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Data forwarder

In telecommunications, a data forwarder or data forwarding device is a device that receives data from one data link and retransmits data representing the same information, using proper format and link protocols, to another data link. In the packet networks, this act of forwarding is referred to as packet forwarding and is performed by network switches or network routers.

For example, in the tactical communications a data forwarder may forward data between: More than one source notes that routers and bridges can function as data forwarding devices. Another source suggests, in fact, that gateways are a "special type of data-forwarding device".
 * links that are identical, i.e., TADIL B to TADIL B,
 * links that are similar, i.e., TADIL A to TADIL B, or
 * links that are dissimilar, i.e., TADIL A to TADIL J.

title: Data forwarder


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Datagram

A datagram is a basic transfer unit associated with a packet-switched network. Datagrams are typically structured in header and payload sections. Datagrams provide a connectionless communication service across a packet-switched network. The delivery, arrival time, and order of arrival of datagrams need not be guaranteed by the network.

History
In the early 1970s, the term datagram was created by combining the words data and telegram by the CCITT rapporteur on packet switching, Halvor Bothner-By. While the word was new, the concept had already a long history.

In 1964, Paul Baran described, in a RAND Corporation report, a hypothetical military network having to resist a nuclear attack. Small standardized message blocks, bearing source and destination addresses, were stored and forwarded in computer nodes of a highly redundant meshed computer network. "The network user who has called up a virtual connection to an end station and has transmitted messages ... might also view the system as a black box providing an apparent circuit connection".

In 1967, Donald Davies published a seminal article in which he introduced the packet and packet switching. His proposed core network is similar to the one proposed by Paul Baran though developed independently. He assumes that "all users of the network will provide themselves with some kind of error control". His target is a "common-carrier communication network". To support remote access to computer services by user terminals, which at that time were transmitted character by character, he included, at the network periphery, interface computers that convert character flows into packet flows and vice versa. In 1970, Lawrence Roberts and Barry D. Wessler published an article about ARPANET, the first multi-node packet-switching network. An accompanying paper described its switching nodes (the IMPs) and its packet formats. The network core performed datagram switching as in Baran's and Davies' model, but the service offered to hosts by the network was connection oriented. A reliable message transfer service was thus offered to user computers, thus greatly simplifying the network design. This made the ARPANET what would come to be called a virtual circuit network.

Roberts presented the idea of packet switching to the communication professionals and faced anger and hostility. Before ARPANET was operating, they argued that the router buffers would quickly run out. After the ARPANET was operating, they argued packet switching would never be economic without the government subsidy. Baran faced the same rejection and thus failed to convince the military to construct a packet-switching network.

In 1973, Louis Pouzin presented his design for CYCLADES, the first large-scale network implementing the pure Davies datagram model. The CYCLADES team has thus been the first to tackle the highly complex problem of providing user applications a reliable virtual circuit service while using the end-to-end principle in a network service known to possibly produce non-negligible datagram losses and reordering. Although Pouzin's concern "in a first stage is not to make breakthrough [sic] in packet switching technology, but to build a reliable communications tool for Cyclades", two members of his team, Hubert Zimmerman and Gérard Le Lann, made significant contributions to the design of Internet's TCP that Vint Cerf, its main designer, acknowledged.

In 1981, the Defense Advanced Research Projects Agency (DARPA) issued the first specification the Internet Protocol (IP). It introduced a major evolution of the datagram concept: fragmentation. With fragmentation, some parts of the global network may use large packet size (typically local area networks to minimize processing overhead), while some others may impose smaller packet sizes (typically wide area networks to minimize response time). Network nodes may fragment a datagram into several smaller packets.

In 1999, the Internet Engineering Task Force (IETF) sanctioned the use of the already largely deployed network address translation (NAT) whereby each public address can be shared by several private devices. With it, the forthcoming Internet Address exhaustion was delayed, leaving enough time to introduce IPv6, the new generation of Internet Protocol supporting longer addresses. The initial principle of full end to end network transparency to datagrams was for this relaxed: NAT nodes had to manage per-connection states, making them in part connection oriented.

In 2015, the IETF upgraded its informational 1998 that datagram switching nodes perform active queue management (AQM), to make it a  stronger and more detailed best current practice recommendation through the publication of. While the initial datagram queueing model was simple to implement and needed no more tuning than queue lengths, support of more sophisticated and parametrized mechanisms were found necessary "to improve and preserve Internet performance" (RED, ECN etc.). Further research on the subject was also called for, with a list of identified items.

Definition
The term datagram is defined as follows: ""A self-contained, independent entity of data carrying sufficient information to be routed from the source to the destination computer without reliance on earlier exchanges between this source and destination computer and the transporting network.""

- RFC 1594

A datagram needs to be self-contained without reliance on earlier exchanges because there is no connection of fixed duration between the two communicating points as there is, for example, in most voice telephone conversations.

Datagram service is often compared to a mail delivery service; the user only provides the destination address but receives no guarantee of delivery, and no confirmation upon successful receipt. Datagram service is therefore considered unreliable. Datagram service routes datagrams without first creating a predetermined path. Datagram service is therefore considered connectionless. There is also no consideration given to the order in which it and other datagrams are sent or received. In fact, many datagrams in the same group can travel along different paths before reaching the same destination in a different order.

Structure
Each datagram has two components, a header and a data payload. The header contains all the information sufficient for routing from the originating equipment to the destination without relying on prior exchanges between the equipment and the network. Headers may include source and destination addresses as well as type and length fields. The payload is the data to be transported. This process of nesting data payloads in a tagged header is called encapsulation.

Internet Protocol
The Internet Protocol (IP) defines standards for several types of datagrams. The internet layer is a datagram service provided by an IP. For example, UDP is run by a datagram service on the internet layer. IP is an entirely connectionless, best effort, unreliable, message delivery service. TCP is a higher-level protocol running on top of IP that provides a reliable connection-oriented service.

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

title: Datagram


text:
Data integrity

Data integrity is the maintenance of, and the assurance of, data accuracy and consistency over its entire life-cycle. It is a critical aspect to the design, implementation, and usage of any system that stores, processes, or retrieves data. The term is broad in scope and may have widely different meanings depending on the specific context even under the same general umbrella of computing. It is at times used as a proxy term for data quality, while data validation is a prerequisite for data integrity.

Definition
Data integrity is the opposite of data corruption. The overall intent of any data integrity technique is the same: ensure data is recorded exactly as intended (such as a database correctly rejecting mutually exclusive possibilities). Moreover, upon later retrieval, ensure the data is the same as when it was originally recorded. In short, data integrity aims to prevent unintentional changes to information. Data integrity is not to be confused with data security, the discipline of protecting data from unauthorized parties.

Any unintended changes to data as the result of a storage, retrieval or processing operation, including malicious intent, unexpected hardware failure, and human error, is failure of data integrity. If the changes are the result of unauthorized access, it may also be a failure of data security. Depending on the data involved this could manifest itself as benign as a single pixel in an image appearing a different color than was originally recorded, to the loss of vacation pictures or a business-critical database, to even catastrophic loss of human life in a life-critical system.

Physical integrity
Physical integrity deals with challenges which are associated with correctly storing and fetching the data itself. Challenges with physical integrity may include electromechanical faults, design flaws, material fatigue, corrosion, power outages, natural disasters, and other special environmental hazards such as ionizing radiation, extreme temperatures, pressures and g-forces. Ensuring physical integrity includes methods such as redundant hardware, an uninterruptible power supply, certain types of RAID arrays, radiation hardened chips, error-correcting memory, use of a clustered file system, using file systems that employ block level checksums such as ZFS, storage arrays that compute parity calculations such as exclusive or or use a cryptographic hash function and even having a watchdog timer on critical subsystems.

Physical integrity often makes extensive use of error detecting algorithms known as error-correcting codes. Human-induced data integrity errors are often detected through the use of simpler checks and algorithms, such as the Damm algorithm or Luhn algorithm. These are used to maintain data integrity after manual transcription from one computer system to another by a human intermediary (e.g. credit card or bank routing numbers). Computer-induced transcription errors can be detected through hash functions.

