network protocols handbook.pdf

298Protocols GuideIBM ProtocolsProtocol NameQLLC: Qualified Logic LinkControlProtocol DescriptionQualified Logical Link Control (QLLC) is an IBM-defined datalink-layerprotocol that allows SNA data to be transported acrossX.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 linkcontrol information. This information is relevant for SNA controlbetween the two systems communicating with each other butis of no concern to X.25 link control. These qualified packetshelp SNA to determine who is calling whom between the twocommunicating systems and indicate such items as maximummessage size.QLLC commands are implemented in X.25 packets with theuse of the Q-bit. X.25 packets containing QLLC primitives aretypically 5 bytes, or the length of the X.25 packet header, plus 2bytes of QLLC control information. After the QLLC connection isestablished, the X.25 connection’s unique virtual circuit is usedto forward data traffic. LLC (Logical Link Control) is a subsetof HDLC (High Level Data Link Control). SDLC (SynchronousData Link Control) and QLLC are also subsets of HDLC.Typical QLLC network architecture is shown below:QDISC Disconnect.QUA Unnumbered Acknowledgement.QDM Disconnect Mode.QFRMR Frame Reject.QTEST Test.QRD Request Disconnect.QXID Exchange Identification.QSM Set Mode.Related protocolsSNA, APPN, X.25, HDCL, SDCLSponsor SourceQLLC is an IBM protocol.QLLC SessionTCP SessionLLC2 SessionX.25NetworkIPNetworkTokenRing3174Router A Router B FEPFigure 2-25: QLLC Network ArchitectureQLLC 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 negotiationProtocol StructureQLLC has the same frame structure as HDLC with the followingframe types:QRRReceive Ready.