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XRT79L71
PRELIMINARY
119
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
A Special Note about the Information Payload when transmitting standard Bellcore GR-499-CORE type
LAPD Messages
It is important to note that the user must write in a specific octet value into the first byte position within the
Transmit LAPD Message buffer located at Address 0x11B0 within the XRT79L71. The value of this octet
depends upon the type of LAPD Message frame/PMDL Message that the user wishes to transmit. Table 22 presents a list of the various types of LAPD Message frames/PMDL Messages that are supported by the
XRT79L71 and the corresponding octet value that the user must write into the first octet position within the
Transmit LAPD Message buffer.
NOTE: To transmit a LAPD Message and information payload that is of a size other than 76 or 82 bytes, then there are no
restrictions on the value that should be written to the first byte within the information payload.
Frame Check Sequence Bytes
The 16 bit FCS (Frame Check Sequence) is calculated over the LAPD Message Header and Information
Payload bytes, by using the CRC-16 polynomial, x16 + x12 + x5 + 1. Afterwards, this FCS value is inserted
into the two-octet FCS value position, within the LAPD Message frame. The LAPD Receiver at the remote
terminal will use the FCS bytes in order to verify that it has received a given LAPD Message in an un-erred
manner.
Please see
Section 4.3.3.3 on how the Receive LAPD Controller block handles and processes
incoming LAPD Message frames.
Operation of the Transmit LAPD Controller
As mentioned earlier, the LAPD Transmitter permits the user to transmit either of the following basic types of
LAPD Messages.
Standard (e.g., 76 or 82 byte size) LAPD Messages
Variable Length (e.g., up to 82 byte size) LAPD Messages
4.2.4.1
Transmitting Standard-type (76 or 82 byte size) LAPD Messages
The user can (1) write the contents of an outbound PMDL Message, into the Transmit LAPD Message Buffer,
and (2) command the LAPD Transmitter to begin the transmission of this PMDL Message by executing the
following steps.
STEP 1 - Make sure that the XRT79L71 has been configured to operate in the C-Bit Parity Framing
format.
This is accomplished by reading out the contents of the Framer Operating Mode Register (Address = 0x1100)
and verifying that Bit 6 (DS3/E3*) is set to "1" and that Bit 2 (Frame Format) is set to "0" as illustrated below.
TABLE 22: THE LAPD MESSAGE TYPE AND THE CORRESPONDING VALUE OF THE FIRST BYTE, WITHIN THE
INFORMATION PAYLOAD FOR STANDARD 76 OR 82 BYTE MESSAGES
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