originally intended mostly for the transmission of speech, encompassing an audio frequency range of 300 to 3400 hertz (commercial bandwidth). However, as long-distance trunks were gradually converted from analog to digital operation, the idea of being able to pass data through the local loop (by using frequencies above the voiceband) took hold, ultimately leading to DSL.

The local loop connecting the telephone exchange to most subscribers has the capability of carrying frequencies well beyond the 3400 Hz upper limit of POTS. Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unused bandwidth of the local loop by creating 4312.5 Hz wide channels starting between 10 and 100 kHz, depending on how the system is configured. Allocation of channels continues to higher frequencies (up to 1.1 MHz for ADSL) until new channels are deemed unusable. Each channel is evaluated for usability in much the same way an analog modem would on a POTS connection. More usable channels equate to more available bandwidth, which is why distance and line quality are a factor (the higher frequencies used by DSL travel only short distances).

The pool of usable channels is then split into two different frequency bands for upstream and downstream traffic, based on a preconfigured ratio. This segregation reduces interference. Once the channel groups have been established, the individual channels are bonded into a pair of virtual circuits, one in each direction. Like analog modems, DSL transceivers constantly monitor the quality of each channel and will add or remove them from service depending on whether they are usable. Once upstream and downstream circuits are established, a subscriber can connect to a service such as an Internet service provider or other network services, like a corporate MPLS network.

The underlying technology of transport across DSL facilities uses modulation of high-frequency carrier waves, an analog signal transmission. A DSL circuit terminates at each end in a modem which modulates patterns of bits into certain high-frequency impulses for transmission to the opposing modem. Signals received from the far-end modem are demodulated to yield a corresponding bit pattern that the modem passes on, in digital form, to its interfaced equipment, such as a computer, router, switch, etc.

Unlike traditional dial-up modems, which modulate bits into signals in the 300–3400 Hz audio baseband, DSL modems modulate frequencies from 4000 Hz to as high as 4 MHz. This frequency band separation enables DSL service and plain old telephone service (POTS) to coexist on the same cables, known as voice grade cables. On the subscriber's end of the circuit, inline DSL filters are installed on each telephone to pass voice frequencies but filter the high-frequency signals that would otherwise be heard as hiss. Also, nonlinear elements in the phone could otherwise generate audible intermodulation and may impair the operation of the data modem in the absence of these low-pass filters. DSL and RADSL modulations do not use the voice-frequency band so high-pass filters are incorporated in the circuitry of DSL modems filter out voice frequencies.

Because DSL operates above the 3.4 kHz voice limit, it cannot pass through a loading coil, which is an inductive coil that is designed to counteract loss caused by shunt capacitance (capacitance between the two wires of the twisted pair). Loading coils are commonly set at regular intervals in POTS lines. Voice service cannot be maintained past a certain distance without such coils. Therefore, some areas that are within range for DSL service are disqualified from eligibility because of loading coil placement. Because of this, phone companies endeavor to remove loading coils on copper loops that can operate without them. Longer lines that require them can be replaced with fiber to the neighborhood or node (FTTN).

Most residential and small-office DSL implementations reserve low frequencies for POTS, so that (with suitable filters and/or splitters) the existing voice service continues to operate independently of the DSL service. Thus POTS-based communications, including fax machines and dial-up modems, can share the wires with DSL. Only one DSL modem can use the subscriber line at a time. The standard way to let multiple computers share a DSL connection uses a router that establishes a connection between the DSL modem and a local Ethernet, powerline, or Wi-Fi network on the customer's premises.

The theoretical foundations of DSL, like much of communication technology, can be traced back to Claude Shannon's seminal 1948 paper, "A Mathematical Theory of Communication". Generally, higher bit rate transmissions require a wider frequency band, though the ratio of bit rate to symbol rate and thus to bandwidth are not linear due to significant innovations in digital signal processing and digital modulation methods.

Naked DSL
Naked DSL is a way of providing only DSL services over a local loop. It is useful when the customer does not need the traditional telephony voice service because voice service is received either on top of the DSL services (usually VoIP) or through another network (E.g., mobile telephony). It is also commonly called an unbundled network element (UNE) in the United States; in Australia it is known as a unconditioned local loop (ULL); in Belgium it is known as "raw copper" and in the UK it is known as Single Order GEA (SoGEA).

It started making a comeback in the United States in 2004 when Qwest started offering it, closely followed by Speakeasy. As a result of AT&T's merger with SBC, and Verizon's merger with MCI, those telephone companies have an obligation to offer naked DSL to consumers.

Typical setup
On the customer side, a DSL modem is hooked up to a phone line. The telephone company connects the other end of the line to a DSLAM, which concentrates a large number of individual DSL connections into a single box. The DSLAM cannot be located too far from the customer because of attenuation between the DSLAM and the user's DSL modem. It is common for a few residential blocks to be connected to one DSLAM.

The above figure is a schematic of a simple DSL connection (in blue). The right side shows a DSLAM residing in the telephone company's telephone exchange. The left side shows the customer premises equipment with an optional router. The router manages a local area network which connects PCs and other local devices. The customer may opt for a modem that contains both a router and wireless access. This option (within the dashed bubble) often simplifies the connection.

Exchange equipment
At the exchange, a digital subscriber line access multiplexer (DSLAM) terminates the DSL circuits and aggregates them, where they are handed off to other networking transports. The DSLAM terminates all connections and recovers the original digital information. In the case of ADSL, the voice component is also separated at this step, either by a filter or splitter integrated in the DSLAM or by specialized filtering equipment installed before it. Load coils in phone lines, used for extending their range in rural areas, must be removed to allow DSL to operate as they only allow frequencies of up to 4000 Hz to pass through phone cables.

Customer equipment

The customer end of the connection consists of a DSL modem. This converts data between the digital signals used by computers and the analog voltage signal of a suitable frequency range which is then applied to the phone line.

In some DSL variations (for example, HDSL), the modem connects directly to the computer via a serial interface, using protocols such as Ethernet or V.35. In other cases (particularly ADSL), it is common for the customer equipment to be integrated with higher-level functionality, such as routing, firewalling, or other application-specific hardware and software. In this case, the equipment is referred to as a gateway.

Most DSL technologies require the installation of appropriate DSL filters at the customer's premises to separate the DSL signal from the low-frequency voice signal. The separation can take place either at the demarcation point, or with filters installed at the telephone outlets inside the customer premises. It is possible for a DSL gateway to integrate the filter, and allow telephones to connect through the gateway.

Modern DSL gateways often integrate routing and other functionality. The system boots, synchronizes the DSL connection and finally establishes the internet IP services and connection between the local network and the service provider, using protocols such as DHCP or PPPoE.

Protocols and configurations
Many DSL technologies implement an Asynchronous Transfer Mode (ATM) layer over the low-level bitstream layer to enable the adaptation of a number of different technologies over the same link.

DSL implementations may create bridged or routed networks. In a bridged configuration, the group of subscriber computers effectively connect into a single subnetwork. The earliest implementations used DHCP to provide the IP address to the subscriber equipment, with authentication via MAC address or an assigned hostname. Later implementations often use Point-to-Point Protocol (PPP) to authenticate with a user ID and password.

Transmission modulation methods
Transmission methods vary by market, region, carrier, and equipment.
 * Discrete multitone modulation (DMT), the most common kind, also known as Orthogonal frequency-division multiplexing (OFDM)
 * Trellis-coded pulse-amplitude modulation (TC-PAM), used for HDSL2 and SHDSL
 * Carrierless amplitude phase modulation (CAP), deprecated in 1996 for ADSL, used for HDSL
 * Two-binary, one-quaternary (2B1Q), used for IDSL and HDSL

DSL technologies
DSL technologies (sometimes collectively summarized as xDSL) include:
 * Symmetric digital subscriber line (SDSL), umbrella term for xDSL where the bitrate is equal in both directions.
 * ISDN digital subscriber line (IDSL), ISDN-based technology that provides a bitrate equivalent to two ISDN bearer and one data channel, 144 kbit/s symmetric over one pair
 * High-bit-rate digital subscriber line (HDSL), ITU-T G.991.1, the first DSL technology that used a higher frequency spectrum than ISDN, 1,544 kbit/s and 2,048 kbit/s symmetric services, either on 2 or 3 pairs at 784 kbit/s each, 2 pairs at 1,168 kbit/s each, or one pair at 2,320 kbit/s
 * High-bit-rate digital subscriber line 2/4 (HDSL2, HDSL4), ANSI, 1,544 kbit/s symmetric over one pair (HDSL2) or two pairs (HDSL4)
 * Symmetric digital subscriber line (SDSL), specific proprietary technology, up to 1,544 kbit/s symmetric over one pair
 * Single-pair high-speed digital subscriber line (G.SHDSL), ITU-T G.991.2, standardized successor of HDSL and proprietary SDSL, up to 5,696 kbit/s per pair, up to four pairs
 * Asymmetric digital subscriber line (ADSL), umbrella term for xDSL where the bitrate is greater in one direction than the other.
 * ANSI T1.413 Issue 2, up to 8 Mbit/s and 1 Mbit/s
 * G.dmt, ITU-T G.992.1, up to 10 Mbit/s and 1 Mbit/s
 * G.lite, ITU-T G.992.2, more noise and attenuation resistant than G.dmt, up to 1,536 kbit/s and 512 kbit/s
 * Asymmetric digital subscriber line 2 (ADSL2), ITU-T G.992.3, up to 12 Mbit/s and 3.5 Mbit/s
 * Asymmetric digital subscriber line 2 plus (ADSL2+), ITU-T G.992.5, up to 24 Mbit/s and 3.5 Mbit/s
 * Very-high-bit-rate digital subscriber line (VDSL), ITU-T G.993.1, up to 52 Mbit/s and 16 Mbit/s
 * Very-high-bit-rate digital subscriber line 2 (VDSL2), ITU-T G.993.2, an improved version of VDSL, compatible with ADSL2+, sum of both directions up to 200 Mbit/s. G.vector crosstalk cancelling feature (ITU-T G.993.5) can be used to increase range at a given bitrate, e.g. 100 Mbit/s at up to 500 meters.
 * G.fast, ITU-T G.9700 and G.9701, up to approximately 1 Gbit/s aggregate uplink and downlink at 100m. Approved in December 2014, deployments planned for 2016.
 * XG-FAST, allows for up to 10 Gbps on copper twisted pair lines, but only for lengths up to 30 meters. Real-world tests have shown 8 Gbps on 30-meter long twisted pair lines.
 * Bonded DSL Rings (DSL Rings), a shared ring topology at 400 Mbit/s
 * Cable/DSL gateway
 * Etherloop Ethernet local loop
 * High-speed voice and data link
 * Rate-Adaptive Digital Subscriber Line (RADSL), designed to increase range and noise tolerance by sacrificing upstream speed
 * Uni-DSL (Uni digital subscriber line or UDSL), technology developed by Texas Instruments, backwards compatible with all DMT standards
 * Hybrid Access Networks combine existing xDSL deployments with a wireless network such as LTE to increase bandwidth and quality of experience by balancing the traffic over the two access networks.

