Introduction
Northwestern University has acquired and installed an enterprise videoconferencing system manufactured by Radvision. The Radvision system is called a ViaIP (See Figure 1 below). The system consists of an Enhanced Communication Server (ECS-200) Gatekeeper, a Multipoint Control Unit (MCU-100), a Gateway, and a Data Collaboration Server (DCS). For a detailed discussion of the interoperability of client terminals, gatekeepers, MCUs, and gateways see Understanding Videoconferencing--H.323 Basics.

Gatekeeper
The Radvision ECS gatekeeper allows H.323 endpoints (client terminals, the MCU, and the gateway) to register with it, and the gatekeeper sets up calls between endpoints when requested. The IP address of the gatekeeper that must be configured into endpoints in order for them to register with the gatekeeper is 129.105.253.250. The ECS-200 is capable of concurrently registering 1,000 endpoints and is capable of handling 200 concurrent calls. These calls can be any combination of point-to-point calls or calls from a client terminal to a MCU session.

Multipoint Control Unit (MCU)
The Radvision MCU will support 100 concurrent users, each running at 128 Kbps. At Northwestern University, we typically use videoconferences that encode the audio/video streams at 384 Kbps. At this higher rate, the MCU supports approximately 70 concurrent users. This means that we could have one large videoconference with 70 sites connected to that videoconference, or 10 videoconferences with 7 users connected to each videoconference, or any combination of videoconferences and sites each running at 384 Kbps as long as the aggregate number of sites does not exceed 70.

Gateway
The Radvision gateway allows H.320-based videoconferencing client terminals that use ISDN telecommunication lines rather than the Internet for connectivity to join a H.323, Internet-based videoconference. The gateway also allows users to make telephone calls into the gateway and participate as an audio only user in the videoconference. The ViaIP gateway is connected to a single ISDN PRI line (telephone number 847 467 0001); a second PRI can be added. The capacity of the ISDN PRI line is such that it can simultaneously handle three H.320 terminals running at 384 Kbps plus five telephone users. While the gateway does have the capability to allow H.323 users to make ISDN calls to H.320 sites, we currently have this capability disabled.

Data Communication Server (DCS)
The Radvision ViaIP has the capability to support T.120 calls (See "Understanding Videoconferencing"- "Data Sharing" for a detailed description of T.120 and data sharing); this capability is called Data Communication Server (DCS). Some MCU sessions (such as service 70) are configured to support DCS. Client terminals that connect to these MCU sessions and are capable of supporting T.120 (like the Polycom units) can share applications, a white board, and text messaging through the videoconference. This capability does not use the video streams, but instead uses a separate IP path through the network. If the client terminal is a Polycom ViaVideo unit, the data sharing will occur using Netmeeting on the same PC that runs the ViaVideo client. If the client terminal is a Polycom Viewstation, the Viewstation will be configured with the IP address of a PC that must run Netmeeting.