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XRT79L71
PRELIMINARY
99
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
In order to fully understand the role of the Transmit Overhead Data Input Interface, some background
information needs to be discussed first.
As mentioned in
Section 4.1, the DS3 frame consists of 4760 bits. Of these bits, 4704 bits are payload bits and
the remaining 56 bits are overhead bits. The XRT79L71 has been designed to handle and process both the
payload type and overhead type of bits for each DS3 frame. Within the XRT79L71, the Transmit Payload Data
Input Interface Block (which was discussed in considerable detail in
Section 4.2.1) has been designed to
accept the payload data from the System-Side Terminal Equipment. Likewise, the Transmit Overhead Data
Input Interface block has been designed to handle and process the overhead bits.
Accepting and Inserting DS3 Overhead Bits via the Transmit Overhead Data Input Interface
By default, the Transmit DS3 Framer block will be configured to internally generate and insert all of the
overhead bits within its outbound DS3 data-stream. More specifically, the Transmit DS3 Framer block will
internally generate the DS3 overhead bits by doing all of the following, as presented below in Table 17 and
Table 18 for C-Bit Parity and the M23 Framing formats, respectively.
TABLE 17: HOW THE TRANSMIT DS3 FRAMER BLOCK INTERNALLY GENERATES EACH OF THE OVERHEAD BITS - C-
BIT PARITY APPLICATIONS
BIT NAME
BIT DESCRIPTION
HOW OVERHEAD BIT IS INTERNALLY GENERATED BY THE TRANSMIT DS3
FRAMER BLOCK
X-Bits (2)
FERF/Yellow Alarm Indicator
Bits
Either Software Controlled or automatically set to "0" whenever the cor-
responding Receive DS3 Framer block declares the LOS, LOF/OOF or
AIS defect condition.
F1 Bits (14)
F-Frame Framing Alignment
bits that are of the value "1"
Set to the value of "1".
F0 Bits (14)
F-Frame Framing Alignment
bits that are of the value "0".
Set to the value of "0".
M1 Bit (1)
M-Frame Framing Alignment
bits that are of the value "1"
Set to the value of "1".
M0 Bits (2)
M-Frame Framing Alignment
bits that are of the value "0"
Set to the value of "0".
P-bits (2)
Parity Bits
Transmit DS3 Framer block computes the even parity value over the
payload bits within a given DS3 frame. The results of this calculation
are inserted into the two P-bit positions within the very next DS3 frame.
CP-bits (3)
Path Parity Bits
Transmit DS3 Framer block computes the even parity value over the
payload bits within a given DS3 frame. The results of this calculation
are inserted into the three CP-bit positions within the very next DS3
frame.
AIC bit (1)
Application Identical Channel
Set to the value "1" in order to denote C-bit Parity framing format.
UDL bits (9)
User Data Link Bits
Set to the value "1".
DL bits (3)
Path Maintenance Data Link
(PMDL) bits
These bits carry the PMDL/LAPD Message that is generated by the
LAPD Transmitter within the Transmit Section of the XRT79L71.
However, if the Transmit LAPD Controller block is not being used, then
these bits will each be set to "1".