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 Communication Managers, or telephony endpoint devices. Additionally, call legs are router-centric. 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 3-23 illustrates the four call legs involved in an end-to-end call between two voice-enabled routers.
Figure 3-23 Dial Peers and Call Legs
An end-to-end call consists of four call legs: two from the source router’s perspective and two from the destination router’s perspective. To complete an end-to-end call from either side and send voice packets back and forth, you must configure all four dial peers. Dial peers are used only 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 3-24.
Figure 3-24 End-to-End Calls
A call is segmented into call legs, and a dial peer is associated with each call leg. The process for call setup, as diagrammed in Figure 3-24, is the following:
■ The POTS call arrives at R1, and an inbound POTS dial peer is matched.
■ 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).
■ R1 uses the dialed string to match an outbound VoIP dial peer.
■ 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).
■ The voice network call request arrives at R2, and an inbound VoIP dial peer is matched.
■ 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.
■ R2 uses the dialed string to match an outbound POTS dial peer.
■ 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 they match. For example, if a call is originating and terminating at the same site and is not crossing through slow-speed WAN links, 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 might require compression with a specific coder-decoder (codec). In addition, this call might require that voice activity detection (VAD) be turned on and will need to receive preferential treatment by specifying a higher priority level.
Cisco voice-enabled routers support five types of dial peers, including POTS, VoIP, Voice over Frame Relay (VoFR), Voice over ATM (VoATM), and Multimedia Mail over IP (MMoIP). However, this topic focuses on POTS and VoIP dial peers, which are the fundamental dial peers used in constructing a VoIP network:
■ POTS dial peers: Connect to a traditional telephony network, such as the PSTN or a PBX, or to a telephony edge device such as a telephone or fax machine. POTS dial peers perform these functions:
■ Provide an address (telephone number or range of numbers) for the edge network or device.
■ Point to the specific voice port that connects the edge network or device.
■ VoIP dial peers: Connect over an IP network. VoIP dial peers perform these functions:
■ Provide a destination address (telephone number or range of numbers) for the edge device located across the network.
■ Associate the destination address with the next-hop router or destination router, depending on the technology used.
In Figure 3-25, 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 voice-enabled 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 voice-enabled router network address.
Follow these steps to enable a router to complete a VoIP call:
■ Configure a compatible dial peer on the source router that specifies the recipient destination address.
■ Configure a POTS dial peer on the recipient router that specifies which voice port the router uses to forward the voice call.
Figure 3-25 Dial Peers
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:
Step 1. Configure a POTS dial peer at each router or gateway where edge telephony devices connect to the network.
Step 2. Use the destination-pattern command in dial-peer configuration mode to configure the telephone number.
Step 3. Use the port command in dial-peer configuration mode to specify the physical voice port that the POTS telephone is connected to.
The dial-peer type will be specified as POTS because the edge device is directly connected to a voice port, and the signaling must be sent from this port to reach the device. Two basic parameters need to be specified for the device: the telephone number and the voice port. When a PBX is connecting to the voice port, a range of telephone numbers can be specified.
Figure 3-26 shows a POTS dial peer. Example 3-9 illustrates proper POTS dial-peer configuration on the Cisco voice-enabled router shown in Figure 3-26. The dial-peer voice 1 pots command notifies the router that dial peer 1 is a POTS dial peer with a tag of 1. The tag is a number that is locally significant to the router. Although the tag does not need to match the phone number specified by the destination-pattern command, many administrators recommend configuring a tag that does match a dial-peer’s phone number to help make the configuration more intuitive. The destination-pattern 7777 command notifies the router that the attached telephony device terminates calls destined for telephone number 7777. The port 1/0/0 command notifies the router that the telephony device is plugged into module 1, VIC slot 0, and voice port 0.
Figure 3-26 POTS Dial Peer
Example 3-9 Configuration for Dial Peer 1 on Router 1
Practice Scenario 1: POTS Dial Peer Configuration
To practice the configuration of a POTS dial peer, consider a scenario. In this scenario, assume that a data center exists at the R1 site and executive offices at the R2 site. Using the diagram shown in Figure 3-27, create POTS dial peers for the four telephones shown.
Figure 3-27 Practice Scenario 1
Note that three configuration commands are required for R1, and nine configuration commands are required for R2. You can write the commands in the space provided here or use a separate sheet of paper. The suggested solution follows.
Practice Scenario 1 Suggested Solution
Although your choice of dial-peer tags might vary, the following offers a suggested solution to Practice Scenario 1:
