Document ID: 14151
Updated: Jun 01, 2005
Contents
Introduction
This document provides information on a frequently asked line coding question.
Q. Does Cisco have an official preference as to the line coding to use on DS-3 lines? Options are Cbit or M 13. There does not seem to be a reason why it would not work with either. I am assuming that it is similar to DS-1, in which case line coding is transparent to the router and we do not care, right?
A. It does not matter what the coding is since the Cisco interface does not actually "see" the line. Our interface is the T3 DSU, which has to worry about the formatting and coding on the network line.
The vendor of the DSU (e.g. T3 Plus) might not support both. If the DSU supports both, Cbit is preferred for improved maintenance options.
Cbit is used when you want to use inband signaling for things like network management, similar to the Facility Data Link (FDL) of the T1. It is a 21-bit field. Cbit is part of the M13 frame, which is like the T1 frame, except it is called an M frame. The "13" represents a DS1 to DS3 multiplexer, hence M13.
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