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Synchronization In Telecommunications
Many services running on modern digital telecommunications networks require accurate synchronization for correct operation. For example, if telephone exchanges are not synchronized, then bit slips will occur and degrade performance. Telecommunication networks rely on the use of highly accurate primary reference clocks which are distributed network-wide using synchronization links and synchronization supply units. Ideally, clocks in a telecommunications network are synchronous, controlled to run at identical rates, or at the same mean rate with a fixed relative phase displacement, within a specified limited range. However, they may be mesochronous in practice. In common usage, mesochronous networks are often described as ''synchronous''. History Synchronization in communications was a hard problem for Alexander Bain in the development of the teleprinter. Thomas Edison achieved synchronization in his stock ticker with a clunky but effective ''unison mechanism'' to resynchron ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Telecommunications Network
A telecommunications network is a group of Node (networking), nodes interconnected by telecommunications links that are used to exchange messages between the nodes. The links may use a variety of technologies based on the methodologies of circuit switching, message switching, or packet switching, to pass messages and signals. Multiple nodes may cooperate to pass the message from an originating node to the destination node, via multiple network hops. For this routing function, each node in the network is assigned a network address for identification and locating it on the network. The collection of addresses in the network is called the address space of the network. Examples of telecommunications networks include computer networks, the Internet, the public switched telephone network (PSTN), the global Telex network, the aeronautical ACARS network, and the wireless radio networks of cell phone telecommunication providers. Network structure this is the structure of network genera ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Holdover In Synchronization Applications
Two independent clocks, once synchronized, will walk away from one another without limit. To have them display the same time it would be necessary to re-synchronize them at regular intervals. The period between synchronizations is referred to as holdover and performance under holdover relies on the quality of the reference oscillator, the PLL design, and the correction mechanisms employed. Importance The quote above suggests that one can think of holdover in synchronization applications as analogous to running on backup power. Modern wireless communication systems require at least knowledge of frequency and often knowledge of phase as well in order to work correctly. Base stations need to know what time it is, and they usually get this knowledge from the outside world somehow (from a GPS Time and Frequency receiver, or from a synchronization source somewhere in the network they are connected to). But if the connection to the reference is lost then the base station will be on its ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Phase-locked Loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is fixed relative to the phase of an input signal. Keeping the input and output phase in lockstep also implies keeping the input and output frequencies the same, thus a phase-locked loop can also track an input frequency. Furthermore, by incorporating a frequency divider, a PLL can generate a stable frequency that is a multiple of the input frequency. These properties are used for clock synchronization, demodulation, frequency synthesis, clock multipliers, and signal recovery from a noisy communication channel. Since 1969, a single integrated circuit can provide a complete PLL building block, and nowadays have output frequencies from a fraction of a hertz up to many gigahertz. Thus, PLLs are widely employed in radio, telecommunications, computers (e.g. to distribute precisely timed clock signals in microprocessors), grid-tie inverters (electronic power converters used ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Asynchronous Communication
In telecommunications, asynchronous communication is transmission of data, generally without the use of an external clock signal, where data can be transmitted intermittently rather than in a steady stream. Any timing required to recover data from the communication symbols is encoded within the symbols. The most significant aspect of asynchronous communications is that data is not transmitted at regular intervals, thus making possible variable bit rate, and that the transmitter and receiver clock generators do not have to be exactly synchronized all the time. In asynchronous transmission, data is sent one byte at a time and each byte is preceded by start and stop bits. Physical layer In asynchronous serial communication in the physical protocol layer, the data blocks are code words of a certain word length, for example octets ( bytes) or ASCII characters, delimited by start bits and stop bits. A variable length space can be inserted between the code words. No bit syn ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Plesiochronous System
In telecommunications, a plesiochronous system is one where different parts of the system are almost, but not quite, perfectly synchronised. According to ITU-T standards, a pair of signals are plesiochronous if their significant instants occur at nominally the same rate, with any variation in rate being constrained within specified limits. A sender and receiver operate plesiosynchronously if they operate at the same nominal clock frequency but may have a slight clock frequency mismatch, which leads to a drifting phase.S. Johnson, S. Scott: A Supercomputer System Interconnect and Scalable IOS, 14th IEEE Symposium on Mass Storage Systems, 1995, Footnote on p.358 The mismatch between the two systems' clocks is known as the plesiochronous difference. In general, plesiochronous systems behave similarly to synchronous systems, except they must employ some means in order to cope with "sync slips", which will happen at intervals due to the plesiochronous nature of the system. The most com ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Isochronous Signal
