XRT72L13 Datasheet, PDF (242/370 Pages) Exar Corporation – M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC

XRT72L13
M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC
REV. P1.0.6
áç
PRELIMINARY
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 59
and Figure 60 , 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 59 and Figure 60 , 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 59 and Figure 60 ,
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 insert 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, the two P-bits are set to zero if an
even number of "ones" is found in the payload of the
previous DS3 frame. For information on how the Re-
ceive DS3 Framer handles P-bits, please see Section
3.3.2.6.1.
4.1.3 Alarm and Signaling-Related Overhead
Bits
The Alarm Indication Signal (AIS) Pattern (C-Bit Pari-
ty Framing Format only)
The Alarm Indication Signal (AIS) pattern is an alarm
signal that is inserted into the "outbound" DS3 stream
when a failure is detected by the “Local" Terminal.
The Transmit DS3 Framer will generate the AIS pat-
tern as defined in ANSI.T1.107a-1990, which is de-
scribed as follows.
VALID M-BITS, F-BITS, AND P-BITS
• All C-bits are zeros
• All X-bits are set to "1"
• A repeating "1010..." pattern is written into the pay-
load of the DS3 frames.
Consequently, no user (or "payload") data will be
transmitted while the Transmit Section of the chip is
transmitting the AIS pattern.
The IDLE Condition Signal
The IDLE Condition signal is used to indicate that the
DS3 channel is functionally sound, but has not yet
been assigned any traffic. The Transmit Section will
transmit the IDLE Condition signal as defined in ANSI
T1.107a-1990, which is described as follows.
• Valid M-bits, F-bits, and P-bits
• The three CP-bits (F-frame #3) are zeros
• The X-bits are set to "1"
• A repeating "1100.." pattern is written into the pay-
load of the DS3 frames.
FEAC - Far End Alarm & Control (Only available
for the C-bit Parity Frame Format)
The third C-bit (C13 or FEAC) in the first F-frame is
used as the "Far End Alarm and Control" (FEAC)
channel between the "Near-End" DS3 terminal and
the "Remote" DS3 terminal. The FEAC channel car-
ries:
• Alarm and Status Information
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