S

sag

A momentary drop in the voltage provided by a power source.

SAP (service access point)

A field in a frame that tells the receiving host which protocol the frame is intended for.

SAP (Service Advertising Protocol)

A NetWare protocol that is used on an IPX network. SAP maintains server information tables, listing each service that has been advertised to it, and provides this information to any nodes that attempt to locate a service.

SAP (Service Advertising Protocol) agent

A router or another node on an IPX network that maintains a server information table. This table lists each service that has been advertised to it and provides this information to any nodes that attempt to locate a service.

SAS (Single Attached Station)

In an FDDI system, a device that is attached to only one of the two rings. Compare with DAS.

SCSI (Small Computer System Interface)

A technology defined by a set of standards originally published by ANSI for use with devices on a bus known as a SCSI bus.

SCSI-1

The first set of ANSI standards for small computer systems that called for up to seven devices, known as targets, to be connected to a computer known as an initiator.

SCSI-2

The second set of ANSI standards for small computer systems, published in 1994. An upgrade to the SCSI-1 standard, SCSI-2 provides a synchronous data transfer rate of 2.5Mbps to 10Mbps for an 8-bit data bus and 5Mbps to 20Mbps for a 16-bit (or wide) bus.

SCSI-3

The SCSI standard most widely used today. SCSI-3 splits the 400-plus pages of documents used to describe SCSI-2 into a series of smaller documents. SCSI-3 does not define any particular performance or transfer rate but rather is a set of documents that define the architecture of the updated SCSI specification.

SCSI bus

The high-speed channel between the SCSI devices on a chain. The SCSI bus architecture contains a multithreaded I/O interface that can process multiple I/O requests at the same time.

SCSI bus termination

The use of a set of electrical resistors called terminators at the extreme ends of the SCSI bus to reflect the electrical impulses being transmitted across the bus.

SCSI ID

A number ranging from 0 to 15 that is assigned to a SCSI device to identify the device and its priority when two or more devices are competing for the right to send data on the bus.

secondary name server

A type of DNS server that gets its zone data from another DNS name server that has authority in that zone.

Security log

A log located in the Windows NT 4/2000 Event Viewer that provides information on audit events that the administrator has determined to be security related. These events include logons, attempts to log on, attempts to access areas that are denied, and attempts to log on outside normal hours.

security policy

In general terms, a written policy that defines the rules and regulations pertaining to the security of company data and the use of computer systems. More specifically, the policy configuration on a server system or a firewall that defines the security parameters for a system.

segment

A physical section of a network. Also, a unit of data that is smaller than a packet.

server

A network node that fulfills service requests for clients. Usually referred to by the type of service it performs, such as file server, communications server, or print server.

server-based application

An application that is run from a network share rather than from a copy installed on a local computer.

server-based networking

A network operating system that is dedicated to providing services to workstations, or clients.

See also [client/server networking]

service pack

A software update that fixes multiple known problems and in some cases provides additional functionality to an application or operating system.

session

A dialog between two computers.

session layer

Layer 5 of the OSI model, which establishes, manages, and terminates sessions between applications on different nodes.

shared system

The infrastructure component that is routed directly into the backbone of an internetwork for optimal systems access. It provides connectivity to servers and other shared systems.

shell

An interface, graphical or otherwise, that enables the functionality of an operating system.

SLIP (Serial Line Internet Protocol)

A protocol that uses encapsulation to allow TCP/IP to be transmitted over asynchronous lines, such as standard telephone lines. Previously used for most Internet access, SLIP has been largely replaced by PPP because of SLIP's lack of error-checking capabilities.

SMDS (Switched Multimegabit Data Service)

The physical-layer implementation for data transmission over public lines at speeds between 1.544Mbps (T1) and 44.736Mbps, using cell relay and fixed-length cells. Defined in IEEE 802.6.

SMP (symmetrical multiprocessing)

The utilization of multiple processors on a single system.

SMTP (Simple Mail Transfer Protocol)

An Internet protocol that is used for the transfer of messages and attachments.

SNAP (Subnetwork Access Protocol)

An Internet protocol that specifies a standard method of encapsulating IP datagrams and ARP messages on a network.

