XRT72L56 Datasheet, PDF (304/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L56
SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
REV. P1.1.2
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PRELIMINARY
• Compute the 16-bit Frame Check Sequence (FCS)
value of the LAPD Message frame (e.g, of the
LAPD Message header and Payload bytes) and
insert this value into the FCS value octet positions
within the LAPD Message frame.
• Append a trailer Flag Sequence octet to the end of
the LAPD Message frame (following the 16-bit FCS
octets).
• Fragment the resulting LAPD Message frame into
bits and begin inserting these bits into the “N” bit-
field within each outbound E3 frame.
• Complete the transmission of the overhead bytes,
information payload byte, FCS value, and the trail-
ing Flag Sequence octets via the Transmit E3
Framer block.
Once the LAPD Transmitter has completed its trans-
mission of the LAPD Message frame, the Framer will
generate an Interrupt to the MIcroprocessor/Micro-
controller (if enabled). Afterwards, the LAPD Trans-
mitter will either halt its transmission of LAPD Mes-
sage frames or will proceed to retransmit the LAPD
Message frame, repeatedly at one-second intervals.
In between these transmissions of the LAPD Mes-
sage frames, the LAPD Transmitter will be sending a
continuous stream of Flag Sequence bytes. The
LAPD Transmitter will continue this behavior until the
user has disabled the LAPD Transmitter by writing a
“1” into bit 3 (No Data Link) within the Tx E3 Configu-
ration register.
NOTE: In order to prevent the user’s data (e.g., the PMDL
Message within the LAPD Message frame) from mimicking
the Flag Sequence byte or an ABORT Sequence, the LAPD
Transmitter will parse through the PMDL Message data and
insert a “0” into this data, immediately following the detec-
tion of five (5) consecutive “1’s” (this stuffing occurs while
the PMDL message data is being read in from the Transmit
LAPD Message frame. The Remote LAPD Receive (See
Section 4.3.5) will have the responsibility of checking the
newly received PMDL messages for a string of five (5) con-
secutive “1’s” and removing the subsequent “0” from the
payload portion of the incoming LAPD Message.
Figure 118 presents a flow chart diagram.
Figure 118 depicts the procedure (in white boxes)
that the user should use in order to transmit a PMDL
message via the LAPD Transmitter, when the LAPD
Transmitter is configured to retransmit the LAPD Mes-
sage frame, repeatedly at One-Second intervals.
This figure also indicates (via the Shaded boxes)
what the LAPD Transmitter circuitry will do before and
during message transmission.
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