h a repeater that merely amplifies the transiting optical signals. Optical amplifiers are largely bit rate independent, hence it should be possible to upgrade the capacity of a line system by merely 42.10 References changing the LTEs, and not the repeaters. This idea is particularly exciting for undersea line systems where a major part of the total system cost lies in the repeaters. Up till now, these have always been Bellamy, John (1991) Digital Telephony, 2nd Edition, Wiley, ISBN 0471 62056 4. constrained to operate at a fixed bit rate. Flood, J.E. and Cockrane, P. (eds.)(1991) Transmission Systems, Peter Peregrinus, ISBN 0 86341 148 7. 42.8.4 Optical switching Ferguson, S. (1991) A common demnominator for Broadband, Tele- communications, August. The use of optical switching, together with various forms of Wave- Mathews, M.S. and Newbombe, P.J. (1991) The Synchronous Digi- length Division Multiplexing (WDM), will lead to a requirement for tal Hierarchy, IEE Review, May and June. yet another layer in the SDH hierarchy beyond the STM section Sexton, M.J. (1990) New directions in Transport Networks, Tele- layer. Real high speed optical switching, together with wavelength communications, October. This page intentionally left blank Signalling systems Samuel Welch OBE MSc (Eng.) CEng FIEE Telecommunications Consultant Formerly Head of Signalling, BT. Contents Analogue network signalling 43/3 Common channel signalling 43/7 Digital network signalling 43/8 References 43/11 This page intentionally left blank Analogue network signalling 43/3 Incoming Outgoing Analogue network signalling lines lines Inlet Switchback Outlet The basic signalling on subscriber lines comprises the following: 1. Supervisory, the on-hook, off-hook conditions of the sub- scribers. Common 2. Address information from the caller which may be decadic control Register Marker dialled pulses or push-button signals. equipment These signalling functions are transferred over the switched net- Translator work by signalling systems. Call processing can be pe