An Introduction to Video Conferencing
How to Make Visual Communication Work Within and Between Companies
May 28, 2009
Video conferencing has improved as more efficient coding techniques are introduced and high-definition life size systems become available. However, getting it to work, particularly between companies, is quite hard.
Telephony has a standardised addressing system and universal interoperability - any phone can call any other on the telephone network. Video conferencing has not yet reached this nirvana. Two types of transmission network are available, and equipment manufacturers often make sure that best results are only possible when working between terminals made by the same vendor.
Choosing How to Connect a Video Conference Terminals - IP or ISDN?
Historically, video conferencing systems used ISDN connections based on the H.320 standards. These a direct digital connections to the public switched telephone network. They have the following advantages
- available world-wide
- dedicated fixed bandwidth path - no interference from other calls or data
- unique numbering scheme already allocated by telephone network
- works well between different companies
- no exposure of company data network to the inbound caller
however, ISDN has significant disadvantages, including
- high cost of getting a circuit
- high call costs, often time and distance-dependent
- limited bandwidth of 64kbps per channel. Video often needs 3 or 6 ISDN channels, increasing cost and complexity
For video conferencing within a company, using the corporate IP data network that connects the computers together makes a good alternative, and this often connects the company offices globally. Video conferencing over IP networks using H.323 or SIP has some great advantages
- company-wide availability
- low cost
- no time and distance costs
- high bandwidth potential, capable of good pictures and sound
Disadvantages include
- few company networks permit connections to other companies without the co-operation of both companies IT and security departments at a very senior level
- IP addresses change, so a gatekeeper is needed to locate terminals even in point to point calls
- network capacity depends on other data traffic and signal impairments can happen
- Video conferencing data rates can congest WAN links on branch office sites
For communication between different parts of one company IP networks are usually preferred. A gatekeeper is often used to allocate terminal addresses so that they are fixed. For communication between enterprises ISDN is a way of solving connectivity problems for occasional calls.
If a lot of conferencing calls are expected then there are solutions to connect IP calls, either via a third-party service provider or using devices like session border controllers to route calls across corporate networks via WAN links. It is possible to route calls between IP and ISDN networks using devices called gateways, but implementing this is generally a job for IT professionals. Many conferencing terminals offer both IP and ISDN connections and this is often the easiest solution for users wanting to access both types of network.
Video Conferencing Interoperability
Once the problems of getting a transmission path to the far end have been sorted out there remains the issue of compatibility between conferencing terminals. ISDN terminals using H.320 and IP terminals using H.323 will usually get a connection with sound and vision, though not always at the highest quality possible if the terminals are from different vendors.
Multi-way Video Conferences
A multipoint control unit (MCU) in the network can allow more than two parties to participate in a video conference. The video picture can either switch between whoever is currently speaking, or all participants can be seen at the same time by subdividing the screen area in a "Hollywood Squares" arrangement.
Video Conferencing Terminals
Terminals come in three classes. Desktop systems work on a PC with a webcam and use the IP network of the PC for transmission. A PC running the popular Skype application is an example of this sort of terminal. Skype is proprietary and therefore does not interwork with H323 terminals.
Next are small standalone units, which usually work with a TV or other display. These are designed for small meeting rooms. At the upper end of the market are large room systems which are usually custom-implemented. These can offer HD resolutions. Finally, at the top end of the market are high-definition systems like Cisco Telepresence, LifeSize, HP Halo. These generally only work well between similar terminals. Vendors often indicate they have interworking solutions for H323 terminals to connect to high-end systems. However, because these normally need to add an extra code/decode process delay and signal quality can suffer so such solutions should be tested if interworking is essential to the intended usage.
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