XRT7250 Datasheet, PDF (148/463 Pages) Exar Corporation – DS3/E3 FRAMER IC

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DS3/E3 FRAMER IC
XRT7250
REV. 1.1.1
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 41), is interpreted as being a stuff bit. In the
C-bit Parity framing format, the C bits take on different
roles, as presented in Table 17.
TABLE 17: 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 40
and Figure 41, 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
is also presented in Figure 40 and Figure 41, which
contains bit-fields that are designated as: M0, M1,
and M0 (where M0 = "0", and M1 = "1").
Each F-frame contains four F-bits, which also aid in
synchronization between the Local and the remote
DS3 terminals. Therefore, each DS3 M-frame con-
sists of a total of 28 F-bits. These F-bits exhibit a re-
peating pattern of "1001" within each F-frame. This
fact is also presented in Figure 40 and Figure 41,
which contains bit-fields that are designated as: F1,
F0, F0, and F1 (where F0 = "0", and F1 = "1").
Each of these bit-fields will be used by the Receive
DS3 Framer block, within the remote terminal equip-
ment, to perform Frame Acquisition and Frame Main-
tenance functions.
NOTE: For more information on how the Receive DS3
Framer uses these bit-fields, please see Section 3.3.2.
4.1.2 Performance Monitoring/Error Detection
Bits (Parity)
The DS3 Frame uses numerous bit fields to support
performance monitoring of the transmission link be-
tween the Local Transmitting Terminal and the Re-
mote Receiving Terminal. The DS3 frame can con-
tain two types of parity bits, depending upon the fram-
ing format chosen. P-bits are available in both the
M13 and C-bit Parity Formats. However, the C-bit
Parity format also includes additional CP-Parity bits.
P-Bits (Applies to M13 and C-Bit Parity Frame For-
mats)
Each DS3 M-frame consists of two (2) P-bits. These
two P-bits carry the parity information of the previous
DS3 frame for performance monitoring. These two P-
bits must be identical, within a given DS3 frame. The
Transmit Section will compute the even parity over all
4704 payload bits within a given DS3 frame, and in-
sert the resulting parity information in the P-bit fields
of the very next DS3 frame. The two P-bits are set to
"1" if the payload of the previous DS3 frame consists
of an odd number of "ones" in the frame. Conversely,
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