This report is also available as an Acrobat file.
5. TRANSMISSION MEDIA
5.1. Copper Conductors
Copper conductors twisted together are the basis of the telephone network. As used in the analogue telephone network, with modems turning digital data into analogue tones, the data rate is limited to around 28 kbps. The introduction of the Integrated Services Digital Network (ISDN) led to the use of improved modulation and coding schemes. In standard ISDN the user can access up to 128 kbps of data at distances up to 100 metres from an ISDN socket [BT94].
Local Area networks also use copper cable twisted pairs. Ethernet running at 10 Mbps can also operate at up to 100 metres or more, depending on the grade of the cable. These physical networks will be available and economic to operate for many years. For this reason considerable effort has gone into upgrading the network protocols that can be used on them for high bit rate real time multimedia. See section 7.0 below.
A recent development is Asymmetric Digital Subscriber Lines (ADSL) [Bellcore92] technology which is aimed at using two wire copper loops at data rates of 1.544 Mbps in the network to user direction and about 600 kbps from the user to network. This is achieved by using better modulation and coding techniques. The driving force behind this technology is the delivery of on demand compressed VHS quality video to the home by Telecommunication operators wishing to compete with cable TV operators. If this technology becomes commercially available (it is undergoing trials) then the opportunity exists to deliver several ISDN B channels or even low speed ATM to the subscribers premises.
In the short term this technology is unlikely to be useful to the academic community.
5.2. Coaxial Cable
Coaxial cable is still an effective medium to deliver multimedia. Cable TV networks use coaxial cable. BT uses coaxial cable to deliver some of its Kilostream services, and Local Area Ethernet networks can operate over coaxial cable to the 10BASE5 and 10BASE2 specification. In general coaxial cable enables longer distance transmission at higher data rates than twisted pair cable, but is more expensive. One video conferencing provider has taken advantage of the potential additional bit rate available over coaxial cable and designed a proprietary video conferencing system for use over a LAN (C-Phone). Because of the higher cost of coaxial cable new LAN installations which are to be based on copper technology most often use twisted pairs.
5.3. Optical Fibre
Optical fibre transmission has been a strong enabling factor in the creation of a high capacity digital network on which networked multimedia applications will depend. On campus, the provision of optical fibre enable the creation of a high capacity backbones to Local Area Networks. However optical fibre provision to the staff desktop or student seat is still more of an exception than a rule due to the higher costs. In principal optical fibre can carry almost any type of traffic, at high data rates. It should be noted that there are different type of optical fibre, single mode, multi-mode, with different sizes, and attenuation characteristics. When upgrading from one technology to another, e.g. FDDI to ATM this may become important.
5.4. Radio Systems
Radio systems are likely to impact on multimedia in two ways. Firstly radio technology operating at microwave frequencies is available to provide wireless local area networks. Use of this technology for delivery of real time multimedia material should be treated carefully, because radio links are susceptible to fading, interference, random delays etc. For non real time use this technology is likely to perform as well as current Ethernet LANs. Secondly the introduction of digital mobile systems termed 'GSM' from a French acronym means that mobile users will have access to the same digital networks as fixed users. So in theory a mobile user will be able to connect to an ISDN users application, or an ATM users, with the only difference being one of speed. At present the bit rate of the mobile link is about 9.6 kbps. This may increase. With improvement in compression techniques the possibility of video conferencing on the move arises. Whether academics will need to adapt their video conferencing equipment to communicate with students stuck an the bus and late for a lecture is a matter for speculation!
Virtual Environments Visualisation