XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
PRELIMINARY
217
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.
N
OTE
:
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.