CTC Union ETU01-Plus, Руководство пользователя

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Chapter 1. Introduction
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1.3 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 consist of 32 time slots of eight bits each, which carry 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.
Multi-Frame Alignment Signal (MFAS)
MFAS framing uses Channel Associated Signaling (CAS) to transmit A/B/C/D bit
signaling information for each of 30 channels. This method uses the 32 time slot
frame format with time slot 0 for the FAS and time slot 16 for the Multi-Frame
Alignment Signal and the Channel Associated Signaling.
E1 line signal
The basic E1 line signal is coded using the Alternate Mark Inversion (AMI) or
HDB3 rule.
In the AMI format, "ones" are alternately transmitted as positive and negative
pulse, whereas "zeros" are transmitted as a zero voltage level. AMI is not used in
most 2.048Mbps transmissions because synchronization loss occurs during long
strings of data zeros."
In the HDB3 format, a string of four consecutive zeros is replaced with a substitute
string of pulses containing an intentional bipolar violation. The HDB3 code
substitutions provide high pulse density so that the receiving equipment is able to
maintain synchronization with the received signal.