Expand Configuration and Management Manual (G06.24+)

Planning a Network Design
Expand Configuration and Management Manual—523347-008
3-8
When to Use a Single-Line Expand Line-Handler
Process
When to Use a Single-Line Expand Line-Handler Process
Single-line Expand line-handler processes are less expensive and require somewhat
less processing time than multi-line paths. However, they lack the fault-tolerance that
multi-line paths and multi-CPU paths provide.
When to Use a Multi-Line Path
A path that consists of more than one line is called a multi-line path. A multi-line path
can consist of up to eight parallel lines. The major benefits of configuring a multi-line
path are
•
Fault-tolerance is increased. If one or more lines in a multi-line path fail, the
Expand subsystem automatically reroutes data over the remaining lines in the
path. You can also attach lines in the path to different hardware communications
devices for an additional level of fault-tolerance.
•
Bandwidth is increased. The Expand subsystem simultaneously transmits data
over all the lines in a multi-line path, thus increasing overall bandwidth.
•
Multiple communications methods can be mixed in a multi-line path. You can mix
direct-connect lines, X.25 connections, and SNAX connections in the same multi-
line path. You cannot mix satellite-connect and Expand-over-IP lines with other line
types. Expand-over-ServerNet and Expand-over-FOX connections cannot be part
of a multi-line path.
The major disadvantages of configuring a multi-line path are
•
Overhead is increased. The Expand subsystem uses a packet-queueing algorithm
to select the best line in a multi-line path on which to queue the next packet. This
algorithm requires additional processing time, which is not required by Expand
line-handler processes that manage a single line.
•
Out-of-sequence (OOS) packet buffering is increased. The frequency of OOS
packets increases when packets are sent over a path that consists of lines of
varying speeds. For example, if the multi-line path contains both 9600 and 56K
byte lines, it is likely that frames traveling on the fast line are received at the
destination ahead of the frames traveling on the slower line. If many OOS packets
are received, the receiving node may require an OOS buffer space that is larger
than the default buffer size. When multipacket frames are used, this situation can
cause frames to be discarded at the destination if the maximum allowable OOS
window is exceeded. For these reasons, you should not configure a path with lines
that vary in speed by more than four to one.
Figure 3-1 illustrates a multi-line configuration with eight dedicated lines and two
ServerNet wide area network (SWAN) concentrators. Four lines are attached to each
SWAN concentrator.