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 (e.g., Ethernet's [[Ethernet_frame#End_of_frame_–_physical_layer|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]]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.

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.

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[[Image:Frame Synced Stream.jpg|frame|none|Frame synchronized PCM stream — telemetry application]]

The transmitter and the receiver must agree ahead of time on which frame [[synchronization in telecommunications|synchronization]] scheme they will use.

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.