x.25(page3)


 * 4.0	X.25 Call Set-Up State Diagram**

In a X.25 network there are several scenarios which can happen when two DTEs wish to initiate a connection. The following diagrams describe: the Networks internal state machine, the networks method for handling different scenarios.



As we can see when a DTE receives a call (INCOMING CALL signaling packet) or initiates a call (CALL REQUEST signaling packet) if there is a situation when the other also occurs we go into call collision in the other case we have a successful creation of the connection.


 * 4.1	X.25 Collision Handling**

As we can see when both DTEs initiate a call together the network (the DCEs) will decide on one of the requests dump the second request, finally it will send both DTEs an INCOMING CALL signaling packet.


 * 4.2	X.25 Call Set-Up**

Successful call set-up (without collision): In accordance with the first diagram when there is no collision the initiator sends a CALL REQUEST signaling packet the receiver receives a INCOMING CALL signaling packet and replies with a CALL ACCEPTED signaling packet, finally the initiator receives a CALL CONNECTED signaling packet and a new connection was set-up.

**5.0	X.25 Virtual Circuits**

A virtual circuit is a logical connection created to ensure reliable communication between two network devices. A virtual circuit denotes the existence of a logical, bidirectional path from one DTE device to another across an X.25 network. Physically, the connection can pass through any number of intermediate nodes, such as DCE devices and PSEs. Multiple virtual circuits (logical connections) can be multiplexed onto a single physical circuit (a physical connection). Virtual circuits are demultiplexed at the remote end, and data is sent to the appropriate destinations. Figure 17-3 illustrates four separate virtual circuits being multiplexed onto a single physical circuit.

Two types of X.25 virtual circuits exist: switched and permanent. Switched virtual circuits (SVCs) are temporary connections used for sporadic data transfers. They require that two DTE devices establish, maintain, and terminate a session each time the devices need to communicate. Permanent virtual circuits (PVCs) are permanently established connections used for frequent and consistent data transfers. PVCs do not require that sessions be established and terminated. Therefore, DTEs can begin transferring data whenever necessary because the session is always active. The basic operation of an X.25 virtual circuit begins when the source DTE device specifies the virtual circuit to be used (in the packet headers) and then sends the packets to a locally connected DCE device. At this point, the local DCE device examines the packet headers to determine which virtual circuit to use and then sends the packets to the closest PSE in the path of that virtual circuit. PSEs (switches) pass the traffic to the next intermediate node in the path, which may be another switch or the remote DCE device. When the traffic arrives at the remote DCE device, the packet headers are examined and the destination address is determined. The packets are then sent to the destination DTE device. If communication occurs over an SVC and neither device has additional data to transfer, the virtual circuit is terminated.

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