Communications Systems Glossary

Sampling rate.  The sampling rate is the rate at which samples are taken form an analogue waveform during the analogue-to-digital conversion process.

Selective-reject automatic repeat request (Selective-reject ARQ).  Selective-reject automatic repeat request (selective-reject ARQ) is an error control technique in which the transmitter only retransmits blocks that are in error.

Serial transmission.  Serial transmission is the transmission of bits one after the other.

Server.  A network server, (more traditionally called a file server), is normally a fast processor with large storage and memory capacity, which manages the network and the network file system and provides a number of functions such as: access to printers, disk space, application programs, back-up tape drives, as well as running the network operating system (NOS).

Session layer (Layer 5).  The session layer is Layer 5 of the ISO OSI 7-layer model.

Shielded twisted pair (STP).  Shielded twisted pair cable is a form of twisted pair cable in which a separate shielding conductor surrounds each pair.

Shift.  The shift is the amount by which the FSK carrier frequency changes from 1 (mark) to 0 (space) and vice versa.

Sideband.  A sideband is a band of frequencies either side of the carrier in amplitude modulation (AM).

Sidelobe.  A slidelobe is the response of a receive antenna to a signal from other than its main axis (and similarly for a transmit antenna).

Signal-to-noise ratio (SNR).  The signal-to-noise ratio (SNR) is ratio of the signal power to the noise power.

Simplex transmission.  Simplex transmission is a mode of data transmission in which a transmission path can carry information in one direction only-from the source to the sink.

Single mode fibre.  A single-mode fibre is a fibre optic cable in which the light is guided down the centre of a very narrow core (approximately 8 microns thick). Supports the fastest optical fibre transmission.

Single sideband (SSB).  When only one sideband is transmitted, the transmission is called single-sideband suppressed carrier (SSBSC) or more commonly SSB.

Sink.  The sink converts the electrical signal from the receiver back into the form of original message information.

Sliding window.  Sliding window is a method of flow control in which a source may send more than one packet without acknowledgement from the sink.

SNMP (Simple network management protocol).  The SNMP (simple network management protocol) is used to pass network-management information in TCP/IP networks.

Source.  The source is the originator of the information that is to be exchanged via the communication system.

Spectrum.  The spectrum is the range of electromagnetic frequencies.

Spread spectrum.  A spread-spectrum signal occupies a bandwidth much greater than that of the message signal it conveys.

Star topology.  A star topology is a network topology in which all nodes are connected to a central node through which all data are transmitted.

Star-wired local area network (Star-wired LAN).  A star-wired local area network is a local area network (LAN) that has a physical star topology even though it may employ a logical bus or ring topology.

Static routing.  Static routing is a form of routing in which paths between nodes are always used in a pre-defined way.

Statistical time division multiplexing (STDM).  Statistical time division multiplexing is a form of time-division multiplexing (TDM) that provides improved channel-sharing efficiency by allowing some of the multiplexed channels to increase in data rate above their share if other stations are not using theirs to the full capacity. Also known as a stat mux.

Stop-and-wait automatic repreat request (Stop-and-wait ARQ).  Stop-and-wait automatic repreat request (stop-and-wait ARQ) is an error correction technique in which the sender transmits a block of data and then waits for an acknowledgment before transmitting the next block.

Sub-sampling.  Sub-sampling is the process of sampling a signal that has already been sampled to reduce the sampling frequency.

Super high frequency (SHF).  The super-high-frequency (SHF) band is a radio-frequency (RF) band in the range 3-30 GHz.

Switched virtual circuit (SVC).  A switched virtual circuit (SVC) provides a semi-permanent link established across a packet-switched network for the duration of the session between source and sink.

Symmetric encryption.  Symmetric encryption is a form of cryptography in which encryption and decryption are performed using the same key.

Synchronous transmission.  Synchronous transmission is data transmission in which source and sink have a common view of time and both run clocks that are continually synchronised by coordinating pulses extracted from the incoming data stream.

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