In production systems, these techniques are used together to ensure various degrees of data integrity. For example, a computer file system may be configured on a fault-tolerant RAID array, but might not provide block-level checksums to detect and prevent silent data corruption. As another example, a database management system might be compliant with the ACID properties, but the RAID controller or hard disk drive's internal write cache might not be.

Logical integrity
This type of integrity is concerned with the correctness or rationality of a piece of data, given a particular context. This includes topics such as referential integrity and entity integrity in a relational database or correctly ignoring impossible sensor data in robotic systems. These concerns involve ensuring that the data "makes sense" given its environment. Challenges include software bugs, design flaws, and human errors. Common methods of ensuring logical integrity include things such as check constraints, foreign key constraints, program assertions, and other run-time sanity checks.

Physical and logical integrity often share many challenges such as human errors and design flaws, and both must appropriately deal with concurrent requests to record and retrieve data, the latter of which is entirely a subject on its own.

If a data sector only has a logical error, it can be reused by overwriting it with new data. In case of a physical error, the affected data sector is permanently unusable.

Databases
Data integrity contains guidelines for data retention, specifying or guaranteeing the length of time data can be retained in a particular database (typically a relational database). To achieve data integrity, these rules are consistently and routinely applied to all data entering the system, and any relaxation of enforcement could cause errors in the data. Implementing checks on the data as close as possible to the source of input (such as human data entry), causes less erroneous data to enter the system. Strict enforcement of data integrity rules results in lower error rates, and time saved troubleshooting and tracing erroneous data and the errors it causes to algorithms.

Data integrity also includes rules defining the relations a piece of data can have to other pieces of data, such as a Customer record being allowed to link to purchased Products, but not to unrelated data such as Corporate Assets. Data integrity often includes checks and correction for invalid data, based on a fixed schema or a predefined set of rules. An example being textual data entered where a date-time value is required. Rules for data derivation are also applicable, specifying how a data value is derived based on algorithm, contributors and conditions. It also specifies the conditions on how the data value could be re-derived.

Types of integrity constraints
Data integrity is normally enforced in a database system by a series of integrity constraints or rules. Three types of integrity constraints are an inherent part of the relational data model: entity integrity, referential integrity and domain integrity.

 * Entity integrity concerns the concept of a primary key. Entity integrity is an integrity rule which states that every table must have a primary key and that the column or columns chosen to be the primary key should be unique and not null.
 * Referential integrity concerns the concept of a foreign key. The referential integrity rule states that any foreign-key value can only be in one of two states. The usual state of affairs is that the foreign-key value refers to a primary key value of some table in the database. Occasionally, and this will depend on the rules of the data owner, a foreign-key value can be null. In this case, we are explicitly saying that either there is no relationship between the objects represented in the database or that this relationship is unknown.
 * Domain integrity specifies that all columns in a relational database must be declared upon a defined domain. The primary unit of data in the relational data model is the data item. Such data items are said to be non-decomposable or atomic. A domain is a set of values of the same type. Domains are therefore pools of values from which actual values appearing in the columns of a table are drawn.
 * User-defined integrity refers to a set of rules specified by a user, which do not belong to the entity, domain and referential integrity categories.

If a database supports these features, it is the responsibility of the database to ensure data integrity as well as the consistency model for the data storage and retrieval. If a database does not support these features, it is the responsibility of the applications to ensure data integrity while the database supports the consistency model for the data storage and retrieval.

Having a single, well-controlled, and well-defined data-integrity system increases:

 * stability (one centralized system performs all data integrity operations)
 * performance (all data integrity operations are performed in the same tier as the consistency model)
 * re-usability (all applications benefit from a single centralized data integrity system)
 * maintainability (one centralized system for all data integrity administration).

Modern databases support these features (see Comparison of relational database management systems), and it has become the de facto responsibility of the database to ensure data integrity. Companies, and indeed many database systems, offer products and services to migrate legacy systems to modern databases.

Examples
An example of a data-integrity mechanism is the parent-and-child relationship of related records. If a parent record owns one or more related child records all of the referential integrity processes are handled by the database itself, which automatically ensures the accuracy and integrity of the data so that no child record can exist without a parent (also called being orphaned) and that no parent loses their child records. It also ensures that no parent record can be deleted while the parent record owns any child records. All of this is handled at the database level and does not require coding integrity checks into each application.

File systems
Various research results show that neither widespread filesystems (including UFS, Ext, XFS, JFS and NTFS) nor hardware RAID solutions provide sufficient protection against data integrity problems.

Some filesystems (including Btrfs and ZFS) provide internal data and metadata checksumming that is used for detecting silent data corruption and improving data integrity. If a corruption is detected that way and internal RAID mechanisms provided by those filesystems are also used, such filesystems can additionally reconstruct corrupted data in a transparent way. This approach allows improved data integrity protection covering the entire data paths, which is usually known as end-to-end data protection.

Data integrity as applied to various industries

 * The U.S. Food and Drug Administration has created draft guidance on data integrity for the pharmaceutical manufacturers required to adhere to U.S. Code of Federal Regulations 21 CFR Parts 210–212. Outside the U.S., similar data integrity guidance has been issued by the United Kingdom (2015), Switzerland (2016), and Australia (2017).
 * Various standards for the manufacture of medical devices address data integrity either directly or indirectly, including ISO 13485, ISO 14155, and ISO 5840.
 * In early 2017, the Financial Industry Regulatory Authority (FINRA), noting data integrity problems with automated trading and money movement surveillance systems, stated it would make "the development of a data integrity program to monitor the accuracy of the submitted data" a priority. In early 2018, FINRA said it would expand its approach on data integrity to firms' "technology change management policies and procedures" and Treasury securities reviews.
 * Other sectors such as mining and product manufacturing are increasingly focusing on the importance of data integrity in associated automation and production monitoring assets.
 * Cloud storage providers have long faced significant challenges ensuring the integrity or provenance of customer data and tracking violations.

title: Data integrity


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Data link

A data link is a means of connecting one location to another for the purpose of transmitting and receiving digital information (data communication). It can also refer to a set of electronics assemblies, consisting of a transmitter and a receiver (two pieces of data terminal equipment) and the interconnecting data telecommunication circuit. These are governed by a link protocol enabling digital data to be transferred from a data source to a data sink.

Types
There are at least three types of basic data-link configurations that can be conceived of and used:
 * Simplex communications, most commonly meaning all communications in one direction only.
 * Half-duplex communications, meaning communications in both directions, but not both ways simultaneously.
 * Duplex communications, communications in both directions simultaneously.

Aviation
In civil aviation, a data-link system (known as Controller Pilot Data Link Communications) is used to send information between aircraft and air traffic controllers for example when an aircraft is too far from the ATC to make voice radio communication and radar observations possible. Such systems are used for aircraft crossing the Atlantic and Pacific oceans. One such system, used by Nav Canada and NATS over the North Atlantic, uses a five-digit data link sequence number confirmed between air traffic control and the pilots of the aircraft before the aircraft proceeds to cross the ocean. This system uses the aircraft's flight management computer to send location, speed and altitude information about the aircraft to the ATC. ATC can then send messages to the aircraft regarding any necessary change of course.

In unmanned aircraft, land vehicles, boats, and spacecraft, a two-way (full-duplex or half-duplex) data-link is used to send control signals, and to receive telemetry.

title: Data link


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Data service unit

A data service unit, sometimes called a digital service unit, is a piece of telecommunications circuit terminating equipment that transforms digital data between telephone company lines and local equipment. The device converts bipolar digital signals coming ultimately from a digital circuit and directly from a Channel service unit (CSU), into a format (e.g. RS- 530) compatible with the piece of data terminal equipment (DTE) (e.g. a router) to which the data is sent. The DSU also performs a similar process in reverse for data heading from the DTE toward the circuit. The telecommunications service a DSU supports can be a point-to-point or multipoint operation in a digital data network.

