IBM BC-203 - Manual (Page 20)

Overview of IBM Networking
IBM Network Media Translation
BC-222
Cisco IOS Bridging and IBM Networking Configuration Guide
QLLC Conversion
Qualified Logical Link Control (QLLC) is a data link protocol defined by IBM that allows SNA data to
be transported across X.25 networks. (Although IBM has defined other protocols for transporting SNA
traffic over an X.25 network, QLLC is the most widely used.) Figure 92 illustrates how QLLC
conversion provides data link layer support for SNA communication.
Figure 92 SNA Data Link Layer Support
As shown in Figure 93, any devices in the SNA communication path that use X.25, whether end systems
or intermediate systems, require a QLLC implementation.
Figure 93 SNA Devices Running QLLC
As shown in Figure 94, the QLLC conversion feature eliminates the need to install the X.25 software on
local IBM equipment. A device attached locally to a Token Ring network can communicate through a
router running the QLLC Conversion feature with a remote device attached to an X.25 network using
QLLC. Typically, the locally attached device is an FEP, an AS 400, or a PS/2, and the remote device is
a terminal controller or a PS/2. In this case, only the remote device needs an X.25 interface and the FEP
can communicate with the terminal controller as if it were directly attached via a Token Ring network.
Figure 94 Router Running QLLC Conversion Feature
More elaborate configurations are possible. The router that implements QLLC conversion need not be
on the same Token Ring network as the FEP. As shown in Figure 95, QLLC/LLC2 conversion is possible
even when an intermediate IP WAN exists between the router connected to the X.25 network and the
router connected to the Token Ring.
SNA
SDLC SDLLC LLC LNX QLLC
X.25
S3028
Upper layers
Data link layer
Running QLLC Running QLLC
S3029
X.25 network
S3030
Running QLLC
Token
Ring
X.25 network