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.
