XRT74L73 Datasheet, PDF (213/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

PRELIMINARY
XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
The LAPD Transmitter (within the Transmit DS3
HDLC Controller Block) allows the user to transmit
Path Maintenance Data Link (PMDL) messages to
the remote terminal via the outbound DS3 Frames.
In this case the message bits are inserted into and
carried by the 3 DL bit fields of F-Frame #5 within
each DS3 M-frame. The on-chip LAPD transmitter
supports both the 76 byte and 82 byte length mes-
sage formats, and the Framer IC allocates 88 bytes
of on-chip RAM (e.g., the Transmit LAPD Message
buffer) to store the message to be transmitted. The
message format complies with ITU-T Q.921 (LAP-D)
protocol with different addresses and is presented be-
low in Figure 69 .
FIGURE 69. LAPD MESSAGE FRAME FORMAT
Flag Sequence (8 bits)
SAPI (6-bits)
TEI (7 bits)
Control (8-bits)
C/R EA
EA
76 or 82 Bytes of Information (Payload)
FCS - MSB
FCS - LSB
Flag Sequence (8-bits)
Where: Flag Sequence = 0x7E
SAPI + CR + EA = 0x3C or 0x3E
TEI + EA = 0x01
Control = 0x03
The following sections 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 for two purposes
1. To denote the boundaries of the LAPD Message
Frame, and
2. To function as the Idle Pattern (e.g., Transmit
HDLC Controller block transmits a continuous
stream of flag sequence octets, whenever no
LAPD Message is being transmitted).
SAPI - Service Access Point Identifier
The SAPI bit-fields are assigned the value of 001111b
or 15 (decimal).
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 terminal. However,
since the Framer IC transmits data in a point-to-point
manner, the TEI value is unimportant.
Control
The Control identifies the type of frame being trans-
mitted. There are three general types of frame for-
mats: Information, Supervisory, and Unnumbered.
The Framer assigns the Control byte the value 0x03.
Hence, the Framer will be transmitting and receiving
Unnumbered LAPD Message frames.
Information Payload
The Information Payload is the 76 bytes or 82 bytes
of data (e.g., the PMDL Message) that the has been
written into the on-chip Transmit LAPD Message buff-
er (which is located at addresses 0x86 through
0xDD).
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 Ad-
dress = 0x86, within the Framer). The value of this
octet depends upon the type of LAPD Message
frame/PMDL Message that the user wishes to trans-
mit. Table 40 presents a list of the various types of
LAPD Message frames/PMDL Messages that are
supported by the XRT74L73 Framer and the corre-
sponding octet value that the user must write into the
first octet position within the Transmit LAPD Message
buffer.
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