34.1 IP NETWORKS AND MULTIMEDIA COMMUNICATION

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Packet networks are now becoming widespread. Every corporate office now has a LAN and a WAN based on IP (such as IP over ATM or IP over Frame Relay). If this network infrastructure can be used not just for data services, but for voice, fax, and video services as well, there will be tremendous cost reduction.

If the multimedia PCs in an organization can be used for voice communication, we can do away with expensive PBX systems. With desktop video cameras, video conferencing and intranet-based learning are possible.

Voice/fax/video communication over IP networks results in tremendous cost savings for organizations because billing is based on flat rates and not based on the distance.

The Internet can be used for telephone conversations, fax transmission, and video conferencing. This will create tremendous cost savings because we pay flat rates for Internet connectivity, unlike PSTN where the billing is done based on the distance.

However, packet-based networks provide technical challenges for real-time transmission of multimedia content.

34.1.1 Limitations of IP Networks

The TCP/IP protocol suite was developed mainly for data applications in the initial days of the Internet. To use the Internet for audio, video, and fax applications, real-time transmission of data is required. The TCP/IP architecture has inherent limitations for supporting real-time transmission. In a TCP/IP network, there is no guarantee that all packets will be received at the destination with constant delay and in sequence. This lack of guarantee of the quality of service is not acceptable for applications such as voice, fax, and video where real-time transmission is a must. Loss of packets can be minimized through the acknowledgements in the TCP layer, but the delay will be very high if a few packets are lost and the destination has to wait until these packets are received. The variable delay also causes problems in two-way conversations. Hence, to support real-time applications using multimedia, a new set of protocols is required.

Networks running TCP/IP protocol stack are not well suited for real-time multimedia communication because the desired quality of service is not guaranteed in these networks.

Another problem with voice and video communication over IP networks is in terms of the data rates. The access speed for most Internet users is generally limited to 28.8 or 33.6 or at most 56kbps. This data rate is low as compared to the PCM coding rate of 64kbps. To overcome this problem, low bit rate coding of voice and video signals is done. Obviously when we reduce the coding rate, we compromise on the quality.

To achieve an acceptable quality of service for multimedia communication over IP networks, a new protocol architecture is defined with which these problems are overcome.

Note

For voice and video transmission over IP networks, low bit rate coding techniques are used to overcome the limitations of low access speeds.

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