Call Rerouting Alternatives

If a call has been rejected by a CAC mechanism due to insufficient network resources, there needs to be some alternate route in place to establish the call. In the absence of an alternate route, the caller will hear a reorder tone. The reorder tone is called a fast-busy tone in North America, and is known as overflow tone or equipment busy in other parts of the world. This tone is often intercepted by Public Switched Telephone Network (PSTN) switches or PBXs with an announcement such as "All circuits are busy, please try your call again later."

Figure 8-2 illustrates an originating gateway, router R1, with CAC configured to reroute a call to the PSTN when insufficient network resources are available to route the call over the WAN link.

Figure 8-2 Legacy VoIP Network with CAC

IP Network Supports 2 Calls Max!

Figure 8-2 Legacy VoIP Network with CAC

IP Network Supports 2 Calls Max!

Call 3

CAC Reroutes the Third Call Through the PSTN

Call 3

CAC Reroutes the Third Call Through the PSTN

In a legacy VoIP environment, also known as a toll-bypass environment, the configuration of the originating gateway determines where the call is rerouted. The following scenarios can be configured:

• Alternate WAN path—The call can be rerouted to take advantage of an alternate WAN link if such a path exists. This is accomplished by configuring a second VoIP dial peer with a higher preference than the primary VoIP dial peer. When the primary VoIP dial peer rejects the call, the second VoIP dial peer is matched, causing the call to use the alternate WAN link.

• Alternate PSTN path—The call can be rerouted to take advantage of an alternate timedivision multiplexing (TDM) network path if such a path exists. This is accomplished by configuring a plain old telephone service (POTS) dial peer and a physical TDM interface connected to a PSTN circuit or a PBX interface. When the primary VoIP dial peer rejects the call, the POTS dial peer is matched, causing the call to use the alternate PSTN link.

• Return to originating switch—The call can be returned to the originating TDM switch to leverage any existing rerouting capabilities within the originating switch. How this is accomplished depends on the interface type providing the connectivity between the originating switch and originating gateway:

— Common channel signaling (CCS): CCS trunks, such as Primary Rate ISDN (PRI) and Basic Rate ISDN (BRI), separate the signaling and voice conversations into two distinct channels. The signaling channel is referred to as the

D channel, and the voice conversation is known as the bearer channel. This separation of channels gives the originating gateway the capability to alert the originating switch in the event that insufficient network resources are available to place the call. This allows the originating switch to tear down the connection and resume handling of the call with an alternate path.

— Channel-associated signaling (CAS): CAS trunks, such as E&M and T1 CAS, combine the signaling and voice conversations in a single channel. The originating gateway has no means of alerting the originating switch if insufficient network resources are available to place the call. For the originating gateway to return the initial call to the originating switch, a second channel must be used to reroute the voice conversation back to the originating switch. This process, known as hairpinning, causes the initial call channel and the second rerouted channel to remain active during the life of the voice conversation.

An IP telephony environment uses much of the same concepts as a legacy VoIP environment to handle CAC. However, an additional layer of control is added by the introduction of the CallManager cluster, which keeps the state of voice gateways and the availability of network resources in a central location. In an IP telephony environment, the configuration of the CallManager cluster in conjunction with the voice gateways determines whether, and where, a call is rerouted in the event of a reject due to insufficient network resources.

Figure 8-3 illustrates an IP telephony solution with CAC configured to reroute a call to the PSTN when there is insufficient network resources to route the call over the WAN link.

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