XRT74L73 Datasheet, PDF (414/488 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
TABLE 92: THE LAPD MESSAGE TYPE AND THE CORRESPONDING VALUE OF THE FIRST BYTE, WITHIN THE
INFORMATION PAYLOAD
LAPD MESSAGE TYPE
VALUE OF FIRST BYTE, WITHIN
INFORMATION PAYLOAD OF MESSAGE
MESSAGE SIZE
CL Path Identification
0x38
76 bytes
IDLE Signal Identification
0x34
76 bytes
Test Signal Identification
0x32
76 bytes
ITU-T Path Identification
0x3F
82 bytes
Frame Check Sequence Bytes
The 16 bit FCS (Frame Check Sequence) is calculat-
ed over the LAPD Message Header and Information
Payload bytes, by using the CRC-16 polynomial, x¹⁶
+ x¹² + x⁵ + 1.
Operation of the LAPD Transmitter
If a message is to be transmitted via the LAPD Trans-
mitter then, the information portion (or the body) of
the message must be written into the Transmit LAPD
Message Buffer, which is located at 0x86 through
0xDD in on-chip RAM via the Microprocessor Inter-
face. Afterwards, the user must do three things:
1. Specify the length of LAPD message to be trans-
mitted.
2. Specify which bit-field (within the E3 frame) that
the LAPD Message frame is to be transported on
(e.g., either the “GC” or the “NR” byte).
3. Specify whether the LAPD Transmitter should
transmit this LAPD Message frame only once, or
an indefinite number of times at One-Second
intervals.
4. Enable the LAPD Transmitter.
5. Initiate the Transmission of the PMDL Message.
Each of these steps will be discussed in detail.
STEP 1 - Specify the type of LAPD Message frame
to be Transmitted (within the Transmit LAPD Mes-
sage Buffer)
The user must write in a specific octet value into the
first octet position within the Transmit LAPD Buffer
(e.g., at Address Location 0x86 within the Framer IC).
This octet is referred to as the LAPD Message Frame
ID octet. The value of this octet must correspond to
the type of LAPD Message frame that is to be trans-
mitted. This octet will ultimately be used by the Re-
mote Terminal Equipment in order to help it identify
the type of LAPD message frame that it is receiving.
Table 92 lists these octets and the corresponding
LAPD Message types.
STEP 2 - Write the PMDL Message into the re-
maining part of the Transmit LAPD Message Buff-
er.
The user must now write in his/her PMDL Message
into the remaining portion of the Transmit LAPD Mes-
sage buffer (e.g., addresses 0x87 through 0x135
within the Framer IC).
STEP 3 - Specifying the Length of the LAPD Mes-
sage
One of two different sizes of LAPD Messages can be
transmitted, by writing the appropriate data to bit 1
within the Tx E3 LAPD Configuration Register. The
bit-format of this register is presented below.
TRANSMIT E3 LAPD CONFIGURATION REGISTER (ADDRESS = 0X33)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT2
Not Used
Auto
Retransmit
Not Used
R/O
R/O
R/O
R/O
R/W
R/O
0
0
0
0
X
0
The relationship between the contents of bit-fields 1
and the LAPD Message size is given in Table 93 .
BIT 1
TxLAPD
Msg Length
R/W
X
BIT 0
TxLAPD
Enable
R/W
X
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