ecame clear that it was of the order of 100km to 150km. The length of junction systems is possible to realise the potential of fibre transmission this rapid and around 2km to 25km. Therefore the benefits of using fibre in long virtually complete acceptance of the new technology is quite un- haul transmission are not apparent in the junction network. Never- usual. The impact of fibre optic communications lies not SO much in theless, fibre optic systems are used in the junction network because that they can do better than other technologies but they have altered they are able to operate without repeaters in the congested environ- the way the communication issues of the next century will be ment of the urban areas. approached. The majority of optical systems currently in operation use the 1300nm window but the requirements for higher capacity, that is 53.6.1 Submarine fibre optic systems 2.4Gbit/s and higher, and the trend for un-repeatered transmission ensured that future systems will operate at 1550nm. The technology The impact of fibre communications in submarine communications needed for this migration is DFB lasers and long wavelength APDs. has been greater than that on terrestrial applications. In submarine They are both beginning to be available in volume and at prices applications not only the medium has changed, from coaxial cable which will lead to cost effective systems. The future of fibre networks 53/25 The future of fibre networks standard was completed there was significant interest expressed by the CCITT standards body, resulting in a modification to SONET to In the early 1980's an attempt was made in North America to make it adaptable to CCITT plesiochronous bit rates. The standard address the shortcomings of the asynchronous multiplexing struc- issued by CCITT became known as SDH (Synchronous Digital ture then in use. The main task was to try and avoid the multiplexed Hierarchy). signal 'hiding' its contents amongst control and stuffing bits. The only way to extract information in this format was t