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  Glossary - S
# A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Source Address (network)
(Hardware) address of the device that has initiated the connection.

Source MAC Address (physical layer)

System Application Architecture (network)
Set of architectural rules defined by IBM to come to applications may be independent of hardware and network resources.

Signaling AAL (ATM)
Service-specific parts of the AAL protocol responsible for signaling. Its specifications, being developed by ITU-T, were adopted from N-ISDN.

Service Access Point (network, standard)
Physical interface between the layers in the OSI model through which lower layers provide services to the higher layers passing over the Protocol Data Units (PDUs).

Subnetwork Attachment Point
The unique address maintained by a subnetwork for each of the DTEs attached to it.

Segmentation and reassembly sublayer (ATM)
The functions of the lower half of the AAL layer that segment user traffic into 48-byte payloads and reassemble 48-byte payloads into user traffic. Converts PDUs into appropriate lengths and formats them to fit the payload of an ATM cell. It adds any necessary header or trailer bits to the data and passes the 48-octet to the ATM layer. Each AAL type has it sown SAR format. At the destination end-station, SAR extracts the payloads from the cells and converts them back into PDUs, which can be used by applications higher up the protocol stack. See also CS.

Segmentation and reassembly protocol data unit (ATM)
Information that has passed through SAR and been loaded into ATM cells that is ready to be forwarded to the TC sublayer of the ATM physical layer for actual transmission. It comprises of the SAR-PDU payload and any control information that the SAR sublayer might add.

Statistic Bit Rate (ATM)
(ATM)Acronym used by ITU-T I.371 "ATM Transfert Capability". Equivalent of nrt-VBR (ATM Forum).

Structured Cabling (cabling)

Structured Connector (physical layer)

Strick and Click (physical layer, optical cable)

Message integrity checking system that uses an principle close to a CRC calculation system. The CRC is provided as a signature for the message.

Service Control Point (network, ISDN)
Central point in Intelligent Network (such as ISDN) for call control database. An SCP could contain the table lookup information for 800 call service routing. See also Service Transfer Point.

Sustainable Cell Rate (ATM)
ATM performance parameter that characterizes a bursty source and specifies the maximum average rate at which cells can be sent over a given virtual connection ( VC). It can be defined as the ratio of the MBS to the minimum burst interarrival time.

Small Computer Serial / System Interface (hardware)
Physical interface standard between high speed external devices, such as disks and CD-ROMs, and desktop systems.

Synchronous Digital Hierarchy (network, communications)
International form (ITU-T) of SONET that designates signal interfaces for very high-speed digital transmission over optical fiber links. SDH is built on blocks of 155.52 Mbit/s. See SONET, STM-n, OC-n, SOH, Payload.

Specification and Description Language (network)
Defined by the ITU-T (recommendation Z100) to provide a tool for unambiguous specification and description of the behaviour of telecommunications systems. The area of application also includes process control and real-time applications. SDL provides a Graphic Representation (SDL/GR) and a textual Phrase Representation (SDL/PR), which are equivalent representations of the same semantics. A system is specified as a set of interconnected abstract machines which are extensions of the Finite State Machine (FSM).
The basis for description of behaviour is communicating Extended State Machines that are represented by processes. Communication is represented by signals and can take place between processes or between processes and the environment of the system model. Some aspects of communication between processes are closely related to the description of system structure.
An Extended State Machine consists of a number of states and a number of transitions connecting the states. One of the states is designated the initial state.
The SDL Forum Society Web site:
what is SDL?.

Structure Definition Language
Used internally by DEC to define and generate the symbols used for VAX/VMS internal data structures in various languages.

System Description Language
language used by the Eiffel/S implementation of Eiffel to assemble clusters into a system.

Synchronous Data Link Control (network)
An IBM layer 2 protocol used in IBM SNA for point-to-point and multi-point communications. Similar to HDLC.

Symmetric Digital Subscriber Line (communications, protocol, network)
See also ADSL.

Structured Data Transfer (ATM, communications)

Service Data Unit (network)
Data unit exchanged between 2 adjancent or counterpart layers of the OSI model to make the service correspond to these layers.

Simple and Efficient Adaptation Layer (ATM)
The original name and recommendation for AAL5.

