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X.25 virtual circuits are logical links rather than physical links as in circuit switching networks. They
include two categories:
• PVCs—Which are suitable for frequent, constant data transmission.
• SVCs—Which are suitable for bursty data transmission.
Each virtual circuit is uniquely identified by a virtual circuit ID. Each packet sent by a DTE carries a
virtual circuit ID, which enables the DCEs in the switched network to forward the packet correctly to
its destination.
X.25 switching
You can use devices supporting X.25 switching as small-sized X.25 PSEs to protect the investment
in X.25. Figure 123 describes the relationshi
ps among LAPB, X.25, and X.25 switching.
Figure 123 Relation between LAPB, X.25, and X.25 switching
In addition to X.25, LAPB can carry IP when two devices are directly connected without an X.25
network in between. With an X.25 switching module, a device can work as a small-sized PSE to
directly forward packets without using the upper layer service.
Hardware compatibility with LAPB and X.25
LAPB and X.25 are not available on the following routers:
• MSR900.
• MSR93X.
X.25 is not available on the following routers:
• MSR1000.
• MSR30-11E.
• MSR30-11F.
Configuring LAPB
LAPB parameters
LAPB frame numbering modes
Two LAPB frame numbering modes are available: modulo 8 and modulo 128. Data frames (I frames)
are numbered cyclically in sequence in the range of 0 to modulo – 1.
All standard LAPB implementations support modulo 8 (the basic mode), which is sufficient for most
links.
LAPB window size K
LAPB window size K indicates the maximum number of outstanding frames for acknowledgement on
the DTE or DCE within any period.
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