Interphase Tech iSPAN - Manual (Page 206)

X.25 Protocol
186 Interphase Corporation
Layer 2: Frame Level Layer
The Frame Level layer is an implementation of the ISO HDLC
standard called Link Access Procedure Balanced (LAPB).
This layer provides an error-free link between two connected
devices.
Layer 3: Network Layer
The Network layer provides communication between devices
that are connected to a common network. In the case of X.25,
this layer is referred to as the X.25 packet.
Logical Channels
The user end of the network is known as Data Terminal
Equipment (DTE) and the carrier's equipment is Data Circuit-
terminating Equipment (DCE). The X.25 protocol layer
permits a DTE user on an X.25 network to communicate with
a number of remote DTEs simultaneously.
Connections occur on logical channels of two types:
• Switched virtual circuits (SVCs)
• Permanent virtual circuits (PVCs)
SVCs
SVCs are much like telephone calls; a connection is
established, data is transferred, and then the connection is
released. Each DTE on the network is given a unique DTE
address, which can be used much like a telephone number.