XRT74L73 Datasheet, PDF (179/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

PRELIMINARY
XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
FRAMER OPERATING MODE REGISTER (ADDRESS = 0X00)
BIT 7
BIT 6
BIT 5
BIT 4
R/W
R/W
R/W
R/W
x
1
x
0
Table 32 lists the relationship between the value of
the this bit-field and the resulting DS3 Frame Format.
TABLE 32: THE RELATIONSHIP BETWEEN THE CONTENT
OF BIT 2, (C-BIT PARITY*/M13) WITHIN THE FRAMER
OPERATING MODE REGISTER AND THE RESULTING DS3
FRAMING FORMAT
BIT 2
DS3 FRAME FORMAT
0
C-Bit Parity
1
M13
NOTE: This bit setting also configures the frame format for
both the Transmit and Receive Section of the XRT74L73.
Each of the two DS3 Frame Formats, as presented in
Figure 47 and Figure 48 , constitute an M-frame (or a
full DS3 Frame). Each M-frame consists of 7 - 680 bit
F-frames (sometimes referred to as, subframes). In
Figure 47 and 48 , each F-frame is represented by
BIT 3
BIT2
BIT 1
BIT 0
R/W
R/W
R/W
R/W
x
x
x
x
the individual rows of payload and overhead bits.
Each F-frame can be further divided into 8 blocks of
85 bits, with 84 of the 85 bits available for payload in-
formation and the remaining one bit used for frame
overhead.
Differences Between the M13 and C-Bit Parity
Frame Formats
The frame formats for M13 and C-bit Parity are very
similar. However, the main difference between these
two framing formats is in the use of the C-bits. In the
M13 Format, the C-bits reflect the status of stuff-op-
portunities that either were or were not used while
multiplexing the 7 DS2 signals into this DS3 signal. If
two of the three stuff bits, within a F-frame, are "1",
then the associated stuff bit, Si (not shown in
Figure 48 ), is interpreted as being a stuff bit. In the
C-bit Parity framing format, the C bits take on differ-
ent roles, as presented in Table 33 .
TABLE 33: C-BIT FUNCTIONS FOR THE C-BIT PARITY DS3 FRAME FORMAT
C - BIT
FUNCTION OF C-BITS WHILE IN THE C-BIT PARITY FRAMING FORMAT
C11
AIC (C-Bit Parity Mode)
C12
NA (Reserved for Network Application)
C13
FEAC (Far End Alarm & Control)
C21, C22, C23 (UDL) User Data Link (undefined for DS3 Frame)
C31,C32, C33 CP (Path) Parity Bits
C41, C42, C43 FEBE (Far End Block Error) Indicators
C51, C52, C53 (DL) Path Maintenance Data Link
C61, C62, C63, (UDL) User Data Link (undefined for DS3 Frame)
C71, C72, C73
Definition of the DS3 Frame Overhead Bits
In general, the DS3 Frame Overhead Bits serve the
following three purposes:
1. Support Frame Synchronization between the
Local and Remote DS3 Terminals
2. Provide parity bits in order to facilitate perfor-
mance monitoring and error detection.
3. Support the transmission of Alarms, Status, and
Data Link information to the Remote DS3 Termi-
nal.
The Overhead bits supporting each of these purpos-
es are further defined below.
4.1.1 Frame Synchronization Bits (Applies to
both M13 and C-bit Parity Framing Formats)
Each DS3 Frame (M-frame) contains a total of 31 bits
that support frame synchronization. Each DS3 M-
frame contains three M-bits. According to Figure 47
and Figure 48 , these M-bits are the first bits in F-
frames 5, 6 and 7. These three bits appear in each
M-frame with the repeating pattern of "010". This fact
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