The Role of IP-to-IP Gateways

A voice gateway joins a VoIP network and the PSTN. A gatekeeper joins separate segments of the same VoIP network. An IP-to-IP gateway (IPIPGW), often called a Session Border Controller, joins independent VoIP or Video over IP networks. It acts as a border device, allowing users in different administrative domains to exchange voice and video using IP, rather than through the PSTN. The call media can either flow through the gateway, or directly between endpoints.

For example, an Internet Telephony Service Provider (ITSP) can use IPIPGWs to route IP voice traffic through another ITSP network. An IPIPGW can provide billing information to the ITSP. IPIPGWs can allow an ITSP to offer its customers end-to-end VoIP service between each other, or between remote offices of the same company. This would allow the exchange of IP calls between CallManager, H.323, and SIP networks.

Figure 1-3 shows an example of two companies that frequently conduct videoconferences between them. They use an IPIPGW to hide the details of each network, while still allowing communication. The H.323 video systems at each location communicate with the IPIPGW, rather than with each other directly. To each network, it looks as if the call signaling originates at the IPIPGW. The IPIPGW acts as a Session Border Controller (SBC) between the two islands, controlling video conferencing between them.

Figure 1-3. IP-to-IP Gateway Example

In Figure 1-3, a Voice Infrastructure and Applications (VIA) zone is used, along with a gatekeeper. This zone acts as a transit zone between the two networks. The gatekeeper is especially configured to route calls appropriately to the IPIPGW.

You can install the IP-to-IP gateway Cisco IOS feature set on many Cisco multiservice routers. The following are some features of an IP-to-IP gateway:

• Interconnecting segments of the same or different VoIP networks using different signaling types, such as H.323 and SIP
• Interconnecting segments of the same or different VoIP networks using different media types
• Billing abilities
• Coder/decoder (codec) control
• Call admission control
• Security