AAL
See ATM Adaptation Layer.
Access control method
This is the main distinguishing feature between different LAN technologies. It regulates each workstation's physical access to the cable (transmission medium), and determines the order in which nodes gain access so that each user gets efficient service. Access methods include token passing, which is used in token ring and FDDI, and Carrier Sense Multiple Access with Collision Detection (CSMA/CD), which is employed by Ethernet.
Active Monitor for Token Ring
Node on a token ring network (usually a server) which generates the token and sends the token down the ring. The Active Monitor keeps track of the token and makes sure that it does not get lost or corrupted. The active monitor also purges the ring and generates a new token (when necessary), initiates and monitors neighbor notification and maintains the master clock.
Address Mask
Used to select bits from an Internet address for subnet addressing. The mask is 32 bits long and selects the network portion of the Internet address and one or more bits of the local portion. Sometimes called Subnet Mask.
Adjusted Ring Length (ARL)
Calculated to insure that if there is a ring failure, the longest ring path is still within specifications. Generally associated with token ring, Adjusted Ring Length insures that the secondary ring can still function properly in the event of a failure on the shortest trunk cable.
Agent
The portion of the system in the client-server model that performs information preparation and exchange on behalf of a client or server application.
American National Standards Institute (ANSI)
A US standards body. ANSI is a member of the International Organization for Standardization (ISO).
ANSI
See American National Standards Institute.
API
See Applications Program Interface.
Applications Program Interface (API)
Software designed to make a computer's facilities accessible to an application program; all operating systems and network operating systems have APIs. In a networking environment it is essential that various machine's APIs are compatible, otherwise programs would be exclusive to the machines in which they reside. As networking has developed, some APIs have become de facto standards, including NetBIOS and DOS 3.1.
ARQ
See Automatic Repeat Request.
ASM
See ATM Switching Module.
Asynchronous
A method of transmitting data whereby each byte is clocked separately. One start bit and one or more stop bits are added to the beginning and end of each character. Asynchronous transmission is the most rudimentary form of data communication, as the originating and recipient machines do not have to be in sync. It is commonly used for low-speed transmission, as with a PC's serial port.
Asynchronous Transfer Mode (ATM)
A high speed, connection-oriented switching and multiplexing technology for transmitting information across a wide area or local area network. ATM divides information into fixed-length cells capable of transmitting different types of traffic simultaneously including voice, video and data. It is asynchronous in that information streams can be sent independently, without a common clock. ATM can be described logically in three planes. The user plane coordinates the interface between user protocols (such as IP or SMDS) and ATM. The management plane coordinates the layers of the ATM stack. The control plane coordinates signaling, setting up and tearing down virtual circuits. ATM is the high bandwidth networking standard for the late 1990s.
ATM
See Asynchronous Transfer Mode.
ATM Adaptation Layer (AAL)
Divides data communications into cells, giving ATM the versatility to carry many different types of data, from constant-rate voice data to highly bursty messages generated by LANs) all within the same cell format.
ATM Forum
An international consortium of hundreds of companies and users chartered to accelerate the use of ATM products and services by developing specifications and promoting the technology. The ATM Forum is not a standards body, but works in cooperation with standards bodies such as ANSI and ITU.
ATM LAN Emulation (LANE)
A set of protocols developed by the ATM Forum which allows an ATM network to emulate the services provided to applications by traditional LANs. ATM LAN Emulation is needed because ATM is connection-oriented and point-to-point, and therefore unicast, whereas LANs broadcast and all stations need to hear all network transmissions to work within the token passing or CSMA/CD structures.
ATM Peer To Peer Connection
A virtual channel or virtual path connection.
ATM Switching Module (ASM)
Module for a Xylan OmniSwitch which supports various types of ATM traffic. The ASM provides a SAR (Segmentation and Reassembly) function between the frames which pass on an OmniSwitch's backplane and the cells needed in an ATM network.
