Table of Contents
- 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.
- Rate at which samples of a particular waveform amplitude are taken.
- 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.
- 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.
- 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.
- 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.
- Bus technology used in Sun SPARC-based workstations and servers. The SBus specification has been adopted by the IEEE as a new bus standard.
- 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.
- 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.
- 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.
- 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.
- See SDLC broadcast in the Cisco Systems Terms and Acronyms section.
- See SDLC Transport in the Cisco Systems Terms and Acronyms section.
- See SDLLC in the Cisco Systems Terms and Acronyms section.
- 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.
- SMDS DSU. DSU for access to SMDS via HSSIs and other serial interfaces.
- service data unit. Unit of information from an upper-layer protocol that defines a service request to a lower-layer protocol.
- 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.
- See secondary station.
- 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.
- 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 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.
- 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.
- 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.
segmentation and reassembly
- 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.
Sequenced Packet Exchange
- See SAR.
Sequenced Packet Protocol
- See SPX.
Sequenced Routing Update Protocol
- See SPP.
serial clock transmit external
- See SRTP.
Serial Line Internet Protocol
- See SCTE.
- See SLIP.
- Method of data transmission in which the bits of a data character are transmitted sequentially over a single channel. Compare with parallel transmission.
- See STUN in the Cisco Systems Terms and Acronyms section.
Server Message Block
- Node or software program that provides services to clients. See also back end, client, and front end.
service access point
- See SMB.
Service Advertisement Protocol
- See SAP.
service data unit
- See SAP.
- See SDU.
service profile identifier
- Interface between non-SNA devices and NetView that sends alerts from equipment unknown to the SNA environment.
Service Specific Connection Oriented Protocol
- See SPID.
service specific convergence sublayer
- See SSCOP.
- See SSCS.
- 1. Related set of communications transactions between two or more network devices.
2. In SNA, a logical connection enabling two NAUs to communicate.
- 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.
- 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.
- Supervisory frame. One of three SDLC frame formats. See also I-frame and U-frame.
- 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.
- See traffic shaping.
- Cable that has a layer of shielded insulation to reduce EMI.
shortest path first algorithm
- See STP.
- See SPF.
- Routing that minimizes distance or path cost through application of an algorithm.
- Process of sending a transmission signal over a physical medium for purposes of communication.
Signaling System 7
- 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.
signal quality error
- See SS7.
- See SQE.
silicon switching engine
- See silicon switching in the Cisco Systems Terms and Acronyms section.
Silicon Switch Processor
- See SSE in the Cisco Systems Terms and Acronyms section.
simple and efficient AAL
- See SSP in the Cisco Systems Terms and Acronyms section.
Simple Gateway Monitoring Protocol
- See SEAL.
Simple Mail Transfer Protocol
- See SGMP.
Simple Multicast Routing Protocol
- See SMTP.
Simple Network Management Protocol
- See SMRP.
- See SNMP.
- 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 attachment station
- See SAC.
- See SAS.
single-route explorer packet
- 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.
- See spanning explorer packet.
- Network using equipment from only one vendor. Single-vendor networks rarely suffer compatibility problems. See also multivendor network.
sliding window flow control
- 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.
- 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.
- 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.
- 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.
- source MAC. MAC address specified in the Source Address field of a packet. Compare with DMAC. See also MAC address.
- Server Message Block. File-system protocol used in LAN Manager and similar NOSs to package data and exchange information with other systems.
SMDS Interface Protocol
- Switched Multimegabit Data Service. High-speed, packet-switched, datagram-based WAN networking technology offered by the telephone companies. See also CBDS.
- See SIP.
- Structure of Management Information. Document (RFC 1155) specifying rules used to define managed objects in the MIB. See also MIB.
- See traffic shaping.
- 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.
- Station Management. ANSI FDDI specification that defines how ring stations are managed.
- Simple Mail Transfer Protocol. Internet protocol providing electronic mail services.
SNA Distribution Services
- 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 Network Interconnection
- See SNADS.
- See SNI.
- 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.
- 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.
- 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.
- 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.
- Authentication scheme that enables an intelligent network device to validate SNMP requests.
- 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.
- 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.
- 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.
