Dial peers are the primary construct used to implement a dial plan on Cisco voice gateways. Dial peers are used to determine which calls are routed and the path a call should take. Dial peers are also used to set configurable options, such as the codec or the Dual-tone Multifrequency (DTMF) relay method to be used for the call.
You can configure two types of voice dial peers in a Cisco gateway:
Inbound Versus Outbound Dial Peers
Each call that a Cisco voice gateway processes requires two dial peers, as shown in Figure 9-2. These dial peers are defined as the inbound and the outbound dial peer from the perspective of the gateway.
Figure 9-2. Inbound Versus Outbound Dial Peers
From the gateway perspective, you can view a call as two distinct call legs associated with the inbound and outbound dial peers.
When Cisco introduced voice gateways, each call matched a POTS dial peer and a VoIP dial peer. One call leg was always associated with a voice port, and one call leg was always associated with the data network. This enabled the toll bypass implementation shown in Figure 9-3. Over time, time-division multiplexing (TDM) switching capabilities were added to the gateways, allowing both incoming and outgoing call legs to be POTS. And with the addition of the IP-to-IP gateway (IPIPGW) feature, both incoming and outgoing call legs can be VoIP.
Figure 9-3. Toll Bypass Topology
For more information on the IPIPGW feature, see Chapter 18, "Cisco Multiservice IP-to-IP Gateway."
The toll bypass topology uses exactly two gateways in the call path. The gateway that is closest to the device placing the call is designated the originating gateway (OG), and the gateway closest to the device being called is designated the terminating gateway (TG). It is now much more common for an enterprise to have a single voice gateway connecting a Cisco CallManager to the PSTN or a PBX. It is also possible to have multiple gateways (as opposed to routers) in the call path using IPIPGW. In these new possible topologies, the terms OG and TG can be confusing because one, two, or many gateways can be in the call path. Many documents on Cisco.com still refer to OG and TG when describing call legs and dial peer matching. In the CallManager case, a gateway is considered a TG when CallManager originates the call, and it is considered an OG when CallManager is the destination. Although the terms OG and TG are now somewhat arbitrary, understanding the origin of these terms is important to deciphering Cisco documentation. When analyzing call flows, however, it is typically easier to consider each gateway independently and look at the incoming and outgoing legs that are specific to that gateway instead of applying arbitrary terms.
Part I: Voice Gateways and Gatekeepers
Gateways and Gatekeepers
Part II: Gateways
Media Gateway Control Protocol
Session Initiation Protocol
Connecting to the PSTN
Connecting to PBXs
Connecting to an IP WAN
Influencing Path Selection
Configuring Class of Restrictions
SRST and MGCP Gateway Fallback
Using Tcl Scripts and VoiceXML
Part III: Gatekeepers
Part IV: IP-to-IP Gateways
Cisco Multiservice IP-to-IP Gateway
Appendix A. Answers to Chapter-Ending Review Questions