  1. architecture: A collection of pages in a memory management system.
  2. programming: A separately relocatable section of an executable program. Unix executables have a text segment (executable machine instructions), a data segment (initialised data) and a bss segment (uninitialised data).
  3. networking: Network segment.
  4. To experience a segmentation fault. Confusingly, the stress is often put on the first syllable, like the noun "segment", rather than the second like mainstream verb "segment". This is because it is actually a noun shorthand that has been verbed.

Selector (ATM)
The last octet of the ATM Address that is currently undefined in the UNI 3.0 specification.

Signaling Endpoint (network)

  1. A program which provides some service to other (client) programs. The connection between client and server is normally by means of message passing, often over a network, and uses some protocol to encode the client's requests and the server's responses. The server may run continuously (as a daemon), waiting for requests to arrive or it may be invoked by some higher level daemon which controls a number of specific servers (inetd on Unix). There are many servers associated with the Internet, such as those for Network File System, Network Information Service (NIS), Domain Name System (DNS), FTP, news, finger, Network Time Protocol. On Unix, a long list can be found in /etc/services or in the NIS database "services". See client-server.
  2. A computer which provides some service for other computers connected to it via a network. The most common example is a file server which has a local disk and services requests from remote clients to read and write files on that disk, often using Sun's Network Filing System (NFS) protocol or Novell Netware on IBM PCs.

Service Layer (network)
Intermediate layer between the Component Interface (CI) and Management Interface (MI) layers. It resides in the system's memory.

Service Types (ATM)
There are four service types CBR, VBR, UBR and ABR. CBR and VBR are guaranteed services while UBR and ABR are described as best effort services.

Severely Errored Seconds

Super Frame

Study Group (standard, network)
Technical working group of the ITU-T. Different groups are designated by a unique number.

Standard Generalized Markup Language

Simple Gateway Management Protocol (network, management)

Shareware (software)
Software available for downloading on the Internet that you can try before you buy. Users who want to continue to use the program are expected to pay a registration fee (rarely more than U.S. $100). In return they get documentation, technical support, and any updated versions.

SMDS Interest Group (network, ATM)
An industry forum active in producing specifications in the area of SMDS. It has also joined some of the ATM Forum activities.

Signaling (ATM)
The standard process, based on ITU-T Q.93B, used to establish ATM point-to-point, point-to-multipoint, and multipoint-to-multipoint connections.

SMDS Interface Protocol (network, SMDS)
Formal name given to each layer of the SMDS Network Interface. SIP1 is the physical layer, SIP2 is the cell layer including DQDB, and SIP3 is DXI.

Sustained Information Rate (ATM, SMDS)
A flow controlmechanism used in SMDS.

Serial Line IP (network, Internet)
A protocol used to run IP over serial lines, such as telephone circuits or RS-232 cables, interconnecting two systems. SLIP is defined in RFC 1055, but is not an Internet Standard. It is being replaced by PPP.

see also Time Slot (TS).

Smartphone (hardware, phone)
An Intelligent phone.

Server Message Block (network)
Protocol that allows local network stations to exchange messages, especially service messages to manage the common management tasks on a local network (opening or closing files, locking...).

Switched Multimegabit Digital Service (network)
A connectionless, MAN service, based on 53-byte packets, that targeted the interconnection of different LANs into a switched public network. SMDS was developed by Bellcore as a public carrier service. See SNI, SIP.

Single Mode Fiber

SMDS Interest Group (SMDS, standard, vendor consortium)
Vendor consortium organisation on SMDS.

Structure of Management Information (network)
The rules used to define the objects that can be accessed via a network management protocol. This protocol is defined in STD 16, RFC 1155. See also Management Information Base.

Smiley face (network, Internet)
This odd symbol is one of the ways a person can portray "mood" in the very flat medium of computers by using "smiley faces".
This is "metacommunication", and there are literally hundreds of such symbols, from the obvious to the obscure. For example :-) expresses "happiness". Don't see it? Tilt your head to the left 90 degrees. Smiles are also used to denote sarcasm.