ATM User to User Connection
An association established by the ATM layer to support direct communication between two or more ATM service users.
ATM-ARP
Resolves MAC to ATM address translation.
Attachment Unit Interface (AUI)
Defined in the IEEE 802.1 specification as the interface between an Ethernet MAU and DTE.
Attenuation
The progressive weakening of a signal as it travels away from its point of origin.
AUI
See Attachment Unit Interface.
Automatic Repeat Request (ARQ)
A type of error correction ensuring a transmitting device automatically resends any data containing errors.
Autonomous System
Internet (TCP/IP) terminology for a collection of gateways (routers) that fall under one administrative entity and cooperate using a common Interior Gateway Protocol (IGP). See subnetwork.
AutoTracker Group
A collection of ports in one or more OmniSwitches which contains a set of VLANs which share a single Spanning Tree. No data can pass between AutoTracker Groups except through a routing process.
AutoTracker Virtual LAN
A Virtual LAN provided by Xylan's AutoTracker function.
Available Bit Rate (ABR)
The bit rate left after the predictive and guaranteed service traffic (CBR/VBR) are served. ABR provides a dynamically negotiated rate.
Audio-Visual Service Specific Convergence Sublayer (AVSSCS)
The ATM Forum is working on a method of transmitting video over ATM Adaptation Layer 5 using Available Bit Rate services. AVSSCS is a proposed convergence protocol for accomplishing this.
CAC
See Connection Admission Control (CAC).
Carrier Sense
The signal provided by the physical layer to the access sublayer which indicates whether one or more stations are currently transmitting.
Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
A contention based network access method in which any computer may attempt to communicate at any time. Since there is no centralized force controlling the medium, a DTE must first sense whether the channel is open. If the channel is open the DTE then transmits. If two computers sense a channel is open and transmit at the same time the result is a collision, after which there is a random pause determined individually by each transmitting machine. Each machine then senses the line again and if it is available retransmits.
Cell
ATM Layer Protocol Data Unit (PDU).
Cell Loss Priority (CLP)
A one bit field in the ATM cell header that determines whether or not a given cell should be dropped by network equipment during periods of congestion. This explicit loss priority can be set by the source node or the network. A CLP which equals zero receives high network priority while a CLP which equals one is dropped during periods of congestion.
Cell Loss Ratio (CLR)
The ratio of discarded cells to cells that are successfully transmitted. Specifically, CLR equals the number of discarded cells over the number of transmitted cells.
Checksum
A computed value which is dependent on the contents of a packet. This value is sent along with the packet when it is transmitted. The receiving system computes a new checksum based on the received data and compares this value with the one sent with the packet. If the two values are the same, the data was received correctly.
Circuit Switching
A communications method whereby a circuit is held open and maintained only while the sender and recipient are communicating, as opposed to a dedicated circuit which is held open regardless of whether data is being sent.
CLNP
See Connectionless Network Protocol.
CLP
See Cell Loss Priority.
CLR
See Cell Loss Ratio.
CLTP
See Connectionless Transport Protocol.
CMIP
See Common Management Information Protocol.
Coaxial Cable
Formerly common in Ethernet networks. Coax used in Ethernet generally comes in a thick or thin variety. Coaxial cable is generally copper based with insulation and a jacket.
Collapsed Backbone
A network architecture in which a building backbone is concentrated in arouter or switch.
Collision
Concurrent Ethernet transmissions from two or more DTEs on the same segment.
Common Management Information Protocol (CMIP)
The OSI network management protocol. See also SNMP.
Concatenation
In general, a packet combination process.
Congestion Control
Mechanisms that control traffic flow so switches and end-stations are not overwhelmed. ATM defines several different schemes. Among them are GFC (Generic Flow Control), CLP field in cell headers and the EFCI (explicit forward congestion indicator) bit in the PTI (payload type identifier). More sophisticated mechanisms are needed to deal with congestion in large ATM networks carrying different types of traffic.