In telecommunications, an isochronous signal is a signal in which the time interval separating any two significant instants is equal to the unit interval or a multiple of the unit interval. Variations in the time intervals are constrained within specified limits. "Isochronous" is a characteristic of one signal, while " synchronous" indicates a relationship between two or more signals. See also * Synchronization in telecommunications * Synchronous network * Mesochronous network * Plesiochronous system * Asynchronous system The primary focus of this article is asynchronous control in digital electronic systems. In a synchronous system, operations ( instructions, calculations, logic, etc.) are coordinated by one, or more, centralized clock signals. An asynchro ... References Telecommunications engineering Synchronization {{Telecomm-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Synchronous Network
Many services running on modern digital telecommunications networks require accurate synchronization for correct operation. For example, if telephone exchanges are not synchronized, then bit slips will occur and degrade performance. Telecommunication networks rely on the use of highly accurate primary reference clocks which are distributed network-wide using synchronization links and synchronization supply units. Ideally, clocks in a telecommunications network are synchronous, controlled to run at identical rates, or at the same mean rate with a fixed relative phase displacement, within a specified limited range. However, they may be mesochronous in practice. In common usage, mesochronous networks are often described as ''synchronous''. History Synchronization in communications was a hard problem for Alexander Bain in the development of the teleprinter. Thomas Edison achieved synchronization in his stock ticker with a clunky but effective ''unison mechanism'' to resynchron ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Caesium Standard
The caesium standard is a primary frequency standard in which the photon absorption by transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency. The first caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK and promoted worldwide by Gernot M. R. Winkler of the United States Naval Observatory. Caesium atomic clocks are one of the most accurate time and frequency standards, and serve as the primary standard for the definition of the second in the International System of Units (SI), the modern metric system. By definition, radiation produced by the transition between the two hyperfine ground states of caesium-133 (in the absence of external influences such as the Earth's magnetic field) has a frequency, , of exactly . That value was chosen so that the caesium second equaled, to the limit of measuring ability in 1960 when it was adopted, the existing standard ephemeris second ba ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Synchronous Digital Hierarchy
Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs). At low transmission rates, data can also be transferred via an electrical interface. The method was developed to replace the plesiochronous digital hierarchy (PDH) system for transporting large amounts of telephone calls and data traffic over the same fiber without the problems of synchronization. SONET and SDH, which are essentially the same, were originally designed to transport circuit mode communications, e.g. DS1, DS3, from a variety of different sources. However, they were primarily designed to support real-time, uncompressed, circuit-switched voice encoded in PCM format. The primary difficulty in doing this prior to SONET/SDH was that the synchronization sources of these various circuits were different. This meant that ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Time Deviation
Time deviation (TDEV), also known as \sigma_x(\tau), measures the time stability of a clock source's phase over an observation interval, expressed as a standard deviation of the time variations. This indicates the time instability of the signal source. This is a scaled variant of frequency stability of Allan deviation. It is commonly defined from the modified Allan deviation, but other estimators may be used. Time variance (TVAR), symbolised as \sigma_x^2(\tau), is the time stability of phase versus observation interval tau. It is a scaled variant of modified Allan variance. TDEV is a metric often used to determine an aspect of the quality of timing signals in telecommunication applications and is a statistical analysis of the phase stability of a signal over a given period. Measurements of a reference timing signal will refer to its TDEV and maximum time interval error (MTIE) values, comparing them to specified masks or goals. Definition The most common estimator uses the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Maximum Time Interval Error
Maximum time interval error (MTIE) is the maximum error committed by a clock under test in measuring a time interval for a given period of time. It is used to specify clock stability requirements in telecommunications standards. MTIE measurements can be used to detect clock instability that can cause data loss on a communications channel. Measurement A given dataset (clock waveform) is first compared to some reference. Phase error (usually measured in nanoseconds) is calculated for an observation interval. This phase shift is known as time interval error (TIE). MTIE is a function of the observation interval. An observation interval window moved across the dataset. Each time the peak-to-peak distance between the largest and smallest TIE in that window is noted. This distance varies as the window moves, being maximal for some window position. This maximal distance is known as MTIE for the given observation interval. Plotting MTIE vs. different observation interval duration gives a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
