IBM BC-201 manual Comparing Qllc Conversion to Sdllc, BC-222

Overview of IBM Networking

IBM Network Media Translation

Figure 97 QLLC Conversion Between a Single 37x5 and Multiple 3x74s across an Arbitrary WAN

Without local

acknowledgment LLC2 session QLLC/X.25 session

With local LLC2 session TCP session QLLC/X.25 session acknowledgment

VR1

					VR2
	Token	T0	Arbitrary		X.25
37x5
	Ring		WAN	S0	Virtual
			Router A	Router B	ring

3270

3x74

3x74

51924

3270

How communication sessions are established over the communication link varies depending on whether or not LLC2 local acknowledgment has been configured on Router A’s Token Ring interface. In both cases, the SNA session extends end-to-end and the QLLC/X.25 session extends from Router B to the 3x74 cluster controller. If LLC2 local acknowledgment has not been configured, the LLC2 session extends from the 37x5 FEP across the Token Ring network and the arbitrary WAN to Router B. In contrast, when LLC2 local acknowledgment has been configured, the LLC2 session extends from the 37x5 FEP Router A, where it is locally terminated. A TCP session is then used across the arbitrary WAN to Router B.

Comparing QLLC Conversion to SDLLC

Although the procedures you use to configure QLLC are similar to those used to configure SDLLC, there are structural and philosophical differences between the point-to-point links that SDLC uses and the multiplexed virtual circuits that X.25 uses.

The most significant structural difference between QLLC conversion and SDLLC is the addressing. To allow a device to use LLC2 to transfer data, both SDLLC and QLLC provide virtual MAC addresses. In SDLLC, the actual MAC address is built by combining the defined virtual MAC (whose last byte is 0x00) with the secondary address used on the SDLC link; in this way, SDLLC supports multidrop. In QLLC conversion, multidrop is meaningless, so the virtual MAC address represents just one session and is defined as part of the X.25 configuration. Because one physical X.25 interface can support many simultaneous connections for many different remote devices, you only need one physical link to the X.25 network. The different connections on different virtual circuits all use the same physical link.

The most significant difference between QLLC conversion and SDLLC is the fact that a typical SDLC/SDLLC operation uses a leased line. In SDLC, dial-up connections are possible, but the maximum data rate is limited. In QLLC, both switched virtual circuits (SVCs) and permanent virtual circuits (PVCs) are available, but the favored use is SVC. While the router maintains a permanent connection to the X.25 network, a remote device can use each SVC for some bounded period of time and then relinquish it for use by another device. Using a PVC is very much like using a leased line.

	Cisco IOS Bridging and IBM Networking Configuration Guide
BC-222	78-11737-02