S

SAC

single-attached concentrator. FDDI or CDDI concentrator that connects to the network by being cascaded from the master port of another FDDI or CDDI concentrator.

sampling rate

Rate at which samples of a particular waveform amplitude are taken.

SAP

1. service access point. Field defined by the IEEE 802.2 specification that is part of an address specification. Thus, the destination plus the DSAP define the recipient of a packet. The same applies to the SSAP. See also DSAP and SSAP.
2. Service Advertisement Protocol. IPX protocol that provides a means of informing network clients, via routers and servers, of available network resources and services. See also IPX.

SAR

segmentation and reassembly. One of the two sublayers of the AAL CPCS, responsible for dividing (at the source) and reassembling (at the destination) the PDUs passed from the CS. The SAR sublayer takes the PDUs processed by the CS and, after dividing them into 48-byte pieces of payload data, passes them to the ATM layer for further processing. See also AAL, ATM layer, CPCS, CS, and SSCS.

SAS

1. single attachment station. Device attached only to the primary ring of an FDDI ring. Also known as a Class B station. Compare with DAS. See also FDDI.
2. statically assigned socket. A socket that is permanently reserved for use by a designated process. In an AppleTalk network, SASs are numbered 1 to 127; they are reserved for use by specific socket clients and for low-level built-in network services.

satellite communication

Use of orbiting satellites to relay data between multiple earth-based stations. Satellite communications offer high bandwidth and a cost that is not related to distance between earth stations, long propagation delays, or broadcast capability.

SBus

Bus technology used in Sun SPARC-based workstations and servers. The SBus specification has been adopted by the IEEE as a new bus standard.

SCR

sustainable cell rate. Parameter defined by the ATM Forum for ATM traffic management. For VBR connections, SCR determines the long-term average cell rate that can be transmitted. See also VBR.

SCTE

serial clock transmit external. Timing signal that DTE echoes to DCE to maintain clocking. SCTE is designed to compensate for clock phase shift on long cables. When the DCE device uses SCTE instead of its internal clock to sample data from the DTE, it is better able to sample the data without error even if there is a phase shift in the cable. See also phase shift.

SDH

Synchronous Digital Hierarchy. European standard that defines a set of rate and format standards that are transmitted using optical signals over fiber. SDH is similar to SONET, with a basic SDH rate of 155.52 Mbps, designated at STM-1. See also SONET and STM-1.

SDLC

Synchronous Data Link Control. SNA data link layer communications protocol. SDLC is a bit-oriented, full-duplex serial protocol that has spawned numerous similar protocols, including HDLC and LAPB. See also HDLC and LAPB.

SDLC broadcast

See SDLC broadcast in the Cisco Systems Terms and Acronyms section.

SDLC Transport

See SDLC Transport in the Cisco Systems Terms and Acronyms section.

SDLLC

See SDLLC in the Cisco Systems Terms and Acronyms section.

SDSL

single-line digital subscriber line. One of four DSL technologies. SDSL delivers1.544 Mbps both downstream and upstream over a single copper twisted pair. The use of a single twisted pair limits the operating range of SDSL to 10,000 feet. Compare with ADSL, HDSL, and VDSL.

SDSU

SMDS DSU. DSU for access to SMDS via HSSIs and other serial interfaces.

SDU

service data unit. Unit of information from an upper-layer protocol that defines a service request to a lower-layer protocol.

SEAL

simple and efficient AAL. Scheme used by AAL5 in which the SAR sublayer segments CS PDUs without adding additional fields. See also AAL, AAL5, CS, and SAR.

secondary

See secondary station.

secondary ring

One of the two rings making up an FDDI or CDDI ring. The secondary ring is usually reserved for use in the event of a failure of the primary ring. Compare to primary ring.

secondary station

In bit-synchronous data link layer protocols such as HDLC, a station that responds to commands from a primary station. Sometimes referred to simply as a secondary. See also primary station.