Symetric Multi Processor (computer, parallel)
Two or more similar processors connected via a high-bandwidth link and managed by one operating system, where each processor has equal access to I/O devices. This is in contrast to the "compute server" kind of parallel processor where a front-end processor handles all I/O to disks, terminals and local area network etc.
The processors are treated more or less equally, with application programs able to run on any or perhaps all processors in the system, interchangeably, at the operating system's discretion. Simple MP usually involves assigning each processor to a fixed task (such as managing the file system), reserving the single main CPU for general tasks.
OS/2 currently supports so-called HMP (Hybrid Multiprocessing), which provides some elements of symmetric multiprocessing, using add-on IBM software called MP/2. OS/2 SMP was planned for release in late 1993.

Symbol Manipulation Program
Steven Wolfram's earlier symbol manipulation program, before he turned to Mathematica.

Simple Mail Transfer Protocol (network, Internet, messaging)
The protocol standard developed to support electronic mail (e-mail) services.

Sequence Number (ATM)
Part of the header of the SAR-PDU (2 bits in AAL1, 4 bits in AAL3/4) it is used as a sequence counter for detecting lost, out-of-sequence ormisinserted SAR-PDUs.

Systems Network Architecture (network)
A host-based hierarchical network architecture introduced by IBM, where logical channels are created between end-points. Logical paths through the network are established in advance (static routing) and the network is tightly controlled by centralized host computers. See also APPN, PIU, SDLC.

SubNetwork Attachment Point (network)
See IEEE 802.1a.

SMDS Network Interface (network, SMDS)
Generic name for all layers of the interface between an end use and the SMDS network. See PHY, DQDB, DXI. SNA Network Interconnection (IBM): A facility that provides cross-network communication between two or more independent SNA networks.

Simple Network Management Protocol (network, Internet)
An IETF-defined standard for handling management information. It is normally found as an application on top of the user datagram protocol (UDP). SNMP is the IETF standard management protocol for TCP/IP networks. See CMIP, LMI, ILMI.

Sequence Number Protection (ATM)
A 4-bit field in the header of the AAL1 SAR-PDU that contains the CRC and the parity bit fields. It is used to detect error in the SN field. The SAR sublayer is responsible for this CRC calculation.

Senior Officers Group for Information Systems Security (security)
Expert group that works on security and cryptography issues.

Section Overhead (network)
Control bytes added to STS-1 or STM-1 frames, providing functions such as OA&M facilities, frame alignment, protection switching, etc.

Synchronous Optical Network
An ANSI (developed by ECSA++) international suite of standards for transmitting digital information over optical interfaces. "Synchronous" indicates that all component portions of the SONET signal can be tied to a single reference clock from 51.84 Mbps to 13.22Gbps. Effective for ISDN services including ATM. It has been recognized as the North American standard for SDH.

Service Provider (network)
A commercial vendor that sells connections to the Internet and usually, related services, to organizations seeking a Net presence.

Span (communications)
Full-duplex digital transmission line between two digital facilities.

Simple Protocol for ATM Network Signalling (ATM, signaling, protocol)
A protocol supported by FORE Systems switches that provides SVC tunelling capability over a PVC network.

Standard Promotion and Application Group (ATM)

Scalable Processor Architecture Reduced instructionset Computer

Synchronous Payload Envelope (sonet)

Subscriber Premises Network (network)

Smart PVC

Switched or Semi-Permanent Virtual Connection (ATM)
A PVC-type connection where SVCs are used for call set-up and (automatic) rerouting. It is also called smart PVC.

Structured Query Langage
Language used to query a relational database. It was first invented by IBM, and was first nomalized by the ANSI (American National Standard Institut) in 1989.

Switching System (communications, network, ATM)
A set of one or more systems that act together and appear as single switch for the purposes of PNNI routing.

Signaling System Number 7 (network, signaling, protocol)
A common channel signaling standard developed by CCITT. It was designed to provide the internal control and network intelligence needed in ISDNs. A protocol suite used for communication with, and control of, telephone central office switches and their attached processors.

Service Specific Coordination Function (ATM)
Part of the SSCS portion of the SAAL. Among other functions it provides a clear interface for relaying user data and providing independence from the underlying sublayers. See also SSCOP.

Service Specific Connection-Oriented Protocol (ATM)
Part of the SSCSportion of the SAAL. SSCOP is an end-to-end protocol that provides error detection and correction by retransmission, status reporting between the sender and the receiver, while it guarantees delivery integrity. See also SSCF.

