ATM connections are established as either permanent virtual circuits (PVCs) or switched virtual circuits (SVCs). As their name implies, PVCs are always present, whereas SVCs must be established each time a connection is set up.
To set up a connection, a signaling circuit is used first. A signaling circuit is a pre-defined circuit (with VPI = 0 and VCI = 5) that is used to transfer signaling messages, which are in turn used for making and releasing calls or connections. If a connection request is successful, a new set of VPI and VCI values are allocated on which the parties that set up the call can send and receive data.
Six message types are used to establish virtual circuits, each message occupying one or more cells and containing the message type, length, and parameters. The following table lists these message types.
|
Message |
Significance if sent by host |
Significance if sent by the network |
|
SETUP |
Requests that a call be established |
Indicates an incoming call |
|
CALL PROCEEDING |
Acknowledges the incoming call |
Indicates the call request will be attempted |
|
CONNECT |
Indicates acceptance of the call |
Indicates the call was accepted |
|
CONNECT ACK |
Acknowledges acceptance of the call |
Acknowledges making the call |
|
RELEASE |
Requests that the call be terminated |
Terminates the call |
|
RELEASE ACK |
Acknowledges releasing the call |
Acknowledges releasing the call |
The sequence for establishing and releasing a call is as follows:
The host sends a SETUP message on the signaling circuit.
The network responds by sending a CALL PROCEEDING message to acknowledge receiving the request.
Along the route to the destination, each switch receiving the SETUP message acknowledges it by sending the CALL PROCEEDING message.
When the SETUP message reaches its final destination, the receiving host responds by sending the CONNECT message to accept the call.
The network sends a CONNECT ACK message to acknowledge receiving the CONNECT message.
Along the route back to the sender, each switch that receives the CONNECT message acknowledges it by sending CONNECT ACK.
To terminate the call, a host (either the caller or the receiver) sends a RELEASE message, causing the message to propagate to the other end of the connection, and then releasing the circuit. Again, the message is acknowledged at each switch along the way.
In ATM networks, users can set up point-to-multipoint (P/MP) calls, with one sender and multiple receivers. A P/MP VC allows an endpoint called the root node to exchange data with a set of remote endpoints called leaves. To set up a point-to-multipoint call, a connection to one of the destinations is set up in the usual way. Once the connection is established, users can send the ADD PARTY message to attach a second destination to the VC returned by the previous call. To add receivers, users can then send additional ADD PARTY messages.
This process is similar to a user dialing multiple parties to set up a telephone conference call. One difference is that an ATM P/MP call doesn't allow data to be sent by parties towards the root (or the originator of the call). This is because the ATM Forum Standard UNI 3.1 restricts data flow on P/MP VCs to be from the root towards the leaves only.