Section DCC

Section Data Communications Channel. In OSS, a 192-kbps data communications channel embedded in the section overhead for OAM&P traffic between two SONET network elements. See also OAM&P and SONET.

security management

One of five categories of network management defined by ISO for management of OSI networks. Security management subsystems are responsible for controlling access to network resources. See also accounting management, configuration management, fault management, and performance management.

seed router

A router in an AppleTalk network that has the network number or cable range built in to its port descriptor. The seed router defines the network number or cable range for other routers in that network segment and responds to configuration queries from nonseed routers on its connected AppleTalk network, allowing those routers to confirm or modify their configurations accordingly. Each AppleTalk network must have at least one seed router. See also nonseed router.

segment

1. Section of a network that is bounded by bridges, routers, or switches.
2. In a LAN using a bus topology, a segment is a continuous electrical circuit that is often connected to other such segments with repeaters.
3. Term used in the TCP specification to describe a single transport layer unit of information. The terms datagram, frame, message, and packet are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.

segmentation and reassembly

See SAR.

Sequenced Packet Exchange

See SPX.

Sequenced Packet Protocol

See SPP.

Sequenced Routing Update Protocol

See SRTP.

serial clock transmit external

See SCTE.

Serial Line Internet Protocol

See SLIP.

serial transmission

Method of data transmission in which the bits of a data character are transmitted sequentially over a single channel. Compare with parallel transmission.

serial tunnel

See STUN in the Cisco Systems Terms and Acronyms section.

server

Node or software program that provides services to clients. See also back end, client, and front end.

Server Message Block

See SMB.

service access point

See SAP.

Service Advertisement Protocol

See SAP.

service data unit

See SDU.

service point

Interface between non-SNA devices and NetView that sends alerts from equipment unknown to the SNA environment.

service profile identifier

See SPID.

Service Specific Connection Oriented Protocol

See SSCOP.

service specific convergence sublayer

See SSCS.

session

1. Related set of communications transactions between two or more network devices.
2. In SNA, a logical connection enabling two NAUs to communicate.

session layer

Layer 5 of the OSI reference model. This layer establishes, manages, and terminates sessions between applications and manages data exchange between presentation layer entities. Corresponds to the data flow control layer of the SNA model. See also application layer, data link layer, network layer, physical layer, presentation layer, and transport layer.

SF

Super Frame. Common framing type used on T1 circuits. SF consists of 12 frames of 192 bits each, with the 193rd bit providing error checking and other functions. SF has been superseded by ESF, but is still widely used. Also called D4 framing. See also ESF.

S-frame

Supervisory frame. One of three SDLC frame formats. See also I-frame and U-frame.

SGMP

Simple Gateway Monitoring Protocol. Network management protocol that was considered for Internet standardization and later evolved into SNMP. Documented in RFC 1028. See also SNMP.

shaping

See traffic shaping.

shielded cable

Cable that has a layer of shielded insulation to reduce EMI.

shielded twisted-pair

See STP.

shortest path first algorithm

See SPF.

shortest-path routing

Routing that minimizes distance or path cost through application of an algorithm.

signaling

Process of sending a transmission signal over a physical medium for purposes of communication.

signaling packet

Generated by an ATM-connected device that wants to establish a connection with another such device. The signaling packet contains the ATM NSAP address of the desired ATM endpoint, as well as any QOS parameters required for the connection. If the endpoint can support the desired QOS, it responds with an accept message, and the connection is opened. See also QOS.

Signaling System 7

See SS7.

signal quality error

See SQE.

silicon switching

See silicon switching in the Cisco Systems Terms and Acronyms section.

silicon switching engine

See SSE in the Cisco Systems Terms and Acronyms section.

Silicon Switch Processor

See SSP in the Cisco Systems Terms and Acronyms section.

simple and efficient AAL

See SEAL.

Simple Gateway Monitoring Protocol

See SGMP.

Simple Mail Transfer Protocol

See SMTP.

Simple Multicast Routing Protocol

See SMRP.

Simple Network Management Protocol

See SNMP.

simplex

Capability for transmission in only one direction between a sending station and a receiving station. Broadcast television is an example of a simplex technology. Compare with full duplex and half duplex.

single-attached concentrator

See SAC.

single attachment station

See SAS.

single-mode fiber

Fiber-optic cabling with a narrow core that allows light to enter only at a single angle. Such cabling has higher bandwidth than multimode fiber, but requires a light source with a narrow spectral width (for example, a laser). Also called monomode fiber. See also multimode fiber.

single-route explorer packet

See spanning explorer packet.

single-vendor network

Network using equipment from only one vendor. Single-vendor networks rarely suffer compatibility problems. See also multivendor network.