The line-length limitations from telephone exchange to subscriber impose severe limits on data transmission rates. Technologies such as VDSL provide very high-speed but short-range links. VDSL is used as a method of delivering triple play services (typically implemented in fiber to the curb network architectures).

Terabit DSL, is a technology that proposes the use of the space between the dielectrics (insulators) on copper twisted pair lines in telephone cables, as waveguides for 300 GHz signals that can offer speeds of up to 1 terabit per second at distances of up to 100 meters, 100 gigabits per second for 300 meters, and 10 gigabits per second for 500 meters. The first experiment for this was carried out with copper lines that were parallel to each other, and not twisted, inside a metal pipe meant to simulate the metal armoring in large telephone cables.

title: Digital subscriber line


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Digital transmission group

In telecommunication, a digital transmission group is a group of digitized voice or data channels or both with bit streams that are combined into a single digital bit stream for transmission over communications media.

Digital transmission groups usually are categorized by their maximum capacity, not by a specific number of channels. However, the maximum digital transmission group capacity must be equal to or greater than the sum of the individual multiplexer input channel capacities.

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

title: Digital transmission group


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Digroup

In the mathematical subject of algebra, a digroup is a generalization of a group that has two one-sided product operations, $$\vdash$$ and $$\dashv$$, instead of the single operation in a group. Digroups were introduced independently by Felipe (2006), Kinyon (2007), and Liu (2004).

To explain digroups, consider a group. In a group there is one operation, such as addition in the set of integers; there is a single "unit" element, like 0 in the integers, and there are inverses, like $$-x$$ in the integers, for which both the following equations hold: $$(-x)+x=0$$ and $$x+(-x)=0$$. A digroup replaces the one operation by two operations that interact in a complicated way, as stated below. A digroup may also have more than one "unit", and an element $$x$$ may have different inverses for each "unit". This makes a digroup vastly more complicated than a group. Despite that complexity, there are reasons to consider digroups, for which see the references.

Definition
A digroup is a set D with two binary operations, $$\vdash$$ and $$\dashv$$, that satisfy the following laws (e.g., Ongay 2010):
 * Associativity:
 * $$\vdash$$ and $$\dashv$$ are associative,
 * $$(x \vdash y) \vdash z = (x \dashv y) \vdash z,$$
 * $$x \dashv (y \dashv z) = x \dashv (y \vdash z),$$
 * $$(x \vdash y) \dashv z = x \vdash (y \dashv z).$$

 * Bar units: There is at least one bar unit, an $$e \in D$$, such that for every $$ x \in D,$$
 * $$e \vdash x = x \dashv e = x.$$
 * The set of bar units is called the halo of D.

 * Inverse: For each bar unit e, each $$ x \in D$$ has a unique e-inverse, $$x_e^{-1} \in D$$, such that
 * $$x \vdash x_e^{-1} = x_e^{-1} \dashv x = e.$$

Generalization
A generalized digroup or g-digroup is a generalization due to Salazar-Díaz, Velásquez, and Wills-Toro (2016), in which each element has a left inverse and a right inverse instead of one two-sided inverse.

title: Digroup


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Direct access

Direct access may refer to:
 * DirectAccess, a network technology in Windows 7 and Windows Server 2008 R2, and Windows 8 and Windows Server 2012
 * Direct access (computing), a concept in computer science
 * Direct Access Archive, a proprietary file format
 * Direct access storage device, a secondary computer storage device
 * Direct Access Test Unit, special numbers used to test telephone exchanges
 * Direct access trading, a technology for stock trading

title: Direct access


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Direct connect

Direct connect may refer to:

 * Direct Connect (protocol), a file sharing client and protocol
 * A protocol used by the program AOL Instant Messenger
 * Sprint Direct Connect, a brand name used by Sprint Corporation for its digital push-to-talk service, similar to a walkie-talkie
 * Direct Connect is an Australian company related to Lumo Energy

title: Direct connect


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Direct distance dialing

Direct distance dialing (DDD) is a telecommunication service feature in North America by which a caller may, without operator assistance, call any other user outside the local calling area. Direct dialing by subscribers typically requires extra digits to be dialed as prefixes to the directory telephone number of the destination. International Direct Distance Dialing (IDDD) extends the system beyond the geographic boundaries of the North American Numbering Plan (NANP).

History
The first direct-dialed long-distance telephone calls were possible in the New Jersey communities of Englewood and Teaneck. Customers of the ENglewood 3, ENglewood 4 and TEaneck 7 exchanges, who could already dial telephone numbers in the New York City area, could place calls to eleven major cities across the United States by dialing the three-digit area code and the seven-digit directory number. Local telephone numbers still consisted of the first two letters of the central office name and five digits. On November 10, 1951, Englewood mayor M. Leslie Denning made the first customer-dialed long-distance call, to Mayor Frank Osborne of Alameda, California.

The destinations, and their area codes, equipped with a long-distance toll-switch at that time were:

 * 617: Boston, Massachusetts
 * 312: Chicago, Illinois
 * 216: Cleveland, Ohio
 * 313: Detroit, Michigan
 * 414: Milwaukee, Wisconsin
 * 415: Oakland, California
 * 215: Philadelphia, Pennsylvania
 * 412: Pittsburgh, Pennsylvania
 * 401: Providence, Rhode Island
 * 916: Sacramento, California
 * 318: San Francisco, California

Other areas could not yet be included in DDD as they did not have the necessary toll switching equipment, or because they still did not use a seven-digit local numbering plan. Montreal, Quebec, and Toronto, Ontario, in Canada, for example, had a mix of six- and seven-digit telephone numbers from 1951 to 1957, and did not have DDD until 1958. Whitehorse, Yukon, had seven-digit numbers starting in 1965, but the necessary switching equipment was not in place until 1972.

San Francisco required the special area code 318 due to temporary routing requirements. San Francisco and Oakland each had their own separate toll-switches, so calls had to be routed accordingly depending on the final destination. As the telephone equipment used at the time could only handle three-digit translation, the temporary use of area code 318 was required to distinguish between the two areas. Area code 318 was temporarily used to specify San Francisco and areas north of the Golden Gate, while calls with destinations in Oakland and the East Bay continued to use area code 415. When the electromechanical card-translator box became available sometime during 1952–53, six-digit translation became possible and the use of area code 318 was no longer required. Area code 318 was reclaimed for future use (now used as an area code for northern Louisiana), and the entire San Francisco Bay Area returned to using area code 415.

Hardware
The No. 4 Crossbar switching system had been introduced in the early 1940s to switch four-wire circuits and replace the incoming operator. With semiautomatic operation analogous to the early days of the panel switch, the operator in the originating city used a multifrequency keypad to dial an access code to connect to the correct city and to send the seven-digit number to incoming equipment at the terminating city. This design was further refined to serve DDD.

The card sorter of the 4A/CTS (Number 4A Crossbar / Card Translator System) allowed six-digit translation of the central office code number dialed by the customer. This determined the proper trunk circuits to use, where separate circuit groups were used for different cities in the same area code, as in the case of Oakland and San Francisco. The new device used metal cards similar in principle to computer punched cards, and they were rapidly scanned as they fell past a light beam. CTS machines were called 4A (Advanced) if the translator was included in the original installation, and 4M (Modified) if it was added later. A 1970s version of 4XB, the 4A/ETS, used a computer to translate. For international dialing, Traffic Service Position System (TSPS) provided the extra computer power.

The reach of DDD was limited due to the inefficiency and expense of switching equipment, and the limited ability to process records of completed calls. An early obstacle was that the majority of switching systems did not provide Automatic Number Identification (ANI). Common control switches, such as the 1XB switch, were fairly quickly retrofitted to provide ANI, and most 5XB switches were initially installed with ANI services. Panel switch were eventually retrofitted, as were some step-by-step systems that were not scheduled for immediate replacement. Even if a switch had ANI, it could not identify callers on party lines. This was only partly overcome by tip-party identification for two-party lines. As the cost of subscriber line carrier declined, party lines were gradually phased out.

