hich the digit timeslots used for speech transmission are Singing. Noise in the telephone circuit due to a net amount of gain also periodically used for signalling. Also known as speech in a four wire segment of circuit. digit signalling. Single mode fibre. Optical fibre designed to propagate signals of Roll call polling. A polling technique where the central station a single wavelength of light only. periodically polls each outstation, based on a list which it Skin effect. The effect in which current flowing at high frequency maintains of their location. See also hub polling. in a conductor is confined to a thin surface layer. This causes an Routing. Technique for sending a message to a specified address. increase in resistance at high frequencies. Smart card. A pocket sized card which has local processing fa- cilities. Scattering. The loss or diffusion of light in a optical fibre due to Solar outage. The term used to describe the disruption of a satel- defects. lite's transmission due to radiation from the sun, caused when Scotopic. Response curve of the human eye at low levels of illumi- the sun passes within the field of view of the satellite's antenna. nation. See also photopic. Source code. A computer programme as it is initially written, Scrambler. Usually refers to a device which recodes a signal, so usually in a higher level language. Once source code is com- that it does not contain long sequences of zeros, but to set rules piled it becomes object code and is in the form a computer can so that the signal can be reconstructed at the receiving end. operate on. Glossary of terms 62/31 Space division switching. Telephony switching technique which Talkoff. Term used to describe the disconnection of a channel due maintains a separate path through it for each call. to speech being interpreted as an in-channel disconnect control Speed dialling. The system in which a shortened address code is signal. used to call often used numbers. Talk time. Maximum time for which a mobile can be used without Spot beam. Focuss