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LAPM
Link Access Procedure for Modems (LAPM) is part of the V.42 error correction protocol for modems.ITU-T Recommendation V.42Error-correcting procedures for DCEs using asynchronous-to-synchronous conversion/ref> LAPM is an error control protocol defined in ITU-T recommendations V.42. Like many data link layer protocols, it is a variant of HDLC. Like the Microcom Networking Protocols that preceded it, LAPM uses cyclic redundancy checking (CRC) and retransmission of corrupted data (ARQ) to ensure data reliability. Error control protocols such as LAPM use frames of variable lengths, each beginning with a header and ending with a frame check sequence trailer (a cyclic redundancy check). Correctly received packets are acknowledged, and unacknowledged packets are automatically retransmitted ( ARQ). The larger the payload included in each frame, the smaller the relative cost of transmitting the header and trailer bits. Usually LAPM adds only about 5 percent framing overhead. This is ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
HDLC
High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The standard for HDLC is ISO/IEC 13239:2002. HDLC provides both connection-oriented and connectionless service. HDLC can be used for point-to-multipoint connections via the original master-slave modes Normal Response Mode (NRM) and Asynchronous Response Mode (ARM), but they are now rarely used; it is now used almost exclusively to connect one device to another, using ''Asynchronous Balanced Mode'' (ABM). History HDLC is based on IBM's SDLC protocol, which is the layer 2 protocol for IBM's Systems Network Architecture (SNA). It was extended and standardized by the ITU as LAP (Link Access Procedure), while ANSI named their essentially identical version ADCCP. The HDLC specification does not specify the full semantics of the frame fields. This allows other fully compliant standards to be deri ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Microcom Networking Protocol
The Microcom Networking Protocols, almost always shortened to MNP, is a family of error-correcting protocols commonly used on early high-speed (2400 bit/s and higher) modems. Originally developed for use on Microcom's own family of modems, the protocol was later openly licensed and used by most of the modem industry, notably the "big three", Telebit, USRobotics and Hayes. MNP was later supplanted by v.42bis, which was used almost universally starting with the first v.32bis modems in the early 1990s. Overview Although Xmodem was introduced 1977, as late as 1985 ''The New York Times'' described XMODEM first, then discussed ''MNP'' as a leading contender, and that 9600 baud modems "are beginning to make their appearance." By 1988, the ''Times'' was talking about 9600 and 19.2K, and that "At least 100 other brands of modems follow" MNP (compared to Hayes' use of LAP-B). Error correction basics Modems are, by their nature, error-prone devices. Noise on the telephone line, a common ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Error Correction
In information theory and coding theory with applications in computer science and telecommunication, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communication channels. Many communication channels are subject to channel noise, and thus errors may be introduced during transmission from the source to a receiver. Error detection techniques allow detecting such errors, while error correction enables reconstruction of the original data in many cases. Definitions ''Error detection'' is the detection of errors caused by noise or other impairments during transmission from the transmitter to the receiver. ''Error correction'' is the detection of errors and reconstruction of the original, error-free data. History In classical antiquity, copyists of the Hebrew Bible were paid for their work according to the number of stichs (lines of verse). As the prose books of the Bible were hardly ever writ ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Asynchronous Serial Communication
Asynchronous serial communication is a form of serial communication in which the communicating endpoints' interfaces are not continuously synchronized by a common clock signal. Instead of a common synchronization signal, the data stream contains synchronization information in form of start and stop signals, before and after each unit of transmission, respectively. The start signal prepares the receiver for arrival of data and the stop signal resets its state to enable triggering of a new sequence. A common kind of start-stop transmission is ASCII over RS-232, for example for use in teletypewriter operation. Origin Mechanical teleprinters using 5-bit codes (see Baudot code) typically used a stop period of 1.5 bit times.Dead link: 2015-Oct-03 Very early electromechanical teletypewriters (pre-1930) could require 2 stop bits to allow mechanical impression without buffering. Hardware which does not support fractional stop bits can communicate with a device that uses 1.5 bit tim ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Lempel Ziv