Service Specific Convergence Sublayer. (ATM)
One of the two components of the Convergence Sublayer (CS) of the AAL that is particular to the traffic service class to be converted. It is developed to support certain user applications such as LAN Emulation, transport of high-quality video, database management.

Server Side Include (network, Internet,www)
The facility provided by several HTTP servers, e.g. (NCSA httpd), to replace certain HTML tags in one HTML file with the contents of another file at the time when the file is sent out by the server, i.e. it is an HTML macro. It is also used to display the date the document was last updated.
It can be used, so that information such as last date modified, file size, author etc. can be automatically included.

Secure Socket Layer (security, Internet, protocol, cryptography)
A protocol designed by Netscape Communications Corporation to provide secure communications on the Internet. It enables encrypted, authenticated communications across the Internet. SSL is layered beneath application protocols such as HTTP, SMTP, Telnet, FTP, Gopher, and NNTP and is layered above the connection protocol TCP/IP.
SSL used mostly (but not exclusively) in communications between web browsers and web servers. It is used by the HTTPS access method. URLs that begin with ³https² indicate that an SSL connection will be used.
SSL provides 3 important things: Privacy, Authentication, and Message Integrity.
In an SSL connection each side of the connection must have a Security Certificate, which each side's software sends to the other. Each side then encrypts what it sends using information from both its own and the other side's Certificate, ensuring that only the intended recipient can de-crypt it, and that the other side can be sure the data came from the place it claims to have come from, and that the message has not been tampered with.

Single Segment Message (network)
A message that constitutes a single PDU.

Service Switching Point (network, ISDN)
Intelligent Network Term for the Class 4/5 Switch. The SSP has an open interface to the IN for switching signaling, control and handoff.

Segment Type (ATM)
A 2-bit field in the SAR-PDUheader of the AAL3/4 that indicates whether the SAR-PDU is a BOM, COM, EOM or SSM.

Stick and Turn or Straight Tip

Standards are necessary for interworking, portability, and reusability. They may be de facto standards for various communities, or officially recognised national or international standards.
Andrew S. Tanenbaum once said, "The nice thing about standards is that there are so many of them to choose from", a reference to the fact that competing standards become a source of confusion, division, obsolescence, and duplication of effort instead of an enhancement to the usefulness of products.
Some bodies concerned in one way or another with computing standards are IAB (RFC and STD), ISO, ANSI, DoD, ECMA, IEEE, IETF, OSF. See also STD.

StarLAN-10 is AT&T's variety of Ethernet over twisted-pair cabling. Older StarLAN-10 is not 100% 10BaseT compliant, as it does not provide link integrity to the AUI. However, many 10BaseT interfaces can be configured to work with StarLAN-10 hubs, alongside StarLAN-10 NICs. Beware, though, that the original StarLAN-10 is NOT in any way compatible with 10BaseT, and worse, there seems to be no way to tell other than trying it to see what happens.
The current StarLAN products supported by AT&T/NCR are fully 802.3 compliant. This includes the SmartHUB model E, SmartHUB model B, SmartHUB XE, and the other fiber and wire SmartHUB models.

State Tansition Diagram

Internet standard (standard, Internet)
A subseries of Request For Comments () that specify Internet standards. The official list of Internet standards is STD-1. See also For Your Information.

Internet Standard 1
The Internet Architecture Board official list of Internet standards.

Statistical Time-Division Multiplexing (network, communications)
Same as ATDM.

Synchronous Time-Division Multiplexing (network, communications)
A TDM scheme where the interleaved time slots are preassigned to the users.

Synchronous Transfer Mode (network)
A packet switching approach where time is divided in time slots assigned to single channels during which users can transmit periodically. Basically, time slots denote allocated(fixed) parts of the total available bandwidth. See also TDM.

Synchronous Transport Module-1 (ATM, network)
An ITU-T-definedSDH physical interface for digital transmission in ATM at the rate of 155.52 Mbps over OC-3 optical fiber. See SONET.

Synchronous Transport Module-n (where 'n' is an integer) (ATM, network)
An ITU-T-definedSDH physical interface for digital transmission in ATM over optical fiber (OC-'n x 3') by multiplexing 'n' STM-1 frames (e.g., STM-4 at 622.08 Mbps, STM-16 at 2.488 Gbps). There is a direct equivalence between the STM-n and the SONET STS-3n transmission rates.