Form and purpose
A DSU is a two or more port device; one port is called the WAN (wide area network) port and the other is called a DTE port. The purpose of the DSU is to transfer serial data synchronously between the WAN port and the DTE ports. If more than one DTE port is used, the DSU assigns the DTE data according to time slots (channels) on the WAN side.

On the WAN side, the DSU, via a CSU, interfaces with a digital carrier such as DS1 or DS3 or a low speed Digital Data Service. On the DTE side, the DSU provides control lines, timing lines and appropriate physical and electrical interface. To maintain the synchronous relationship between the ports, the DSU manages timing by slaving ports to the bit rate of another or to its internal clock. Typically, the DTE port provides timing to the data terminal equipment while the WAN port dictates the rate.

DSUs usually include some maintenance capabilities. At minimum, they can loop data back at either the WAN or DTE ports, or at both. When only one port is looped back, the data received at that port is simultaneously sent back toward the port and passed in normal fashion to the other port. Most DSUs also allow various data patterns to be generated and monitored to measure error rate of the communication link. A DSU may be a separate piece of equipment, or may be combined in a CSU/DSU.

title: Data service unit


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Data signaling rate

In telecommunication, data signaling rate (DSR), also known as gross bit rate, is the aggregate rate at which data passes a point in the transmission path of a data transmission system.

Properties

 * 1) The DSR is usually expressed in bits per second.
 * 2) The data signaling rate is given by $$\sum_{i = 1}^{m} \frac{\log_2 {n_i} }{T_i}$$ where m is the number of parallel channels, ni is the number of significant conditions of the modulation in the i-th channel, and Ti is the unit interval, expressed in seconds, for the i-th channel.
 * 3) For serial transmission in a single channel, the DSR reduces to (1/T)log2n; with a two-condition modulation, i. e. n = 2, the DSR is 1/T, according to Hartley's law.
 * 4) For parallel transmission with equal unit intervals and equal numbers of significant conditions on each channel, the DSR is (m/T)log2n; in the case of a two-condition modulation, this reduces to m/T.
 * 5) The DSR may be expressed in bauds, in which case, the factor log2ni in the above summation formula should be deleted when calculating bauds.
 * 6) In synchronous binary signaling, the DSR in bits per second may be numerically the same as the modulation rate expressed in bauds. Signal processors, such as four-phase modems, cannot change the DSR, but the modulation rate depends on the line modulation scheme, in accordance with Note 4. For example, in a 2400 bit/s 4-phase sending modem, the signaling rate is 2400 bit/s on the serial input side, but the modulation rate is only 1200 bauds on the 4-phase output side.

Maximum rate
The maximum user signaling rate, synonymous to gross bit rate or data signaling rate, is the maximum rate, in bits per second, at which binary information can be transferred in a given direction between users over the telecommunications system facilities dedicated to a particular information transfer transaction, under conditions of continuous transmission and no overhead information.

For a single channel, the signaling rate is given by $$SCSR = \frac{\log_2{n} }{T}$$, where SCSR is the single-channel signaling rate in bits per second, T is the minimum time interval in seconds for which each level must be maintained, and n is the number of significant conditions of modulation of the channel.

In the case where an individual end-to-end telecommunications service is provided by parallel channels, the parallel-channel signaling rate is given by $$PCSR = \sum_{i = 1}^{m} \frac{\log_2 {n_i} }{T_i}$$, where PCSR is the total signaling rate for m channels, m is the number of parallel channels, Ti is the minimum interval between significant instants for the I-th channel, and ni is the number of significant conditions of modulation for the I-th channel.

In the case where an end-to-end telecommunications service is provided by tandem channels, the end-to-end signaling rate is the lowest signaling rate among the component channels.

title: Data signaling rate


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Date-time group

In communications messages, a date-time group (DTG) is a set of characters, usually in a prescribed format, used to express the year, the month, the day of the month, the hour of the day, the minute of the hour, and the time zone, if different from Coordinated Universal Time (UTC). The order in which these elements are presented may vary. The DTG is usually placed in the header of the message. One example is " (UTC)"; while another example is "".

The DTG may indicate either the date and time a message was dispatched by a transmitting station or the date and time it was handed into a transmission facility by a user or originator for dispatch.

The DTG may be used as a message identifier if it is unique for each message.

Military Date Time Group
A form of DTG is used in the US Military's message traffic (a form of Automated Message Handling System). In US military messages and communications (e.g., on maps showing troop movements) the format is DD HHMM (SS) Z MON YY. Although occasionally seen with spaces, it can also be written as a single string of characters. Three different formats can be found: Z references the military identifier of time zone:
 * DDHHMMSSZmmmYY - Full time (used for software timestamps)
 * DDHHMMZmmmYY - shortened time (used e.g. for timestamps manually written)
 * DDHHMMZ - short time (e.g. used for planning)
 * UTC-12: Y (e.g., Baker Island)
 * UTC-11: X (American Samoa, Niue)
 * UTC-10: W (Honolulu, HI)
 * UTC-9: V (Juneau, AK)
 * UTC-8: U (PST, Los Angeles, CA)
 * UTC-7: T (MST, Denver, CO)
 * UTC-6: S (CST, Dallas, TX)
 * UTC-5: R (EST, New York, NY)
 * UTC-4: Q (Halifax, Nova Scotia)
 * UTC-3: P (Buenos Aires, Argentina; Rio de Janeiro, Brazil)
 * UTC-2: O (South Georgia and the South Sandwich Islands)
 * UTC-1: N (Azores)
 * UTC+-0: Z (Zulu time)
 * UTC+1: A (France)
 * UTC+2: B (Athens, Greece)
 * UTC+3: C (Arab Standard Time, Iraq, Bahrain, Kuwait, Saudi Arabia, Yemen, Qatar, as well as Moscow in Russia)
 * UTC+4: D (Oman, the UAE)
 * UTC+5: E (Pakistan, western Kazakhstan, Tajikistan, Uzbekistan, and Turkmenistan)
 * UTC+6: F (eastern Kazakhstan, Bangladesh)
 * UTC+7: G (Thailand)
 * UTC+8: H (Beijing, China, Singapore)
 * UTC+9: I (Tokyo, Japan)
 * UTC+10: K (Brisbane, Australia)
 * UTC+11: L (Sydney, Australia)
 * UTC+12: M (Wellington, New Zealand)

Examples
Example 1: 051100Z represents the 5th day of the current month 11:00 (UTC).

Example 2: 091630Tjul11 represents 9 July 2011 4:30 pm (MST).

Example 3: represents the current time of refresh:  (UTC).

title: Date-time group


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DB (car)

Deutsch-Bonnet (DB), is a brand of sports cars created in 1937 by Charles Deutsch and René Bonnet and disappeared in 1962.

The D.B Coupés, in racing or customer versions, were intensively involved in rallying and on international circuits: 24 Hours of Le Mans, 12 Hours of Sebring, 12 Hours of Reims, TdF Automobile (notably with Prince Rainier of Monaco at the wheel, who made the brand famous).

History

DB (until 1947 known as Deutsch-Bonnet) was a French automobile maker between 1938 and 1961, based in Champigny-sur-Marne near Paris. The firm was founded by Charles Deutsch and René Bonnet, an offshoot of the Deutsch family's existing coachbuilding shop which had been taken over by Bonnet in 1932. Immediately before the war the partners concentrated on making light-weight racing cars, but a few years after the war, starting with the presentation of a Panhard based cabriolet at the 1950 Paris Motor Show, the company also began to produce small road-going sports cars. By 1952 the company no longer had its own stand at the Paris Motor Show, but one of their cars appeared as a star attraction on the large Panhard stand, reflecting the level of cooperation between the two businesses.

The company was defunct by 1961, as Deutsch and Bonnet's differing design philosophies hamstrung further cooperation. The number of DB's built is not certain; estimates of up to 2,000 cars are mentioned but more conservative numbers are closer to one thousand.