SIP

SMDS Interface Protocol. Used in communications between CPE and SMDS network equipment. Allows the CPE to use SMDS service for high-speed WAN internetworking. Based on the IEEE 802.6 DQDB standard. See also DQDB.

sliding window flow control

Method of flow control in which a receiver gives transmitter permission to transmit data until a window is full. When the window is full, the transmitter must stop transmitting until the receiver advertises a larger window. TCP, other transport protocols, and several data link layer protocols use this method of flow control.

SLIP

Serial Line Internet Protocol. Standard protocol for point-to-point serial connections using a variation of TCP/IP. Predecessor of PPP. See also CSLIP and PPP.

slotted ring

LAN architecture based on a ring topology in which the ring is divided into slots that circulate continuously. Slots can be either empty or full, and transmissions must start at the beginning of a slot.

SMAC

source MAC. MAC address specified in the Source Address field of a packet. Compare with DMAC. See also MAC address.

SMB

Server Message Block. File-system protocol used in LAN Manager and similar NOSs to package data and exchange information with other systems.

SMDS

Switched Multimegabit Data Service. High-speed, packet-switched, datagram-based WAN networking technology offered by the telephone companies. See also CBDS.

SMDS Interface Protocol

See SIP.

SMI

Structure of Management Information. Document (RFC 1155) specifying rules used to define managed objects in the MIB. See also MIB.

smoothing

See traffic shaping.

SMRP

Simple Multicast Routing Protocol. Specialized multicast network protocol for routing multimedia data streams on enterprise networks. SMRP works in conjunction with multicast extensions to the AppleTalk protocol.

SMT

Station Management. ANSI FDDI specification that defines how ring stations are managed.

SMTP

Simple Mail Transfer Protocol. Internet protocol providing electronic mail services.

SNA

Systems Network Architecture. Large, complex, feature-rich network architecture developed in the 1970s by IBM. Similar in some respects to the OSI reference model, but with a number of differences. SNA is essentially composed of seven layers. See data flow control layer, data link control layer, path control layer, physical control layer, presentation services layer, transaction services layer, and transmission control layer.

SNA Distribution Services

See SNADS.

SNA Network Interconnection

See SNI.

SNADS

SNA Distribution Services. Consists of a set of SNA transaction programs that interconnect and cooperate to provide asynchronous distribution of information between end users. One of three SNA transaction services. See also DDM and DIA.

SNAP

Subnetwork Access Protocol. Internet protocol that operates between a network entity in the subnetwork and a network entity in the end system. SNAP specifies a standard method of encapsulating IP datagrams and ARP messages on IEEE networks. The SNAP entity in the end system makes use of the services of the subnetwork and performs three key functions: data transfer, connection management, and QOS selection.

SNI

1. Subscriber Network Interface. Interface for SMDS-based networks that connects CPE and an SMDS switch. See also UNI.
2. SNA Network Interconnection. IBM gateway connecting multiple SNA networks.

SNMP

Simple Network Management Protocol. Network management protocol used almost exclusively in TCP/IP networks. SNMP provides a means to monitor and control network devices, and to manage configurations, statistics collection, performance, and security. See also SGMP and SNMP2.

SNMP communities

Authentication scheme that enables an intelligent network device to validate SNMP requests.

SNMP2

SNMP Version 2. Version 2 of the popular network management protocol. SNMP2 supports centralized as well as distributed network management strategies, and includes improvements in the SMI, protocol operations, management architecture, and security. See also SNMP.

SNPA

subnetwork point of attachment. A data link layer address (such as an Ethernet address, X.25 address, or Frame Relay DLCI address). SNPA addresses are used to configure a CLNS route for an interface.

socket

1. Software structure operating as a communications end point within a network device.
2. An addressable entity within a node connected to an AppleTalk network; sockets are owned by software processes known as socket clients. AppleTalk sockets are divided into two groups: SASs, which are reserved for clients such as AppleTalk core protocols, and DASs, which are assigned dynamically by DDP upon request from clients in the node. An AppleTalk socket is similar in concept to a TCP/IP port.

socket client

A software process or function implemented in an AppleTalk network node.

socket listener

Software provided by a socket client to receive datagrams addressed to the socket. See also socket client.

socket number

An 8-bit number that identifies a socket. A maximum of 254 different socket numbers can be assigned in an AppleTalk node.

