XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
PRELIMINARY
REV. P1.0.1
180
Table 32 lists the relationship between the value of
the this bit-field and the resulting DS3 Frame Format.
N
OTE
:
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
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 .
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
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
x
1
x
0
x
x
x
x
FRAMER OPERATING MODE REGISTER (ADDRESS = 0X00)
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
T
ABLE
32: T
HE
R
ELATIONSHIP
BETWEEN
THE
CONTENT
OF
B
IT
2, (C-B
IT
P
ARITY
*/M13)
WITHIN
THE
F
RAMER
O
PERATING
M
ODE
R
EGISTER
AND
THE
RESULTING
DS3
F
RAMING
F
ORMAT
B
IT
2
DS3 F
RAME
F
ORMAT
0
C-Bit Parity
1
M13
T
ABLE
33: C-
BIT
F
UNCTIONS
FOR
THE
C-
BIT
P
ARITY
DS3 F
RAME
F
ORMAT
C - B
IT
F
UNCTION
OF
C-
BITS
WHILE
IN
THE
C-B
IT
P
ARITY
F
RAMING
F
ORMAT
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,
C71, C72, C73
(UDL) User Data Link (undefined for DS3 Frame)