Introduction to Tandem X.25 Capabilities

Packet-Layer Components
What is X.25?
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Permanent Virtual Circuit (PVC) — A permanent virtual circuit provides a
permanent logical channel assigned exclusively to the calling DTE. A PVC
automatically connects a DTE to a predetermined destination. Devices do not
need permission to transmit to each other. No control packets are required to
establish or end the call. A PVC is analogous to a private or leased point-to-point
line.
Logical Channel (LCN) — The physical medium that transmits communications
through the network is capable of carrying multiple circuits at the same time. To
make efficient use of the medium, it is divided into logical channels The LCN
provides the logical connection between the user DTE and the packet exchange at
either end of the network. Thus, a different LCN is assigned to a single virtual
circuit on each side of the network cloud.
Logical channels make it possible for the physical medium, which carries the
electrical signals from one location to another, to carry multiple calls at the same
time. Any number of logical channels may be assigned, up to a maximum of 4095.
For permanent virtual circuits, logical channel numbers are assigned when the
service subscription is negotiated.
For virtual calls, a logical channel group number and a logical channel number are
assigned during the call-setup phase. The range of channels to be used for
switched virtual circuits is assigned when the service subscription is negotiated.
The range of virtual circuits is subdivided into ranges of numbers for the different
types of services that might be required: such as, one-way incoming, two-way, or
one-way outgoing. The separate groups of logical channels for incoming and
outgoing calls greatly reduce the probability of collision on a particular channel
(Figure 1-10). Note that the rules for assigning logical channels do not restrict the
number of channels in each category. The numbers do not have to be consecutive,
and gaps in the numbers are allowed.