SONET

Synchronous Optical Network. High-speed (up to 2.5 Gbps) synchronous network specification developed by Bellcore and designed to run on optical fiber. STS-1 is the basic building block of SONET. Approved as an international standard in 1988. See also SDH, STS-1, and STS-3c.

source address

Address of a network device that is sending data. See also destination address.

source MAC

See SMAC.

source-route bridging

See SRB.

source-route translational bridging

See SR/TLB.

source-route transparent bridging

See SRT.

source service access point

See SSAP.

Southeastern Universities Research Association Network

See SURAnet.

SP

See SP in the Cisco Systems Terms and Acronyms section.

span

Full-duplex digital transmission line between two digital facilities.

SPAN

See SPAN in the Cisco Systems Terms and Acronyms section.

spanning explorer packet

Follows a statically configured spanning tree when looking for paths in an SRB network. Also known as a limited-route explorer packet or a single-route explorer packet. See also all-routes explorer packet, explorer packet, and local explorer packet.

spanning tree

Loop-free subset of a network topology. See also spanning-tree algorithm and Spanning-Tree Protocol.

spanning-tree algorithm

Algorithm used by the Spanning-Tree Protocol to create a spanning tree. Sometimes abbreviated STA. See also spanning tree and Spanning-Tree Protocol.

Spanning-Tree Protocol

Bridge protocol that utilizes the spanning-tree algorithm, enabling a learning bridge to dynamically work around loops in a network topology by creating a spanning tree. Bridges exchange BPDU messages with other bridges to detect loops, and then remove the loops by shutting down selected bridge interfaces. Refers to both the IEEE 802.1 Spanning-Tree Protocol standard and the earlier Digital Equipment Corporation Spanning-Tree Protocol upon which it is based. The IEEE version supports bridge domains and allows the bridge to construct a loop-free topology across an extended LAN. The IEEE version is generally preferred over the Digital version. Sometimes abbreviated STP. See also BPDU, learning bridge, MAC address learning, spanning tree, and spanning-tree algorithm.

sparse mode PIM

See PIM sparse mode.

speed matching

Feature that provides sufficient buffering capability in a destination device to allow a high-speed source to transmit data at its maximum rate, even if the destination device is a lower-speed device.

SPF

shortest path first algorithm. Routing algorithm that iterates on length of path to determine a shortest-path spanning tree. Commonly used in link-state routing algorithms. Sometimes called Dijkstra's algorithm. See also link-state routing algorithm.

SPID

service profile identifier. Number that some service providers use to define the services to which an ISDN device subscribes. The ISDN device uses the SPID when accessing the switch that initializes the connection to a service provider.

split-horizon updates

Routing technique in which information about routes is prevented from exiting the router interface through which that information was received. Split-horizon updates are useful in preventing routing loops.

spoofing

1. Scheme used by routers to cause a host to treat an interface as if it were up and supporting a session. The router spoofs replies to keepalive messages from the host in order to convince that host that the session still exists. Spoofing is useful in routing environments such as DDR, in which a circuit-switched link is taken down when there is no traffic to be sent across it in order to save toll charges. See also DDR.
2. The act of a packet illegally claiming to be from an address from which it was not actually sent. Spoofing is designed to foil network security mechanisms such as filters and access lists.

spooler

Application that manages requests or jobs submitted to it for execution. Spoolers process the submitted requests in an orderly fashion from a queue. A print spooler is a common example of a spooler.

SPP

Sequenced Packet Protocol. Provides reliable, connection-based, flow-controlled packet transmission on behalf of client processes. Part of the XNS protocol suite.

SPX

Sequenced Packet Exchange. Reliable, connection-oriented protocol that supplements the datagram service provided by network layer (Layer 3) protocols. Novell derived this commonly used NetWare transport protocol from the SPP of the XNS protocol suite.

SQE

signal quality error. Transmission sent by a transceiver back to the controller to let the controller know whether the collision circuitry is functional. Also called heartbeat.

SRAM

Type of RAM that retains its contents for as long as power is supplied. SRAM does not require constant refreshing, like DRAM. Compare with DRAM.

