____________________________________________________ DS34T101, DS34T102, DS34T104, DS34T108
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10.11.1.2 E1 Framing Formats
E1 frames contain 32 8-bit channels. The first DS0 of each frame is used to carry overhead bits for frame
alignment, alarm indication and node-to-node communication. The other 31 DS0 channels are available to carry
voice and data. In many applications the 17
th channel of each frame carries voice-channel signaling information
and other overhead.
Successive frames in an E1 signal alternate between FAS frames (i.e. frames containing the Frame Alignment
Signal in the first DS0) and NFAS frames (i.e. frames that don’t contain the FAS). In FAS frames, the lower seven
bits of the first DS0 contain the FAS sequence 0011011. Receive framers can detect the FAS and use it to find
frame boundaries in an incoming E1 signal.
In most applications E1 frames are grouped into 16-frame CRC-4 multiframes each composed of two sub-
multiframes (SMF). The CRC-4 multiframe framing pattern is shown in
Table 10-37. As shown in that table, the
MSb of the first DS0 in frames 1, 3, 5, 7, 9 and 11 contains a fixed multiframe alignment pattern of 001011.
Receive frames can detect this pattern and use it to find CRC-4 multiframe boundaries in an incoming E1 signal.
The C1-C4 bits each sub-multiframe convey a cyclic redundancy check 4 (CRC-4) computed over that SMF in the
previous multiframe. The register E1 and E2 bits in SMF II allow a node to indicate CRC-4 errors detected in the
received E1 signal. The A bits in NFAS frames are alarm bits and are also known as RAI (remote alarm indication).
The SaX bits in NFAS frames are data channels. Collectively all of the Sa4 bits are one data channel. Similarly,
Sa5, Sa6, Sa7 and Sa8 bits form four other data channels. These channels can be used in various application-
specific ways. ITU-T G.704 specifies one use for synchronization status messaging.
See ITU-T G.704 for full details of E1 frame and multiframe formats.
Table 10-37. E1 CRC-4 Multiframe Framing Pattern
Bits 1 to 8 of the Frame
Sub-
Multi-
Frame
CRC-4
Frame #
Type
1
2
3
4
5
6
7
8
0
FAS
C1
0
1
0
1
NFAS
0
1
A
Sa4
Sa5
Sa6
Sa7
Sa8
2
FAS
C2
0
1
0
1
3
NFAS
0
1
A
Sa4
Sa5
Sa6
Sa7
Sa8
4
FAS
C3
0
1
0
1
5
NFAS
1
A
Sa4
Sa5
Sa6
Sa7
Sa8
6
FAS
C4
0
1
0
1
I
7
NFAS
0
1
A
Sa4
Sa5
Sa6
Sa7
Sa8
8
FAS
C1
0
1
0
1
9
NFAS
1
A
Sa4
Sa5
Sa6
Sa7
Sa8
10
FAS
C2
0
1
0
1
11
NFAS
1
A
Sa4
Sa5
Sa6
Sa7
Sa8
12
FAS
C3
0
1
0
1
13
NFAS
E1
1
A
Sa4
Sa5
Sa6
Sa7
Sa8
14
FAS
C4
0
1
0
1
II
15
NFAS
E2
1
A
Sa4
Sa5
Sa6
Sa7
Sa8
10.11.1.3 Framer/Formatter Configuration
Registers that are related to setting up the framer and formatter are shown in the following table.
Table 10-38. Registers Related to Setting Up the Framer and Formatter
Register Name
Description
Functions
Page
Transmit Master Mode Register
Tx E1/T1 mode, reset, initialization
Transmit Control Register 1 (T1 Mode)
Tx source of the F-Bit
Transmit Control Register 1 (E1 Mode)
Tx source of FAS and Si bits
Transmit Control Register 2 (T1 Mode)
Tx F-Bit corruption, SLC-96 enable