Qualified Logical Link Control (QLLC) is only for SNA support. It permits the use of additional link control information that SNA needs, but which X.25 does not. QLLC architecture specifies the mapping between Synchronous Data Link Control (SDLC) frames and X.25 packets.
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 must be used in the following situations:
Some systems can control the segmenting and unsegmenting of messages themselves, rather than leaving it to the X.25 protocol . These systems use additional control on top of X.25 to perform this. The in AIX Technical Reference/6000; does not support this additional control of Logical Link Control (LLC) headers, embedded in X.25 packets, to segment messages. The system leaves it to the X.25 protocol to handle segmenting.
Note: The X.25 ports that are to be used for SNA must have COMIO emulation configured on them. This produces ports such as x25s0 which are used by QLLC. For example, the X.25 port, sx25a0, cannot be used directly by SNA or QLLC.