XRT7250 datasheet(324/463 Pages) EXAR

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

305

The receipt of the Flag Sequence octet can mean

one of two things.

1. This Flag Sequence byte may be marking the

beginning or end of an incoming LAPD Message

frame.

2. The Received Flag Sequence octet could be just

one of many Flag Sequence octets that are trans-

mitted via the E3 Transport Medium, during idle

periods between the transmission of LAPD Mes-

sage frames.

The LAPD Receiver will negate the Flag Present bit

as soon as it has received an octet that is something

other than the Flag Sequence octet. Once this hap-

pens, the LAPD Receiver should be receiving either

octet # 2 of the incoming LAPD Message, or an

ABORT Sequence (e.g., a string of seven or more

consecutive “1’s”). If this next set of data is an

ABORT Sequence, then the LAPD Receiver will as-

sert the RxABORT bit-field (Bit 6) within the Rx E3

LAPD Status Register, as depicted below.

However, if this next octet is Octet #2 of an incoming

LAPD Message frame, then the LAPD Receiver is be-

ginning to receive a LAPD Message frame.

As the LAPD Receiver receives this LAPD Message

frame, it is reading in the LAPD Message frame oc-

tets, from “N” bit-fields within each incoming E3

frame. Secondly, it is reassembling these bits into a

LAPD Message frame.

Once the LAPD Receiver has received the complete

LAPD Message frame, then it will proceed to perform

the following five (5) steps.

1. PMDL Message Extraction

The LAPD Receiver will extract out the PMDL Mes-

sage, from the newly received LAPD Message frame.

The LAPD Receiver will then write this PMDL Mes-

sage into the Receive LAPD Message buffer within

the Framer IC.

NOTE: As the LAPD Receiver is extracting the PMDL Mes-

sage, from the newly received LAPD Message frame, the

LAPD Receiver will also check the PMDL data for the

occurrence of stuff bits (e.g., “0’s” that were inserted into

the PMDL Message by the Remote LAPD Transmitter, in

order to prevent this data from mimicking the Flag

Sequence byte or an ABORT Sequence), and remove them

prior to writing the PMDL Message into the Receive LAPD

Message Buffer. Specifically, the LAPD Receiver will

search through the PMDL Message data and will remove

any “0” that immediately follows a string of 5 consecutive

“1’s”.

NOTE: For more information on how the LAPD Transmitter

inserted these stuff bits, please see Section 4.2.3.1.

2. FCS (Frame Check Sequence) Word Verification

The LAPD Receiver will compute the CRC-16 value

of the header octets and the PMDL Message octets,

within this LAPD Message frame and will compare it

with the value of the two octets, residing in the FCS

word-field of this LAPD Message frame. If the FCS

value of the newly received LAPD Message frame

matches the locally-computed CRC-16 value, then

the LAPD Receiver will conclude that it has received

this LAPD Message frame in an error-free manner.

However, if the FCS value does not match the locally-

computed CRC-16 value, then the LAPD Receiver

will conclude that this LAPD Message frame is erred.

RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0

Not Used

RxABORT

RxLAPDType[1:0]

RxCR

Type

RxFCS

Error

End of

Message

Flag

Present

0000

001

RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0

Not Used

RxABORT

RxLAPDType[1:0]

RxCR

Type

RxFCS

Error

End of

Message

Flag

Present

1000

000

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