As this and other improved technologies became available, as well as Automatic Message Accounting (AMA) computers to process the long-distance records into customer bills, the reach of DDD was slow in the 1950s, but quickened in the early 1960s. Electronic switching systems allowed electronic processing of the dialed digits, referring to electronic memories to determine call routing, and this has reached the state of the art, with digital telephone exchanges which are basically specialized computers that route voice traffic from one "peripheral" to another as digital data. Call routing can now be done based on the area code, central office code and even the first two digits of the line number, although routing based on digits past the central office code is usually limited to cases of competitive local exchange carriers, number pooling and number portability.

IDDD
In the 1960s, with the domestic conversion still underway, plans were laid to extend Direct Distance Dialing beyond North America (including a number of the Caribbean Islands). Some subscribers could already directly dial transatlantic telephone calls to certain destinations as early as in 1957 over the recently completed Atlantic cable to England. A new systematic extension of Direct Distance Dialing was developed and was introduced as International Direct Distance Dialing (IDDD) in March 1970.

With so much new equipment already working that could not handle more than the requisite ten-digit telephone numbers of DDD, the new system was based on designs by which most toll offices did not have to store and forward the whole international telephone number. Gateway offices were set up in New York, London and Paris, connected to the ordinary automatic toll network. The New York gateway was at 32 Avenue of the Americas. The new LT1 5XB switch on the tenth floor of 435 West 50th Street received new originating registers and outgoing senders able to handle fifteen-digit telephone numbers, with appropriate modifications to completing markers and other equipment. Other 5XB switches in the next few years were installed with IDDD as original equipment, and in the 1970s ESS offices also provided the service.

The key to the new system was two-stage multi-frequency pulsing. The outgoing sender sent its Class 4 toll center an off-hook signal as usual, received a wink as usual as a "proceed to send" signal, and outpulsed only a special three-digit (later six-digit) access code. The toll center picked a trunk through the long-distance network to the gateway office, which sent a second wink to the originating office, which then sent the whole dialed number. Thus the toll switching system needed no modification except at the gateway. The international trunks used Signaling System No. 5, a "North Atlantic" version of the North American multi-frequency signaling system, with minor modifications including slightly higher digit rate. European MF systems of the time used compelled signaling, which would slow down too much on a long transoceanic connection.

In the 1970s, toll centers were modified by adding the Traffic Service Position System (TSPS). With these new computers in place, digit storage in the toll system was no longer a problem. End offices were less extensively modified, and sent all their digits in a single stream. TSPS handled the gateway codes and other complexities of toll connections to the gateway office.

Equivalent service in the United Kingdom
In the United Kingdom and other parts of the Commonwealth of Nations, an equivalent service to direct distance dialing is subscriber trunk dialing (STD), and ISD for international subscriber trunk dialing. Queen Elizabeth II inaugurated STD on 5 December 1958, when she dialed a call from Bristol to Edinburgh and spoke to the Lord Provost.

title: Direct distance dialing


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Direct-sequence spread spectrum

In telecommunications, direct-sequence spread spectrum (DSSS) is a spread-spectrum modulation technique primarily used to reduce overall signal interference. The direct-sequence modulation makes the transmitted signal wider in bandwidth than the information bandwidth. After the despreading or removal of the direct-sequence modulation in the receiver, the information bandwidth is restored, while the unintentional and intentional interference is substantially reduced.

Swiss inventor, Gustav Guanella proposed a "means for and method of secret signals". With DSSS, the message symbols are modulated by a sequence of complex values known as spreading sequence. Each element of the spreading sequence, a so-called chip, has a shorter duration than the original message symbols. The modulation of the message symbols scrambles and spreads the signal in the spectrum, and thereby results in a bandwidth of the spreading sequence. The smaller the chip duration, the larger the bandwidth of the resulting DSSS signal; more bandwidth multiplexed to the message signal results in better resistance against narrowband interference.

Some practical and effective uses of DSSS include the code-division multiple access (CDMA) method, the IEEE 802.11b specification used in Wi-Fi networks, and the Global Positioning System.

Transmission method
Direct-sequence spread-spectrum transmissions multiply the symbol sequence being transmitted with a spreading sequence that has a higher rate than the original message rate. Usually, sequences are chosen such that the resulting spectrum is spectrally white. Knowledge of the same sequence is used to reconstruct the original data at the receiving end. This is commonly implemented by the element-wise multiplication with the spreading sequence, followed by summation over a message symbol period. This process, despreading, is mathematically a correlation of the transmitted spreading sequence with the spreading sequence. In an AWGN channel, the despreaded signal's signal-to-noise ratio is increased by the spreading factor, which is the ratio of the spreading-sequence rate to the data rate.

While a transmitted DSSS signal occupies a wider bandwidth than the direct modulation of the original signal would require, its spectrum can be restricted by conventional pulse-shape filtering.

If an undesired transmitter transmits on the same channel but with a different spreading sequence, the despreading process reduces the power of that signal. This effect is the basis for the code-division multiple access (CDMA) method of multi-user medium access, which allows multiple transmitters to share the same channel within the limits of the cross-correlation properties of their spreading sequences.

Benefits

 * Resistance to unintended or intended jamming
 * Sharing of a single channel among multiple users
 * Reduced signal/background-noise level hampers interception
 * Determination of relative timing between transmitter and receiver

Uses

 * The United States GPS, European Galileo and Russian GLONASS satellite navigation systems; earlier GLONASS used DSSS with a single spreading sequence in conjunction with FDMA, while later GLONASS used DSSS to achieve CDMA with multiple spreading sequences.
 * DS-CDMA (Direct-Sequence Code Division Multiple Access) is a multiple access scheme based on DSSS, by spreading the signals from/to different users with different codes. It is the most widely used type of CDMA.
 * Cordless phones operating in the 900 MHz, 2.4 GHz and 5.8 GHz bands
 * IEEE 802.11b 2.4 GHz Wi-Fi, and its predecessor 802.11-1999. (Their successor 802.11g uses both OFDM and DSSS)
 * Automatic meter reading
 * IEEE 802.15.4 (used, e.g., as PHY and MAC layer for Zigbee, or, as the physical layer for WirelessHART)
 * Radio-controlled model Automotive, Aeronautical and Marine vehicles
 * Spread spectrum radar for covertness and resistance to jamming and spoofing

title: Direct-sequence spread spectrum


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Disengagement originator

In telecommunication, a disengagement originator is the user or execution unit that initiates a disengagement attempt. The disengagement originator may be the originating user, the destination user, or the communications system. The communications system may deliberately originate the disengagement because of preemption, or inadvertently due to system malfunction.

title: Disengagement originator


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Dispersion-limited operation

A dispersion-limited operation is an operation of a communications link in which signal waveform degradation attributable to the dispersive effects of the communications medium is the dominant mechanism that limits link performance. The dispersion is the filter-like effect which a link has on the signal, due to the different propagation speeds of the eigenmodes of the link. Practically, this means that the waveform at the input will be different from the waveform at the output of the link.

Note that the amount of allowable degradation is dependent on the quality of the receiver. In fiber optic communications, dispersion-limited operation is often confused with distortion-limited operation.

title: Dispersion-limited operation


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Distortion

In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal representing sound or a video signal representing images, in an electronic device or communication channel.

Distortion is usually unwanted, and so engineers strive to eliminate or minimize it. In some situations, however, distortion may be desirable. For example, in noise reduction systems like the Dolby system, an audio signal is deliberately distorted in ways that emphasize aspects of the signal that are subject to electrical noise, then it is symmetrically "undistorted" after passing through a noisy communication channel, reducing the noise in the received signal. Distortion is also used as a musical effect, particularly with electric guitars.

The addition of noise or other outside signals (hum, interference) is not considered distortion, though the effects of quantization distortion are sometimes included in noise. Quality measures that reflect both noise and distortion include the signal-to-noise and distortion (SINAD) ratio and total harmonic distortion plus noise (THD+N).

Electronic signals
In telecommunication and signal processing, a noise-free system can be characterised by a transfer function, such that the output $$y(t)$$ can be written as a function of the input $$x$$ as

 * $$y(t) = F(x(t))$$

When the transfer function comprises only a perfect gain constant A and perfect delay T

 * $$y(t) = A\cdot x(t-T)$$

the output is undistorted. Distortion occurs when the transfer function F is more complicated than this. If F is a linear function, for instance a filter whose gain and/or delay varies with frequency, the signal suffers linear distortion. Linear distortion does not introduce new frequency components to a signal but does alter the balance of existing ones.

This diagram shows the behaviour of a signal (made up of a square wave followed by a sine wave) as it is passed through various distorting functions.

 * 1) The first trace (in black) shows the input.  It also shows the output from a non-distorting transfer function (straight line).
 * 2) A high-pass filter (green trace) distorts the shape of a square wave by reducing its low frequency components.  This is the cause of the "droop" seen on the top of the pulses. This "pulse distortion" can be very significant when a train of pulses must pass through an AC-coupled (high-pass filtered) amplifier. As the sine wave contains only one frequency, its shape is unaltered.
 * 3) A low-pass filter (blue trace) rounds the pulses by removing the high frequency components. All systems are low pass to some extent. Note that the phase of the sine wave is different for the lowpass and the highpass cases, due to the phase distortion of the filters.
 * 4) A slightly non-linear transfer function (purple), this one gently compresses the peaks of the sine wave, as may be typical of a tube audio amplifier. This generates small amounts of low order harmonics.
 * 5) A hard-clipping transfer function (red) generates high order harmonics. Parts of the transfer function are flat, which indicates that all information about the input signal has been lost in this region.