- A software process or function implemented in an AppleTalk network node.
- Software provided by a socket client to receive datagrams addressed to the socket. See also socket client.
- An 8-bit number that identifies a socket. A maximum of 254 different socket numbers can be assigned in an AppleTalk node.
- 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.
- Address of a network device that is sending data. See also destination address.
- See SMAC.
source-route translational bridging
- See SRB.
source-route transparent bridging
- See SR/TLB.
source service access point
- See SRT.
Southeastern Universities Research Association Network
- See SSAP.
- See SURAnet.
- See SP in the Cisco Systems Terms and Acronyms section.
- Full-duplex digital transmission line between two digital facilities.
spanning explorer packet
- See SPAN in the Cisco Systems Terms and Acronyms section.
- 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.
- Loop-free subset of a network topology. See also spanning-tree algorithm and Spanning-Tree Protocol.
- Algorithm used by the Spanning-Tree Protocol to create a spanning tree. Sometimes abbreviated STA. See also spanning tree and Spanning-Tree Protocol.
sparse mode PIM
- 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.
- See PIM sparse mode.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Sequenced Packet Protocol. Provides reliable, connection-based, flow-controlled packet transmission on behalf of client processes. Part of the XNS protocol suite.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Sequenced Routing Update Protocol. Protocol that assists VINES servers in finding neighboring clients, servers, and routers. See also RTP (Routing Table Protocol).
- Signaling System 7. Standard CCS system used with BISDN and ISDN. Developed by Bellcore. See also CCS.
- 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).
- 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.
- 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.
- Service Specific Connection Oriented Protocol. A data link protocol that guarantees delivery of ATM signaling packets.
- 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.
- See SSE in the Cisco Systems Terms and Acronyms section.
statically assigned socket
- 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.
- See SAS.
- See spanning-tree algorithm.
- See protocol stack.
- Set of rules or procedures that are either widely used or officially specified. See also de facto standard and de jure standard.
- Device placed in standby mode on a Token Ring network in case an active monitor fails. See also active monitor and ring monitor.
- CSMA/CD LAN, based on IEEE 802.3, developed by AT&T.
- 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.
- See asynchronous transmission.
- 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.
- Route that is explicitly configured and entered into the routing table. Static routes take precedence over routes chosen by dynamic routing protocols.
- See SMT.
statistical time-division 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.
- See statistical multiplexing.
- See statistical multiplexing.
store and forward packet switching
- 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.
- 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.
Structure of Management Information
- 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.
- See SMI.
- 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.
- 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.
- 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.
- Network that has only a single connection to a router.
- Portion of an SNA network that consists of a subarea node and any attached links and peripheral nodes.
- SNA communication controller or host that handles complete network addresses.
- See STUN in the Cisco Systems Terms and Acronyms section.
- In broadband terminology, a frequency-based subdivision creating a separate communications channel.
- One of a number of virtual interfaces on a single physical interface.
- See subnetwork.
- Portion of an IP address that is specified as the subnetwork by the subnet mask. See also IP address, subnet mask, and subnetwork.
- 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 Access Protocol
- 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 point of attachment
- See SNAP.
Subscriber Network Interface
- See SNPA.
- See SNI.
- 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.
- See SF.
- See RP in the Cisco Systems Terms and Acronyms section.
sustainable cell rate
- 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.
- See SCR.
- 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.
- 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 Multimegabit Data Service
- LAN implemented with LAN switches. See LAN switch.
Switched Port Analyzer
- See SMDS.
switched virtual circuit
- See span.
switched virtual connection
- See SVC.
- See SVC.
- See Switch Processor in the Cisco Systems Terms and Acronyms section.
- See SSP.
Synchronous Data Link Control
- Establishment of common timing between sender and receiver.
Synchronous Digital Hierarchy
- See SDLC.
Synchronous Optical Network
- See SDH.
- See SONET.
Synchronous Transport Module level 1
- 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 Signal level 1
- See STM-1.
Synchronous Transport Signal level 3, concatenated
- See STS-1.
- See STS-3c.
- system generation. Process of defining network resources in a network.
system services control points
- See sysgen.
Systems Network Architecture
- See SSCP.
- See SNA.
Cisco Systems Inc.