Chapter 1. Introduction
11
May 2011
1.4 Typical System Applications
General
In a typical application (Figure 1-1), the ETU-01A is used in a point-to-point connection. The
synchronous data channels of each host are connected over an E1 line.
Figure 1-1 Point-to-Point Application
Fractional E1 data service is based on the assumption that the user data rate is a fraction of the
available E1 bandwidth, in multiples of 56K or 64K.
Figure 1-2 Fractional E1, Cascade Application
In the above figure, the available E1 Time Slots are divided (Fractional E1) and cascaded
(multiplexed). Various data interfaces are employed in each ETU-01A to provide connections
between hosts, LANs and servers across the E1 transmission facility.
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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