QLLC: Qulified Logic Link Control for SNA over X.25

Qualified Logical Link Control (QLLC) is an IBM-defined data-link-layer protocol that allows SNA data to be transported across X.25 networks. When SNA is used over X.25, it uses the qualifier-bit (Q-bit) in the X.25 packet header to indicate special link control information. This information is relevant for SNA control between the two systems communicating with each other, but it is of no concern to X.25 link control. These qualified packets help SNA to determine who is calling whom between the two communicating systems and indicate such items as maximum message size.

QLLC commands are implemented in X.25 packets with the use of the Q-bit. X.25 packets containing QLLC primitives are typically 5 bytes, or the length of the X.25 packet header, plus 2 bytes of QLLC control information. After the QLLC connection is established, the X.25 connection's unique virtual circuit is used to forward data traffic. LLC (Logical Link Control) is a subset of HDLC (High Level Data Link Control). SDLC (Synchronous Data Link Control) and QLLC are also subsets of HDLC.

QLLC supports the following X.25 optional facilities:

• Modulo 8/128 packet sequence numbering
• Closed user groups
• Recognized private operating agencies

• Network user identification
• Reverse charging
• Packet-size negotiation
• Window-size negotiation
• Throughput class negotiation

Typical QLLC network architecture:

QLLC has the same frame structure as HDLC with the following frame types:

QLLC is an IBM protocol.