XR72L52 Datasheet, PDF (287/480 Pages) Exar Corporation – TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
XRT72L52
REV. 1.0.1
FIGURE 105. 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
Comprise the 4 HEADER Bytes
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.
The user must write this value (0x7E) at address 0x86.
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 termi-
nal out of multiple possible terminal. However, since the Framer IC transmits data in a point-to-point manner,
the TEI value is unimportant.
The user must write 0x3C or 0x3E at address 0x87 and 0x01 at address ox88
Control
The Control identifies the type of frame being transmitted. There are three general types of frame formats: In-
formation, Supervisory, and Unnumbered. The Framer assigned the Control byte the value 03h. Hence, the
Framer will be transmitting and receiving Unnumbered LAPD Message frames.
The user must write 0x03 at address 0x89.
Information Payload
The Information Payload is the 76 bytes or 82 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 addresses 0x8A through 0xDB).
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 = 0x8A). The value of this octet depends upon the type of
LAPD Message frame/PMDL Message that the user wishes to transmit. Table 49 presents a list of the various
types of LAPD Message frames/PMDL Messages that are supported by the XRT72L52 Framer device and the
corresponding octet value that the user must write into the first octet position within the Transmit LAPD Mes-
sage buffer.
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