Call Progress

Each form of connectivity has a method for establishing connections. The method differs depending on the protocol used and whether the connection is designed to be permanent or temporary. Permanent connections tend to focus on keepalives to make sure the connection is still up. After all, once a Frame Relay or DSL circuit is established, it isn't brought down too often.

Temporary circuits are those that are meant to be up for definite periods of time. A connection might last for 30 minutes, 8 hours, or "Until the database has been uploaded," but there is usually a definite stopping point. That stopping point might even be "When there hasn't been important traffic crossing the link in a 5-minute window."

Call Setup

Setting up an ISDN call is a process with distinct steps. Communication needs to occur in the correct order for the call to be completed. Figure 7.3 walks you through this process.

Figure 7.3. ISDN call setup.

The figure shows four separate sets of communications, with anywhere from one to three segments. We can see that router A starts the conversation. Router A begins with a SETUP message to the local switch, indicating which device it wants to call. The switch responds with a SETUP ACKNOWLEDGE, indicating it received the SETUP message. The switch follows with a CALL PROCEEDING message, stating that it is beginning the next stage of call setup.

The call is forwarded to switch B through processes that are beyond the scope of the discussion here. The switch that is local to router B sends router B a SETUP message, indicating that another device wants to communicate. If the router finds this acceptable, it responds with a CALL PROCEEDING message. Once the process at the called router is ongoing, that router sends an ALERTING message to its local switch. This ALERTING message is passed across the WAN to the calling device. The ALERTING message is a progress indicator message, telling the calling device that communications will commence.

Once the called device is ready, it forwards a CONNECT message to its local switch. This message is forwarded to the calling device, telling it that the called device is ready to communicate. The calling device then forwards a CONNECT ACKNOWLEDGEMENT across the WAN to the called party.

Call Teardown

Once a call is made, it will probably be necessary to disconnect the call at some time. Figure 7.4 shows the call teardown process.

Figure 7.4. ISDN call teardown.

Once a device decides to disconnect, router B in the example, it informs the local switch. The local switch passes the DISCONNECT message to the remote router, indicating that the call is now over. The disconnecting router tells the local switch, via a RELEASE message, that it is now ready to terminate communication. The router on the far end generates a RELEASED message once it receives the DISCONNECT; this message is then passed back to the device that terminated the connection. Once router B receives the RELEASED message, it acknowledges with a RELEASE COMPLETE message, which is the last occurrence of communication between the devices.