XRT72L53 Datasheet, PDF (384/467 Pages) Exar Corporation – THREE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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PRELIMINARY
XRT72L53
THREE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
REV. P1.1.8
FIGURE 174. 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 denote the boundaries of the LAPD Message
Frame.
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 ter-
minal 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 assigned the Control byte the value 03h.
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 user has writ-
ten into the on-chip Transmit LAPD Message buffer
(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 oc-
tet depends upon the type of LAPD Message frame/
PMDL Message that the user wishes to transmit.
Table 77 presents a list of the various types of LAPD
Message frames/PMDL Messages that are supported
by the XRT72L53 Framer device and the correspond-
ing octet value that the user must write into the first
octet position within the Transmit LAPD Message
buffer.
TABLE 77: THE LAPD MESSAGE TYPE AND THE CORRESPONDING VALUE OF THE FIRST BYTE, WITHIN THE
INFORMATION PAYLOAD
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
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