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Multiple SVCs

An issue that often comes up is what happens between the carriers in the X.25 cloud. The
possibility exists that there could be multiple carriers and very low throughput, because it is
hard to determine what path an X.25 circuit setup will take through the carrier's cloud.

The window size between carriers might be a low number, such as the default, which is 2. And,
depending on your packet sizes, which could be as low as 128, your effective window is
probably very small from end to end. Small window sizes mean low throughput, no matter what
the bandwidth is. You can address this by setting up multiple switched virtual circuits between
the routers.

As shown in Figure 10-9, you should use several SVCs between two destinations if you want
to push more data through the X.25 cloud. (When the window closes on the first VC, you can
still push data through the second VC.) This means that you'll be using more RAM.

Figure 10-9

Multiple SVCs

This is all done over one physical connection. You should check the frame size because larger
is not always better. Naturally, if you are sending larger packets over noisy lines, and a large
packet gets corrupted, you will be retransmitting a lot, so smaller packets might be better over
noisy lines.

NOTE

A maximum of eight SVCs per protocol per destination are allowed.

Physical connection

Carrier 1

Window size 2

Physical connection

Carrier 2

8 Virtual connections over 1 physical connection
Each router can send 16 packets
Carriers use a Window size of 2 by default

87200333.book Page 341 Wednesday, August 22, 2001 2:53 PM