SRB

source-route bridging. Method of bridging originated by IBM and popular in Token Ring networks. In a SRB network, the entire route to a destination is predetermined, in real time, prior to the sending of data to the destination. Contrast with transparent bridging.

SRT

source-route transparent bridging. IBM bridging scheme that merges the two most prevalent bridging strategies, SRB and transparent bridging. SRT employs both technologies in one device to satisfy the needs of all ENs. No translation between bridging protocols is necessary. Compare with SR/TLB.

SR/TLB

source-route translational bridging. Method of bridging where source-route stations can communicate with transparent bridge stations with the help of an intermediate bridge that translates between the two bridge protocols. Compare with SRT.

SRTP

Sequenced Routing Update Protocol. Protocol that assists VINES servers in finding neighboring clients, servers, and routers. See also RTP (Routing Table Protocol).

SS7

Signaling System 7. Standard CCS system used with BISDN and ISDN. Developed by Bellcore. See also CCS.

SSAP

source service access point. The SAP of the network node designated in the Source field of a packet. Compare to DSAP. See also SAP (service access point).

SSCP

system services control points. Focal points within an SNA network for managing network configuration, coordinating network operator and problem determination requests, and providing directory services and other session services for network end users.

SSCP-PU session

Session used by SNA to allow an SSCP to manage the resources of a node through the PU. SSCPs can send requests to, and receive replies from, individual nodes in order to control the network configuration.

SSCOP

Service Specific Connection Oriented Protocol. A data link protocol that guarantees delivery of ATM signaling packets.

SSCS

service specific convergence sublayer. One of the two sublayers of any AAL. SSCS, which is service dependent, offers assured data transmission. The SSCS can be null as well, in classical IP over ATM or LAN emulation implementations. See also AAL, ATM layer, CPCS, CS, and SAR.

SSE

See SSE in the Cisco Systems Terms and Acronyms section.

SSP

1. Switch-to-Switch Protocol. Protocol specified in the DLSw standard that routers use to establish DLSw connections, locate resources, forward data, and handle flow control and error recovery. See also DLSw.
2. Silicon Switch Processor. See SSP in the Cisco Systems Terms and Acronyms section.

statically assigned socket

See SAS.

STA

See spanning-tree algorithm.

stack

See protocol stack.

standard

Set of rules or procedures that are either widely used or officially specified. See also de facto standard and de jure standard.

standby monitor

Device placed in standby mode on a Token Ring network in case an active monitor fails. See also active monitor and ring monitor.

StarLAN

CSMA/CD LAN, based on IEEE 802.3, developed by AT&T.

star topology

LAN topology in which end points on a network are connected to a common central switch by point-to-point links. A ring topology that is organized as a star implements a unidirectional closed-loop star, instead of point-to-point links. Compare with bus topology, ring topology, and tree topology.

start-stop transmission

See asynchronous transmission.

startup range

A range of values (from 65280 to 65534) from which an AppleTalk node selects the network number part of its provisional address if it has not saved another network number.

static route

Route that is explicitly configured and entered into the routing table. Static routes take precedence over routes chosen by dynamic routing protocols.

Station Management

See SMT.

statistical multiplexing

Technique whereby information from multiple logical channels can be transmitted across a single physical channel. Statistical multiplexing dynamically allocates bandwidth only to active input channels, making better use of available bandwidth and allowing more devices to be connected than with other multiplexing techniques. Also referred to as statistical time-division multiplexing or stat mux. Compare with ATDM, FDM, and TDM.

statistical time-division multiplexing

See statistical multiplexing.

stat mux

See statistical multiplexing.

STM-1

Synchronous Transport Module level 1. One of a number of SDH formats that specifies the frame structure for the 155.52-Mbps lines used to carry ATM cells. See also SDH.

store and forward packet switching

Packet-switching technique in which frames are completely processed before being forwarded out the appropriate port. This processing includes calculating the CRC and checking the destination address. In addition, frames must be temporarily stored until network resources (such as an unused link) are available to forward the message. Contrast with cut-through packet switching.

STP

1. shielded twisted-pair. Two-pair wiring medium used in a variety of network implementations. STP cabling has a layer of shielded insulation to reduce EMI. Compare with UTP. See also twisted pair.
2. See Spanning-Tree Protocol.