Connection Admission Control
Two mechanisms used to control the set up of virtual circuits. Overbooking, which allows one connection to exceed permissible traffic limits assumes that other active connections are not using the maximum available resources. Full booking limits network access once maximum resources are committed and only adds connections that specify acceptable traffic parameters.
Connection Oriented
The model of interconnection in which communication proceeds through three well-defined phases: connection establishment, data transfer, connection release. Examples X.25, Internet TCP and OSI Transport Protocol.
Connectionless
The model of interconnection in which communication takes place without formal connection establishment. Examples: Internet IP, UDP, CLTP, and OSI CLNP. Some LANS are connection-oriented.
Connectionless Network Protocol (CLNP)
The OSI equivalent to Internet IP, sometimes called ISO IP. It is the OSI protocol for providing OSI Connectionless Network Service (datagram service).
Connectionless Transport Protocol (CLTP)
The OSI equivalent of UDP. It provides for end-to-end transport data addressing (via Transport Selector) and error control (via checksum), but cannot guarantee delivery or provide flow control.
Connector
Hardware installed on cable ends to provide physical and optical cable attachment to a transmitter, receiver or another cable.
Controller
A device which acts as the electrical and logical interface between a host system and a local area network. Often it is a plug-in addition to the equipment and involves software as well as hardware. In standard (thick) Ethernet, the controller is attached to the network bus by way of a transceiver. For Thin net, the controller and transceiver are usually combined - a device between the host and terminals that relays information between them.
Converter
A converter is a repeater that also converts from one media type to another, for example from fiber to token ring or visa versa. Sometimes referred to as a media adapter.
Core Gateway
On the Internet, a key router (gateway, in older Internet terminology). All networks on the Internet must provide a path from a core gateway to the network.
Corporate Utility Networks
Private networks which carry all or nearly all of a company s voice data and video traffic. They take advantage of the price benefits of buying bandwidth in quantity. The bandwidth is then usually divided up among the applications using T1 time division (TDM) circuit multiplexers. Some have used statistical multiplexors (X.25, or more efficient vendor proprietary protocols) for allocating the bandwidth. These private networks can be designed for higher reliability and greater security than public networks.
CRC
See Cyclical Redundancy Check.
CRF(VC) (Virtual Channel Connection Related Function)
A way of saying switch, concentrator, or other network equipment that handles VC/VPs.
CSMA/CD
See Carrier Sense Multiple Access with Collision Detection.
Cyclical Redundancy Check (CRC)
An error checking mechanism for link layer data transmissions. Polynomial calculations are performed using on the number of bits in the message. The bits are then sent along with the data to its recipient. The recipient checks the data it receives and repeats the calculation. If there are any discrepancies between the results of the two calculations, the recipient requests the originator to resend the data.
Data Carrier Equipment (DCE)
The equipment that enables a DTE to communicate over a telephone line or data circuit. The DCE establishes, maintains, and terminates a connection as well as performing the conversions necessary for communications.
Data Terminal Equipment (DTE)
Generally any device receiving and/or originating data on a network. Within the switching context, DTE applies to computers and terminals as distinguished from packet switching network to which they are connected.
Datagram
A self-contained, independent entity of data carrying sufficient information to be routed from its source to the destination computer without reliance on earlier exchanges between the source, the destination computer and the transporting network. See also: Frame, Packet.
DCE
See Data Carrier Equipment.
Dedicated End Station Switching (DESS)
The capability to enhance a workstation's performance by allowing full bandwidth of a legacy LAN connection to the workstation.
DESS
See Dedicated End Station Switching.
Deterministic Multiplexing
Multiplexing whereby each channel takes a predetermined space. Digital systems use Time Division Multiplexing (TDM) while analog systems use Frequency Division Multiplexing.
Domain
In networking, the technical term for a subdivision of the hosts on a network. The division can be physical, as in separate building LANs, or logical, as in giving the hosts in a particular administrative area their own name group even though they are on the same network.