Synchronous Transport Module 'n' concatenated (where 'n' is an integer) (ATM, network)
SDH standards for transmission over optical fiber (OC-'n x 3') by multiplexing 'n' STM-1 frames (e.g., STM-4 at 622.08 Mbps, STM-16 at 2.488 Gbps) but treating the information fields as a single concatenated payload. See SONET.

Shielded Twisted Pair (network, hardware)
Generic name for any cable constructed with two insulated copper wires twisted together and wrapped by a protective jacket shield. See also UTP.

Signal Transfer Point (network, ISDN)
The point in the Intelligent Network(such as ISDN) where call control is handed off or transferred between the Service Switching Point and Service Control Point. Signaling in and out of the STP utilizes SS7 protocol.

Synchronous Transport Signal (switching, transport)

Synchronous Transport Signal-1 (network)
SONET signal standard for optical transmission over OC-1 optical fiber at 51.84 Mbps. See SDH.

Synchronous Transport Signal-'n'(where 'n' is an integer) (network)
SONET standards for transmission over OC-n optical fiber by multiplexing 'n' STS-1 frames (e.g., STS-3 at 155.52 Mbps, STS-12 at 622.08 Mbps, STS-48 at 2.488 Gbps).

Synchronous Transport Signal 'n' concatenated (where 'n' is an integer) (network)
SONET standards for transmission over OC-n optical fiber by multiplexing 'n' STS-1 frames (e.g., STS-3 at 155.52 Mbps, STS-12 at 622.08 Mbps, STS-48 at 2.488 Gbps) but treating the information fields as a single concatenated payload.

Switched Virtual Connection (network)
A connection that is set up and taken down dynamically through signaling. See also PVC.

Switched Virtual Circuit (network)
A virtual circuit (X.25), virtual connection (Frame Relay) or virtual channel connection (ATM) that has been established dynamically in response to a signaling request message.

Switched Virutal Channel (or Circuit) Connection (ATM)

Standard Vector Graphic (graphic)
Graphic standard based on Cascading Style Sheets (CSS) and XML. The goal is to have a vector graphic system but lighter than the other current vector graphic standards. Thus, SVG should offer to:
  • handle images with a very sharp accuracy,
  • overlay images (thanks to CSS),
  • use each element with indexing capabilities (thanks to XML),
  • distort text following a curve,
  • use layers, transparency and animations.
It is a merging between PGML (Precision Graphics Markup Language) and VML (Vector Markup Language).

Super Video Graphics Array| Adapter | Accelerator (hardware)
A video display standard created by VESA(Video Electronics Standards Association) for IBM PC compatible personal computers. The resolution is 800 x 600 4 bit pixels. Each pixel can therefore be one of 16 colours.
In actual usage, however, this does not seem to be a single standard. Not even many standards, just a designation for graphics cards with more than the minimal VGA standard features.

Switched Virutal Path (or Circuit) Connection (ATM)

Swap (operating system, software)
To move a program from fast-access memory to a slow-access memory ("swap out"), or vice versa ("swap in"). The term often refers specifically to the use of a hard disk (or a swap file) as virtual memory or "swap space".

Switch, ATM (network, ATM)
An ATM device responsible for switching the cells. There exist various switch architectures, which can be classified according to different aspects (i.e. buffering, switch matrix, interconnection design, division multiplexing).
Equipment used to connect and distribute communications between a trunk line or backbone and individual nodes.

Switch Fabric
The central functional block of the ATM switch, which is responsible for buffering and routing the incoming cells to the appropriate output ports.

Switched Ethernet (network)
A method to make a shared-media LAN protocol function as a switched protocol.

Switched LAN
Emerging technology that replaces the shared bus backplane of Ethernet hubs and the shared ring backplane of Token Ring hubs with a switching backplane. Connectivity is provided by switching sender traffic directly to the port of the addressed destination device. Provides potentially higher throughput, scalable capacity, and simpler configuration support. Does not require any changes to access wiring or adapter cards.

Synchronous (communications)
Signals that are sourced from the same timing reference. These may have the same frequency.

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