Light-weight engineering
The business produced light sports cars, originally in steel or aluminium but subsequently with fibreglass bodies mainly powered by Panhard flat-twin engines, most commonly of 610, 744, or 848 cc. Deutsch was a "theoretical engineer who had a natural instinct for aerodynamics," while Bonnet was a more "pragmatic mechanical engineer".

The fibreglass bodies covered a tubular central beam chassis made from steel, with front wheel drive and four wheel independent suspension directly lifted from the Panhard donors. Until 1952 all DBs had been intended for competition purposes only.

Racing origins
Bonnet had been promised a works drive in an Amilcar Pégase in the 1936 French Grand Prix for sports cars, but when this failed to materialise they set about building their own racer. The 1938 alloy-bodied DB1 roadster was a special, built using the remains of a Citroën Traction Avant 11CV. The construction took seventeen months. A series of numbered successors followed. The close-roofed 1.5-litre DB2's career was hindered by the war and was sold later, without Deutsch ever using it. The DB3 was a monocoque project developed during the war, but was never built, as the improved pontoon-bodied DB4 took preference. With a central beam chassis with a forked cradle for the 1.5 litre Traction 7A-based engine (originally intended for the DB2) it was finished in July 1945, with most of the work having been carried out in secret during the occupation. The very similar 2-litre DB5 was finished soon thereafter. Their two specials both placed in the first postwar race in France, in Paris in 1945, being the only post-war cars entered. An open-wheeled DB7 appeared in 1947 (preceded by the heavy and large DB6 which saw very little action), after which the Automobiles Deutsch & Bonnet was officially formed.

Neither single-seater DB was at all successful, but they did show Deutsch - who had hitherto preferred dependable standard units - that a tuned engine would become necessary. DB thus moved into the performance parts market, developing and offering a four-speed conversion for Citroëns and an overhead camshaft head - developed with the aid of engine specialists Maurice Sainturat and Dante Giacosa. The DB8 appeared in 1948, and won two concours d'élegances before partaking in any competitions. Their early cars were all built using Citroën parts, but supply was troublesome and DB soon moved on to using Panhard technology. This relationship came about as Deutsch was an officer of independent racer's club AGACI. When this organization decided to begin a Mouvement Racer 500, modelled on the British Formula 3, Deutsch offered club members the design of a racing car using a Panhard 500 engine. One member asked to have DB build such a car, and after it made a star appearance at the 1949 Paris Salon Panhard was happy to support the construction of about fifteen more. The formula expired in 1951, with the DB Panhard 500 never competitive abroad.

DB was very active in competition, especially in Le Mans 24 Hours and other long distance racing. Nearly all DBs, even the road cars, were designed with competition foremost in mind. In 1952, a DB Speedster was entered in the 12 Hours of Sebring and won its class handsomely, beginning its career in the United States market. Steve Lansing and Ward Morehouse were the drivers.

At the 1954 Le Mans DB entered five cars and were also involved with Panhards "Monopole" racers. René Bonnet himself, together with racing legend Élie Bayol, finished tenth overall and best of the DBs. The other Panhard-engined also finished (in 16th), while three Renault-engined central-seater DB designs all failed to complete the race. The Renault-engined designs had been created as a concession to pressure from DB's customers, but they did very badly in the race, in part because of a shortage of preparation time for what was an unknown entity to Deutsch and Bonnet. In either case, DB proceeded to focus on Panhard designs exclusively.

Road cars
The 1949 DB8 was bodied by Antem of Belgium and shown at the 1949 Paris Salon. While a handsome (winning two concours d'élegances) and modern design, Citroën refused to allow the provision of parts for series production. After DB began to depend on Panhard for engines, Antem was again commissioned to make a cabriolet with the intent to build a small series of street cars. 3750 mm long, the car weighed 500 kg and used the Dyna's 750 cc flat-two and much of the suspension and drivetrain. As with most DBs, it had a central frame with two outliers. An 850 cc version was also offered, a model which could reach 140 rather than the 125–130 km/h of the smaller one. Naturally, Panhard developed a racing barquette version (called the Tank) of the Antem cabriolet. These competed at Le Mans 1951 as well as several other races. About twenty Antem cabriolets were built in 1951, but DB chose to let it die in favor of a coupé version of the same ("Coach" in French). A few DB-Antem Coach were built, mostly for competition. These had bodywork designed by Deutsch, and again mainly relied on Dyna underpinnings and a central steel-tube frame.

The steel-bodied, Frua-designed 1952 "Mille Miles" (celebrating class victories at the Mille Miglia) was a mini-GT with a 65 hp Panhard two-cylinder. It was somewhat expensive, and at the 1953 Paris Salon a Chausson-designed DB Coach in fibreglass, although it did not enter production until 1954. The HBR 4/5 model (1954–1959) was the partners' most successful project to date, with several hundred of the little cars produced between 1954 and 1959. This was followed by the Le Mans convertible and hardtop, which was shown in 1959 and built by DB until 1962, and continued until 1964 by René Bonnet. About 660 of the Mille Miles/Coach/HBR were built, and 232 DB Le Mans (not including the Bonnet-built cars). Later versions could be equipped with engines of 1 and 1.3 litres, and superchargers were also available. No two cars may have been alike, as they were built according to customer specifications from a wide range of options.

More racing success
Deutsch's very efficient and influential aerodynamic designs allowed DB race cars to reach impressive top speeds despite the small Panhard flat-twin engine. DB's received class victories at Le Mans (three times), Sebring (twice), and Mille Miglia (four times). DB even managed an outright win in the handicapped 1954 Tourist Trophy sports car race, with Laureau and Armagnac driving. DB always showed strongly in the "Index of Performance", a category especially suitable for DB's small-engined, aerodynamic little racers. The Index of Performance is perhaps best known at the Le Mans 24 hours competition, but the category also existed at many French automobile races of the era, such as the Tour de France. DBs were also successful in American SCCA racing, where they racked up an impressive number of victories in the H-sports category.

Disagreement and the end of the partnership
Deutsch and Bonnet disagreed whether they should build cars of front-wheel drive or mid-engined design. There was also disagreement on which engines to use. Charles Deutsch, wanting to stick to Panhard engines, left DB in 1961 to found his own firm (CD). Bonnet founded Automobiles René Bonnet, producing mid-engined cars equipped with Renault power units: this business was later to become part of Matra Automobiles. Deutsch ended up an engineering consultant.

title: DB (car)


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Weighting filter

A weighting filter is used to emphasize or suppress some aspects of a phenomenon compared to others, for measurement or other purposes.

Audio applications
In each field of audio measurement, special units are used to indicate a weighted measurement as opposed to a basic physical measurement of energy level. For sound, the unit is the phon (1 kHz equivalent level).

Sound
Sound has three basic components, the wavelength, frequency, and speed. In sound measurement, we measure the loudness of the sound in decibels (dB). Decibels are logarithmic with 0 dB as the reference. There are also a range of frequencies that sounds can have. Frequency is the number of times a sine wave repeats itself in a second. Normal auditory systems can usually hear between 20 and 20,000 Hz. When we measure sound, the measurement instrument takes the incoming auditory signal and analyzes it for these different features. Weighting filters in these instruments then filter out certain frequencies and decibel levels depending on the filter. A weighted filters are most similar to natural human hearing. This allows the sound level meter to determine what decibel level the incoming sound would likely be for a normal hearing human's auditory system.

Loudness measurements
In the measurement of loudness, for example, an A-weighting filter is commonly used to emphasize frequencies around 3–6 kHz where the human ear is most sensitive, while attenuating very high and very low frequencies to which the ear is insensitive. The aim is to ensure that measured loudness corresponds well with subjectively perceived loudness. A-weighting is only really valid for relatively quiet sounds and for pure tones as it is based on the 40-phon Fletcher–Munson equal-loudness contour. The B and C curves were intended for louder sounds (though they are less used) while the D curve is used in assessing loud aircraft noise (IEC 537). B curves filter out more medium loudness levels when compared to an A curves. This curve is rarely ever used in the assessment or monitoring of noise levels anymore. C curves differ from both A and B in the fact that they filter less of the lower and higher frequencies. The filter is a much flatter shape and is used in sound measurement in especially loud and noisy environments. A weighted curves follow a 40 phon curve while C weighted follows a 100 phon curve. The three curves differ not in their measurement of exposure levels, but in the frequencies measured. A weighted curves allow more frequencies equal to or less than 500 Hz through, which is most representative of the human ear.