The transfer function of an ideal amplifier, with perfect gain and delay, is only an approximation. The true behavior of the system is usually different. Nonlinearities in the transfer function of an active device (such as vacuum tubes, transistors, and operational amplifiers) are a common source of non-linear distortion; in passive components (such as a coaxial cable or optical fiber), linear distortion can be caused by inhomogeneities, reflections, and so on in the propagation path.

Amplitude distortion
Amplitude distortion is distortion occurring in a system, subsystem, or device when the output amplitude is not a linear function of the input amplitude under specified conditions.

Harmonic distortion
Harmonic distortion adds overtones that are whole number multiples of a sound wave's frequencies. Nonlinearities that give rise to amplitude distortion in audio systems are most often measured in terms of the harmonics (overtones) added to a pure sinewave fed to the system. Harmonic distortion may be expressed in terms of the relative strength of individual components, in decibels, or the root mean square of all harmonic components: Total harmonic distortion (THD), as a percentage. The level at which harmonic distortion becomes audible depends on the exact nature of the distortion. Different types of distortion (like crossover distortion) are more audible than others (like soft clipping) even if the THD measurements are identical. Harmonic distortion in radio frequency applications is rarely expressed as THD.

Frequency response distortion
Non-flat frequency response is a form of distortion that occurs when different frequencies are amplified by different amounts in a filter. For example, the non-uniform frequency response curve of AC-coupled cascade amplifier is an example of frequency distortion. In the audio case, this is mainly caused by room acoustics, poor loudspeakers and microphones, long loudspeaker cables in combination with frequency dependent loudspeaker impedance, etc.

Phase distortion
This form of distortion mostly occurs due to electrical reactance. Here, all the components of the input signal are not amplified with the same phase shift, hence making some parts of the output signal out of phase with the rest of the output.

Group delay distortion
Can be found only in dispersive media. In a waveguide, phase velocity varies with frequency. In a filter, group delay tends to peak near the cut-off frequency, resulting in pulse distortion. When analog long distance trunks were commonplace, for example in 12 channel carrier, group delay distortion had to be corrected in repeaters.

Correction of distortion
As the system output is given by y(t) = F(x(t)), then if the inverse function F−1 can be found, and used intentionally to distort either the input or the output of the system, then the distortion is corrected.

An example of a similar correction is where LP/vinyl recordings or FM audio transmissions are deliberately pre-emphasised by a linear filter, the reproducing system applies an inverse filter to make the overall system undistorted.

Correction is not possible if the inverse does not exist—for instance if the transfer function has flat spots (the inverse would map multiple input points to a single output point). This produces an uncorrectable loss of information. Such a situation can occur when an amplifier is overdriven—causing clipping or slew rate distortion when, for a moment, the amplifier characteristics alone and not the input signal determine the output.

Cancellation of even-order harmonic distortion
Many symmetrical electronic circuits reduce the magnitude of even harmonics generated by the non-linearities of the amplifier's components, by combining two signals from opposite halves of the circuit where distortion components that are roughly the same magnitude but out of phase. Examples include push-pull amplifiers and long-tailed pairs.

Teletypewriter or modem signaling
In binary signaling such as FSK, distortion is the shifting of the significant instants of the signal pulses from their proper positions relative to the beginning of the start pulse. The magnitude of the distortion is expressed in percent of an ideal unit pulse length. This is sometimes called bias distortion.

Telegraphic distortion is a similar and older problem, distorting the ratio between mark and space intervals.

Audio distortion

With respect to audio, distortion refers to any kind of deformation of an output waveform compared to its input, usually clipping, harmonic distortion, or intermodulation distortion (mixing phenomena) caused by non-linear behavior of electronic components and power supply limitations. Terms for specific types of nonlinear audio distortion include: crossover distortion and slew-induced distortion (SID).

Other forms of audio distortion are non-flat frequency response, compression, modulation, aliasing, quantization noise, wow and flutter from analog media such as vinyl records and magnetic tape. The human ear cannot hear phase distortion, except that it may affect the stereo imaging.

In most fields, distortion is characterized as unwanted change to a signal. Distortion in music is often intentionally used as an effect when applied to an electric guitar signal in styles of rock music such as heavy metal and punk rock.

Distortion in art
In the art world, a distortion is any change made by an artist to the size, shape or visual character of a form in order to express an idea, convey a feeling, or enhance visual impact. Such distortions or "abstractions" primarily refer to purposeful deviations from photorealistic perspective or from realistic proportionality. Examples include "The Weeping Woman" by Picasso and "The Adoration of the Shepherds" by El Greco, whose human subject matters are irregularly and (as is often with physical distortions) asymmetrically proportioned in a way that is not possible in standard perspective.

Optics
In optics, image/optical distortion is a divergence from rectilinear projection caused by a change in magnification with increasing distance from the optical axis of an optical system.

Map projections
In cartography, a distortion is the misrepresentation of the area or shape of a feature. The Mercator projection, for example, distorts by exaggerating the size of regions at high latitude.

title: Distortion


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Distortion-limited operation

In telecommunication, distortion-limited operation is the condition prevailing when distortion of a received signal, rather than its attenuated amplitude (or power), limits performance under stated operational conditions and limits.

Note: Distortion-limited operation is reached when the system distorts the shape of the waveform beyond specified limits. For linear systems, distortion-limited operation is equivalent to bandwidth-limited operation.

title: Distortion-limited operation


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Distributed database

A distributed database is a database in which data is stored across different physical locations. It may be stored in multiple computers located in the same physical location (e.g. a data centre); or maybe dispersed over a network of interconnected computers. Unlike parallel systems, in which the processors are tightly coupled and constitute a single database system, a distributed database system consists of loosely coupled sites that share no physical components.

System administrators can distribute collections of data (e.g. in a database) across multiple physical locations. A distributed database can reside on organised network servers or decentralised independent computers on the Internet, on corporate intranets or extranets, or on other organisation networks. Because distributed databases store data across multiple computers, distributed databases may improve performance at end-user worksites by allowing transactions to be processed on many machines, instead of being limited to one.

Two processes ensure that the distributed databases remain up-to-date and current: replication and duplication.

 * 1) Replication involves using specialized software that looks for changes in the distributive database. Once the changes have been identified, the replication process makes all the databases look the same. The replication process can be complex and time-consuming, depending on the size and number of the distributed databases. This process can also require much time and computer resources.
 * 2) Duplication, on the other hand, has less complexity. It identifies one database as a master and then duplicates that database. The duplication process is normally done at a set time after hours. This is to ensure that each distributed location has the same data.  In the duplication process, users may change only the master database. This ensures that local data will not be overwritten.

Both replication and duplication can keep the data current in all distributive locations.

Besides distributed database replication and fragmentation, there are many other distributed database design technologies. For example, local autonomy, synchronous, and asynchronous distributed database technologies. The implementation of these technologies can and do depend on the needs of the business and the sensitivity/confidentiality of the data stored in the database and the price the business is willing to spend on ensuring data security, consistency and integrity.

When discussing access to distributed databases, Microsoft favors the term distributed query, which it defines in protocol-specific manner as "[a]ny SELECT, INSERT, UPDATE, or DELETE statement that references tables and rowsets from one or more external OLE DB data sources". Oracle provides a more language-centric view in which distributed queries and distributed transactions form part of distributed SQL.

Architecture
There are 3 main architecture types for distributed databases:

 * Shared-memory: very rarely used
 * Shared-disk
 * Shared-nothing

In the shared-memory and shared-disk architectures, the data is not partitioned, but it has to be in a shared-nothing architecture.

Shared-disk architecture is more common for cloud databases than for on-premise.

Historically, shared-nothing was the first architecture to be implemented on the cloud, before the advent of shared cloud storage made shared-disk possible.

In practice, different layers of the database can have different architectures. It is now common to have a compute layer with a shared nothing architecture, and a storage layer with a shared disk architecture. This is for instance the case of Snowflake and AWS Aurora.

List of shared-nothing databases

 * IBM Db2
 * Greenplum
 * Netezza
 * Teradata
 * Vertica

List of shared-disk databases

 * AWS Aurora
 * Neon
 * Snowflake

title: Distributed database


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Distributed-queue dual-bus

In telecommunication, a distributed-queue dual-bus network (DQDB) is a distributed multi-access network that (a) supports integrated communications using a dual bus and distributed queuing, (b) provides access to local or metropolitan area networks, and (c) supports connectionless data transfer, connection-oriented data transfer, and isochronous communications, such as voice communications.

IEEE 802.6 is an example of a network providing DQDB access methods.

Concept of operation
The DQDB medium access control (MAC) algorithm is generally credited to Robert Newman who developed this algorithm in his PhD thesis in the 1980s at the University of Western Australia. To appreciate the innovative value of the DQDB MAC algorithm, it must be seen against the background of LAN protocols at that time, which were based on broadcast (such as IEEE 802.3 Ethernet) or a ring (like IEEE 802.5 Token Ring and FDDI). The DQDB may be thought of as two token rings, one carrying data in each direction around the ring. This improves reliability which is important in Metropolitan Area Networks (MAN), where repairs may take longer than in a LAN and Wi-Fi because the damage may be inaccessible.