Downlink
The connection between one or more LAN segments and the bridge/router in a collapsed backbone configuration.
DS-1 (Digital [transmission] System 1)
A term which refers to the 1.44 Mbps (US) or 2.108 Mbps (Europe) digital signal carried on a T1 facility.
DS-3 (Digital [transmission] System 3)
A term which refers to the 45 Mbps transmission rate carried on a T3 facility.
DTE
See Data Terminal Equipment.
Duplex
A technique allowing bi-directional simultaneous transmission along a channel. Often referred to as full duplex.
Duplex (two-position) connectors
Used as the physical connections between stations on the FDDI ring to connect fiber optic cables. The connectors are polarized to prevent the transmitting and receiving fibers from becoming inadvertently interchanged.
Dynamic Routing
A procedure for sending messages across a network by which line failure or overload results in message rerouting.
EGP
See Exterior Gateway Protocol.
Encapsulation
The technique used by layered protocols in which a layer adds header information to the protocol data unit (PDU) from the layer above.
Enterprise Hub
Hubs that have high-bandwidth backplanes which can support multiple networks in a single chassis. Expansion modules can be added to the hub which increase the number of available ports, add bridging, routing, network management capabilities or support wide area communications. Also known as third generation hubs.
ESM
See Ethernet Switching Module.
Ethernet
A 10 Mbps CSMA/CD standard originally developed by Xerox to run on thick coaxial cabling. Ethernet has evolved and now runs on thin coaxial, thick coaxial, fiber, and twisted pair cabling and is the most popular standard in use for LAN networking.
Ethernet Switching Module (ESM)
A module in the Xylan OmniSwitch which supports Ethernet traffic. Several types of ESMs are available supporting the various Ethernet physical media including 10BaseT, 10Base2, 10Base5 and 10BaseFL.
Exterior Gateway Protocol (EGP)
A routing protocol used by gateways in two-level internets. EGP is used in the Internet core system. See Core Gateway.
Fast Packet Switching
An emerging, packet-oriented, digital technology that differs from traditional packet switching in a number of ways. The most obvious differentiator is that it transmits all data in a single packet format whether the information is video, voice or data. Fast packet switching uses short, fixed length packets (cells) and is capable of speeds between 100,000 and 1,000,000 packets/second.
Fault Tolerance
The ability of a device to prevent or recover from network and mechanical failures. Key elements of fault tolerance include hot-swap capabilities, out of band management, redundant load-sharing power supplies, passive backplanes, and redundant cooling systems.
FDDI
A network based on a backbone of dual counter-rotating 100 Mbps fiber optic rings. One of the rings is normally designated as the primary ring the other is the secondary ring. This even holds true if one of the point-to-point fiber optic segments becomes disabled. The counter-rotating rings are connected to single-fiber slave rings through concentrators. Bypassing inactive stations is accomplished with fiber optic switches. The concentrator can electronically bypass stations on the slave ring.
FDDI Switching Module (FSM)
A module in the Xylan OmniSwitch which supports FDDI traffic. Several types of FSMs are available, which support the various FDDI physical media, including multimode fiber optic, single mode fiber optic, and unshielded twisted pair cable.
FDM
See Frequency Division Multiplexing.
File Transfer Protocol (FTP)
The Internet Protocol (and program) used to transfer fields between hosts.
Flooding
Transmission of a frame to all devices in a segment or ring (in routed networks) or a virtual LAN (in a virtual LAN-based network). Flooding is performed on broadcasts, multicasts, and frames whose destination address is unknown.
Flow Control
A routine for controlling the transfer of data between two points on a network. This avoids data loss when a recipient's buffer is full. Buffers play an essential role in overall flow control on a network.
Fragmentation
The process in which a protocol data unit is broken into smaller pieces to fit the requirements of a given physical network. The reverse process is termed reassembly.