Loudness measurements with weighting filters
There are a variety of reasons for measuring sound. This includes following regulations to protect worker's hearing, following noise ordinances, in telecommunications, and many more. At the basis of sound measurement is the idea of breaking down an incoming signal based on its different properties. Every incoming sinusoidal wave of sound has a frequency and amplitude. Using this information, a sound level can be deduced from the root-sums-of-squares of the amplitudes of all the incoming auditory information. Whether using a sound level meter or a noise dosimeter, the processing is somewhat similar. With a calibrated sound level meter, the incoming sounds are going to be picked up by the microphone and then measured by the internal electronic circuits. The sound measurement that the device outputs can be filtered through an A, B, or C weighting curve. The curve used will have slight effects on the resulting decibel level.

Telecommunications
In the field of telecommunications, weighting filters are widely used in the measurement of electrical noise on telephone circuits, and in the assessment of noise as perceived through the acoustic response of different types of instrument (handset). Other noise-weighting curves have existed, e.g. DIN standards. The term psophometric weighting, though referring in principle to any weighting curve intended for noise measurement, is often used to refer to a particular weighting curve, used in telephony for narrow-bandwidth voiceband speech circuits.

Environmental noise measurement
A-weighted decibels are abbreviated dB(A) or dBA. When acoustic (calibrated microphone) measurements are being referred to, then the units used will be dB SPL (sound pressure level) referenced to 20 micropascals = 0 dB SPL.

The A-weighting curve has been widely adopted for environmental noise measurement, and is standard in many sound level meters (see ITU-R 468 weighting for a further explanation).

A-weighting is also in common use for assessing potential hearing damage caused by loud noise, though this seems to be based on the widespread availability of sound level meters incorporating A-Weighting rather than on any good experimental evidence to suggest that such use is valid. The distance of the measuring microphone from a sound source is often "forgotten", when SPL measurements are quoted, making the data useless. In the case of environmental or aircraft noise, distance need not be quoted as it is the level at the point of measurement that is needed, but when measuring refrigerators and similar appliances the distance should be stated; where not stated it is usually one metre (1 m). An extra complication here is the effect of a reverberant room, and so noise measurement on appliances should state "at 1 m in an open field" or "at 1 m in anechoic chamber". Measurements made outdoors will approximate well to anechoic conditions.

A-weighted SPL measurements of noise level are increasingly to be found on sales literature for domestic appliances such as refrigerators and freezers, and computer fans. Although the threshold of hearing is typically around 0 dB SPL, this is in fact very quiet indeed, and appliances are more likely to have noise levels of 30 to 40 dB SPL.

Audio reproduction and broadcasting equipment

Human sensitivity to noise in the region of 6 kHz became particularly apparent in the late 1960s with the introduction of compact cassette recorders and Dolby-B noise reduction. A-weighted noise measurements were found to give misleading results because they did not give sufficient prominence to the 6 kHz region where the noise reduction was having greatest effect, and sometimes one piece of equipment would even measure worse than another and yet sound better, because of differing spectral content.

ITU-R 468 noise weighting was therefore developed to more accurately reflect the subjective loudness of all types of noise, as opposed to tones. This curve, which came out of work done by the BBC Research Department, and was standardised by the CCIR and later adopted by many other standards bodies (IEC, BSI/) and,, is maintained by the ITU. Noise measurements using this weighting typically also use a quasi-peak detector law rather than slow averaging. This also helps to quantify the audibility of bursty noise, ticks and pops that might go undetected with a slow rms measurement.

ITU-R 468 noise weighting with quasi-peak detection is widely used in Europe, especially in telecommunications, and in broadcasting particularly after it was adopted by the Dolby corporation who realised its superior validity for their purposes. Its advantages over A-weighting seem to be less well appreciated in the US and in consumer electronics, where the use of A-weighting predominates—probably because A-weighting produces a 9 to 12 dB "better" specification, see specsmanship. It is commonly used by broadcasters in Britain, Europe, and former countries of the British Empire such as Australia and South Africa.

Though the noise level of 16-bit audio systems (such as CD players) is commonly quoted (on the basis of calculations that take no account of subjective effect) as −96 dB relative to FS (full scale), the best 468-weighted results are in the region of −68 dB relative to Alignment Level (commonly defined as 18 dB below FS) i.e. −86 dB relative to FS.

The use of weighting curves is in no way to be regarded as 'cheating', provided that the proper curve is used. Nothing of relevance is being 'hidden', and even when, for example, hum is present at 50 or 100 Hz at a level above the quoted (weighted) noise floor this is of no importance because our ears are very insensitive to low frequencies at low levels, so it will not be heard. A-weighting is often used to compare and qualify ADCs, for instance, because it more accurately represents the way noise shaping hides dither noise in the ultrasonic range.

Other applications of weighting
In the measurement of gamma rays or other ionising radiation, a radiation monitor or dosimeter will commonly use a filter to attenuate those energy levels or wavelengths that cause the least damage to the human body, while letting through those that do the most damage, so that any source of radiation may be measured in terms of its true danger rather than just its 'strength'. The sievert is a unit of weighted radiation dose for ionising radiation, which supersedes the older unit the REM (roentgen equivalent man).

Weighting is also applied to the measurement of sunlight when assessing the risk of skin damage through sunburn, since different wavelengths have different biological effects. Common examples are the SPF of sunscreen, and the UV index.

Another use of weighting is in television, where the red, green and blue components of the signal are weighted according to their perceived brightness. This ensures compatibility with black and white receivers, and also benefits noise performance and allows separation into meaningful luminance and chrominance signals for transmission.

title: Weighting filter


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DBm

dBm or dBmW (decibel-milliwatts) is a unit of power level expressed using a logarithmic decibel (dB) scale respective to one milliwatt (mW). It's a quick-and-dirty metric commonly used by radio, microwave and fiber-optical communication technicians & engineers to measure the power of system transmissions on a log scale, which can express both very large and very small values in a short form. dBW is a similar unit measured relative to one watt (1,000 mW), rather than a milliwatt.

The decibel (dB) is a dimensionless unit, used for quantifying the ratio between two values, such as signal-to-noise ratio. The dBm is also dimensionless, but since it compares to a fixed reference value, the dBm rating is an absolute one.

The dBm is not a part of the International System of Units (SI) and therefore is discouraged from use in documents or systems that adhere to SI units. (The corresponding SI unit is the watt.) However, the unit decibel (dB), without the 'm' suffix, is permitted for relative quantities, but not accepted for use directly alongside SI units. Ten decibel-milliwatts may be written 10 dB (1 mW) in SI.

In audio and telephony, dBm is typically referenced relative to the 600-ohm impedance commonly used in telephone voice networks, while in radio-frequency work dBm is typically referenced relative to a 50-ohm impedance.

Unit conversions
A power level of 0 dBm corresponds to a power of 1 milliwatt. A 10 dB increase in level is equivalent to a ten-fold increase in power. Therefore, a 20 dB increase in level is equivalent to a 100-fold increase in power. A 3 dB increase in level is approximately equivalent to doubling the power, which means that a level of 3 dBm corresponds roughly to a power of 2 mW. Similarly, for each 3 dB decrease in level, the power is reduced by about one half, making −3 dBm correspond to a power of about 0.5 mW.

To express an arbitrary power P in mW as x in dBm, the following expression may be used:
 * $$\begin{align}

x &= 10 \log_{10} \frac{P}{1~\text{mW}} \end{align}$$ Conversely, to express an arbitrary power level x in dBm, as P in mW:
 * $$\begin{align}

P &= 1~\text{mW} \cdot 10^{\frac{x}{10}} \end{align}$$

Table of examples
Below is a table summarizing useful cases:

Standards
The signal intensity (power per unit area) can be converted to received signal power by multiplying by the square of the wavelength and dividing by 4π (see Free-space path loss).

