An overview of Video Multi-Conference Units (MCU)


This article gives a small introduction to a Multi-Conference Unit in Video Conferencing systems, the necessity to have a separate hardware based multi-conferencing unit,  the features and functionalities supported by them, managed multi-conferencing systems, gateways and gatekeepers.

What is a Multi-Conference Unit?

To understand this, you ought to know what is a Point to Point Video Conferencing system. Basically, when you want to see and speak to someone who is far away, you could use a video conferencing system to have a live face to face meeting with them, of course sitting from your office. A VC system consists of a Codec (Processor, Interfaces for ISDN/IP etc.), Display Units (TV, LCD Monitor etc.), Camera (Generally attached with the codec, some are detachable), Microphones and Speakers. This systems ought to be there with the other person with whom you wish to communicate. It is not necessary that both are from the same manufacturer, as most of them follow standards and are very much interoperable.

These Video Conferencing codec’s allow you to do simple multi-party calls. You are in a conference with your regional office one, and you also want your regional office two to participate in the same conference (from a different place), you could dial both of them and have a simple multi-conference. Most of the codec’s support 4-Party Conference (including yours) or 5-Party Conference, with the basic unit itself. Now, what if you want to have a 12-Party conference? That’s what a multi-conference unit is for.

Features of a Multi-Conference Unit:

The MCU is a hardware unit and some of the bigger ones are Chassis based systems where the media, recording, gateway and interface units can be added via blades and may have redundant hot swappable fan units and power supplies. Any standard video conference system can connect to the MCU for multi-conference by dialling in, or scheduling a call from the MCU. Some MCU’s also support entry through a Video Auto-Attendant or Interactive Voice Response when an endpoint unit calls. Depending on the model, they support 12-Party, 24-Party end points, etc. The bigger ones can support a few hundred end points and they can be cascaded to support even more. Some vendors also support (may require additional hardware, blades) recording, archiving, streaming and playback of video sessions.

The endpoints can call an MCU on any of the following – H.320(ISDN), H.323(IP), SIP(Video Phones),HD(High Definition video). MCU supports Transcoding – The various video conference systems connecting to it can have different data rates, different audio or video algorithms, different standards, different frame rates, etc. The MCU can still negotiate the best possible connectivity with each of them. MCU can also support telepresence calls.

When so many sites are present in a single video conference session, it is vital that there are display options. MCU’s support flexible continuous presence where display layout is divided so that the speaker or main site is on the centre and the other sites are in small PIP displays in the bottom or/and sides. So, bigger displays are better to clearly view a large number of sites simultaneously or multiple displays can be arranged. Alternatively, only the site that is speaking can be displayed on the screen and other sites can be displayed when they start speaking. This can be done by a technique called voice activated camera switching. The best thing, if you want complete control is to take the chair control where you decide who is on the main screen – you might just want the HQ on the main screen always with the branches mostly listening.

Multi-Conference Service Providers:

The Multi-Conferencing facility can be procured on-demand (temporarily) from large service providers who invest in big MCU’s and extend hourly/no. of calls/no. of sites based billing systems. So, when a multi-conference call is required, you can book it with them and use their system to call you or your endpoints can call them to set up a Multi-Conference. Some providers also have their video conference booths where you can go and call other sites, if you don’t have a video conference end point or the bandwidth required for VC Sessions.

Gateways and Gatekeepers:

Gateways: It is a network device that allows H.323 (IP) based video conferencing units to connect to  H.320 (ISDN) based video conferencing units. The MCU’s might use this to provide multi protocol support. They have LAN ports as well as ISDN/PRI ports.

Gatekeepers: They provide addressing service for H.323(IP) based video conferencing units and optionally can also impose bandwidth restrictions. The addressing service is required for a video conference unit to dial another VC unit by using the video conference IP address (which is generally static). Otherwise, the IP address might be changed at the gateway (NAT/DHCP) and the receiving end video conference unit cannot identify who is calling.

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