Frame
A unit of information in a layer two protocol. In LANs, a frame is a MAC-layer unit containing control information and an entire layer three packet. The term packet is sometimes used to mean a frame, but frame is never used to describe a layer three packet.
Frame Relay
An interface designed to provide high speed frame transmission with minimum delay and efficient use of bandwidth. It operates at level two, the Data Link layer, in the OSI seven layer model, and is used in public and private wide area networks gradually replacing X.25.
Frequency Division Multiplexing (FDM)
Method by which the available transmission frequency range is divided into narrower bands; each of these bands, is used for a separate channel. This allows several signals to be sent over the same transmission medium.
FTP
See File Transfer Protocol.
Full Duplex
A communications method in which each end can simultaneously transmit and receive.
Gateway
A combination of hardware and software that interconnects otherwise incompatible networks or networking devices.
Half Duplex
A communications method in which one end transmits while the other receives, then the process is reversed.
Head End
A central point in a broadband network that receives signals on one set of frequency bands and retransmits them on another set of frequencies. The head end is viewed as a central hub. Every transmission on a broadband network must go through the head end.
Header
Control information added to the beginning of a transmitted message containing essential information such as the packet or block address, destination, message number and routing instructions. In an ATM cell, the header is the five bytes that supply addressing and control information, including generic flow control, virtual path identifier, virtual circuit identifier, payload type and cell-loss priority.
Header Error Check (HEC)
An 8-bit Cyclic Redundancy Code (CRC) computed on all fields in an ATM header; capable of detecting single bit and certain multiple bit errors. HEC is used by the Physical Layer for cell delineation.
Hub
The center of a star topology network or cabling system.
Hybrid network
A LAN consisting of a number of topologies and access methods. For example, a network that includes both token ring and Ethernet.
MAC
See Media Access Control.
MAC Address
The hardware address of a device connected to a shared medium. See also: Media Access Control, Ethernet, token ring.
MAC Layer
A sublayer of data link that deals with the issues specific to a particular type of LAN (i.e. 802.3, 802.5, etc.).
MAC-Layer Switching
LAN data routed through a network based on the source and destination addresses contained in the MAC header of the frame.
MAN
See Metropolitan Area Network.
Management Bus
A bus in the Xylan OmniSwitch which provides communication between the Management Processor Module and various Switching Modules.
Management Information Base (MIB)
A collection of objects that can be accessed via a network management protocol. See SNMP and CMIP.
Management Processor Module (MPM)
A module in the Xylan OmniSwitch which provides central functions. The MPM module in the OmniSwitch includes an SNMP agent which communicates with remote network management stations.
Matrix (physical-layer) Switching
Ports in a network hub physically switched from one network connection to another. Physical-layer switching occurs in hardware, rather than software. Disadvantages to Matrix switching are evident in fault tolerance and adaptation to add-ins, moves and changes.
Maximum Lobe Length (MLL)
Term used in token ring to describe the maximum allowable distance between a node and a wire center on a single ring network.
Maximum Tranmission Unit (MTU)
The maximum number of octets that can be carried by the network in one frame or packet.
Media Access Control
Generic term for the way in which workstations gain access to transmission media. Most widely used in reference to LANs.
Meta-Signaling
In Broadband ISDN, it is accepted that the User to Network Interface will include point-to-multipoint configurations. In multipoint configurations, a separate, independent virtual channel for signaling is required between the network and each of the devices on the multipoint interface. The meta-signaling channel is a pre-established management channel which will be used to establish the point-to-point signaling virtual circuits between the network and each of the individual devices on a multipoint interface. The meta-signaling channel will operate on virtual circuit one within each virtual path. Meta-signaling is not required on point-to-point interfaces. In those cases, the signaling protocol (Q.2931) will operate on virtual circuit five.
Metropolitan Area Network
A network spanning a geographical area greater than a LAN, but less than a WAN. IEEE 802.6 specifies protocols and cabling for a MAN, though these standards are changing with the emergence of ATM.