LZ77 and LZ78 are the two lossless data compression algorithms published in papers by Abraham Lempel and Jacob Ziv in 1977 and 1978. They are also known as LZ1 and LZ2 respectively. These two algorithms form the basis for many variations including LZW, LZSS, LZMA and others. Besides their academic influence, these algorithms formed the basis of several ubiquitous compression schemes, including GIF and the DEFLATE algorithm used in PNG and ZIP. They are both theoretically dictionary coders. LZ77 maintains a sliding window during compression. This was later shown to be equivalent to the ''explicit dictionary'' constructed by LZ78—however, they are only equivalent when the entire data is intended to be decompressed. Since LZ77 encodes and decodes from a sliding window over previously seen characters, decompression must always start at the beginning of the input. Conceptually, LZ78 decompression could allow random access to the input if the entire dictionary were known in ad ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
British Telecom
BT Group plc (trade name, trading as BT and formerly British Telecom) is a British Multinational corporation, multinational telecommunications holding company headquartered in London, England. It has operations in around 180 countries and is the largest provider of Landline, fixed-line, Internet access, broadband and Mobile telephony, mobile services in the UK, and also provides Pay television, subscription television and Information technology, IT services. BT's origins date back to the founding in 1846 of the Electric Telegraph Company, the world's first public telegraph company, which developed a nationwide communications network. BT Group as it came to be started in 1912, when the General Post Office, a government department, took over the system of the National Telephone Company becoming the monopoly telecoms supplier in the United Kingdom. The Post Office Act of 1969 led to the GPO becoming a public corporation. The ''British Telecom'' brand was introduced in 1980, and be ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Latency (engineering)
Latency, from a general point of view, is a time delay between the cause and the effect of some physical change in the system being observed. Lag, as it is known in gaming circles, refers to the latency between the input to a simulation and the visual or auditory response, often occurring because of network delay in online games. Latency is physically a consequence of the limited velocity at which any physical interaction can propagate. The magnitude of this velocity is always less than or equal to the speed of light. Therefore, every physical system with any physical separation (distance) between cause and effect will experience some sort of latency, regardless of the nature of the stimulation at which it has been exposed to. The precise definition of latency depends on the system being observed or the nature of the simulation. In communications, the lower limit of latency is determined by the medium being used to transfer information. In reliable two-way communication ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Automatic Repeat-request
Automatic repeat request (ARQ), also known as automatic repeat query, is an error-control method for data transmission that uses acknowledgements (messages sent by the receiver indicating that it has correctly received a packet) and timeouts (specified periods of time allowed to elapse before an acknowledgment is to be received) to achieve reliable data transmission over an unreliable communication channel. If the sender does not receive an acknowledgment before the timeout, it re-transmits the packet until it receives an acknowledgment or exceeds a predefined number of retransmissions. Variations of ARQ protocols include Stop-and-wait ARQ, Go-Back-N ARQ, and Selective Repeat ARQ. All three protocols usually use some form of sliding window protocol to help the sender determine which (if any) packets need to be retransmitted. These protocols reside in the data link or transport layers (layers 2 and 4) of the OSI model. Examples The Transmission Control Protocol uses a vari ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Payload (computing)
In computing and telecommunications, the payload is the part of transmitted data that is the actual intended message. Headers and metadata are sent only to enable payload delivery. In the context of a computer virus or worm, the payload is the portion of the malware which performs malicious action. The term is borrowed from transportation, where ''payload'' refers to the part of the load that ''pays'' for transportation. Networking In computer networking, data to be transmitted is the payload. It is almost always encapsulated in some type of frame format, composed of framing bits and a frame check sequence. Examples are Ethernet frames, Point-to-Point Protocol (PPP) frames, Fibre Channel frames, and V.42 modem frames. Programming In computer programming, the most common usage of the term is in the context of message protocols, to differentiate the protocol overhead from the actual data. For example, a JSON web service response might be: The string ''Hello, worl ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Communications Protocol
A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchronization of communication and possible error recovery methods. Protocols may be implemented by hardware, software, or a combination of both. Communicating systems use well-defined formats for exchanging various messages. Each message has an exact meaning intended to elicit a response from a range of possible responses pre-determined for that particular situation. The specified behavior is typically independent of how it is to be implemented. Communication protocols have to be agreed upon by the parties involved. To reach an agreement, a protocol may be developed into a technical standard. A programming language describes the same for computations, so there is a close analogy between protocols and programming languages: ''protocols are ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
