SAR (Segmentation and Reassembly)A method of breaking up arbitrarily sized packets.

SAS (Subscriber Authorization System). A Conditional Access sub-system that translates data about the customer into an Entitlement Management Message, at the request of the Subscriber Management System. It also ensures that the customer's security module receives the necessary authorization to view programs.

Satellite Bus. Provides power, mechanical structure, propulsion, orbit control and onboard management for a satellite.

Satellite communications. The use of satellites to transmit radio signals around the globe.

Satellite Payload. The components of a satellite which receive and amplify the uplink signal change the uplink frequency to the downlink frequency and provide the output RF power for the downlink.

SCH (Synchronization Channel). The SCH is a special channel reserved within a cellular mobile phone network for synchronization purposes. The SCH is broadcast by the base station to enable the mobile terminals within its cell synchronize themselves with the network. This involves alignment in terms of the operating frequency and system time as well as other important system information such as system identification numbers.

Schematic. A diagram that illustrates the electronic components of a system.

Scrambling. The deliberate distortion of transmitted signals to make them unintelligible. Scrambled signals are descrambled by a device at the destination.

SDH (Synchronous Digital Hierarchy). The ITU-TS International standard for transmitting information over optical fiber.

SDMA (Space Division Multiple Access). SDMA is a multiple access technique based upon sharing the access to the network on a geographical basis whereby the service area is divided into non-overlapping cells containing low-power transmitters.

SDT (Service Description Table). A DVB SI table that contains data on what services are available.

Seamless. A term used in telecommunications to describe the integration of a number of different networks used to switch a single call. This interconnection appears 'seamless' to any subscriber because it is not apparent to them when a call is being switched to another network.

Selective cells. Selective cells (also known as sectored cells) are used to restrict the transmitted signal within a particular area.

Sequencing. The sequential numbering of packets in a message so they can be reassembled in the correct order at the destination address.

Server. A device (or application) that provides services to its clients based on submitted requests. A server may either accept or reject a connection request from a client.

Service provisioning>. Part of the customer care system of a network. Contains a database of each subscriber and the services subscribed to. Requests for registration, or alterations to existing services are forwarded to the service provisioning systems.

Session. The connection of two devices for communication.

Session layer. The fifth layer of the OSI model, which is concerned with establishing a connection between communication devices. This layer is also responsible for the termination of a conversation/session when the devices have finished communicating.

Service area. The geographic area served by a single carrier.

Set-Top Box. See IRD.

SGSN: Serving GPRS Support Node. The SGSN is a network component within the GPRS system.

Shadowing. An air transmission problem that occurs when obstacles are in the path of the radio signal and causes it to vary in strength as the mobile station is roaming in the network.

SI (Service Information). A DVB set of tables that describe the delivery system, content and scheduling/timing of broadcast data streams, for example the EPG.

SID (System Identification Number). The SID is used during registration of the mobile station to allow it determine if it is operating in its home system or in a foreign system.

SIM (Subscriber Identity Module). A card that the mobile subscriber inserts into the mobile terminal. It contains a code that uniquely identifies an individual subscriber to the network. The SIM card allows the subscriber to use the GSM service regardless of the mobile terminal, that is, a subscriber can remove the SIM card from terminal A, insert it into terminal B, and use any services the SIM card allows on terminal B.

Simplex circuit. A transmission channel that carries signals in only one direction at a time.

SMATV (Satellite Master Antenna TV). Community, domestic TV networks that are low-cost. They may serve apartment blocks and combine satellite and other channels over one cable network.

SMG: Special Mobile Group. SMG is a committee within ETSI that oversees the development of mobile standards.

SMS (Short Messaging Service). A service that allows mobile subscribers to both send and receive alphanumeric messages up to 160 characters long.

SMS (Subscriber Management System).A subsystem of Conditional Access sub-system that manages customer information and requests Entitlement Management Messages from the Subscriber Authorization System.

SNR (Signal-to-Noise Ratio). The ratio of a wanted signal to the background interfering signal or noise, measured in dBs. SINR (I is Interference) is similar.

Solar Eclipse. Satellites obtain their power mainly from solar cells. However, at certain times of the year, they enter the daily shadow of the earth. During this time, some transponders may have to be switched off.

Solar Outage. As a satellite receiver antenna is pointed towards the satellite, it also points at the hot sun. This results in thermal noise being picked up by the antenna.

SONET (Synchronous Optical Network). An ANSI standard for transmitting information over optical fiber. This standard is used or accepted in the United States and Canada and is a variation of the SDH international standard.

Spectrum. The complete range of electromagnetic waves.

Speed dial. A feature of both cellular and landline telephones that store frequently used telephone numbers which can be activated by pressing a key.

Spread spectrum. A method of transmitting a signal by spreading it over a wide frequency band. Used initially by the military, it is becoming increasingly popular in commercial telecommunications as a way of helping to secure the signal, because spreading it over a wide spectrum makes it difficult to intercept and difficult to jam.

Standby time. The amount of time a fully charged mobile terminal can be left on before its battery runs down.

Standing wave. A wave phenomenon resulting from interference.

Station Keeping. A satellite mechanism whereby thrusters are fired every few weeks to counteract small forces which modify its orbit, causing it to drift into a "figure-of eight" motion away from its normal orbit.

Subscriber. A paying customer to a network, for example, the GSM network.

Supplementary service>. A supplementary service is an additional, optional service to which a user can subscribe usually for a fee. Examples of supplementary services include call forwarding, call waiting, calling line identification presentation.

SVC (Switched Virtual Circuit).A connection established via signaling. The user defines the endpoints when the call is initiated.

SVCC (Switched Virtual Channel Connection). A Switched Virtual Path Connection is one which is established and taken down dynamically through control signaling. A Virtual Channel Connection (VCC) is an ATM connection where switching is performed on the VPI/VCIfields of each cell.

SVPC (Switched Virtual Path Connection)>. A Switched Virtual Path Connection is one which is established and taken down dynamically through control signaling. A Virtual Path Connection (VPC) is an ATM connection where switching is performed on the VPI field only of each cell.

SWIFT (Society for Worldwide Interbank Financial Telecommunications). The communications network connecting most of the world's banks and financial institutions.