Chapter 3: Routing Calls over Analog Voice Ports

Cisco Press

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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 book 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

Router1#configure terminalRouter1(config)#dial-peer voice 1 potsRouter1(config-dialpeer)#destination-pattern 7777Router1(config-dialpeer)#port 1/0/0Router1(config-dialpeer)#end
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.

R1:

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R2:

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Practice Scenario 1 Suggested Solution

Although your choice of dial-peer tags might vary, the following offers a suggested solution to Practice Scenario 1:

R1:

dial-peer voice 2222 pots        destination-pattern 2222        port 1/0/0

R2:

dial-peer voice 3111 pots        destination-pattern 3111        port 1/0/0dial-peer voice 3112 pots        destination-pattern 3112        port 1/0/1dial-peer voice 3113 pots        destination-pattern 3113        port 1/1/0

Configuring VoIP Dial Peers

The administrator must know how to identify the far-end voice-enabled device that will terminate the call. In a small network environment, the device might be the IP address of the remote device. In a large environment, identifying the device might mean pointing to a Cisco Unified Communications Manager or gatekeeper for address resolution and CAC to complete the call.

Follow these steps to configure VoIP dial peers:

Step 1. Configure the path across the network for voice data.

Step 2. Specify the dial peer as a VoIP dial peer.

Step 3. Use the destination-pattern command to configure a range of numbers reachable by the remote router or gateway.

Step 4. Use the session target command to specify the IP address of the terminating router or gateway.

Step 5. (Optional) As a best practice, use the remote device loopback address as the IP address.

The dial peer specified as a VoIP dial peer alerts the router that it must process a call according to the various dial-peer parameters. The dial peer must then send the call setup information in IP packets for transport across the network. Specified parameters might include the codec used for compression (for example, VAD) or marking the packet for priority service.

The destination-pattern parameter configured for this dial peer is typically a range of numbers reachable via the remote router or gateway.

Because this dial peer points to a device across the network, the router needs a destination IP address to put in the IP packet. The session target parameter allows the administrator to specify either an IP address of the terminating router or gateway or another device. For example, a gatekeeper or Cisco Unified Communications Manager might return an IP address of that remote terminating device.

To determine which IP address a dial peer should point to, Cisco recommends that you use a loopback address. The loopback address is always up on a router as long as the router is powered on and the interface is not administratively shut down. The reason an interface IP address is not recommended is that if the interface goes down, the call will fail, even if an alternate path to the router exists.

Figure 3-28 shows a topology needing a VoIP dial peer configured on Router1. Example 3-10 lists the proper VoIP dial-peer configuration on Router 1, which is a Cisco voice-enabled router. The dial-peer voice 2 voip command notifies the router that dial peer 2 is a VoIP dial peer with a tag of 2. The destination-pattern 8888 command notifies the router that this dial peer defines an IP voice path across the network for telephone number 8888. The session target ipv4:10.18.0.1 command defines the IP address of the router connected to the remote telephony device.

Figure 3-28

VoIP Dial Peers

Example 3-10  Configuration for Dial Peer 2 on Router 1

Router1#configure terminalRouter1(config)#dial-peer voice 2 voipRouter1(config-dialpeer)#destination-pattern 8888Router1(config-dialpeer)#session target ipv4:10.18.0.1Router1(config-dialpeer)#end
Practice Scenario 2: VoIP Dial Peer Configuration

Create VoIP dial peers for each of the R1 and R2 sites based on the diagram presented in Figure 3-29.

Figure 3-29

Practice Scenario 2

R1:

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R2:

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Practice Scenario 2 Suggested Solution

Although your choice of dial-peer tags might vary, the following offers a suggested solution to Practice Scenario 2:

R1:

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R2:

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From this practice scenario, notice how configuration intensive it would be for an administrator to configure a dial peer for each phone number in a VoIP network. Next, consider how wildcards can be used with the destination-pattern command to allow a single dial peer to point to multiple phone numbers.

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