MIB
See Management Information Base.
Microsegmentation
The process of dividing up LAN segments to contain fewer users on a shared media LAN, increasing performance by reducing congestion. It is generally implemented with LAN switches.
Mid-Level Networks
The transit networks that make up the second level of the Internet hierarchy. They connect the sub-networks to the backbone networks. Also known as regionals.
MLL
See Maximum Lobe Length.
MPOA
See Multi Protocol Over ATM.
MTP 3 (Message Transfer Protocol 3)
MTP 3 provides a connectionless message transfer system for passing information across a network. MTP 3 defines the functions and procedures of the signaling system for signaling message handling and signaling network management.
MTU
See Maximum Transfer Unit.
Multi Protocol Over ATM (MPOA)
An emerging ATM protocol developed by the ATM Forum which will, when complete, provide a standard method for the routing of multiple protocols across an ATM network.
Multicast
A form of broadcast where copies of a packet are delivered only to a subset of all possible destinations.
Multicasting
Directing a message or packet to some subset of all stations on a network by the use of a special destination address.
Multiplex
To simultaneously transmit two or more messages on a single channel.
Multiplexor
A device used for division of a transmission facility into two or more subchannels, either by splitting the frequency band into narrower bands or by allotting a common channel to several different transmitting devices one at a time. Also known as a Mux.
Packet
The unit of data sent across a packet-switching network. A packet is always associated with an address header and control information.
Packet Filtering
The ability of a bridge, router or gateway to limit propagation of packets across two or more interconnected networks. Generally the filtered packet's source and destination are known to be on the same network and therefore do not need to pass through the router, bridge or gateway. Packet filtering is typically implemented through a table of observed addresses and their corresponding networks.
Packet Switching
A communications method in which variable length packets are individually routed between hosts. See also: circuit switching, connection oriented, connectionless.
Payload Type
A three bit field in the ATM cell header that indicates whether a cell contains user information or Connection Associated Layer Management information in its payload and also whether a cell experienced congestion.
PCR
See Peak Cell Rate.
PDU
See Protocol Data Unit.
Peak Cell Rate (PCR)
The maximum rate at which cells can be transmitted across a virtual circuit, specified in cells per second and defined by the interval between the transmission of the last bit of one cell and the first bit of the next.
Permanent Virtual Circuits (PVCs)
Links in a connection-oriented network which are established through configuration rather than dynamically.
Phase Jitter
The result of repeaters regenerating a signal which has experienced envelop delay in transmission through electronics and cable.
Point-to-Point Protocol (PPP)
The successor to SLIP, PPP provides router-to-router and host-to-network connections over both synchronous and asynchronous circuits. See SLIP.
Port Based Virtual LAN (PVL)
A virtual LAN which is defined as a set of physical switch ports.
PPP
See Point-to-Point Protocol.
Protocol
A formal description of messages to be exchanged and rules to be followed for two or more systems to exchange information.
Protocol Converter
A device for translating the protocol of one network or device to the corresponding protocol of another network or device. A protocol converter enables equipment with different conventions to communicate with one another.
Protocol Data Units (PDUs)
The data passed from one layer to another.
Proxy
The mechanism whereby one systems fronts for another system in responding to protocol requests. Proxy statements are used in network management to avoid having to implement full protocol stacks in simple devices such as modems.
Proxy ARP
The technique in which one machine, usually a router, answers ARP requests intended for another machine. By faking its identity, the router accepts responsibility for routing packets to the real destination. Proxy ARP allows a site to use a single IP address with two physical networks.
PT
See Payload Type.
PVCs
See Permanent Virtual Circuits.
PVLs
See Port Based Virtual LAN.
Redirect
A packet switching function that routes a call to an alternative network address if the link to the original address is not working. It is carried out by end point switches or higher layer protocols such as ICMP.
Repeater
A device which propagates electrical signals from one segment to another without making routing decisions or providing packet filtering. In the OSI model a repeater is a Physical Layer intermediate system. See bridge and router.
