In recent years, television viewers have been demanding greater choice in the video material they view. In addition, the demand for streaming video to a computer via the Internet has grown strongly. This has led to a need to supply increasing amounts of video material over local communication networks (whether the copper pairs used by telephone network operators or the coaxial cables used by cable television network operators).
In telephony networks this additional demand is being met by the introduction of Digital Subscriber Loop (DSL) technology. As its name suggests, this technology carries digital signals over the local copper loop between a user's home and a local telephone exchange. Data-rates of several megabits per second to the user's home are achievable. The digital signal is conveyed between modems placed at either end of the copper loop—the modem in the user's home usually being incorporated in a set-top box. The advantages of statistical multiplexing have led to the digital signals being organised into packets (whether they be Asynchronous Transfer Mode (ATM) packets or Internet Protocol (IP) packets).
Many cable networks are being upgraded to carry digital signals to user's homes. At least where the signals have been carried over the Internet, those digital signals are again arranged into packets.
Video material requires a data rate which varies between 1.5 Mbps (for a quality comparable to that offered by a video cassette recording) to 20 Mbps (High-Definition Television). In DSL networks, this means that a mechanism is needed to manage contention for the capacity offered over the copper loop leading to the user's home. Although cable networks can carry around sixty video streams simultaneously, their capacity is normally shared by several hundred users, meaning that contention for the capacity on the cable will also need to be managed as cable network users demand more choice in the material they view.
In a conventional telephone network, the problem of contention for scarce telecommunication resources is dealt with by simply preventing a user from receiving (or sending) any traffic unless the necessary capacity to carry that traffic can be reserved beforehand. There are numerous connection admission control schemes of this sort for packet networks too—examples include the Resource Reservation Protocol (RSVP). Although such schemes prevent congestion, connection admission control is notoriously complex for variable bit rate flows such as video flows. One proposal that alleviates this problem to some extent is disclosed in European patent application no. 0 932 282 A2. However, such schemes also suffer from the further disadvantage that data relating to each admitted flow (often referred to as ’state’ in the art) must be kept for the entire duration of the admitted flow.
An alternative to the use of connection admission control in packet networks is to use reactive flow control. These schemes allow users access to communications resources but attempt to cause senders to decrease their sending rate on the onset of congestion. The scheme used for reliable transmission across the Internet (Transmission Control Protocol) is the most common example. This is unsuitable for video flows however, since real-time video servers cannot reduce their sending rate.
In most flow control schemes, all users are adversely affected by the onset of congestion. Some flow control schemes are more sophisticated, classifying traffic into different classes, with some classes being more likely to suffer packet delay or discard than others. In situations where such classification is not available or where most traffic is within one class, alternative solutions must be provided. One such alternative solution which concentrates the adverse effects of ATM cell discard on one IP packet at a time is described in ’Early Selective Packet Discard for Alternating Resource Access of TCP over ATM-UBR’ by Kangsik Cheon and Shivendra S. Panwar, in the Proceedings of IEEE Conference on Local Computer Networks LCN 97, Minneapolis, Minn., Nov. 2-5, 1997.
However, none of the above proposals provides a method of managing contention in a packet network which avoids the complexity of connection admission control schemes without indiscriminately affecting users on the onset of congestion within the capacity allocated to a given class of traffic.