Chapter 1. Introduction
10
November 2001
1.5 E1 signal structure
The E1 line operates at a nominal rate of 2.048Mbps. The
data transferred over the E1 line is organized into frames, with
each E1 frame containing 256 bits. The 256 bits are a total of the
32 time slots, each containing eight bits, carrying the data payload.
E1 transmission utilizes two main types of framing: Frame
Alignment Signal (FAS) and Multi-Frame Alignment Signal
(MFAS). Framing is necessary in order for equipment receiving
the E1 signal to be able to identify and extract the individual
channels. PCM-30 (CAS) transmission system use MFAS framing
along with the FAS framing. PCM-31 (CCS) transmission system
use only FAS framing.
Frame Alignment Signal (FAS)
The 2.048 Mbps frame consists of 32 individual time slots
(numbered 0-31). As described previously, each time slot consists
of an individual 64Kbps channel of data. In the FAS format, time
slot 0 of every other frame is reserved for the frame alignment
signal pattern. Alternate frames contain the FAS Distant Alarm
indication bit and others bits reserved for national and international
use.
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