Call Legs in IOS Gateways

One of the most fundamental concepts in routing a call through an IOS voice gateway is that of a call leg. The notion of a call leg is inexorably tied to that of a dial-peer.

In the case of a VoIP call, two types of dial-peers are used: POTS dial-peers and VoIP dial-peers. A POTS dial-peer configures the characteristics of its corresponding telephony interface and ties a dial string to a specific voice port on the local gateway. Correspondingly, a VoIP dial-peer sets the attributes of the IP connection and ties a dial string to a remote IP device.

In the case of IOS gateways, a VoIP call is logically broken into discrete segments known as call legs. A particular dial-peer is associated with each call leg. A call leg is a logical connection that has local significance only to the voice gateway where the dial-peer is matched.

All VoIP calls must match both an inbound dial-peer and an outbound dial-peer. The inbound dial-peer match corresponds to an inbound call leg. Similarly, an outbound dial-peer match corresponds to an outbound call leg. Both the call direction and the type of dial-peer that is matched completely define the call leg. Figure 12-4 illustrates this gateway-centric concept of a call leg and how it is used in the framework of a VoIP call.

Figure 12-4 Call Legs on an IOS Voice Gateway

Call Direction

Originating

Voice Gateway

Call Leg 1

Terminating Voice

Terminating Voice

Call Leg 2

Call Leg 3

Call Leg 4

Call Leg 2

Call Leg 3

Call Leg 4

Call Leg 1 = Inbound POTS Call Leg Call Leg 2 = Outbound IP Call Leg Call Leg 3 = Inbound IP Call Leg Call Leg 4 = Outbound POTS Call Leg

Figure 12-4 shows that a call leg can be either an inbound call leg or an outbound call leg depending on the direction of the call. For example, call leg 1 is an inbound POTS call leg, whereas call leg 2 is an outbound IP call leg on the originating voice gateway. If the call direction is reversed, so are the inbound and outbound call leg directions.

As a result of the dial-peers that are matched, outbound call legs are used for call routing and setting outbound call attributes, whereas inbound call legs set the call attributes in the reverse direction for that same gateway. When troubleshooting IOS gateways, it is important to always note the call legs and their equivalent dial-peers.

TIP A common cause of fax/modem problems in IOS gateways is the omission of an inbound

IP dial-peer that is properly configured for relay or passthrough. As a best practice, the terminating voice gateway should always have an inbound IP dial-peer that is configured similarly to the outbound IP dial-peer on the originating voice gateway. You should then confirm that after this inbound dial-peer is properly configured on the terminating voice gateway that it is matched by the incoming fax/modem call rather than another dial-peer or the system default, dial-peer 0. For additional information about voice dial-peers and understanding their inbound and outbound matching characteristics, refer to the document titled "Understanding Inbound and Outbound Dial Peers Matching on IOS Platforms" (Document ID: 14074) at Cisco.com.

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