The DQDB standard IEEE 802.6 was developed while ATM (Broadband ISDN) was still in early development, but there was strong interaction between the two standards. ATM cells and DQDB frames were harmonized. They both settled on essentially a 48-byte data frame with a 5-byte header. In the DQDB algorithm, a distributed queue was implemented by communicating queue state information via the header. Each node in a DQDB network maintains a pair of state variables which represent its position in the distributed queue and the size of the queue. The headers on the reverse bus communicated requests to be inserted in the distributed queue so that upstream nodes would know that they should allow DQDB cells to pass unused on the forward bus. The algorithm was remarkable for its extreme simplicity.

Currently DQDB systems are being installed by many carriers in entire cities, with lengths that reach up to 160 km with speeds of a DS3 line (44.736 Mbit/s). Other implementations use optical fiber for a length of up to 100 km and speeds around 150 Mbit/s.

title: Distributed-queue dual-bus


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Distributed switching

Distributed switching is an architecture in which multiple processor-controlled switching units are distributed. There is often a hierarchy of switching elements, with a centralized host switch and with remote switches located close to concentrations of users.

Use in telephony networks
Distributed switching is often used in telephone networks, though it is often called host-remote switching.

In rural areas, population centers tend to be too small for economical deployment of a full-featured dedicated telephone exchange, and distances between these centers make transmission costs relatively high. Normal telephone traffic patterns show that most calling is done between people in a community of interest, in this case a geographical one: the population center. Use of distributed switching allows for the majority of calls that are local to that population center to be switched there without needing to be transported to and from the host switch.

The host switch provides connectivity between the remote switches and to the larger network, and the host may also directly handle some rare and complex call types (conference calling, for example) that the remote itself is not equipped to handle. Host switches also perform OAM&P (Operation, Administration, Maintenance, and Provisioning) functions, including billing, for the entire cluster of the host and its remote switches.

A key capability of a remote switch is the ability to act in emergency standalone (ESA) mode, wherein local calls can still be placed even in the event that the connection between that remote and the host has been lost. In this mode, only local calling is available anyway, so the billing capability of the host switch is not required. ESA is increasingly available on digital loop carrier platforms as well as on purpose-built remote switches in order to improve the scope of their utility.

Use within telecommunications equipment platforms
Many data-centric telecommunications platforms such as routers and Ethernet switches utilize distributed switching on separate cards within an equipment chassis. Even when this is used, it is common to have a centralized switching fabric to interconnect the distributed switches. This architecture has become less common as backplane bus speeds and centralized switch fabric capacities have increased.

title: Distributed switching


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Disturbance voltage

In telecommunication, a disturbance voltage is an unwanted voltage induced in a system by natural or man-made sources.

In telecommunications systems, the disturbance voltage creates currents that limit or interfere with the interchange of information. An example of a disturbance voltage is a voltage that produces (a) false signals in a telephone, (b) Noise (radio) in a radio receiver, or (c) distortion in a received signal.

title: Disturbance voltage


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Diurnal phase shift

In telecommunication, diurnal phase shift is the phase shift of electromagnetic signals associated with daily changes in the ionosphere. The major changes usually occur during the period of time when sunrise or sunset is present at critical points along the path. Significant phase shifts may occur on paths wherein a reflection area of the path is subject to a large tidal range. In cable systems, significant phase shifts can be occasioned by diurnal temperature variance.

title: Diurnal phase shift


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Department of Defense master clock

The Department of Defense master clock is the atomic master clock to which time and frequency measurements for the United States Department of Defense are referenced.

Located in Washington D.C., the U.S. Naval Observatory master clock is designated as the "DOD Master Clock". It is one of the two standard time and frequency references for the U.S. Government in accordance with Federal Standard 1002-A. The other standard time and frequency reference for the U.S. Government is the National Institute of Standards and Technology (NIST) master clock.

The U.S. Naval Observatory also maintains an alternate clock designated "USNO Alternate Master Clock" at Schriever Space Force Base, Colorado.

title: Department of Defense master clock


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Double-ended synchronization

For two connected exchanges in a communications network, a double-ended synchronization (also called double-ended control) is a synchronization control scheme in which the phase error signals used to control the clock at one telephone exchange are derived by comparison with the phase of the incoming digital signal and the phase of the internal clocks at both exchanges.

title: Double-ended synchronization


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Double-sideband reduced-carrier transmission

Double-sideband reduced carrier transmission (DSB-RC): transmission in which (a) the frequencies produced by amplitude modulation are symmetrically spaced above and below the carrier and (b) the carrier level is reduced for transmission at a fixed level below that which is provided to the modulator.

Note: In DSB-RC transmission, the carrier is usually transmitted at a level suitable for use as a reference by the receiver, except for the case in which it is reduced to the minimum practical level, i.e. the carrier is suppressed.


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Double-sideband suppressed-carrier transmission

Double-sideband suppressed-carrier transmission (DSB-SC) is transmission in which frequencies produced by amplitude modulation  (AM) are symmetrically spaced above and below the carrier frequency and the carrier level is reduced to the lowest practical level, ideally being completely suppressed. In the DSB-SC modulation, unlike in AM, the wave carrier is not transmitted; thus, much of the power is distributed between the side bands, which implies an increase of the cover in DSB-SC, compared to AM, for the same power use.

DSB-SC transmission is a special case of double-sideband reduced carrier transmission. It is used for radio data systems. This mode is frequently used in amateur radio voice communications, especially on high-frequency bands

Spectrum
DSB-SC is basically an amplitude modulation wave without the carrier, therefore reducing power waste, giving it a 50% efficiency. This is an increase compared to normal AM transmission (DSB) that has a maximum efficiency of 33.333%, since 2/3 of the power is in the carrier which conveys no useful information and both sidebands containing identical copies of the same information. Single Side Band Suppressed Carrier (SSB-SC) is 100% efficient.

Spectrum plot of a DSB-SC signal:

Generation
DSB-SC is generated by a mixer. The signal produced is the product of the message signal and a carrier signal. The mathematical representation of this process is shown below, where the product-to-sum trigonometric identity is used.

\underbrace{V_m \cos \left( \omega_m t \right)}_{\mbox{Message}} \times \underbrace{V_c \cos \left( \omega_c t \right)}_{\mbox{Carrier}} = \underbrace{\frac{V_m V_c}{2} \left[ \cos\left(\left( \omega_m + \omega_c \right)t\right) + \cos\left(\left( \omega_m - \omega_c \right)t\right) \right]}_{\mbox{Modulated Signal}} $$

Demodulation
For DSBSC, Coherent Demodulation is done by multiplying the DSB-SC signal with the carrier signal (with the same phase as in the modulation process) just like the modulation process. This resultant signal is then passed through a low pass filter to produce a scaled version of the original message signal.

\overbrace{\frac{V_m V_c}{2} \left[ \cos\left(\left( \omega_m + \omega_c \right)t\right) + \cos\left(\left( \omega_m - \omega_c \right)t\right) \right]}^{\mbox{Modulated Signal}} \times \overbrace{V'_c \cos \left( \omega_c t \right)}^{\mbox{Carrier}} $$
 * $$= \left(\frac{1}{2}V_c V'_c\right)\underbrace{V_m \cos(\omega_m t)}_{\text{original message}} + \frac{1}{4}V_c V'_c V_m \left[\cos((\omega_m + 2\omega_c)t) + \cos((\omega_m - 2\omega_c)t)\right]$$

The equation above shows that by multiplying the modulated signal by the carrier signal, the result is a scaled version of the original message signal plus a second term. Since $$\omega_c \gg \omega_m$$, this second term is much higher in frequency than the original message. Once this signal passes through a low pass filter, the higher frequency component is removed, leaving just the original message.

Distortion and attenuation
For demodulation, the demodulation oscillator's frequency and phase must be exactly the same as the modulation oscillator's, otherwise, distortion and/or attenuation will occur.

To see this effect, take the following conditions:
 * Message signal to be transmitted: $$f(t)$$
 * Modulation (carrier) signal: $$V_c\cos(\omega_c t)$$
 * Demodulation signal (with small frequency and phase deviations from the modulation signal): $$V'_c\cos\left[(\omega_c+\Delta\omega)t + \theta\right]$$

The resultant signal can then be given by
 * $$f(t) \times V_c\cos(\omega_c t) \times V'_c\cos\left[(\omega_c+\Delta\omega)t + \theta\right]$$
 * $$=\frac{1}{2}V_c V'_c f(t) \cos\left(\Delta\omega\cdot t+\theta\right) + \frac{1}{2}V_c V'_c f(t) \cos\left[(2\omega_c+\Delta\omega)t+\theta\right]$$
 * $$\xrightarrow{\text{After low pass filter}} \frac{1}{2}V_c V'_c f(t) \cos\left(\Delta\omega\cdot t+\theta\right)$$

The $$\cos\left(\Delta\omega\cdot t+\theta\right)$$ terms results in distortion and attenuation of the original message signal. In particular, if the frequencies are correct, but the phase is wrong, contribution from $$\theta$$ is a constant attenuation factor, also $$\Delta\omega\cdot t$$ represents a cyclic inversion of the recovered signal, which is a serious form of distortion.

How it works
This is best shown graphically. Below is a message signal that one may wish to modulate onto a carrier, consisting of a couple of sinusoidal components with frequencies respectively 800 Hz and 1200 Hz.

The equation for this message signal is $$s(t) = \frac{1}{2}\cos\left(2\pi 800 t\right) - \frac{1}{2}\cos\left( 2\pi 1200 t\right)$$.

The carrier, in this case, is a plain 5 kHz ($$c(t) = \cos\left( 2\pi 5000 t \right)$$) sinusoid—pictured below.

