XRT74L73 Datasheet, PDF (216/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
PRELIMINARY
• Compute the 16 bit Frame Check Sum (FCS) of the
LAPD Message Frame (e.g., of the LAPD Message
header and information payload) and append this
value to the LAPD Message.
• Append a trailer Flag Sequence octet to the end of
the message LAPD (following the 16 bit FCS
value).
• Serialize the composite LAPD message and begin
inserting the LAPD message into the DL bit fields of
each outgoing DS3 Frame.
• Complete the transmission of the frame overhead,
payload, FCS value, and trailer Flag Sequence
octet via the Transmit DS3 Framer.
Once the LAPD Transmitter has completed its trans-
mission of the LAPD Message, the Framer will gener-
ate an interrupt to the local µC/µP (if enabled). After-
wards, the LAPD Transmitter will proceed to retrans-
mit the LAPD Message, repeatedly at one second in-
tervals. During Idle periods (e.g., in between these
transmission of the LAPD Message), 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 "0" to bit 0 (TxLAPD
Enable) within the Tx DS3 LAPD Configuration Reg-
ister. If the LAPD Transmitter is idle, then it will con-
tinuously send the Flag Sequence octets (via the DL
bits of each outbound DS3 Frame) to the remote ter-
minal equipment.
NOTE: In order to prevent the user’s data (e.g., the payload
portion of the LAPD Message Frame) from mimicking the
Flag Sequence byte, the LAPD Transmitter will insert a "0"
into the LAPD data stream immediately following the detec-
tion of five (5) consecutive 1s (this stuffing occurs only while
the information payload is being transmitted). The ’remote’
LAPD Receiver (see Section 4.3.3.2) will have the responsi-
bility of detecting the 5 consecutive 1s and removing the
subsequent "0" from the payload portion of the incoming
LAPD message.
Figure 70 presents a flow chart depicting the proce-
dure (in white boxes) that the user should use in or-
der to transmit a LAPD message. This figure also in-
dicates (via the shaded boxes) what the LAPD Trans-
mitter circuitry will do before and during message
transmission.
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