In United States Department of Defense practice, unweighted measurement is normally understood, applicable to a certain bandwidth, which must be stated or implied.

In European practice, psophometric weighting may be, as indicated by context, equivalent to dBm0p, which is preferred.

In audio, 0 dBm often corresponds to approximately 0.775 volts, since 0.775 V dissipates 1 mW in a 600 Ω load. The corresponding voltage level is 0 dBu, without the 600 Ω restriction. Conversely, for RF situations with a 50 Ω load, 0 dBm corresponds to approximately 0.224 volts, since 0.224 V dissipates 1 mW in a 50 Ω load. In general the relationship between the power level P in dBms and the RMS voltage V in volts across a load of resistance R (typically used to terminate a transmission line with impedance Z) is:
 * $$\begin{align}

V &= \sqrt{R \frac{10^{x/10}}{1000}} \end{align}$$

Expression in dBm is typically used for optical and electrical power measurements, not for other types of power (such as thermal). A listing by power levels in watts is available that includes a variety of examples not necessarily related to electrical or optical power.

The dBm was first proposed as an industry standard in the paper "A New Standard Volume Indicator and Reference Level".

title: DBm


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DBrn

The symbol dBrn or dB(rn) is an abbreviation for decibels above reference noise.

Weighted noise power in dB is referred to 1.0 picowatt. Thus, 0 dBrn = -90 dBm. Use of 144 line, 144-receiver, or C-message weighting, or flat weighting, can be indicated in parentheses.

With C-message weighting, a one-milliwatt, 1000 Hz tone will read +90 dBrn, but the same power as white noise, randomly distributed over a 3 kHz band will read approximately +88.5 dBrn, because of the frequency weighting.

With 144 weightings, a one milliwatt, 1000 Hz white noise tone will also read +90 dBrn, but the same 3 kHz power will only read +82 dBrn, because of the different frequency weighting.

Note: In telecommunications, dBrn adjusted also called dBa denotes "decibels adjusted", i.e. weighted absolute noise power. This is totally unrelated to and not a synonym for dB(A).

title: DBrn


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Data circuit-terminating equipment

A data circuit-terminating equipment (DCE) is a device that sits between the data terminal equipment (DTE) and a data transmission circuit. It is also called data communication(s) equipment  and data carrier equipment. Usually, the DTE device is the terminal (or computer), and the DCE is a modem.

In a data station, the DCE performs functions such as signal conversion, coding, and line clocking and may be a part of the DTE or intermediate equipment. Interfacing equipment may be required to couple the DTE into a transmission circuit or channel and from a transmission circuit or channel into the DTE.

Usage
Although the terms are most commonly used with RS-232, several data communication standards define different types of interfaces between a DCE and a DTE. The DCE is a device that communicates with a DTE device in these standards. Standards that use this nomenclature include:
 * Federal Standard 1037C, MIL-STD-188
 * RS-232
 * Certain ITU-T standards in the V series (notably V.24 and V.35)
 * Certain ITU-T standards in the X series (notably X.21 and X.25)

A general rule is that DCE devices provide the clock signal (internal clocking) and the DTE device synchronizes on the provided clock (external clocking). D-sub connectors follow another rule for pin assignment. DTE devices usually transmit on pin connector number 2 and receive on pin connector number 3. DCE devices are just the opposite: pin connector number 2 receives and pin connector number 3 transmits the signals.

When two devices, that are both DTE or both DCE, must be connected together without a modem or a similar media translator between them, a crossover cable must be used, e.g. a null modem for RS-232 or an Ethernet crossover cable/body

title: Data circuit-terminating equipment


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Degradation

Degradation may refer to:

Science

 * Degradation (geology), lowering of a fluvial surface by erosion
 * Degradation (telecommunications), of an electronic signal
 * Biodegradation of organic substances by living organisms
 * Environmental degradation in ecology
 * Land degradation, a process in which the value of the biophysical environment is affected by a combination of human-induced processes acting upon the land
 * Polymer degradation, as plastics age

Other

 * Elegant degradation, gradual rather than sudden
 * Graceful degradation, in a fault-tolerant system
 * Degradation (knighthood), revocation of knighthood
 * Cashiering, whereby a military officer is dismissed for misconduct
 * Reduction in rank, whereby a military officer is reduced to a lower rank for misconduct
 * Degradation, the former ceremony of defrocking a disgraced priest
 * Degradation, a song by the Violent Femmes, from Add It Up (1981–1993)

title: Degradation


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Degree of isochronous distortion

The degree of isochronous distortion, in data transmission, is the ratio of the absolute value of the maximum measured difference between the actual and the theoretical intervals separating any two significant instants of modulation (or demodulation), to the unit interval. These instants are not necessarily consecutive. This value is usually expressed as a percentage.

The result of the measurement should be qualified by an indication if the period, usually limited, of the observation. For a prolonged modulation (or demodulation), it will be appropriate to consider the probability that an assigned value of the degree of distortion will be exceeded.

title: Degree of isochronous distortion


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Degree of start-stop distortion

In telecommunication, the term degree of start-stop distortion has the following meanings:

 * 1) In asynchronous serial communication data transmission, the ratio of (a) the absolute value of the maximum measured difference between the actual and theoretical intervals separating any significant instant of modulation (or demodulation) from the significant instant of the start element immediately preceding it to (b) the unit interval.
 * 2) The highest absolute value of individual distortion affecting the significant instants of a start-stop modulation.

The degree of distortion of a start-stop modulation (or demodulation) is usually expressed as a percentage. Distinction can be made between the degree of late (positive) distortion and the degree of early (negative) distortion.

title: Degree of start-stop distortion


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Delay

Delay or DeLay may refer to:

People

 * B. H. DeLay (1891–1923), American aviator and movie stunt pilot
 * Dorothy DeLay (1917–2002), American violin instructor
 * Florence Delay (born 1941), French academician and actor
 * Jan Delay, stage name of German musician Jan Phillip Eißfeldt (born 1976)
 * Jason Delay (born 1995), American baseball player
 * Jean Delay (1907–1987), French psychiatrist, neurologist, and writer
 * Paul deLay (1952–2007), American blues musician
 * Tom DeLay (born 1947), American politician
 * Tom Delay (businessman) (born 1959), British businessman
 * Vladislav Delay (born 1976), Finnish musician

Other uses

 * Delay (audio effect), a technology for producing delayed playback of an audio signal
 * Delay (programming), a programming language construct for delaying evaluation of an expression
 * Delay 1968 or Delay, a 1981 compilation album by German experimental rock band Can
 * The Delay, a 2012 Uruguayan film

title: Delay


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Delay line

Delay line may refer to:

 * Propagation delay, the length of time taken for something to reach its destination
 * Analog delay line, used to delay a signal
 * Bi-directional delay line, a numerical analysis technique used in computer simulation for solving ordinary differential equations by converting them to hyperbolic equations
 * Digital delay line, a sequential logic element
 * Delay-line memory, a form of computer memory used on some of the earliest digital computers
 * Detcord delay line loops

title: Delay line


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Delta modulation

Delta modulation (DM or Δ-modulation) is an analog-to-digital and digital-to-analog signal conversion technique used for transmission of voice information where quality is not of primary importance. DM is the simplest form of differential pulse-code modulation (DPCM) where the difference between successive samples is encoded into n-bit data streams. In delta modulation, the transmitted data are reduced to a 1-bit data stream representing either up (↗) or down (↘). Its main features are:

 * The analog signal is approximated with a series of segments.
 * Each segment of the approximated signal is compared to the preceding bits and the successive bits are determined by this comparison.
 * Only the change of information is sent, that is, only an increase or decrease of the signal amplitude from the previous sample is sent whereas a no-change condition causes the modulated signal to remain at the same ↗ or ↘ state of the previous sample.

