telecommunications Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...

, frame synchronization or framing is the process by which, while receiving a stream of fixed-length frames, the receiver identifies the frame boundaries, permitting the

data Data ( , ) are a collection of discrete or continuous values that convey information, describing the quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted for ...

bits within the frame to be extracted for decoding or retransmission. When packets of varying length are sent, it is necessary to have an instantly recognizable packet-end delimiter (e.g., Ethernet's end of stream symbol). Loss of carrier signal can be interpreted as a packet-end delimiter in some cases. When a continuous stream of fixed-length frames are sent, a synchronized receiver can in principle identify frame boundaries forever. In practice, receivers can usually maintain synchronization despite transmission errors;

bit slip In digital transmission, bit slip is the loss or gain of a bit or bits, caused by clock driftvariations in the respective clock rates of the transmitting and receiving devices. One cause of bit slip is overflow of a receive buffer that occu ...

s are much rarer than

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

s. Thus, it is acceptable to use a much smaller frame boundary marker, at the expense of a lengthier process to establish synchronization in the first place. Frame synchronization is achieved when the incoming frame alignment signals are identified (that is, distinguished from data bits), permitting the data bits within the frame to be extracted for decoding or retransmission.

Framing

If the transmission is temporarily interrupted, or a bit slip event occurs, the receiver must re-synchronize. The transmitter and the receiver must agree ahead of time on which frame

synchronization Synchronization is the coordination of events to operate a system in unison. For example, the Conductor (music), conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are sa ...

scheme they will use. Common frame synchronization schemes are: ;Framing bit: A common practice in

, for example in

T-carrier The T-carrier is a member of the series of carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls. The first version, the Transmission System 1 (T1), was introduced in 1962 in the Bell Syst ...

, is to insert, in a dedicated

time slot Broadcast programming is the practice of organizing or ordering (scheduling) of broadcast media shows, typically radio and television, in a daily, weekly, monthly, quarterly, or season-long schedule. Modern broadcasters use broadcast automatio ...

within the frame, a noninformation framing bit that is used for synchronization of the incoming data with the receiver. In a

bit stream A bitstream (or bit stream), also known as binary sequence, is a sequence of bits. A bytestream is a sequence of bytes. Typically, each byte is an 8-bit quantity, and so the term octet stream is sometimes used interchangeably. An octet may ...

, framing bits are predictable (do not carry information), and occur at specified positions in the frame. Correct framing is verified when almost all framing bits (minus a small allowance for transmission errors) have their predicted values. ;Syncword and flag sequence framing: Rather than a single bit, some systems use a multi-bit

syncword In computer networks, a syncword, sync character, sync sequence or preamble is used to synchronize a data transmission by indicating the end of header information and the start of data. The syncword is a known sequence of data used to identif ...

in each frame, or a flag sequence that marks the beginning and end of each frame.

High-Level Data Link Control High-Level Data Link Control (HDLC) is a communication protocol used for transmitting data between devices in telecommunication and networking. Developed by the International Organization for Standardization (ISO), it is defined in the standard ...

and similar systems use flag sequences. ;CRC-based framing: Some telecommunications hardware uses

CRC-based framing CRC-based framing is a kind of frame synchronization used in Asynchronous Transfer Mode (ATM) and other similar protocols. The concept of CRC-based framing was developed by StrataCom, Inc. in order to improve the efficiency of a pre-standard Async ...

, where correct framing is verified when almost all frames have valid CRCs.

Frame synchronizer

telemetry Telemetry is the in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots ''tele'', 'far off', an ...

applications, a ''frame synchronizer'' is used to locate frame boundaries within a serial pulse-code modulated (PCM) binary stream. The frame synchronizer immediately follows the bit synchronizer in most telemetry applications. Without frame synchronization, decommutation is impossible. The frame synchronizer searches the incoming bit-stream for occurrences of the frame synchronization pattern. If the pattern persists for long enough that an accidental match is implausible, the synchronizer declares the data synchronized and available for decoding. If a large number of mis-matches occur, the synchronizer declares a loss of synchronization. The search can be sequential (only consider one starting point at a time), or multiple candidate starting points may be considered at once. Advanced techniques continue searching even while synchronization is established, so that, if synchronization is lost, by the time the loss is noticed a new frame start position has been found. It is not uncommon to have multiple levels of frame synchronization, where a series of frames is assembled into a larger "superframe" or "major frame". Individual frames are then "minor frames" within that superframe. Each frame contains a subframe ID (often a simple counter) which identifies its position within the superframe. A second frame synchronizer establishes superframe synchronization. This allows subcommutation, where some data is sent less frequently than every frame.

References

{{FS1037C MS188

Scientific articles

J. L. Massey. "Optimum frame synchronization ". IEEE trans. comm., com-20(2):115-119, April 1972

R Scholtz. "Frame synchronization techniques", ''IEEE Transactions on Communications'', 1980
* P. Robertson. "Optimal Frame Synchronization for Continuous and Packet Data Transmission", PhD Dissertation, 1995, Fortschrittberichte VDI Reihe 10, Nr. 37
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Data transmission Synchronization

Framing

Frame synchronizer

See also

References

Scientific articles