Telecommunications - QLLC - SNA over X.25

To allow an X25 network to provide all the facilities of the data-link control then a layer must be inserted between path control and X.25. This layer is called logical link control, there are 3 versions of this -

PSH = Physical Services Header

QLLC = Qualified Logical Lin Control

ELLC = Enhanced logical Link Control

Also see following files:

SNA_PRO


3.1 - Qualified Logical Link Control (QLLC)

SNA over X.25

QLLC

QLLC is designed to allow adjacent SNA nodes to connect via a X.25 network.

QLLC carries SDLC unnumbered commands as user data in data packets with Q-

bit=1 (Q packets).

Qualified Data Packet Formats

8 7 6 5 4 3 2 1
General Format Identifier GFI Logical channel group num
Logical Channel Number
P(r) M P(s) 0
QLLC Address Field (coded 'FF' for commands 'NOT FF' for responses)
QLLC Control Field
QXID/QTEST/QFRMR information field - optional

QLLC C-Field I-Field Primary Secondary Peer-to-peer
Function Hex Octet 1 Allowed command response Command Response
QSM 93 NO x x
QDISC 53 NO x x
QXID BF YES x x
QTEST F3 YES x x
QRR F1 NO x x
QRD 53 NO x
QXID BF YES x x
QTEST F3 YES x x
QUA 73 NO x x
QDM 1F NO x x
QFRMR 97 YES x x

CALL USER DATA FORMAT

byte QLLC ELLC PSH
0 Protocol Identifir C3 C6 C2
1 Field Format Identifier FFI FFI FFI
2 ELLC call control indicator CCI
3 Reserved
4
. Connection Identifier
11

Format of FFI . . optional and when present defines the

remainder of the CUD field.

Format of CCI . . x x x x x x x I/R

|

0=initial connection

1=connection recovery

Format of CI . . eight byte in length and permits DTEs to accept or reject incoming calls based on its content.

 


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