Table of Contents
Advertisement
MTT-14B e_Manual D07-00-083P RevA00
4.2.7 2.048 Mbit/s Framing
E1 transmission utilizes two types of framing: FAS (Frame Align-
ment Signal) and MFAS (MultiFrame Alignment Signal). Framing
is necessary so that the equipment receiving the E1 signal is able
to identify and extract the individual channels. PCM-31 uses FAS
framing and PCM-30 uses MFAS with FAS framing.
FAS (Frame Alignment Signal)
The 2.048 Mbit/s frame consists of 32 individual time slots (num-
bered 0-31). As described previously, each time slot consists of
an individual 64 kbit/s channel of data.
In the FAS format, time slot 0 of every other frame is reserved
for the FAS pattern. Alternate frames contain the FAS Distant
Alarm indication bit and other bits reserved for National and
International use. Hence, there are 31 time slots into which data
can be placed as in Figure 61.
One 2.048 Mbit/s Frame
T ime Slot 0
1
2
3
4
E
0
0
1
E
1
A
Sa
Notes:
•
Even Frame: Contains FAS.
•
Odd Frame: Contains NFAS.
•
Sa: This bit is reserved for national use.
•
E: Error indicator bit.
•
A: Remote alarm indicator bit.
•
0011011: Frame alignment signal.
•
(8 bits per timeslot)(8000 frames per second) = 2.048 Mbps
Figure 61 FAS Framing Format
FAS does not accommodate voice channel signalling. The first
bit (c or Si) of these frames is reserved for international use. It
can be used for the CRC-4, Cyclic Redundancy Check-4, when
enhanced performance monitoring is required. Therefore, when
CRC is enabled in TEST CONFIGURATION, these bits depend
upon the CRC calculation and should continually change between
0 and 1. When CRC-4 is not enabled, these bits are set to 1.
In FAS framing, the odd frames do not contain the frame align-
ment signal. The bits are defined as follows:
1
...
31
BITS
5
6
7
8
1
0
1
1
Sa
Sa
Sa
Sa
Page 86 of 101
Advertisement
Table of Contents
