Glossary T

S

S reference point

ISDN reference point that works with a T reference point to convert a four- wire ISDN network to the two-wire ISDN network needed to communicate with the ISDN switches at the network provider.

sampling rate

The rate at which samples of a specific waveform amplitude are collected within a specified period of time.

SAP

(1) Service Access Point: A field specified by IEEE 802.2 that is part of an address specification. See also: DSAP and SSAP. (2) Service Advertising Protocol: The Novell NetWare protocol that supplies a way to inform network clients of resources and services availability on a network, using routers and servers. See also: IPX.

SCR

Sustainable Cell Rate: An ATM Forum parameter used for traffic management, it is the long-term average cell rate for VBR connections that can be transmitted.

scripts

A script predefines commands that should be issued in sequence, typically to complete a connection or accomplish a repetitive task.

SDH

Synchronous Digital Hierarchy: The CCITT/ITU-TS standard for transport of data over fiber. Designed for worldwide use, SDH is partly compatible with SONET. The base rate for transmission is 155.52 Mbps. Widely used in Europe, although the standard is international.

SDLC

Synchronous Data Link Control: A protocol used in SNA Data Link layer communications. SDLC is a bit-oriented, full-duplex serial protocol that is the basis for several similar protocols, including HDLC and LAPB. See also: HDLC and LAPB.

security policy

Document that defines the business requirements and processes that are to be used to protect corporate data. A security policy might be as generic as 'no file transfers allowed' to very specific, such as 'FTP puts allowed only to server X.'

security server

A centralized device that authenticates access requests, typically via a protocol such as TACACS+ or RADIUS. See also: TACACS+ and RADIUS.

serialization delay

The delay associated with frames arriving at an interface. A function of the frame length and the arrival rate.

server

Hardware and software that provide network services to clients.

set-based

Set-based routers and switches use the set command to configure devices. Cisco is moving away from set-based commands and is using the command-line interface (CLI) on all new devices.

Session layer

Layer 5 of the OSI reference model, responsible for creating, managing, and terminating sessions between applications and overseeing data exchange between Presentation layer entities. See also: Application layer, Data Link layer, Network layer, Physical layer, Presentation layer, and Transport layer.

setup mode

Mode that a router will enter if no configuration is found in non-volatile RAM when the router boots. Enables the administrator to configure a router step-by-step. Not as robust or flexible as the command-line interface.

SF

super frame: A super frame (also called a 'D4 frame') consists of 12 frames with 192 bits each, and the 193rd bit providing other functions including error checking. SF is frequently used on T1 circuits. A newer version of the technology is Extended Super Frame (ESF), which uses 24 frames. See also: ESF.

shared memory

A type of switch-fabric where the input and output queues are the same memory blocks but with separate pointers.

shared trees

A method of multicast data forwarding. Shared trees use an architecture in which multiple sources share a common rendezvous point.

signaling packet

An informational packet created by an ATM-connected mechanism that wants to establish a connection with another such mechanism. The packet contains the QoS parameters needed for connection and the ATM NSAP address of the endpoint. The endpoint responds with a message of acceptance if it is able to support the desired QoS, and the connection is established. See also: QoS.

silicon switching

A type of high-speed switching used in Cisco 7000 series routers, based on the use of a separate processor (the Silicon Switch Processor, or SSP). See also: SSE.

simplex

The mode with which data is transmitted. Simplex is a way of transmitting in only one direction. Half duplex transmits in two directions, but only one direction at a time. Full duplex transmits in both directions simultaneously.

SLB

Server Load Balancing: A methodology for load balancing servers. A server farm is allocated a virtual address for clients to connect to and the router shares connections across the farm. See also: HSRP.

sliding window

The method of flow control used by TCP, as well as several Data Link layer protocols. This method places a buffer between the receiving application and the network data flow. The 'window' available for accepting data is the size of the buffer minus the amount of data already there. This window increases in size as the application reads data from it and decreases as new data is sent. The receiver sends the transmitter announcements of the current window size, and it may stop accepting data until the window increases above a certain threshold.