RFC 1577
An Internet Advisory Board protocol which defines a method for routing IP datagrams across an ATM network.
RIF
See Routing Information Field.
Ring
A LAN topology in which each workstation is connected to two other workstations. This forms a loop or ring. Data is sent from workstation to workstation around the loop in the same direction. Each device acts as a repeater by resending messages to other devices. Rings have predictable response time, determined by the number of devices, the more devices, the slower the LAN. Network control is distributed in a ring network. Loss of one device may disable the entire network; however, most rings have a self-healing capacity so that if a node fails, the loop is automatically reconfigured.
Ring Error Monitor for Token Ring
A ring resident function which maintains statistical records of error conditions on the ring operation.
Ring In and Ring Out
The token ring connectors on the MAU that connect the unit to the trunk cabling. The wrap feature is implemented at these interfaces.
Round Trip Delay
A measure of the current delay in a network from request sent to reply received.
Route
The path that network traffic takes from its source to its destination.
Router
A system responsible for making decisions about which of several paths network traffic will follow. To do this it uses a routing protocol to gain information about the network and uses algorithms to choose the best route based on several criteria known as routing metrics. In OSI terminology, a router is a Network Layer intermediate system.
Routing
The process of delivering a message across a network or networks via the most appropriate path.
Routing Domain
A set of routers exchanging routing information within administrative boundaries.
Routing Information Field (RIF)
A field in a token ring or FDDI frame which provides information used by source-routing bridges to move the frame through a network.
SAAL
See Signaling ATM Adaptation Layer.
SAR
See Segmentation and Reassembly.
Segment
An electrically continuous piece of a bus based LAN. Segments can be joined together using repeaters, switches bridges or routers.
Segmentation
Increasing the available bandwidth per node by dividing a network with bridge, switches or routers to decrease the number of nodes on a segment.
Segmentation and Reassembly
The Segmentation and Reassembly Layer is the lower of two sublayers (Convergence Sublayer and SAR) that make up the ATM Adaptation Layer (AAL). The SAR is responsible for mapping data from the AAL Convergence Sublayer into the cell payloads of an ATM cell stream.
Serial Line Internet Protocol (SLIP)
Protocol used in TCP/IP networks for routing packets over a serial connection. Now largely replaced by Point-to-Point Protocol (PPP).
Service Specific Connection Oriented Protocol (SSCOP)
Responsible for providing mechanisms for the establishment, release and monitoring of signaling information exchanged between peer signaling entities.
Shielded Twisted Pair (STP)
Cable resembling coaxial cable that includes one or more sets of cable pairs which have been molded into an insulating material and covered by a braided shielding conductor. STP offers better noise protection than unshielded twisted pair (UTP).
Signaling
Communications between devices to set up calls and tear them down.
Signaling ATM Adaptation Layer (SAAL)
Provides reliable transport of signaling messages (Q.2931) between peer Q.2931 entities. These signaling messages are carried over a PVC.
Simple Network Management Protocol (SNMP)
A protocol designed to manage networking devices.
SNMP
See Simple Network Management Protocol.
Source Route Bridge
A bridge which is capable of processing the Routing Information Field in a token ring or FDDI frame to determine whether or not to forward that particular frame.
Source Routing
A protocol in which participating stations determine the path that frames will follow between them. Source routing is used in some, but not all, token ring and FDDI LANs.
Spanning Tree
A protocol specified in the IEEE 802.1D standard which allows a network to have a topology which contains physical loops. Spanning Tree operates in transparent bridges, often called Spanning Tree bridges, and opens certain paths to prevent packets from looping endlessly on the network.
Spanning Tree Domain
A portion of a network in which a single Spanning Tree operates.
Standby Monitor
Any 802.5 token ring adapter currently attached (active) to the ring which is not the active monitor. Only one active monitor can be present at any one time on the IEE 802.5 ring.