Structure of Management Information

See SMI.

STS-1

Synchronous Transport Signal level 1. Basic building block signal of SONET, operating at 51.84 Mbps. Faster SONET rates are defined as STS-n, where n is a multiple of 51.84 Mbps. See also SONET.

STS-3c

Synchronous Transport Signal level 3, concatenated. SONET format that specifies the frame structure for the 155.52-Mbps lines used to carry ATM cells. See also SONET.

stub area

OSPF area that carries a default route, intra-area routes, and interarea routes, but does not carry external routes. Virtual links cannot be configured across a stub area, and they cannot contain an ASBR. Compare to non-stub area. See also ASBR and OSPF.

stub network

Network that has only a single connection to a router.

subarea

Portion of an SNA network that consists of a subarea node and any attached links and peripheral nodes.

subarea node

SNA communication controller or host that handles complete network addresses.

STUN

See STUN in the Cisco Systems Terms and Acronyms section.

subchannel

In broadband terminology, a frequency-based subdivision creating a separate communications channel.

subinterface

One of a number of virtual interfaces on a single physical interface.

subnet

See subnetwork.

subnet address

Portion of an IP address that is specified as the subnetwork by the subnet mask. See also IP address, subnet mask, and subnetwork.

subnet mask

32-bit address mask used in IP to indicate the bits of an IP address that are being used for the subnet address. Sometimes referred to simply as mask. See also address mask and IP address.

subnetwork

1. In IP networks, a network sharing a particular subnet address. Subnetworks are networks arbitrarily segmented by a network administrator in order to provide a multilevel, hierarchical routing structure while shielding the subnetwork from the addressing complexity of attached networks. Sometimes called a subnet. See also IP address, subnet address, and subnet mask.
2. In OSI networks, a collection of ESs and ISs under the control of a single administrative domain and using a single network access protocol.

Subnetwork Access Protocol

See SNAP.

subnetwork point of attachment

See SNPA.

Subscriber Network Interface

See SNI.

subvector

A data segment of a vector in an SNA message. A subvector consists of a length field, a key that describes the subvector type, and subvector specific data.

Super Frame

See SF.

supervisory processor

See RP in the Cisco Systems Terms and Acronyms section.

SURAnet

Southeastern Universities Research Association Network. Network connecting universities and other organizations in the Southeastern United States. SURAnet, originally funded by the NSF and a part of the NSFNET, is now part of BBN Planet. See also BBN Planet, NSF, and NSFNET.

sustainable cell rate

See SCR.

SVC

switched virtual circuit. Virtual circuit that is dynamically established on demand and is torn down when transmission is complete. SVCs are used in situations where data transmission is sporadic. Called a switched virtual connection in ATM terminology. Compare with PVC.

switch

1. Network device that filters, forwards, and floods frames based on the destination address of each frame. The switch operates at the data link layer of the OSI model.
2. General term applied to an electronic or mechanical device that allows a connection to be established as necessary and terminated when there is no longer a session to support.

switched LAN

LAN implemented with LAN switches. See LAN switch.

Switched Multimegabit Data Service

See SMDS.

Switched Port Analyzer

See span.

switched virtual circuit

See SVC.

switched virtual connection

See SVC.

Switch Processor

See Switch Processor in the Cisco Systems Terms and Acronyms section.

Switch-to-Switch Protocol

See SSP.

synchronization

Establishment of common timing between sender and receiver.

Synchronous Data Link Control

See SDLC.

Synchronous Digital Hierarchy

See SDH.

Synchronous Optical Network

See SONET.

synchronous transmission

Term describing digital signals that are transmitted with precise clocking. Such signals have the same frequency, with individual characters encapsulated in control bits (called start bits and stop bits) that designate the beginning and end of each character. Compare with asynchronous transmission, isochronous transmission, and plesiochronous transmission.

Synchronous Transport Module level 1

See STM-1.

Synchronous Transport Signal level 1

See STS-1.

Synchronous Transport Signal level 3, concatenated

See STS-3c.

sysgen

system generation. Process of defining network resources in a network.

system generation

See sysgen.

system services control points

See SSCP.

Systems Network Architecture

See SNA.