Introduction to Tandem X.25 Capabilities

Packet Types
What is X.25?
065307 Tandem Computers Incorporated 1–19
Packet Types X.25 defines five general types of packets, which perform different packet-level
communications functions.
Call-Setup and Clearing
Data and Interrupt
Flow Control and Reset
Restart
Diagnostic
Each of these general packet types includes one or more specific packet types.
Call Setup Packets Call setup packets are required to establish a switched virtual circuit. Call setup
packets are not used by permanent virtual circuits.
Call Request / Incoming Call Packets
The calling DTE sends a Call Request packet to the local DCE. The DCE forwards this
call as an Incoming Call packet to the destination DTE.
The packet formats of the Call Request and Incoming Call packets are identical.
Call Accepted / Call Connected Packets
In response to an Incoming Call packet, the called DTE accepts the call by transmitting
a Call Accepted packet, which is sent to the calling DTE as a Call Connected packet.
Figure 1-13 illustrates the exchange of packets for establishing a call on a switched
virtual circuit.
Figure 1-13. Call Setup Packets
Network
Incoming Call
Call Connected Call Accepted
Call Request
Calling
DTE
Called
DTE
050
Local
DCE
Remote
DCE