Microwave frequency band in the range of 2 - 4 GHz that corresponds to wavelength from 15 cm to 7.5 cm.

A network that is formed by several interconnected Bluetooth piconets.
Also see Bluetooth and piconet.

Synchronous Connection-Oriented (SCO) link. A point-to-point link between a master and a single slave in a piconet. The master maintains the SCO link by using reserved slots at regular intervals.
Also see Bluetooth, piconet, and ACL link.

Small Computer System Interface. A high speed parallel interface defined by ANSI which is plugged into an ISA socket or a PCI slot on a computer motherboard.  It transfers data at rates of 4 Mbps - 40 Mbps.
SCSI is used to connect computers with peripherals (usually storage devices). Up to 7 peripherals can be connected to one SCSI interface. The newer wide SCSI interface gives higher data transfer rate at 320 Mbps and supports up to 15 peripherals.
Also see ISA and PCI.

Screened Twisted Pair. A twisted pair cable in which each twisted pair is NOT shielded individually but all twisted pairs are protected by outer (overall) shield. ScTP gives better protection to interference than UTP.
See picture. Also see twisted pair. Compare with STP.

Synchronous Digital Hierarchy. An ITU-T standard for multiplexing and demultiplexing of optical signals that run over fiber optic infrastructure. SDH is classified based on its traffic carrying capacity into STM-1 (155 Mbps), STM-4 (622 Mbps), and so on.
SDH is used outside North America (mainly in Europe and Asia). North American counterpart is called SONET. SDH provides transport for any kind of access networks.
Also see multiplexing and SONET.

Spatial (or Space) Division Multiplexing. A technique which multiplexes data streams spatially and transmit the data streams on one sub-channel at the same time.

Spatial (or Space) Division Multiple Access. A technique that allows multiple users that are separated in space to transmit and receive on the same sub-channel at the same time. SDMA is implemented in Adaptive Antenna Systems (AAS).

Symmetric Digital Subscriber Line or Single Pair Digital Subscriber Line. A DSL variant that achieves upstream and downstream data rates up to E1/T1 rates (i.e. 2.048 Mbps /1.544 Mbps) over a telephone cable with coverage of up to about 3 km. ETSI defines SDSL as the European variant of ITU-T G.shdsl standard.
Also see DSL.

The interface on a computer that allows asynchronous transfer of data one bit at a time. Serial port on a PC usually follows RS-232C standard and has DB-9 or DB-25 male connector. Also known as communication or COM port.
See picture. Also see parallel port.

A computer or software that stores or provides resources for other computers (clients) in a network. A server is named after its specific function, e.g. Web server, virtual private server, mail server, print server.
Also see client.

Layer 5 in the OSI model. The Session Layer deals with user identification and session establishment and termination. The Session Layer manages dialogue between end-user application processes. SIP, NetBIOS, Winsock are examples of Session Layer protocols.
Also see OSI model.

Symmetric High-bit-rate Digital Subscriber Line. A DSL variant that achieves upstream and downstream data rates up to 2.3 Mbps over a pair of copper wires or 4.6 Mbps over two pairs of copper wires. SHDSL is a single-pair version of HDSL. SHDSL conforms to ITU-T G.991.2 (G.shdsl) Recommendation.
Also see DSL. Compare with HDSL.

Super High Frequency. Frequency band in the range of 3 - 30 GHz, corresponding to wavelength from 100 mm to 10 mm. This microwave band is used in satellite communications. The UNII band that is used in wireless LAN (i.e. 802.11a) also falls within SHF spectrum.

Communication in one direction, receive or transmit.
Contrast with duplex.

Refers to fiber optic in which light travels in one mode (path) and has smaller core diameter.
See picture. Also see fiber optic. Compare with multimode.

Session Initiation Protocol. A standard protocol that is used to initiate, manage, and terminate interactive user session in multimedia communications over a packet-switched network, such as the Internet. Current SIP implementation is in VoIP, however SIP can handle broader applications because of its multimedia support.
Also see VoIP.

Serial IrDA. An IrDA standard for short range, point-to-point, and half duplex infrared asynchronous serial transmission using infrared that provides maximum data rate of 115.2 kbps.
Also see IrDA and infrared.

Super Low Frequency. Frequency band in the range of 30 - 300 Hz, corresponding to wavelength from 10,000 km to 1,000 km. SLF band overlaps audio frequencies. AC power grids also have frequencies within this range, i.e. 50 Hz and 60 Hz.

Serial Line Internet Protocol. Older encapsulation protocol used for dial-up Internet connection and communications over serial ports. SLIP requires IP address configuration before a connection is established and it doesn't provide error-detection scheme. That's why SLIP has been replaced by PPP which is simpler and more reliable.
Also see PPP.

Small or Medium size Enterprise.

Simple Mail Transfer Protocol. A standard protocol that is used for e-mail transfer between servers over the Internet.

