Applications for Connecting to an IP WAN

Connecting voice gateways over an IP WAN allows you to send voice and video to other locations as IP packets. For businesses that have geographically distributed offices, using IP telephony to call between offices can be more cost effective than making long-distance calls. IP telephony is increasingly becoming a need for businesses that spread their offices globally. It lets you leverage your investment in WAN bandwidth between offices. The WAN connection can be a direct circuit between sites, such as a T1; a virtual circuit, including Frame Relay; ATM permanent virtual circuit (PVC); or a shared connection, as with a Synchronous Optical Network (SONET) ring. Communication between the voice gateways could rely on your service provider, such as with Multiprotocol Label Switching (MPLS), or on the Internet, as when using a virtual private network (VPN) between sites. Satellite links are also an option, provided their speed and reliability are acceptable.

The following are some situations in which IP WAN connections might be appropriate:

• Your company is using VoIP at multiple sites. Sending voice over the WAN connections between them is a way to make intracompany phone calls without using the PSTN. Voice performance is better with a full-mesh topology. In a hub-and-spoke design, spoke-to-spoke phone calls have to go through the hub, adding extra delay to the call. Careful attention to quality of service (QoS) is essential when sending Voice over Frame Relay (VoFR) or Voice over ATM (VoATM). PPP has no built-in QoS mechanisms; therefore, it needs multilevel precedence and preemption (MLPP) to be deployed to ensure that latency and delay requirements for voice are met.
• You want to do "tail end hop-off," which turns a long-distance call into a local one. (In tail end hop-off, you route voice traffic that is bound for a phone in a particular area to your voice gateway in that area. The voice gateway then routes the traffic out to the PSTN as a local call.)
• A company that wants to preserve its PBX investment yet avoid the cost of POTS lines and trunks can take advantage of existing WAN links for voice between sites. In this case, the voice gateway translates between the internal analog voice and the external VoIP. You need to create dial peers that point to the PBX and to the other voice gateways.
• Companies might want to centralize their PSTN connections and require remote sites to route all voice traffic bound to the PSTN through a central site. The central site needs enough PRI connections to handle the calls, and each remote site needs only minimal POTS lines in case the WAN connection is lost or for emergency calls. This centralization is normally done when all the sites are located in the same area.
• Centralizing Cisco CallManagers reduces the financial investment in IP telephony. In a centralized design, Cisco CallManagers communicate with the other voice devices over an IP WAN. Signaling between phones and CallManagers, and between gateways and CallManagers, is sent over the WAN. Voice media traffic flows directly between the IP phones and the gateway at the remote site and might not traverse the WAN.