SLIP

Serial Line Internet Protocol: An industry standard serial encapsulation for point-to- point connections that supports only a single routed protocol, TCP/IP. SLIP is the predecessor to PPP. See also: PPP.

SMDS

Switched Multimegabit Data Service: A packet-switched, datagram-based WAN networking technology offered by telephone companies that provides high speed.

SMTP

Simple Mail Transfer Protocol: A protocol used on the Internet to provide electronic mail services.

SNA

System Network Architecture: A complex, feature-rich network architecture with several variations; created by IBM in the 1970s and composed of seven layers closely resembling the OSI reference model.

SNAP

Subnetwork Access Protocol: SNAP is a frame used in Ethernet, Token Ring, and FDDI LANs. Data transfer, connection management, and QoS selection are three primary functions executed by the SNAP frame.

snapshot routing

Snapshot routing takes a point-in-time capture of a dynamic routing table and maintains it even when the remote connection goes down. This allows the use of a dynamic routing protocol without requiring the link to remain active, which might incur per-minute usage charges.

socket

(1) A software structure that operates within a network device as a destination point for communications. (2) In AppleTalk networks, an entity at a specific location within a node; AppleTalk sockets are conceptually similar to TCP/IP ports.

SOHO

Small office, home office: A contemporary term for remote users.

SONET

Synchronous Optical Network: The ANSI standard for synchronous transmission on fiber-optic media, developed at Bell Labs. It specifies a base signal rate of 51.84Mbps and a set of multiples of that rate, known as Optical Carrier levels, up to 2.5Gbps.

source-route bridging

See: SRB.

source trees

A method of multicast data forwarding. Source trees use the architecture of the source of the multicast traffic as the root of the tree.

SP

Switch Processor: Also known as a 'ciscoBus controller,' it is a Cisco 7000 series processor module acting as governing agent for all CxBus activities.

span

A full-duplex digital transmission line connecting two facilities.

SPAN

Switch Port Analyzer: A feature of the Catalyst 5000 switch, offering freedom to manipulate within a switched Ethernet environment by extending the monitoring ability of the existing network analyzers into the environment. At one switched segment, the SPAN mirrors traffic onto a predetermined SPAN port, while a network analyzer connected to the SPAN port is able to monitor traffic from any other Catalyst switched port.

spanning explorer packet

Sometimes called 'limited-route explorer packet' or 'single-route explorer packet,' it pursues a statically configured spanning tree when searching for paths in a source-route bridging network. See also: all-routes explorer packet, explorer packet, and local explorer packet.

spanning tree

A subset of a network topology, within which no loops exist. When bridges are interconnected into a loop, the bridge, or switch, cannot identify a frame that has been forwarded previously, so there is no mechanism for removing a frame as it passes the interface numerous times. Without a method of removing these frames, the bridges continuously forward them-consuming bandwidth and adding overhead to the network. Spanning trees prune the network to provide only one path for any packet. See also: Spanning Tree Protocol and spanning-tree algorithm.

spanning-tree algorithm (STA)

An algorithm that creates a spanning tree using the Spanning Tree Protocol (STP). See also: spanning tree and Spanning Tree Protocol.

Spanning Tree Protocol (STP)

The bridge protocol (IEEE 802.1D) that enables a learning bridge to dynamically avoid loops in the network topology by creating a spanning tree, using the spanning-tree algorithm. Spanning-tree frames called Bridge Protocol Data Units (BPDUs) are sent and received by all switches in the network at regular intervals. The switches participating in the spanning tree don't forward the frames; instead, they're processed to determine the spanning- tree topology itself. Cisco Catalyst series switches use STP 802.1D to perform this function. See also: BPDU, learning bridge, MAC address, spanning tree, and spanning-tree algorithm.

SPF

Shortest Path First algorithm: A routing algorithm used to decide on the shortest-path spanning tree. Sometimes called 'Dijkstra's algorithm' and frequently used in link-state routing algorithms. See also: link-state routing algorithm.