The modulation is performed by multiplication in the time domain, which yields a 5 kHz carrier signal, whose amplitude varies in the same manner as the message signal.

$$x(t) = \underbrace{\cos\left( 2\pi 5000 t \right)}_\mbox{Carrier} \times \underbrace{\left[\frac{1}{2}\cos\left(2\pi 800 t\right) - \frac{1}{2}\cos\left( 2\pi 1200 t\right)\right]}_\mbox{Message Signal}$$

The name "suppressed carrier" comes about because the carrier signal component is suppressed—it does not appear in the output signal. This is apparent when the spectrum of the output signal is viewed. In the picture shown below we see four peaks, the two peaks below 5000 Hz are the lower sideband (LSB) and the two peaks above 5000 Hz are the upper sideband (USB), but there is no peak at the 5000 Hz mark, which is the frequency of the suppressed carrier.


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Drift

Drift or Drifts may refer to:

Geography

 * Drift or ford (crossing) of a river
 * Drift (navigation), difference between heading and course of a vessel
 * Drift, Kentucky, unincorporated community in the United States
 * In Cornwall, England:
 * Drift, Cornwall, village
 * Drift Reservoir, associated with the village

Science, technology, and physics

 * Directional Recoil Identification from Tracks, a dark-matter experiment
 * Drift (video gaming), a typical game-controller malfunction
 * Drift pin, metalworking tool for localizing hammer blows and for aligning holes
 * Drift (geology), deposited material of glacial origin
 * drift (in mining), a roughly horizontal passage; an adit
 * Drift, linear term of a stochastic process
 * Drift (motorsport), the controlled sliding of a vehicle through a sharp turn, either via over-steering with sudden sharp braking, or counter-steering with a sudden "clutch kick" acceleration
 * Incremental changes:
 * Drift (linguistics), a type of language change
 * Genetic drift, change in allele frequency
 * Drift (telecommunication), long-term change in an attribute of a system or equipment
 * Clock drift - variation in time-keeping
 * Frequency drift - oscillator offset in electrical engineering
 * Concept drift (in data-science and machine-learning applications)

Film and television

 * Drift (film series), 2006–2008 film series by Futoshi Jinno
 * Drift, 2006 TV crime drama film directed by Paul W. S. Anderson
 * Drift, fictional technology system that links the minds of two Jaeger pilots in the 2013 sci-fi film Pacific Rim and its sequel
 * Drift (2013 Australian film), film starring Sam Worthington
 * Drift (2013 Belgian film), art house film
 * Drift (2015 film), Swiss film
 * Drift (2017 film), German film
 * Drift, 2007 experimental short film by Max Hattler
 * Drift (2023 film), a film by Anthony Chen

Books and Publishing

 * The Drift (magazine)
 * Drift (novel), a 2002 Doctor Who novel
 * Drift: The Unmooring of American Military Power, a book by Rachel Maddow
 * Plot drift, when a story deviates unexpectedly from its initial direction, in writing, television, or other media.

Music

 * The Drift (band), American post-rock band
 * Songs:
 * "Drift", 1985 song from work ‘’Secret’’
 * "Drift" (Emily Osment song) (2011)
 * "Drift", end credits song of 2013 film Pacific Rim
 * Albums/EPs:
 * Drift (Flotsam and Jetsam album) (1995)
 * The Drift, album by Scott Walker (2006)
 * Drift (Ken Block album) (2008)
 * Drift (Nosaj Thing album) (2009)
 * The Drift (EP), by Michelle Channel and Arjun Singh (2014)
 * Drift (Erra album) (2016)
 * Drift (Underworld project), ongoing music-and-video experiment by that band (2018-2019)
 * Drift (The Devlins album), 1993
 * Drift, short for drift phonk, a subgenre of phonk

title: Drift


text:
Drop (liquid)

A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the end of a tube or other surface boundary, producing a hanging drop called a pendant drop. Drops may also be formed by the condensation of a vapor or by atomization of a larger mass of solid. Water vapor will condense into droplets depending on the temperature. The temperature at which droplets form is called the dew point.

Surface tension
Liquid forms drops because it exhibits surface tension.

A simple way to form a drop is to allow liquid to flow slowly from the lower end of a vertical tube of small diameter. The surface tension of the liquid causes the liquid to hang from the tube, forming a pendant. When the drop exceeds a certain size it is no longer stable and detaches itself. The falling liquid is also a drop held together by surface tension.

Viscosity and pitch drop experiments
Some substances that appear to be solid, can be shown to instead be extremely viscous liquids, because they form drops and display droplet behavior. In the famous pitch drop experiments, pitch – a substance somewhat like solid bitumen – is shown to be a liquid in this way. Pitch in a funnel slowly forms droplets, each droplet taking about 10 years to form and break off.

Pendant drop test

In the pendant drop test, a drop of liquid is suspended from the end of a tube or by any surface by surface tension. The force due to surface tension is proportional to the length of the boundary between the liquid and the tube, with the proportionality constant usually denoted $$\gamma$$. Since the length of this boundary is the circumference of the tube, the force due to surface tension is given by

 * $$\,F_{\gamma} = \pi d \gamma$$

where d is the tube diameter.

The mass m of the drop hanging from the end of the tube can be found by equating the force due to gravity ($$F_{g} = mg$$) with the component of the surface tension in the vertical direction ($$F_{\gamma} \sin \alpha$$) giving the formula

 * $$\,mg = \pi d \gamma \sin \alpha$$

where α is the angle of contact with the tube's front surface, and g is the acceleration due to gravity.

The limit of this formula, as α goes to 90°, gives the maximum weight of a pendant drop for a liquid with a given surface tension, $$\gamma$$.

 * $$\,mg = \pi d \gamma$$

This relationship is the basis of a convenient method of measuring surface tension, commonly used in the petroleum industry. More sophisticated methods are available to take account of the developing shape of the pendant as the drop grows. These methods are used if the surface tension is unknown.

Drop adhesion to a solid
The drop adhesion to a solid can be divided into two categories: lateral adhesion and normal adhesion. Lateral adhesion resembles friction (though tribologically lateral adhesion is a more accurate term) and refers to the force required to slide a drop on the surface, namely the force to detach the drop from its position on the surface only to translate it to another position on the surface. Normal adhesion is the adhesion required to detach a drop from the surface in the normal direction, namely the force to cause the drop to fly off from the surface. The measurement of both adhesion forms can be done with the Centrifugal Adhesion Balance (CAB). The CAB uses a combination of centrifugal and gravitational forces to obtain any ratio of lateral and normal forces. For example, it can apply a normal force at zero lateral force for the drop to fly off away from the surface in the normal direction or it can induce a lateral force at zero normal force (simulating zero gravity).

Droplet
The term droplet is a diminutive form of 'drop' – and as a guide is typically used for liquid particles of less than 500 μm diameter. In spray application, droplets are usually described by their perceived size (i.e., diameter) whereas the dose (or number of infective particles in the case of biopesticides) is a function of their volume. This increases by a cubic function relative to diameter; thus, a 50 μm droplet represents a dose in 65 pl and a 500 μm drop represents a dose in 65 nanometers.

Speed
A droplet with a diameter of 3 mm has a terminal velocity of approximately 8 m/s. Drops smaller than 1 mm in diameter will attain 95% of their terminal velocity within 2 m. But above this size the distance to get to terminal velocity increases sharply. An example is a drop with a diameter of 2 mm that may achieve this at 5.6 m.

Optics
Due to the different refractive index of water and air, refraction and reflection occur on the surfaces of raindrops, leading to rainbow formation.

Sound
The major source of sound when a droplet hits a liquid surface is the resonance of excited bubbles trapped underwater. These oscillating bubbles are responsible for most liquid sounds, such as running water or splashes, as they actually consist of many drop-liquid collisions.

"Dripping tap" noise prevention
Reducing the surface tension of a body of liquid makes possible to reduce or prevent noise due to droplets falling into it. This would involve adding soap, detergent or a similar substance to water. The reduced surface tension reduces the noise from dripping.

Shape
The classic shape associated with a drop (with a pointy end in its upper side) comes from the observation of a droplet clinging to a surface. The shape of a drop falling through a gas is actually more or less spherical for drops less than 2 mm in diameter. Larger drops tend to be flatter on the bottom part due to the pressure of the gas they move through. As a result, as drops get larger, a concave depression forms which leads to the eventual breakup of the drop.

Capillary length
The capillary length is a length scaling factor that relates gravity, density, and surface tension, and is directly responsible for the shape a droplet for a specific fluid will take. The capillary length stems from the Laplace pressure, using the radius of the droplet.

Using the capillary length we can define microdrops and macrodrops. Microdrops are droplets with radius smaller than the capillary length, where the shape of the droplet is governed by surface tension and they form a more or less spherical cap shape. If a droplet has a radius larger than the capillary length, they are known as macrodrops and the gravitational forces will dominate. Macrodrops will be 'flattened' by gravity and the height of the droplet will be reduced.

Size
Raindrop sizes typically range from 0.5 mm to 4 mm, with size distributions quickly decreasing past diameters larger than 2-2.5 mm.

Scientists traditionally thought that the variation in the size of raindrops was due to collisions on the way down to the ground. In 2009, French researchers succeeded in showing that the distribution of sizes is due to the drops' interaction with air, which deforms larger drops and causes them to fragment into smaller drops, effectively limiting the largest raindrops to about 6 mm diameter. However, drops up to 10 mm (equivalent in volume to a sphere of radius 4.5 mm) are theoretically stable and could be levitated in a wind tunnel. The largest recorded raindrop was 8.8 mm in diameter, located at the base of a cumulus congestus cloud in the vicinity of Kwajalein Atoll in July 1999. A raindrop of identical size was detected over northern Brazil in September 1995.

