General

The purpose of this Recommendation is to describe general principles applicable to the detection and correction or recovery of link transmission errors in public data networks.

Two fundamental objectives of error control procedures are:

- to ensure an incidence of undetected errors that is within acceptably low probability limits;

- to ensure that detected errors are corrected or recovered using an error control procedure consistent with data throughput and sequencing requirements which apply when the error rate of the Physical Layer is within the fully acceptable and the tolerance limits of specified performance.

In the context of the Reference Model of Open Systems Interconnection, it is noted in Recommendation X.200 that each (N) peer protocol should include sufficient control information to enable the (N) entities to detect or recover from error conditions within its purview. Reporting detected but unrecovered errors is a service that must be provided by each layer. Specifically, it is an objective of the Data Link Layer to detect and possibly correct errors which may occur in the Physical Layer.

For any particular error detection arrangement the probability of undetected errors will generally tend to increase:

- with increasing error rate,

- for any given error rate, as the error distribution becomes less random and as the length of error bursts increases,

- with increasing frame length,

- possibly due to scrambling arrangements which may have factors in common with the generating polynomial used for error detection.

Data throughput in the presence of errors depends on the design of the error control procedure, which in turn depends on the following conditions:

- error rate,

- error distribution,

- scrambling and/or multiplexing arrangements insofar as they affect the error distribution or error rate,

- transmission path (propagation) time delay,

- data signalling rate,

- frame length,

- window size,

- buffer memory resources at the sending and receiving end of the link.