SPID

Service Profile Identifier: A number assigned by service providers or local telephone companies and assigned by administrators to a BRI port. SPIDs are used to determine subscription services of a device connected via ISDN. ISDN devices use SPID when accessing the telephone company switch that initializes the link to a service provider.

split horizon

Useful for preventing routing loops, a type of distance-vector routing rule where information about routes is prevented from leaving the router interface through which that information was received.

spoofing

(1) In dial-on-demand routing (DDR), where a circuit-switched link is taken down to save toll charges when there is no traffic to be sent, spoofing is a scheme used by routers that causes a host to treat an interface as if it were functioning and supporting a session. The router pretends to send 'spoof' replies to keep-alive messages from the host in an effort to convince the host that the session is up and running. See also: DDR. (2) The illegal act of sending a packet labeled with a false address, in order to deceive network security mechanisms such as filters and access lists.

spooler

A management application that processes requests submitted to it for execution in a sequential fashion from a queue. A good example is a print spooler.

SPX

Sequenced Packet Exchange: A Novell NetWare transport protocol that augments the datagram service provided by Network layer (layer 3) protocols, it was derived from the Switch- to-Switch Protocol of the XNS protocol suite.

SQE

Signal Quality Error: In an Ethernet network, a message sent from a transceiver to an attached machine that the collision-detection circuitry is working.

SRB

source-route bridging: Created by IBM, the bridging method used in Token Ring networks. The source determines the entire route to a destination before sending the data and includes that information in route information fields (RIF) within each packet. Contrast with: transparent bridging.

SRT

source-route transparent bridging: A bridging scheme developed by IBM, merging source-route and transparent bridging. SRT takes advantage of both technologies in one device, fulfilling the needs of all end nodes. Translation between bridging protocols is not necessary. Compare to: SR/TLB.

SR/TLB

source-route translational bridging: A bridging method that enables source-route stations to communicate with transparent bridge stations aided by an intermediate bridge that translates between the two bridge protocols. Used for bridging between Token Ring and Ethernet. Compare to: SRT.

SS-7 signaling

Signaling System 7: The current standard for telecommunications switching control signaling. This is an out-of-band signaling that establishes circuits and provides billing information.

SSAP

Source Service Access Point: The SAP of the network node identified in the Source field of the packet. See also: DSAP and SAP.

SSE

Silicon Switching Engine: The software component of Cisco's silicon switching technology, hard-coded into the Silicon Switch Processor (SSP). Silicon switching is available only on the Cisco 7000 with an SSP. Silicon-switched packets are compared to the silicon-switching cache on the SSE. The SSP is a dedicated switch processor that offloads the switching process from the route processor, providing a fast-switching solution, but packets must still traverse the backplane of the router to get to the SSP and then back to the exit interface.

SSM

Source Specific Multicast: An extension to the PIM protocol that removes the problem of finding the best server without having to resort to MSDP source discovery. SSM requires the network be running IGMPv3. See also: PIM, MSDP, and IGMPv3.

Stac

A compression method developed by Stacker Corporation for use over serial links.

Standard Image (SI)

The basic operating system used on some Catalyst 2950 switches. See also: Enhanced Image.

standard IP access list

IP access list that uses only the source IP addresses to filter a network.

standard IPX access list

IPX access list that uses only the source and destination IPX address to filter a network.

star topology

A LAN physical topology with endpoints on the network converging at a common central switch (known as a hub) using point-to-point links. A logical ring topology can be configured as a physical star topology using a unidirectional closed-loop star rather than point-to-point links. That is, connections within the hub are arranged in an internal ring. Contrast with: bus topology and ring topology.

state transitions

Digital signaling scheme that reads the 'state' of the digital signal in the middle of the bit cell. If it is five volts, the cell is read as a one. If the state of the digital signal is zero volts, the bit cell is read as a zero.

static route

A route whose information is purposefully entered into the routing table and takes priority over those chosen by dynamic routing protocols.

static VLAN

VLAN that is manually configured port-by-port. This is the method typically used in production networks.

statistical multiplexing

Multiplexing in general is a technique that enables data from multiple logical channels to be sent across a single physical channel. Statistical multiplexing dynamically assigns bandwidth only to input channels that are active, optimizing available bandwidth so that more devices can be connected than with other multiplexing techniques. Also known as 'statistical time-division multiplexing' or 'stat mux.' Contrast with ATDM, FDM, and TDM.