Standardized droplet sizes in medicine
In medicine, this property is used to create droppers and IV infusion sets which have a standardized diameter, in such a way that 1 millilitre is equivalent to 20 drops. When smaller amounts are necessary (such as paediatrics), microdroppers or paediatric infusion sets are used, in which 1 millilitre = 60 microdrops.

title: Drop (liquid)


text:
Drop and insert

In a multichannel transmission system, drop and insert is a process that diverts (drop) a portion of the multiplexed aggregate signal at an intermediate point, and introduces (insert) a different signal for subsequent transmission in the same position.

Drop and insert is practiced, for example, in time-division multiplexing (TDM) when a time slot or frequency band is replaced from another source. The diverted signal may be demodulated or reinserted into another transmission system in the same or another time slot or frequency band. The time slot or frequency band vacated by the diverted signal need not necessarily be reoccupied by another signal. Likewise, a previously unoccupied time slot or frequency band may be occupied by a signal inserted at the drop-and-insert point.

Signals not of interest at the drop-and-insert point are not diverted.

title: Drop and insert


text:
Dropout

Dropout or drop out may refer to:

 * Dropping out, prematurely leaving school, college or university

Film and television

 * Dropout (film), a 1970 Italian drama
 * "The Dropout", a 1970 episode of The Brady Bunch
 * The Dropout (podcast), 2019 true crime podcast
 * The Dropout, a 2022 American miniseries based on the podcast

Games

 * drop out (cards), to fold, i.e. to concede the current hand and take no further part in it

Music

 * "Drop Out" (song), by Lil Pump, 2019
 * "Drop Out," a song by Rocket from the Crypt from the 1995 album Scream, Dracula, Scream!
 * "Drop Out", a song by Converge from the 2004 album You Fail Me
 * "Drop Out", music of Dance Dance Revolution Extreme
 * Drop Out with The Barracudas, by The Barracudas, 1981
 * The Dropouts, an early incarnation of Priestess (band)

Other

 * Dropout (media company), an American subscription media service provider owned by a company of the same name
 * The Dropouts, an American comic strip by Howard Post

Science and technology

 * Dropout (astronomy), a radiation source whose radiation intensity falls off sharply
 * Dropout (bicycle part), a type of fork end
 * Dropout (communications), a momentary loss of signal
 * Dropout (neural networks), a regularization technique for reducing overfitting

title: Dropout


text:
DTE

DTE may refer to:

 * Data terminal equipment, an end instrument used in telecommunication and data transmission
 * Distance to empty, a feature in an automobile electronic instrument cluster
 * Dithioerythritol, a chemical
 * DTE (direct to edit), a digital video recording method
 * DTE Energy, a Detroit, Michigan-based utility
 * Dora the Explorer, a children's animated television show.
 * Dual-Tile encoding, another name for byte pair encoding
 * Directorate of Technical Education, Maharashtra, an Indian state government agency for higher education.
 * Department of Technical Education, a higher education governance body under the government of Kerala, India

title: DTE


text:
Dual access

In telecommunication, the term dual access has the following meanings:

 * 1) The connection of a user to two switching centers by separate access lines using a single message routing indicator or telephone number.
 * 2) In satellite communications, the transmission of two carriers simultaneously through a single communication satellite repeater.

Also, network hardware company D-Link has named technology which allows two simultaneous connections over one cable, for example 1) Internet and 2) provider's local FTP or game servers or IPTV data flow.

title: Dual access


text:
Dual in-line package

In microelectronics, a dual in-line package (DIP or DIL) is an electronic component package with a rectangular housing and two parallel rows of electrical connecting pins. The package may be through-hole mounted to a printed circuit board (PCB) or inserted in a socket. The dual-inline format was invented by Don Forbes, Rex Rice and Bryant Rogers at Fairchild R&D in 1964, when the restricted number of leads available on circular transistor-style packages became a limitation in the use of integrated circuits. Increasingly complex circuits required more signal and power supply leads (as observed in Rent's rule); eventually microprocessors and similar complex devices required more leads than could be put on a DIP package, leading to development of higher-density chip carriers. Furthermore, square and rectangular packages made it easier to route printed-circuit traces beneath the packages.

A DIP is usually referred to as a DIPn, where n is the total number of pins. For example, a microcircuit package with two rows of seven vertical leads would be a DIP14. The photograph at the upper right shows three DIP14 ICs. Common packages have as few as three and as many as 64 leads. Many analog and digital integrated circuit types are available in DIP packages, as are arrays of transistors, switches, light emitting diodes, and resistors. DIP plugs for ribbon cables can be used with standard IC sockets.

DIP packages are usually made from an opaque molded epoxy plastic pressed around a tin-, silver-, or gold-plated lead frame that supports the device die and provides connection pins. Some types of IC are made in ceramic DIP packages, where high temperature or high reliability is required, or where the device has an optical window to the interior of the package. Most DIP packages are secured to a PCB by inserting the pins through holes in the board and soldering them in place. Where replacement of the parts is necessary, such as in test fixtures or where programmable devices must be removed for changes, a DIP socket is used. Some sockets include a zero insertion force (ZIF) mechanism.

Variations of the DIP package include those with only a single row of pins, e.g. a resistor array, possibly including a heat sink tab in place of the second row of pins, and types with four rows of pins, two rows, staggered, on each side of the package. DIP packages have been mostly displaced by surface-mount package types, which avoid the expense of drilling holes in a PCB and which allow higher density of interconnections.

Types of devices

DIPs are commonly used for integrated circuits (ICs). Other devices in DIP packages include resistor networks, DIP switches, LED segmented and bar graph displays, and electromechanical relays.

DIP connector plugs for ribbon cables are common in computers and other electronic equipment.

Dallas Semiconductor manufactured integrated DIP real-time clock (RTC) modules which contained an IC chip and a non-replaceable 10-year lithium battery.

DIP header blocks on to which discrete components could be soldered were used where groups of components needed to be easily removed, for configuration changes, optional features or calibration.

Uses
The original dual-in-line package was invented by Bryant "Buck" Rogers in 1964 while working for Fairchild Semiconductor. The first devices had 14 pins and looked much like they do today. The rectangular shape allowed integrated circuits to be packaged more densely than previous round packages. The package was well-suited to automated assembly equipment; a PCB could be populated with scores or hundreds of ICs, then all the components on the circuit board could be soldered at one time on a wave soldering machine and passed on to automated testing machines, with very little human labor required. DIP packages were still large with respect to the integrated circuits within them. By the end of the 20th century, surface-mount packages allowed further reduction in the size and weight of systems. DIP chips are still popular for circuit prototyping on a breadboard because of how easily they can be inserted and used there.

DIPs were the mainstream of the microelectronics industry in the 1970s and 1980s. Their use has declined in the first decade of the 21st century due to the emerging new surface-mount technology (SMT) packages such as plastic leaded chip carrier (PLCC) and small-outline integrated circuit (SOIC), though DIPs continued in extensive use through the 1990s, and still continue to be used substantially as the year 2011 passes. Because some modern chips are available only in surface-mount package types, a number of companies sell various prototyping adapters to allow those surface-mount devices (SMD) to be used like DIP devices with through-hole breadboards and soldered prototyping boards (such as stripboard and perfboard). (SMT can pose quite a problem, at least an inconvenience, for prototyping in general; most of the characteristics of SMT that are advantages for mass production are difficulties for prototyping.)

For programmable devices like EPROMs and GALs, DIPs remained popular for many years due to their easy handling with external programming circuitry (i.e., the DIP devices could be simply plugged into a socket on the programming device.) However, with In-System Programming (ISP) technology now state of the art, this advantage of DIPs is rapidly losing importance as well.

Through the 1990s, devices with fewer than 20 leads were manufactured in a DIP format in addition to the newer formats. Since about 2000, newer devices are often unavailable in the DIP format.

Mounting
DIPs can be mounted either by through-hole soldering or in sockets. Sockets allow easy replacement of a device and eliminates the risk of damage from overheating during soldering. Generally sockets were used for high-value or large ICs, which cost much more than the socket. Where devices would be frequently inserted and removed, such as in test equipment or EPROM programmers, a zero insertion force socket would be used.

DIPs are also used with breadboards, a temporary mounting arrangement for education, design development or device testing. Some hobbyists, for one-off construction or permanent prototyping, use point-to-point wiring with DIPs, and their appearance when physically inverted as part of this method inspires the informal term "dead bug style" for the method.

Construction
The body (housing) of a DIP containing an IC chip is usually made from molded plastic or ceramic. The hermetic nature of a ceramic housing is preferred for extremely high reliability devices. However, the vast majority of DIPs are manufactured via a thermoset molding process in which an epoxy mold compound is heated and transferred under pressure to encapsulate the device. Typical cure cycles for the resins are less than 2 minutes and a single cycle may produce hundreds of devices.

The leads emerge from the longer sides of the package along the seam, parallel to the top and bottom planes of the package, and are bent downward approximately 90 degrees (or slightly less, leaving them angled slightly outward from the centerline of the package body). (The SOIC, the SMT package that most resembles a typical DIP, appears essentially the same, notwithstanding size scale, except that after being bent down the leads are bent upward again by an equal angle to become parallel with the bottom plane of the package.) In ceramic (CERDIP) packages, an epoxy or grout is used to hermetically seal the two halves together, providing an air and moisture tight seal to protect the IC die inside. Plastic DIP (PDIP) packages are usually sealed by fusing or cementing the plastic halves around the leads, but a high degree of hermeticity is not achieved because the plastic itself is usually somewhat porous to moisture and the process cannot ensure a good microscopic seal between the leads and the plastic at all points around the perimeter. However, contaminants are usually still kept out well enough that the device can operate reliably for decades with reasonable care in a controlled environment.

