Introduction to Tandem X.25 Capabilities

Packet-Layer Components
What is X.25?
065307 Tandem Computers Incorporated 1–13
Figure 1-9 . Packaging Data for Transmission
Data
Packet
Electric
Signals
Frame
Electric
Signals
Data
Packet
Frame
009
Source DTE
Upper Layers
Packet Layer
Frame Layer
Physical Layer
Destination DTE
Upper Layers
Packet Layer
Frame Layer
Physical Layer
Routing The Packet Layer provides two types of virtual circuits for the routing of information
from one location to another: switched virtual circuits (SVCs) and permanent virtual
circuits (PVCs). When you subscribe to a network, you can select the number and type
of circuits that you wish.
The physical medium (such as copper wire) that is used for the circuits is divided into
logical channels (LCNs) at either end of the network. The network maps the two
logical circuits to a virtual circuit as it routes the packet from the point of origin to the
point of destination.
Switched Virtual Circuit (SVC) — A switched virtual circuit, sometimes referred to
as a virtual circuit or virtual call, provides a temporary end-to-end connection
between two DTEs through the network. A virtual circuit is similar to a dial-up
call in a telephone network. The connection lasts only for the duration of the
virtual call. When two locations have completed the information transfer, the call
is cleared, and the logical channels are free to be assigned to another virtual
circuit. A number of control packets (Call Setup and Call Clearing packets) must
be transmitted to initiate and to end a virtual call. The requirement for call
establishment and call disconnect packets extends the time required for the
communication.