Consider a call center environment. Calls coming into a call center need to be distributed among available customer service agents. A hunt group takes calls coming into a single number and logically distributes those calls across hunt group members. This chapter also describes how to configure hunt groups and similarly how to reroute a call across the PSTN during times when an IP WAN connection is unavailable.
Finally, this chapter addresses the configuration of special purpose connections such as private line, automatic ringdown (PLAR) and connections that interconnect existing PBX systems.
Configuring Dial Peers
As a call is set up across the network, the existence of various parameters is checked and negotiated. A mismatch in parameters can cause call failure. Therefore, it is important to understand how routers interpret call legs and how call legs relate to inbound and outbound dial peers. Successful implementation of a VoIP network relies heavily on the proper application of dial peers, the digits they match, and the services they specify. A network designer needs in-depth knowledge of dial peer configuration options and their uses. This section discusses the proper use of digit manipulation and the configuration of dial peers.
Understanding Call Legs
Call legs are logical connections between any two telephony devices, such as gateways, routers, Cisco Unified CallManagers, or telephony endpoint devices. Additionally, call legs are routercentric. When an inbound call arrives, it is processed separately until the destination is determined. Then a second outbound call leg is established, and the inbound call leg is switched to the outbound voice port. The topology shown in Figure 4-1 illustrates the four call legs involved in an endtoend call between two voiceenabled routers.
An endtoend call consists of four call legs: two from the source router's perspective and two from the destination router's perspective. To complete an endtoend call from either side and send voice packets back and forth, you must configure all four dial peers. Dial peers are only used to set up calls. After the call is established, dial peers are no longer employed.
An inbound call leg occurs when an incoming call comes into the router or gateway. An outbound call leg occurs when a call is placed from the router or gateway, as depicted in Figure 4-2.
A call is segmented into call legs, and a dial peer is associated with each call leg. The process for call setup, as diagramed in Figure 4-2, is:
1. The POTS call arrives at R1, and an inbound POTS dial peer is matched.
2. After associating the incoming call to an inbound POTS dial peer, R1 creates an inbound POTS call leg and assigns it a call ID (call leg 1).
3. R1 uses the dialed string to match an outbound VoIP dial peer.
4. After associating the dialed string to an outbound voice network dial peer, R1 creates an outbound voice network call leg and assigns it a call ID (call leg 2).
5. The voice network call request arrives at R2, and an inbound VoIP dial peer is matched.
6. After R2 associates the incoming call to an inbound VoIP dial peer, R2 creates the inbound voice network call leg and assigns it a call ID (call leg 3). At this point, both R1 and R2 negotiate voice network capabilities and applications, if required. The originating router or gateway might request nondefault capabilities or applications. When this is the case, the terminating router or gateway must match an inbound VoIP dial peer that is configured for such capabilities or applications.
7. R2 uses the dialed string to match an outbound POTS dial peer.
8. After associating the incoming call setup with an outbound POTS dial peer, R2 creates an outbound POTS call leg, assigns it a call ID, and completes the call (call leg 4).
Understanding Dial Peers
When a call is placed, an edge device generates dialed digits as a way of signaling where the call should terminate. When these digits enter a router voice port, the router must decide whether the call can be routed and where the call can be sent. The router does this by searching a list of dial peers.
A dial peer is an addressable call endpoint. The address is called a destination pattern and is configured in every dial peer. Destination patterns use both explicit digits and wildcard variables to define one telephone number or range of numbers.
Dial peers define the parameters for the calls that they match. For example, if a call is originating and terminating at the same site and is not crossing through slow-speed WAN links, then the call can cross the local network uncompressed and without special priority. A call that originates locally and crosses the WAN link to a remote site may require compression with a specific coder-decoder (CODEC). In addition, this call may require that voice activity detection (VAD) be turned on and will need to receive preferential treatment by specifying a higher priority level.
- POTS dial peers Connect to a traditional telephony network, such as the public switched telephone network (PSTN) or a PBX, or to a telephony edge device such as a telephone or fax machine.
- VoIP dial peers Connect over an IP network.
In Figure 4-3, the telephony device connects to the Cisco voice-enabled router. The POTS dial peer configuration includes the telephone number of the telephony device and the voice port to which it is attached. The router determines where to forward incoming calls for that telephone number.
The Cisco voiceenabled router VoIP dial peer is connected to the packet network. The VoIP dial peer configuration includes the destination telephone number (or range of numbers) and the next-hop or destination voiceenabled router network address.
Follow these steps to enable a router to complete a VoIP call:
1. Configure a compatible dial peer on the source router that specifies the recipient destination address.
2. Configure a POTS dial peer on the recipient router that specifies which voice port the router uses to forward the voice call.
Configuring POTS Dial Peers
Before the configuration of Cisco IOS dial peers can begin, you must have a good understanding of where the edge devices reside, what type of connections need to be made between these devices, and what telephone numbering scheme is applied to the devices.
Follow these steps to configure POTS dial peers:
1. Configure a POTS dial peer at each router or gateway, where edge telephony devices connect to the network.
2. Use the destination-pattern command in dial peer configuration mode to configure the telephone number.
3. Use the port command in dial peer configuration mode to specify the physical voice port that the POTS telephone is connected to.
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