Support for Gateways

The gateway is another fundamental device in an H.323 network. Without the gateway, there could be no cross-technology communication. Some may mistakenly interchange the terms “gateway” and “gatekeeper,” but they are very different devices.

PRIMARY QUESTIONS

• What is a gateway?
• Why do I need a gateway?
• What connections does ITN need for a gateway?

ANSWERS

What is a gateway? A gateway allows different protocols to talk to each other through the translation of IP protocol to ISDN protocol and vice versa. This device allows an ISDN unit to call an H.323 unit and vice-versa or an H.323 unit to communicate with an ATM video unit. To perform this function, a gateway acts as a transcoder from one type of technology to another. Transcoding may involve changing the physical transport medium of the data being delivered as well as the protocols.

Why do I need a gateway? A gateway is a translator, allowing the end-user to communicate with other end-users that use different voice, video, and data protocols.

What connections does ITN need for a gateway? A gateway would require an IP connection to the network for H.323 communications, typically a 10Mbps connection at full duplex. For an ISDN gateway, connection to the PSTN would consist of a PRI line with 24 64Kbps channels. Smaller ISDN gateways are available and would use multiple ISDN lines instead of a PRI. An ATM gateway would require an ATM interface to the ATM network.

Recommendation: The IPVTF recommends the purchase of a Radvision L2W-323 PRI gateway.

Importance of Gateways

Gateways are important because they let H.323 users continue to use legacy devices until sufficient resources become available to replace outdated hardware. In addition, many endpoints operated by commercial enterprises are still ISDN-based and will most likely continue to remain in service until the nationwide commercial IP network attains enough robustness to support two-way video.

The Radvision gateway, together with the existing V-GATE gateways, will give ITN the flexibility it needs to accommodate existing ISDN and ATM video codecs. All incoming gateway calls would come into a 317 area code. From there, a routing hierarchy using DID, TCS4, Interactive Voice Response, and Default Routing will be employed to route calls to their appropriate H.323 endpoint according to the dial plan set forth later in this document.