![](http://datasheet.mmic.net.cn/Exar-Corporation/XRT79L71IB-F_datasheet_100145/XRT79L71IB-F_241.png)
PRELIMINARY
XRT79L71
226
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
Where, for standard Bellcore GR-499-CORE applications:
Flag Sequence = 0x7E
SAPI + CR + EA = 0x3C or 0x3E
TEI + EA = 0x01
Control = 0x03
The following text defines each of these bit/byte-fields within the LAPD Message Frame Format.
Flag Sequence Byte
The Flag Sequence byte is of the value 0x7E, and is used to denote the boundaries of the LAPD Message
Frame. Additionally, whenever the Receive LAPD Controller is not currently receiving a LAPD Message, it will
instead be receiving a continuous stream of Flag Sequence bytes via the DL bits within each inbound DS3
frame.
SAPI - Service Access Point Identifier
Traditionally, for N-ISDN applications, the SAPI field typically indicates the type of data or service being
supported by the LAPD Message. However, for standard Bellcore GR-499-CORE applications, this parameter
has no meaning and is assigned the value "001111b" or 1510 per Bellcore GR-499-CORE
TEI - Terminal Endpoint Identifier
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 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 0x11C0.
Frame Check Sequence Bytes
FIGURE 96. LAPD MESSAGE FRAME FORMAT
FLAG SEQUENCE (8 BITS)
SAPI (6-bits)
C/R
EA
TEI (7 bits)
EA
Control (8-bits)
76 OR 82 OR ANY-SIZE BYTES OF INFORMATION (PAYLOAD)
FCS - MSB
FCS - LSB
Flag Sequence (8-bits)