Inside the package, the lower half has the leads embedded, and at the center of the package is a rectangular space, chamber, or void into which the IC die is cemented. The leads of the package extend diagonally inside the package from their positions of emergence along the periphery to points along a rectangular perimeter surrounding the die, tapering as they go to become fine contacts at the die. Ultra-fine bond wires (barely visible to the naked human eye) are welded between these die periphery contacts and bond pads on the die itself, connecting one lead to each bond pad, and making the final connection between the microcircuits and the external DIP leads. The bond wires are not usually taut but loop upward slightly to allow slack for thermal expansion and contraction of the materials; if a single bond wire breaks or detaches, the entire IC may become useless. The top of the package covers all of this delicate assemblage without crushing the bond wires, protecting it from contamination by foreign materials.

Usually, a company logo, alphanumeric codes and sometimes words are printed on top of the package to identify its manufacturer and type, when it was made (usually as a year and a week number), sometimes where it was made, and other proprietary information (perhaps revision numbers, manufacturing plant codes, or stepping ID codes.)

The necessity of laying out all of the leads in a basically radial pattern in a single plane from the die perimeter to two rows on the periphery of the package is the main reason that DIP packages with higher lead counts must have wider spacing between the lead rows, and it effectively limits the number of leads which a practical DIP package may have. Even for a very small die with many bond pads (e.g. a chip with 15 inverters, requiring 32 leads), a wider DIP would still be required to accommodate the radiating leads internally. This is one of the reasons that four-sided and multiple rowed packages, such as PGAs, were introduced (around the early 1980s).

A large DIP package (such as the DIP64 used for the Motorola 68000 CPU) has long leads inside the package between pins and the die, making such a package unsuitable for high speed devices.

Some other types of DIP devices are built very differently. Most of these have molded plastic housings and straight leads or leads that extend directly out of the bottom of the package. For some, LED displays particularly, the housing is usually a hollow plastic box with the bottom/back open, filled (around the contained electronic components) with a hard translucent epoxy material from which the leads emerge. Others, such as DIP switches, are composed of two (or more) plastic housing parts snapped, welded, or glued together around a set of contacts and tiny mechanical parts, with the leads emerging through molded-in holes or notches in the plastic.

Variants

Several DIP variants for ICs exist, mostly distinguished by packaging material:
 * Ceramic dual in-line package (CERDIP or CDIP)
 * Plastic dual in-line package (PDIP)
 * Shrink plastic dual in-line package (SPDIP) – A denser version of the PDIP with a 0.07 in (1.778 mm) lead pitch.
 * Skinny dual in-line package (SDIP or SPDIP ) – Sometimes used to refer to a "narrow" 0.300 in. (or 300 mil) wide DIP, normally when clarification is needed e.g. for DIP with 24 pins or more, which usually come in "wide" 0.600 in wide DIP package. An example of a typical proper full spec for a "narrow" DIP package would be 300 mil body width, 0.1 in pin pitch.

EPROMs were sold in ceramic DIPs manufactured with a circular window of clear quartz over the chip die to allow the part to be erased by ultraviolet light. Often, the same chips were also sold in less expensive windowless PDIP or CERDIP packages as one-time programmable (OTP) versions. Windowed and windowless packages were also used for microcontrollers, and other devices, containing EPROM memory. Windowed CERDIP-packaged EPROMs were used for the BIOS ROM of many early IBM PC clones with an adhesive label covering the window to prevent inadvertent erasure through exposure to ambient light.

Molded plastic DIPs are much lower in cost than ceramic packages; one 1979 study showed that a plastic 14 pin DIP cost around US$0.063 and a ceramic package cost US$0.82.

Single in-line

A single in-line package (SIP or SIL package) has one row of connecting pins. It is not as popular as the DIP, but has been used for packaging RAM chips and multiple resistors with a common pin. As compared to DIPs with a typical maximum pin count of 64, SIPs have a typical maximum pin count of 24 with lower package costs.

One variant of the single in-line package uses part of the lead frame for a heat sink tab. This multi-leaded power package is useful for such applications as audio power amplifiers, for example.

Quad in-line

The QIP, sometimes called a QIL package, has the same dimensions as a DIL package, but the leads on each side are bent into an alternating zigzag configuration so as to fit four lines of solder pads (instead of two with a DIL). The QIL design increased the spacing between solder pads without increasing package size, for two reasons:
 * 1) It allowed more reliable soldering. This may seem odd today, given the far closer solder pad spacing in use now, but in the 1970s, the heyday of the QIL, bridging of neighbouring solder pads on DIL chips was an issue at times,
 * 2) QIL also increased the possibility of running a copper track between 2 solder pads. This was very handy on the then standard single sided single layer PCBs.

Lead count and spacing
Commonly found DIP packages that conform to JEDEC standards use an inter-lead spacing (lead pitch) of 0.1 in (JEDEC MS-001BA). Row spacing varies depending on lead counts, with 0.3 in. (7.62 mm) (JEDEC MS-001) or 0.6 inch (15.24 mm) (JEDEC MS-011) the most common. Less common standardized row spacings include 0.4 inch (10.16 mm) (JEDEC MS-010) and 0.9 inch (22.86 mm), as well as a row spacing of 0.3 inch, 0.6 inch or 0.75 inch with a 0.07 inch (1.778 mm) lead pitch.

The former Soviet Union and Eastern bloc countries used similar packages, but with a metric pin-to-pin spacing of 2.5 mm rather than 0.1 in.

The number of leads is always even. For 0.3 inch spacing, typical lead counts are 8, 14, 16, 18, and 28; less common are 4, 6, 20, and 24 lead counts. To have an even number of leads some DIPs have unused not connected (NC) leads to the internal chip, or are duplicated, e.g. two ground pins. For 0.6 inch spacing, typical lead counts are 24, 28, 32, and 40; less common are 36, 42, 48, 52, and 64 lead counts. Some microprocessors, such as the Motorola 68000 and Zilog Z180, used lead counts as high as 64; this is typically the maximum number of leads for a DIP package.

Orientation and lead numbering

As shown in the diagram, leads are numbered consecutively from Pin 1. When the identifying notch in the package is at the top, Pin 1 is the top left corner of the device. Sometimes Pin 1 is identified with an indent or paint dot mark.

For example, for a 14-lead DIP, with the notch at the top, the left leads are numbered from 1 to 7 (top to bottom) and the right row of leads are numbered 8 to 14 (bottom to top).

Leads are skipped on some DIP devices (e.g. segmented LED displays, relays, or devices that replace leads with a heat sink fin). The remaining leads are numbered as if all positions had leads.

In addition to providing for human visual identification of the orientation of the package, the notch allows automated chip-insertion machinery to confirm correct orientation of the chip by mechanical sensing.

Descendants
The SOIC (Small Outline IC), a surface-mount package which is currently very popular, particularly in consumer electronics and personal computers, is essentially a shrunk version of the standard IC PDIP, the fundamental difference which makes it an SMT device being a second bend in the leads to flatten them parallel to the bottom plane of the plastic housing. The SOJ (Small Outline J-lead) and other SMT packages with "SOP" (for "Small Outline Package") in their names can be considered further relatives of the DIP, their original ancestor. SOIC packages tend to have half the pitch of DIP, and SOP are half that, a fourth of DIP. (0.1"/2.54 mm, 0.05"/1.27 mm, and 0.025"/0.635 mm, respectively)

Pin grid array (PGA) packages may be considered to have evolved from the DIP. PGAs with the same 0.1 in pin centers as most DIPs were popular for microprocessors from the early to mid-1980s through the 1990s. Owners of personal computers containing Intel 80286 through P5 Pentium processors may be most familiar with these PGA packages, which were often inserted into ZIF sockets on motherboards. The similarity is such that a PGA socket may be physically compatible with some DIP devices, though the converse is rarely true.

title: Dual in-line package


text:
Duct

The word duct is derived from the Latin word for led/leading. It may refer to:

 * Duct (anatomy), various ducts in anatomy and physiology
 * Tear duct, which carry tears to the eyes
 * Duct (HVAC), for transfer of air between spaces in a structure
 * Duct tape, a kind of adhesive tape
 * Ducted fan, motor for aircraft
 * Electrical bus duct, a metal enclosure for busbars
 * Duct (industrial exhaust), industrial exhaust duct system designed for low pressure-pneumatic convey of gas, fumes, dusts, shavings, and other pollutants from works space to atmosphere after cleaning and removal of contaminants
 * Atmospheric duct, a horizontal layer in the lower atmosphere in which the vertical refractive index gradients are such that radio signals (a) are guided or ducted, (b) tend to follow the curvature of the Earth, and (c) experience less attenuation in the ducts than they would if the ducts were not present
 * Tropospheric ducting, a type of radio propagation in the troposphere that allows signals to travel unusually long distances
 * Earth–ionosphere waveguide, a type of atmospheric duct
 * Surface duct, a sound propagation phenomenon at sea
 * Duct bank, a set of electrical conduits or telecommunications conduits, entering a building underground
 * Duct Publishing, an imprint of the German group VDM Publishing devoted to the reproduction of Wikipedia content
 * Dispatchable Unit Control Table (DUCT) in z/Architecture
 * Flexible Ducting

title: Duct