To achieve high signal-to-noise ratio, delta modulation must use oversampling techniques, that is, the analog signal is sampled at a rate several times higher than the Nyquist rate.

Derived forms of delta modulation are continuously variable slope delta modulation, delta-sigma modulation, and differential modulation. Differential pulse-code modulation is the superset of DM.

Principle
Rather than quantizing the value of the input analog waveform, delta modulation quantizes the difference between the current and the previous step, as shown in the block diagram in Fig. 1.

The modulator is made by a quantizer which converts the difference between the input signal and the integral of the previous steps. In its simplest form, the quantizer can be realized with a comparator referenced to 0 (two levels quantizer), whose output is 1 or -1 if the quantizer's input is positive or negative. The demodulator is simply an integrator (like the one in the feedback loop) whose output rises or falls with each 1 or -1 received. The integrator itself constitutes a low-pass filter.

Transfer characteristics
The two sources of noise in delta modulation are "slope overload", when step size is too small to track the original waveform, and "granularity", when step size is too large. But a 1971 study shows that slope overload is less objectionable compared to granularity than one might expect based solely on SNR measures.

Output signal power
In delta modulation, there is no limit to the number of pulses of the same sign that may occur, so it is capable of tracking slow-changing signals of any amplitude without clipping. However, if the transmitted signal has excessive derivative (abrupt changes) then slope overload occurs and the modulated signal can not track the input signal. E.g. if the input signal is

$$m(t)={A\cos (\omega t)}$$,

the modulated signal (derivative of the input signal) which is transmitted by the modulator is

$$|\dot{m}(t)|_{max}=\omega A$$,

whereas the condition to avoid slope overload is

$$|\dot{m}(t)|_{max}=\omega A<\sigma f_s$$.

So the maximum amplitude of the input signal can be

$$A_{max}={\sigma f_s \over \omega}$$,

where fs is the sampling frequency and ω is the frequency of the input signal and σ is step size in quantization. So Amax is the maximum amplitude that DM can transmit without causing the slope overload and the power of transmitted signal depends on the maximum amplitude.

Bit-rate
If the communication channel is of limited bandwidth, there is the possibility of interference in either DM or PCM. Hence, 'DM' and 'PCM' operate at same bit-rate which is equal to N  times the sampling frequency.

History
The seminal paper combining feedback with oversampling to achieve delta modulation was by F. de Jager of Philips Research Laboratories in 1952. Initial patents include:

 * "Communication system utilizing constant amplitude pulses of opposite polarities" by Maurice Deloraine et. al. (French patent issued 1946, US patent filed 1947).
 * "Differential quantization of communication signals" by C. Chapin Cutler (filed 1950), which describes differential PCM and delta modulation (1-bit DPCM).

Adaptive delta modulation
Adaptive delta modulation (ADM) was first published by Dr. John E. Abate (Bell Labs Fellow) in his doctoral thesis at NJ Institute Of Technology in 1968. ADM was later selected as the standard for all NASA communications between mission control and space-craft.

In the mid-1980s, Massachusetts audio company DBX marketed a commercially unsuccessful digital recording system based on adaptive delta modulation. See DBX 700.

Adaptive delta modulation or Continuously variable slope delta modulation (CVSD) is a modification of DM in which the step size is not fixed. Rather, when several consecutive bits have the same direction value, the encoder and decoder assume that slope overload is occurring, and the step size becomes progressively larger.

Otherwise, the step size becomes gradually smaller over time. ADM reduces slope error, at the expense of increasing quantization error. This error can be reduced by using a low-pass filter. ADM provides robust performance in the presence of bit errors meaning error detection and correction are not typically used in an ADM radio design, it is this very useful technique that allows for adaptive-delta-modulation.

Applications
Contemporary applications of delta modulation includes, but is not limited to, recreating legacy synthesizer waveforms. With the increasing availability of FPGAs and game-related ASICs, sample rates are easily controlled so as to avoid slope overload and granularity issues. For example, the C64DTV used a 32 MHz sample rate, providing ample dynamic range to recreate the SID output to acceptable levels.

SBS Application 24 kbps delta modulation
Delta modulation was used by Satellite Business Systems (SBS) for its voice ports to provide long distance phone service to large domestic corporations with a significant inter-corporation communications need (such as IBM). This system was in service throughout the 1980s. The voice ports used digitally implemented 24 kbit/s delta modulation with Voice Activity Compression (VAC) and echo suppressors to control the half second echo path through the satellite. They performed formal listening tests to verify the 24 kbit/s delta modulator achieved full voice quality with no discernible degradation as compared to a high quality phone line or the standard 64 kbit/s μ-law companded PCM. This provided an eight to three improvement in satellite channel capacity. IBM developed the Satellite Communications Controller and the voice port functions.

The original proposal in 1974, used a state-of-the-art 24 kbit/s delta modulator with a single integrator and a Shindler Compander modified for gain error recovery. This proved to have less than full phone line speech quality. In 1977, one engineer with two assistants in the IBM Research Triangle Park, NC laboratory was assigned to improve the quality.

The final implementation replaced the integrator with a Predictor implemented with a two pole complex pair low-pass filter designed to approximate the long term average speech spectrum. The theory was that ideally the integrator should be a predictor designed to match the signal spectrum. A nearly perfect Shindler Compander replaced the modified version. It was found the modified compander resulted in a less than perfect step size at most signal levels and the fast gain error recovery increased the noise as determined by actual listening tests as compared to simple signal to noise measurements. The final compander achieved a very mild gain error recovery due to the natural truncation rounding error caused by twelve bit arithmetic.

The complete function of delta modulation, VAC and Echo Control for six ports was implemented in a single digital integrated circuit chip with twelve bit arithmetic. A single digital-to-analog converter (DAC) was shared by all six ports providing voltage compare functions for the modulators and feeding sample and hold circuits for the demodulator outputs. A single card held the chip, DAC and all the analog circuits for the phone line interface including transformers.

title: Delta modulation


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Demand assignment

In telecommunication, a demand assignment is a method which several users share access to a communication channel on a real-time basis, i.e., a user needing to communicate with another user on the same network requests the required circuit, uses it, and when the call is finished, the circuit is released, making the circuit available to other users.

Demand assignment is similar to conventional telephone switching, in which common trunks are provided for many users, on a demand basis, through a limited-size trunk group.

title: Demand assignment


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Demand factor

In telecommunication, electronics and the electrical power industry, the term demand factor is used to refer to the fractional amount of some quantity being used relative to the maximum amount that could be used by the same system. The demand factor is always less than or equal to one. As the amount of demand is a time dependent quantity so is the demand factor.

 * $$f_\text{Demand}(t) = \frac{\text{Demand}}{\text{Maximum possible demand}}$$

The demand factor is often implicitly averaged over time when the time period of demand is understood by the context.

Electrical engineering
In electrical engineering the demand factor is taken as a time independent quantity where the numerator is taken as the maximum demand in the specified time period instead of the averaged or instantaneous demand.

 * $$f_\text{Demand} = \frac{\text{Maximum load in given time period}}{\text{Maximum possible load}}$$

This is the peak in the load profile divided by the full load of the device.

Example: If a residence has equipment which could draw 6,000 W when all equipment was drawing a full load, drew a maximum of 3,000 W in a specified time, then the demand factor = 3,000 W / 6,000 W = 0.5

This quantity is relevant when trying to establish the amount of load for which a system should be rated. In the above example, it would be unlikely that the system would be rated to 6,000 W, even though there may be a slight possibility that this amount of power can be drawn. This is closely related to the load factor which is the average load divided by the peak load in a specified time period.

