XRT74L74 Datasheet, PDF (326/498 Pages) Exar Corporation – 4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L74
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.1.1
PRELIMINARY
TABLE 68: THE LAPD MESSAGE TYPE AND THE CORRESPONDING VALUE OF THE FIRST BYTE, WITHIN THE
INFORMATION PAYLOAD
LAPD MESSAGE TYPE
CL Path Identification
IDLE Signal Identification
Test Signal Identification
ITU-T Path Identification
VALUE OF FIRST BYTE, WITHIN INFORMATION
PAYLOAD OF MESSAGE
0x38
0x34
0x32
0x3F
MESSAGE SIZE
76 bytes
76 bytes
76 bytes
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 the user wishes to transmit a message via the
LAPD Transmitter, the information portion (or the
body) of the message must be written into the Trans-
mit LAPD Message Buffer, which is located at 0x86
through 0xDD in on-chip RAM via the Microprocessor
Interface. Afterwards, the user must do five things:
1. Configure the source of the “N” bit (within each
outbound E3 frame, to be the LAPD Transmitter.
2. Specify the length of LAPD message to be trans-
mitted.
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 - Configure the source of the “N” bit (with-
in each outbound E3 frame, to be the LAPD
Transmitter.
This is accomplished by writing the appropriate data
into the TxNSourceSel[1:0] bit-fields, within the TxE3
Configuration Register, as illustrated below.
TXE3 CONFIGURATION REGISTER (ADDRESS = 0X30)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
Tx
BIP-4
Enable
TxASourceSel[1:0]
TxNSourceSel[1:0]
R/W
R/W
R/W
R/W
R/W
0
0
0
0
0
BIT 2
Tx AIS
Enable
R/W
0
BIT 1
Tx LOS
Enable
R/W
0
BIT 0
Tx FAS
Source
Select
R/W
0
Setting TxNSourceSel[1:0] to “10” configures the
Transmit E3 Framer block to use the LAPD Transmit-
ter as the data source for the “N” bits. Hence, the “N”
bit, (within each outbound E3 frame) is now carrying
LAPD Messages to the remote terminal equipment.
STEP 2 - 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 desired to be
transmitted. This octet will ultimately be used by the
Remote Terminal Equipment in order to help it identify
the type of LAPD message frame that it is receiving.
Table 69 lists these octets and the corresponding
LAPD Message types.
STEP 3 - 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).
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