Systems Network Architecture. An earlier network technology that was developed by IBM.

Simple Network Management Protocol. A standard protocol that enables network management and monitoring via the Internet. Although it was developed originally for monitoring network components (such as router and switch), now it can control any object that can connect to the Internet (such as telecom equipment, home electronics, etc).

Signal to Noise Ratio. The ratio of signal strength to noise strength in decibel (dB).

A virtual connection between two applications in a server, represented by port number.

Scalable OFDMA. This technique is used in Mobile WiMAX (IEEE 802.16e) to enable support for diverse spectrum regulations and various usage models. With SOFDMA, WiMAX channel bandwidth is made scalable (from 1.25 MHz up to 20 MHz) by adjusting FFT (Fast Fourier Transform) size at a fixed sub-carrier frequency spacing.
Also see OFDMA.

A special software provided by VoIP operators or independent software vendor for making and receiving VoIP call using a computer that is equipped with sound card, microphone, and speaker. VoIP service using softphone is offered for free by many operators and it serves as an introductory service.
In the coming years, we expect that softphone will be built into familiar applications like e-mail editor and web browser.
Also see VoIP.

Small Office Home Office.

Synchronous Optical NETwork. An ANSI standard for multiplexing and demultiplexing of optical signals that run over fiber optic infrastructure. SONET is classified based on its traffic carrying capacity into OC-1 (51.8 Mbps), OC-3 (155 Mbps), and so on.
SONET is used in North America, the European version is called SDH. SONET is often used in backbone network, providing transport for any kind of access networks.
Also see SDH.

A filter that is used to split voice frequencies from ADSL transmission. Splitter may be separate from ADSL modem or integrated in ADSL modem's inside circuits.
Also see ADSL.

Stateful Packet Inspection. A type of firewall that inspects incoming packets to see if it corresponds to an outgoing request.
Also see firewall.

A technique in which an already modulated signal is modulated a second time in such a way as to produce a waveform which interferes in a barely noticeable way with any other signal operating in the same frequency band. With spread spectrum technique, signal is spread over wider bandwidth than is needed for transmission. This technique reduces the effect of interference.
Also see DSSS and FHSS.

Signal Quality Error. SQE is the IEEE term for collision.
Also see collision.

Service Set IDentifier. A wireless LAN name that is assigned by its owner. Default SSID is usually pre-defined by a wireless access point manufacturer. You must change it to another name if you don't want your network to be shared unknowingly by a passer-by or a neighbor.

Secure Sockets Layer. A protocol that enables secure communications over the Internet by providing authentication using cryptography. SSL was developed by Netscape.
Also see IPSec.

IP Address assigned by an ISP to a subscriber account and stays the same during her subscription to the ISP.
Contrast with dynamic IP address.

Star Topology is a network topology where each node is connected via a point-to-point link to a central connection point (hub). Failure in one node doesn't affect the network since each node has a dedicated and independent connection to the hub. But if the hub goes down, the network will stop working.
Ethernet networks over twisted pair (10BaseT, 100BaseT, and 1000BaseT) implement star topology.
See picture. Also see bus, ring, and mesh topologies.

Synchronous Transport Module level 1.  Refers to a 155 Mbps circuit capacity in SDH. There are higher speed circuits in SDH, i.e. STM-4 (622 Mbps), STM-16 (2.5 Gbps), and STM-64 (10 Gbps).
Also see SDH.

Shielded Twisted Pair. A twisted pair cable in which each twisted pair is shielded individually and all twisted pairs are protected by outer (overall) shield. STP is mostly used in Token Ring networks. STP gives the best protection from interference than UTP and ScTP.
See picture. Also see twisted pair. Compare with ScTP.

An Ethernet cable (i.e. Cat 5, Cat 5e, or Cat 6) that has both ends pinned in the same way, that's according to either EIA/TIA-568A or the other 568B. The order of the 4 wires (out of the cable's 8 wires) is the same at both ends of the cable. Only wires/pins 1, 2, 3, and 6 are used for transmit and receive.
Straight-through cable is used to connect a computer to a hub or a switch and to connect one hub to another if each hub has a built-in uplink port.
Also see Cat 5 cable. Compare with crossover cable.

An IP address that represents the size of a network.
Also see IP address.

A network component that acts as a central connection point like a hub but forwards a packet only to the destination node instead of broadcasting the packet to every node in a network, therefore reduces network traffic. Switch examines the packet MAC address to determine its destination. Switch works at Layer 2 of the OSI model.
In telecommunication, the term Switch refers to the (telephone) Exchange.
Compare with hub. Also see OSI model.

A transmission system in which synchronization is done on a message basis rather than on a character basis. It employs no start and stop bits, but each message is preceded by two or more sync characters. Both transmitting and receiving modem must clock at exactly the same rate.
Compare with asynchronous.