 * $$f_\text{Load} = \frac{\text{Average load}}{\text{Maximum load in given time period}}$$

title: Demand factor


text:
Demand load

In telecommunication, the term demand load can have the following meanings:

 * In general, the total power required by a facility. The demand load is the sum of the operational load (including any tactical load) and nonoperational demand loads. It is determined by applying the proper demand factor to each of the connected loads and a diversity factor to the sum total.
 * At a communications center, the power required by all automatic switching, synchronous, and terminal equipment (operated simultaneously on-line or in standby), control and keying equipment, plus lighting, ventilation, and air- conditioning equipment required to maintain full continuity of communications.
 * The power required for ventilating equipment, shop lighting, and other support items that may be operated simultaneously with the technical load.
 * The sum of the technical demand and nontechnical demand loads of an operating facility.

title: Demand load


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Design objective

Design objective (DO): In communications systems, a desired performance characteristic for communications circuits and equipment that is based on engineering analyses, but (a) is not considered feasible to mandate in a standard, or (b) has not been tested.

DOs are used because applicable systems standards are not in existence.

Examples of reasons for designating a performance characteristic as a DO rather than as a standard are (a) it may be bordering on an advancement in the state of the art, (b) the requirement may not have been fully confirmed by measurement or experience with operating circuits, and (c) it may not have been demonstrated that the requirement can be met considering other constraints, such as cost and size.

A DO is sometimes established in a standard for developmental consideration. A DO may also specify a performance characteristic used in the preparation of specifications for development or procurement of new equipment or systems.

Design is the process of formulation of a plan for satisfaction of human needs

title: Design objective


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Detector (disambiguation)

A detector is a device capable of registering a specific substance or physical phenomenon.

Detector may also refer to:

 * Detector (radio), a device that recovers information from a modulated wave
 * Detector (film), a 2000 Norwegian film
 * USS Detector, two United States Navy ships
 * USS Detector (AMc-75), was a coastal minesweeper launched 29 May 1941
 * USS Detector (AM-429), was a minesweeper launched 5 December 1952

title: Detector (disambiguation)


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Deterministic routing

In telecommunications, deterministic routing is the advance determination of the routes between given pairs of nodes. Examples:
 * 1) In a network where routing is controlled by a telephone switch or network switch, switching in which the routes between given pairs of nodes are pre-programmed, i.e., are determined, in advance of transmission. The routes used to complete a given call through a network are identified, in advance of transmission, in routing tables maintained in each switch database. The tables assign the trunks that are to be used to reach each switch code, area code, and International Access Prefix (IAP), usually with one or two alternate routes.
 * 2) In a non-switched network, the routes used to send a given message through the network are identified in advance in routing tables maintained in a database.

title: Deterministic routing


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Pulse dialing

Pulse dialing is a signaling technology in telecommunications in which a direct current local loop circuit is interrupted according to a defined coding system for each signal transmitted, usually a digit. This lends the method the often used name loop disconnect dialing. In the most common variant of pulse dialing, decadic dialing, each of the ten Arabic numerals are encoded in a sequence of up to ten pulses. The most common version decodes the digits 1 through 9, as one to nine pulses, respectively, and the digit 0 as ten pulses. Historically, the most common device to produce such pulse trains is the rotary dial of the telephone, lending the technology another name, rotary dialing.

The pulse repetition rate was historically determined based on the response time needed for electromechanical switching systems to operate reliably. Most telephone systems used the nominal rate of ten pulses per second, but operator dialing within and between central offices often used pulse rates up to twenty per second.

Early automatic exchanges
Automatic telephone exchange systems were developed in the late 19th and early 20th century. For identification, telephone subscribers were assigned a telephone number unique to each circuit. Various methods evolved to signal the desired destination telephone number for a telephone call directly dialed by the subscriber. An automatic switch-hook was designed by Hilborne Roosevelt.

The first commercial automatic telephone exchange, designed by Almon Brown Strowger, opened in La Porte, Indiana on 3 November 1892, and used two telegraph-type keys on the telephone, which had to be operated the correct number of times to control the vertical and horizontal relay magnets in the exchange. But the use of separate keys with separate conductors to the exchange was not practical. The most common signaling system became a system of using direct-current pulse trains generated in the telephone sets of subscribers by interrupting the single-pair wire loop of the telephone circuit.

Rotary dial
Strowger also filed the first patent for a rotary dial in 1891. The first dials worked by direct, forward action. The pulses were sent as the user rotated the dial to the finger stop starting at a different position for each digit transmitted. Operating the dial error-free required smooth rotary motion of the finger wheel by the user, but was found as too unreliable. This mechanism was soon refined to include a recoil spring and a centrifugal governor to control the recoil speed. The user selected a digit to be dialed by inserting a finger into the corresponding hole and rotated the dial to the finger stop. When released from this position, the dial pulsing contacts were opened and closed repeatedly, thus interrupting the loop current in a pattern on the return to the home position. The exchange switch decoded the pattern for each digit thus transmitted by stepping relays or by accumulation in digit registers.

Pulse rate and coding
In the first electromechanical switching systems, the current pulses generated by the rotary dial on the local loop directly operated electrical stepping switches at the central office. The mechanical nature of these relays generally limited the speed of operation, the pulsing rate, to ten pulses per second.

The specifications of the Bell System in the US required service personnel to adjust dials in customer stations to a precision of 9.5 to 10.5 pulses per second (PPS), but the tolerance of the switching equipment was generally between 8 and 11 PPS. The British (GPO, later Post Office Telecommunications) standard for Strowger switch exchanges has been ten impulses per second (allowable range 7 to 12) and a 66% break ratio (allowable range 63% to 72%).

In most switching systems one pulse is used for the digit 1, two pulses for 2, and so on, with ten pulses for the digit 0; this makes the code unary, excepting the digit 0. Exceptions to this are Sweden, with one pulse for 0, two pulses for 1, and so on, and New Zealand, with ten pulses for 0, nine pulses for 1, etc. Oslo, the capital city of Norway, used the New Zealand system, but the rest of the country did not. Systems that used this encoding of the ten digits in a sequence of up to ten pulses are known as decadic dialing systems.

Some switching systems used digit registers that doubled the allowable pulse rate up to twenty pulses per second, and the inter-digital pause could be reduced as the switch selection did not have to be completed during the pause. These included access lines to the panel switch in the 1920s, crossbar systems, the later version (7A2) of the rotary system, and the earlier 1970s stored program control exchanges.

In some telephones, the pulses may be heard in the receiver as clicking sounds. However, in general, such effects were undesirable and telephone designers suppressed them by mechanical means with off-normal switches on the dial, or greatly attenuated them by electrical means with a varistor connected across the receiver.

Switch-hook dialing
As pulse dialing is achieved by interruption of the local loop, it was possible to dial digits by rapidly tapping, i.e. depressing, the switch hook the corresponding number of times at approximately ten taps per second. However, many telephone makers implemented a slow switch hook release to prevent rapid switching.

In the United Kingdom, it was once possible to make calls from coin-box phones by tapping the switch hook without depositing coins. Unlawfully obtaining a free telephone call was deemed a criminal offence, abstracting electricity from the General Post Office, which operated the telephone system, and several cases were prosecuted.

In popular culture, tapping was shown in the film Red Dragon when prisoner Hannibal Lecter dialed out on a telephone without dialing mechanism. The technique was also used by the character Phantom Phreak in the film Hackers.

Successors
It was recognized as early as the 1910s that push button dialing was faster than with rotary dials, when the first panel switches used keys for dialing at the operator stations. In the 1940s, Bell Laboratories conducted field trials of pushbutton telephones for customer dialing to determine accuracy and efficiency. However, the technology of using mechanical reed relays was too unreliable until transistors transformed the industry. In 1963, the Bell System introduced to the public dual-tone multi-frequency (DTMF) technology under the name Touch-Tone, which was a trademark in the U.S. until 1984. The Touch-Tone system used push-button telephones. In the decades after 1963, rotary dials were gradually phased out on new telephone models in favor of keypads and the primary dialing method to the central office became touchtone dialing. Most central office systems still support rotary telephones today. Some keypad telephones have a switch or configuration method for the selection of tone or pulse dialing.

Mobile telephones and most voice-over-IP systems use out-of-band signaling and do not send any digits until the entire number has been keyed by the user. Many VoIP systems are based on the Session Initiation Protocol (SIP), which uses a form of Uniform Resource Identifiers (URI) for addressing, instead of digits alone.

title: Pulse dialing