Star
A network topology where each node is connected to a central hub. The hub establishes, maintains and terminates all connections between nodes.
Store and Forward
A method of switching messages in which a message from one workstation is received at a device. The device then stores the message, determines its ultimate destination, finds an available circuit and then forwards it. Many routers work in this manner.
STP
See Shielded Twisted Pair.
Subnet
A portion of a network, which may be a physically independent network segment, that shares a network address with other portions of the network and is distinguished by a subnet number.
Subnet Address
The subnet portion of an IP address.
Subnet Mask
See Address Mask.
Subnetwork
A collection of OSI end systems and intermediate systems under the control of a single administrative domain and using a single network access protocol.
Sustainable Cell Rate
The maximum throughput bursty traffic can achieve within a given virtual circuit without risking cell loss.
SVCs
See Switched Virtual Circuits.
Switched Virtual Circuits (SVCs)
Connections in a connection-oriented network which are established dynamically, rather than through network configuration. SVCs are set up through a protocol which operates between a switch and an end station, and between switches
Synchronous
Signals that are sourced from the same timing reference and have the same frequency.
Synchronous Optical Network (SONET)
A set of standards for data communication over fiber optic cable at speeds of 51.84 Mbps and above.
Synchronous Transfer Mode
B-ISDN communications method that transmits a group of different data streams synchronized to a single reference clock.
Synchronous Transmission
A transmission method in which characters are synchronized and controlled by timing signals generated at the sending and receiving stations (as opposed to start/stop communications). Both stations operate continuously at the same frequency and are maintained in a desired phase relationship. Any of several data codes may be used for the transmission, as long as the code uses the required line control characters. (Also called bi-sync or binary synchronous ).
TCP
See Transmission Control Protocol.
TCP/IP
Generally used in reference to the various protocols which support the Internet and many private networks.
TDM
See Time Division Multiplexing.
Time Division Multiplexing (TDM)
A transmission scheme in which signals from various sources are transmitted "simultaneously" by allocating time slices in sequence to each of the signals.
Token
A unique packet that is passed around a token ring or FDDI LAN continuously. As it passes, the token gives each workstation the go-ahead to transmit data.
Token Bus
A LAN with a bus topology that uses token passing as its access method.
Token Passing
An access method in which a token is passed from node to node, thereby granting permission to transmit. A transmitting workstation attaches its message to the token which then conveys the data around the ring until it finds its ultimate destination. When the recipient has accepted the message, it releases the token so that the next workstation that needs to transmit can use it.
Token Ring
A network architecture standardized in IEEE 802.5 by which the workstations on a ring are given leave to transmit data while they are in possession of a token which passes from node to node continuously. Token ring operates at 4 or 16 Mbps.
Token Ring Switching Module (TSM)
A module in the Xylan OmniSwitch which supports token ring traffic.
Topology
Can be either physical or logical. Physical topology describes the physical connections of a network and the geometric arrangement of links and nodes that make up that network. Logical topology describes the possible logical connections between nodes, and indicates which pairs of nodes are able to communicate.
Transit Network
A transit network passes traffic between networks in addition to carrying traffic for its own hosts. It must have paths to at least two other networks.
Transmission Control Protocol (TCP)
A layer-four protocol in the set of protocols which support the Internet and many private networks. TCP provides a guaranteed transport service.
Tree
A LAN topology in which there is only one route between any two of the nodes on the network. The pattern of connections resembles a tree.
Trunk
A single circuit between two points, both of which are switching centers and/or individual distribution points.
Trunking Protocol
A protocol which encapsulates frames in such a way that their virtual LAN assignments can be interpreted at the receiving switch, router or server.
Twisted Pair
Insulated copper wires twisted together with the twists or lays varied in length to reduce potential signal interference between the pairs. They are usually bundled together and wrapped in a cable sheath. New data grade Unshielded Twisted Pair (Category 5) is specified for 100 Mbps transmission.