SNMP (Simple Network Management Protocol)

A protocol that provides network devices with a method to monitor and control network devices; manage configurations, statistics collection, performance, and security; and report network management information to a management console. SNMP is part of the TCP/IP protocol suite.

SNMP agent

A software component that allows a device to communicate with, and be contacted by, an SNMP management system.

SNMP trap

An SNMP utility that sends an alarm to notify the administrator that something within the network activity differs from the established threshold, as defined by the administrator.

socket

A logical interprocess communications mechanism through which a program communicates with another program or with a network.

socket identifier

An 8-bit number that is used to identify the socket and is used by IPX when it needs to address a packet to a particular process running on a server. The developers and designers of services and protocols usually assign socket identifiers. A socket identifier is also known as a socket number.

SONET (Synchronous Optical Network)

A U.S. standard for data transmission that operates at speeds up to 2.4Gbps over optical networks referred to as OC-x, where x is the level.

source address

The address of the host that sent the frame. The source address is contained in the frame so the destination node knows who sent the data.

source-route bridge

A bridge that is used in source-route bridging to send a packet to the destination node through the route specified by the sending node.

spike

An instantaneous, dramatic increase in the voltage output to a device. Spikes are responsible for much of the damage that is done to network hardware components.

SPX (Sequenced Packet Exchange)

A protocol that is used in conjunction with IPX when guaranteed delivery is required. SPX is used mainly in NetWare network environments.

SSL (Secure Sockets Layer)

A method of securely transmitting information to and receiving information from a remote Web site. SSL is implemented through the HTTPS.

STA (Spanning Tree Algorithm)

A standard that is defined by IEEE 802.1 as part of STP to eliminate loops in an internetwork with multiple paths.

static IP address

An IP address that is assigned to a network device manually, as opposed to dynamically via DHCP.

static routing

A routing method in which all routes must be entered into a device manually and in which no route information is exchanged between routing devices on the network. Compare with dynamic routing.

static window

A mechanism used in flow control that prevents the sender of data from overwhelming the receiver. The amount of data that can be buffered in a static window is configured dynamically by the protocol.

station IPX address

A 12-digit number that is used to uniquely identify each device on an IPX network.

storage area network

A subnetwork of storage devices, usually found on high-speed networks and shared by all servers on a network.

store-and-forward

A fast-packet-switching method that produces a higher latency than other switching methods because the entire contents of the packet are copied into the onboard buffers of the switch. CRC calculations are performed before the packet can be passed on to the destination address.

STP (shielded twisted-pair)

Twisted-pair network cable that has shielding to insulate the cable from EMI.

STP (Spanning Tree Protocol)

A protocol that was developed to eliminate the loops caused by the multiple paths in an internetwork. STP is defined in IEEE 802.1.

subdomain

A privately controlled segment of the DNS namespace that exists under other segments of the namespace as a division of the main domain.

subnet

A logical division of a network, based on the address to which all the devices on the network are assigned.

subnet mask

A 32-bit address that is used to mask, or screen, a portion of an IP address to differentiate the part of the address that designates the network and the part that designates the host from one another.

subnetting

The process of dividing an assigned IP address range into smaller clusters of hosts.

supernetting

The process of aggregating IP network addresses and using them as a single network address range.

Supervisor account

In a NetWare network, a default account that has rights to access everything and to assign rights to other users on the network.

surge

A voltage increase that is less dramatic than that of a spike but can last a lot longer. Sometimes referred to as a swell. The opposite of brownout.

surge protector

An inexpensive and simple device that is placed between a power outlet and a network component to protect the component from spikes and surges. Also known as a surge suppresser.

SVC (switched virtual circuit)

A virtual circuit that is established dynamically on demand to form a dedicated link and is then broken when transmission is complete. Known as a switched virtual connection in ATM terminology.

switch

A Layer 2 networking device that is used in twisted-pair networks. A switch forwards frames based on destination addresses.

SYN

A message that is sent to initiate a TCP session between two devices.

synchronous transmission

A digital signal transmission method that uses a precise clocking method and a predefined number of bits sent at a constant rate.

System log

A log that is located in the Windows NT 4/2000 Event Viewer that provides information on events logged by Windows NT/2000 system components. These events include driver failures, device conflicts, read/write errors, timeouts, and bad block errors.