Frame synchronization: Difference between revisions

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{{context|date=February 2013}}

In [[telecommunicationtelecommunications]], '''frame synchronization''' or '''framing''' is the process by which, while receiving a stream of fixed-length [[Frame (networking)|frames]], the receiver identifies the frame boundaries, permitting the [[data]] 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. 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]]s are much rarer than [[bit error]]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.

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Common frame synchronization schemes are:

;Framing bit: A common practice in [[telecommunicationtelecommunications]]s, for example in [[T-carrier]], is to insert, in a dedicated [[Time-division multiplexing|time slot]] within the frame, a noninformation '''framing bit''' that is used for synchronization of the incoming data with the receiver. In a [[bit stream]], 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 each frame, or a '''flag sequence''' that marks the beginning and end of each frame. [[High-Level Data Link Control]] and similar systems use flag sequences.<ref>{{Cite IETF |rfc=1662 |title=PPP in HDLC-like Framing |last=Simpson |first=William A. |date=July 1994 |publisher=Internet Engineering Task Force}}</ref>

;CRC-based framing: Some telecommunications hardware uses [[CRC-based framing]], where correct framing is verified when almost all frames have valid CRCs.