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Multimedia requires high capacity storage systems. But over and above the continuous improvement in storage capacity of magnetic and optical devices multimedia raises issues relating to the format in which audio, video and data should be stored. A VHS tape stores video and audio in a structured analogue form. Digital storage techniques have hitherto used random access techniques, which do not suit the time structured format of audio and video. Since bandwidth is invariably limited in most networks users need to consider the option of local storage of multimedia data for subsequent playback. For instance the Internet is capable at present of delivering only low quality live video and audio. Local storage could enable higher quality playback for the end user. Storage systems are evolving to meet this requirement. For instance the ATML DiskBric is optimised for multimedia data streams and ATM interfaces with a 4 to 8 Gbytes capacity.
Storage devices such as CD ROM's need to be able to provide data at high speeds and in large chunks with low access times. Current CD ROM's can transfer data at around 300 kbyte/sec or higher, hold 600 Mbyte of data and have an access time of around 300 milli-secs. For some applications this is only just adequate. Even hard disc technology is strained by multimedia demands. The IEE working group P1394 is studying the use of 125 microsecond frame based storage and retrieval on disc drives.
Multimedia applications, particularly those using video and images demand large bandwidths. However bandwidth for the foreseeable future will be limited. The limitations arise from the cost of installing optical fibre transmission, terminal equipment complexity and speed, tariffing regimes, switching speeds, and increasing numbers of users sharing equipment and networks. Photonic switching in trunk networks and the use of ATM on optical fibres will provide higher bandwidths
[Midwinte94], but the growth of networks such as the Internet demonstrate that demand will always exceed provision.
Consequently users and applications need to formulate their demands so that bandwidth is used appropriately and efficiently. The current support for higher bandwidth offered by network technologies will be discussed in section 4.0
3.5. Quality of Service
The availability of multimedia resources places new demands on the service that a network must provide. The most important of these are the bit error rate, the packet or cell loss, delay and delay variation. Network resources need to be committed to mulitmedia data streams to
accommodate the peak bit rate, mean bit rate and burstiness of the data stream. Until the advent of ATM networks users have had to live with the characteristics of the network to which they are connected. ATM provides the means to specify requirements in advance through an application. Users of ATM networks will be allocated different bandwidths and quality of service according to the application in use. For instance an audio application would request a circuit with low delay to ensure adequate voice quality. Other networks are also now capable of adapting to user requirements. An ISDN network [Ovum94] can provide additional ISDN
channels on demand for higher data rates.
3.6. Platform Support
Multimedia also makes new demands on the workstations used to reproduce audio and video.Processor speeds operating systems, displays, storage medium and network interfaces and application must all be capable of handling multimedia. The Multimedia PC specification defines a defines a 33 MHz 386SX processor with 4 Mbyte of RAM, a VGA graphics card, a sound card, a large hard disc and standard peripherals as being the minimum level of machine needed. Apple Macintosh and UNIX workstations already come with many of these features. Increasingly multimedia features are being incorporated into computer motherboards to reduce the need for plug-in cards. Considerable investment is needed particularly in education to provide large numbers of multimedia ready machines.
There is no ideal platform for multimedia. All vendors are hoping that their products will benefit from the demand for multimedia. A limiting factor in the use of multimedia over networks will be not only the suitability of the networks but the availability of multimedia machines connected to the networks.
Virtual Environments Visualisation