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XRT79L71
PRELIMINARY
473
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
The TEI bit-fields are assigned the value of 0x00. The TEI field is used in N-ISDN systems to identify a
terminal out of multiple possible terminals. However, since DS3 and E3 data is transmitted in a point-to-point
manner, the TEI value is unimportant in this application.
Control
The Control byte-field identifies the type of frame being transmitted. There are three general types of frame
formats: Information, Supervisory, and Unnumbered. For standard Bellcore GR-499-CORE applications the
user must use the Control byte the value 0x03. Hence, the XRT79L71 will be transmitting and receiving
Unnumbered LAPD Message frames.
Information Payload
The Information Payload is the 76 bytes, 82 bytes or any number of bytes of data (e.g., the PMDL Message)
that the user has written into the on-chip Transmit LAPD Message buffer which is located at Address 0x11B0.
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 62 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 a LAPD Message and information payload is transmitted 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
LAPD Controller block within the 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.
6.2.3.1
Transmitting Standard-type (76 or 82 byte size) LAPD Messages
TABLE 62: 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