XRT7250 datasheet(385/463 Pages) EXAR

385 / 463 page

XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

366

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

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 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

octet depends upon the type of LAPD Message

frame/PMDL Message that the user wishes to trans-

mit. Table 76 presents a list of the various types of

LAPD Message frames/PMDL Messages that are

supported by the XRT7250 Framer device and the

corresponding octet value that the user must write in-

to the first octet position within the Transmit LAPD

Message buffer.

FIGURE 174. LAPD MESSAGE FRAME FORMAT

Flag Sequence (8 bits)

SAPI (6-bits)

C/R

TEI (7 bits)

Control (8-bits)

76 or 82 Bytes of Information (Payload)

FCS - MSB

FCS - LSB

Flag Sequence (8-bits)

TABLE 76: 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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