STM-1

Synchronous Transport Module Level 1. In the European SDH standard, one of many formats identifying the frame structure for the 155.52Mbps lines that are used to carry ATM cells.

store-and-forward

See: store-and-forward packet switching.

store-and-forward packet switching

A technique in which the switch first copies each packet into its buffer and performs a cyclic redundancy check (CRC). If the packet is error-free, the switch then looks up the destination address in its filter table, determines the appropriate exit port, and sends the packet.

STP

(1) shielded twisted-pair: A two-pair wiring scheme, used in many network implementations, that has a layer of shielded insulation to reduce EMI. (2) Spanning Tree Protocol.

stub area

An OSPF area carrying a default route, intra-area routes, and interarea routes, but no external routes. Configuration of virtual links cannot be achieved across a stub area, and stub areas are not allowed to contain an ASBR. See also: non-stub area, ASBR, and OSPF.

stub network

A network having only one connection to a router.

STUN

Serial Tunnel: A technology used to connect an HDLC link to an SDLC link over a serial link.

subarea

A portion of an SNA network made up of a subarea node and its attached links and peripheral nodes.

subarea node

An SNA communications host or controller that handles entire network addresses.

subchannel

A frequency-based subdivision that creates a separate broadband communications channel.

subinterface

One of many virtual interfaces available on a single physical interface.

subnet

See: subnetwork.

subnet address

The portion of an IP address that is specifically identified by the subnet mask as the subnetwork. See also: IP address, subnetwork, and subnet mask.

subnet mask

Also simply known as 'mask,' a 32-bit address mask used in IP to identify the bits of an IP address that are used for the subnet address. Using a mask, the router does not need to examine all 32 bits, only those selected by the mask. See also: address mask and IP address.

subnetwork

(1) Any network that is part of a larger IP network and is identified by a subnet address. A network administrator segments a network into subnetworks in order to provide a hierarchical, multilevel routing structure, and at the same time protect the subnetwork from the addressing complexity of networks that are attached. Also known as a 'subnet.' See also: IP address, subnet mask, and subnet address. (2) In OSI networks, the term specifically refers to a collection of computing devices controlled by only one administrative domain, using a solitary network connection protocol.

subscribed

A host that has sent an IGMP join message is said to have subscribed to the group.

SVC

switched virtual circuit: A dynamically established virtual circuit, created on demand and dissolved as soon as transmission is over and the circuit is no longer needed. In ATM terminology, it is referred to as a switched virtual connection. Compare to: PVC.

switch

(1) In networking, a device responsible for multiple functions such as filtering, flooding, and sending frames. It works by using the destination address of individual frames. Switches operate at the Data Link layer of the OSI model. (2) Broadly, any electronic/mechanical device enabling connections to be established as needed and terminated if no longer necessary.

switch block

An arrangement of layer 2 switches connecting users in the wiring closet into the access of the network.

switch-fabric

The heart of the switch, where the actual switching process takes place. See also: bus, shared memory, and crossbar.

switched Ethernet

A device that switches Ethernet frames between segments by filtering on hardware addresses.

switched LAN

Any LAN implemented by using LAN switches. See also: LAN switch.

switching fabric

The central functional block of any switch design; responsible for buffering and routing the incoming data to the appropriate output ports.

Synchronous Digital Hierarchy

See: SDH.

synchronous transmission

Signals transmitted digitally with precision clocking. These signals have identical frequencies and contain individual characters encapsulated in control bits (called start/stop bits) that designate the beginning and ending of each character. Contrast with: asynchronous transmission and isochronous transmission.