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PRELIMINARY
XRT79L71
306
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
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 42 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: If the LAPD Message and information payload that is to be transmitted 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.
Receive LAPD Controller Block handles and processes incoming LAPD Message frames.
Operation of the Transmit LAPD Controller
As mentioned earlier, the Transmit LAPD Controller 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
The procedure for transmitting these types of LAPD Messages is presented below.
5.2.3.1
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 Transmit LAPD Controller 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 E3, ITU-T G.751 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 "0" and that Bit 2 (Frame Format) is set to "0", as illustrated below.
TABLE 42: 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