Press about Race: Multimedia over IP networks.

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Modern methods of doing business, as well as the territorial remoteness of offices and branches of companies have led to the development of a wide variety of means and methods of communication. The video conferencing service has gained wide popularity, which, unlike a traditional telephone, provides the opportunity for multilateral negotiations, presentations, training and other forms of communication between interlocutors located at almost any distance from each other.

Video conferences organized over IP networks are of particular interest. What are their features?

Pros and cons of the service: via The Internet…

Basic requirements for the communication channel for video conferencing:

• wide bandwidth,

• high quality (minimal number of errors),

• stable transmission of video and audio signals.

Traditionally, these requirements were met by dedicated communication channels with V.35/E1 interfaces or telephone ISDN networks. However, even in modern IP networks, there are sufficiently developed means of ensuring the quality of services (QoS) that allow providing video conferencing.

Unfortunately, it is not yet possible to talk about the successful organization of the "videoconferencing" service over the Internet, primarily because Internet service providers do not provide data transmission services with guaranteed QoS. In some cases (when all participants of the conference are connected to the network of the same operator) this problem can be solved. But usually the signal passes through several networks with different quality characteristics and , alas, it is not necessary to count on any acceptable video quality .

Video conferencing requires a reliable high-speed data transmission network with service level management.

First of all, the video conferencing service can be provided in corporate networks built either on the basis of dedicated channels or using technologies that ensure high quality of service - ATM, Frame Relay, MPLS. The use of IP networks as a transport medium is fraught with technical difficulties here, but this method has undeniable advantages - the service can be organized at almost any workplace, the efficiency of using channels increases. At first glance, such a statement seems paradoxical. After all, transmitting a signal over an IP network requires quite high overhead. With a video codec speed of, say, 512 kbit/s, a channel of 768 kbit/s is required. But the video conference traffic is uneven: when the image in the frame is stationary, it is minimal, and at some moments a bandwidth corresponding to the maximum codec speed is required. During a communication session, far from the entire channel resource is occupied, so it can be used simultaneously for data and voice transmission. To provide the same service on dedicated channels or on the ISDN network, you need to have the bandwidth corresponding to the maximum codec speed, and the transmission of another type of information over the same channel will be impossible.

...and via satellites…

When building large communication networks, it is impossible to do without satellite communication channels, which is especially important for Russia with its vast territory, on a significant part of which "communication off-road" reigns. But when organizing video conferences via satellite channels , the operator faces additional difficulties.

First of all, this is a large delay in signal propagation. By itself, if it remains constant, it does not affect the quality of the signal, but the information does not reach the recipient immediately, but after a fraction of a second, which causes some discomfort when communicating. If there is a loss of information in the satellite channel, then its "correction" takes too much time. This leads to jumps and blurring of the image during a video call session. The use of transport IP networks exacerbates this problem. If only short fragments of the video stream can be lost in a dedicated channel , which will cause minor image defects, then in the case when traffic is transmitted in large packets over an IP network, any communication failure will lead to the loss of the entire packet and, as a result, to significant distortion of information.

When organizing a video conference over a satellite channel , it is necessary to take a particularly critical approach to the quality of signal transmission (probability of errors), monitor possible sources of interference, and be sure to test the equipment used for compatibility with the satellite channel.

In the company's practice, there have been cases when the inconsistency of the synchronization signals of the satellite modem and the equipment of the "last mile" led to a large number of transmission errors and, as a result, to unsatisfactory video quality.

Another problem with the use of satellite networks is the high cost of channels, which in the case of the provision of the "videoconferencing" service becomes critical at all (since a high transmission speed is required, then the rent is considerable).

Traditionally , VSAT stations (antennas with a small aperture) and PAMA channel allocation technology are used to organize satellite networks . This solution has high availability and reliability of communication, allows you to use arbitrary data transfer protocols and work in dedicated line mode. But such an organization of the service requires a permanent lease of an expensive channel with high bandwidth. Therefore, you can turn to DAMA technology ( channel allocation on request), which is in good agreement with the ideology of transmitting information over IP networks. There are different options for using DAMA.

In the simplest case, the channel is established at the request of users for the duration of the video session and is released upon its completion. Only the time of the channel occupation is paid (as when using simple telephone networks). The video conferencing service is provided in a dedicated band (similarly ISDN) or over an IP network. In the latter case , other types of traffic can be transmitted simultaneously (provided that the channel's high bandwidth, at least adequate to the speed of the video codec, is maintained throughout the session).

The satellite network operator can also organize its own IP network, for example, using DialAw@y IP or Skystar 360 technologies (developed by Gilat). All information transfer between points of this network will take place only on top of the IP network, which will allow more efficient distribution of satellite resources between subscribers and will make communication less costly. However, it should be borne in mind that in such networks, all traffic in passes, as a rule, through the operator's central ground node, which doubles the delay by half and exacerbates the problems described above.

The choice of an IP network as a transport medium for the provision of the videoconferencing service seems quite logical. At the same time, higher-speed channels are required than on dedicated lines or on ISDN networks. However, the same channels can be used much more efficiently due to the simultaneous transmission of other types of traffic. A necessary condition for the operation of such a network is the possibility of centralized service quality management, which is possible only on the network of one operator.

If the subscribers are separated by a large distance and borders, which makes it impossible for them to communicate with each other through one operator, then a reasonable alternative is to use ISDN networks. Well, if they are not available, then you can organize a common dedicated network for video conferencing and for providing any other communication services. Satellite communication channels can become an important component of it .

Shestakov Mikhail Leonidovich
Head of the System Expertise Department Race Communications
Basov Alexander Yurievich
Race Communications Product Manager

Inform Courier Svyaz No. 4/2003, pp. 68-69