![](http://datasheet.mmic.net.cn/Exar-Corporation/XRT79L71IB-F_datasheet_100145/XRT79L71IB-F_566.png)
XRT79L71
PRELIMINARY
551
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/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 Receive LAPD Message buffer which is located at Address 0x>>>>.
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 Receive LAPD Controller block
will use the FCS bytes in order to verify that it has received a given LAPD Message in an un-erred manner.
Controller block computes and inserts the FCS values into its outbound LAPD Message frames.
Operation of the Receive LAPD Controller
As mentioned earlier, the Receive LAPD Controller permits the user to receive 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 receiving these types of LAPD Messages is presented below.
6.3.5.1
Receiving Standard-type (76 or 82 byte size) LAPD Messages
The user can (1) configure the Receive LAPD Controller block to extract out the contents of the incoming
PMDL Messages from the incoming DS3 data-stream, and (2) to properly read out the contents of a newly
received message which is being stored in the Receive LAPD Message buffer, by executing the following
steps.
STEP 1 - Make sure that the XRT79L71 has been configured to operate in the E3, ITU-T G.832 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 "1" as illustrated below.
STEP 2 - Select either the NR or the GC byte as the LAPD Channel.
The user can accomplish this by writing the appropriate value into Bit 3 (Receive LAPD from NR Byte), within
the Receive E3 LAPD Control Register, as depicted below.
Framer Operating Mode Register (Address = 0x1100)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
Local Loop-
back
DS3/E3*
Internal LOS
Enable
RESET
Direct
Mapped
ATM
Frame For-
mat
Timing Reference Select
[1:0]
R/W
X
0
X
0
X
1
X