VoIP provides transport of voice over the IP protocol family. IP makes voice globally available regardless of the data link protocol in use (Ethernet, ATM, Frame Relay). With VoIP, enterprises do not have to build separate voice and data networks. Integrating voice and data into a single converged network reduces the costs of owning and managing separate networks.

Figure 15-8 shows a company that has separate voice and data networks. Phones connect to local PBXs, and the PBXs are connected using TDM trunks. Off-net calls are routed to the PSTN. The data network uses LAN switches connected to WAN routers. The WAN for data uses Frame Relay.

Separate operations and management systems are required for these networks. Each system has its corresponding monthly WAN charges and personnel, resulting in additional costs.

Figure 15-7 VoATM Trunks Between PBXs

Figure 15-7 VoATM Trunks Between PBXs

Figure 15-8 Separate Voice and Data Networks

With IP telephony, you can reduce the number of systems, circuits, and support personnel. Figure 15-9 shows a multiservice IP telephony network that employs Ethernet-based phones with server-based call processing with gateway routers. Survivable Remote Site Telephony (SRST) is used for failover or backup to the PSTN if WAN failure occurs. On-net calls travel through the Frame Relay network, and off-net calls are forwarded to the PSTN. The PSTN link is also used if voice overflow or congestion occurs on the WAN network. Calls are then routed to the PSTN.

Figure 15-9 Converged VoIP Network

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