label swapping

Routing algorithm used by APPN in which each router that a message passes through on its way to its destination independently determines the best path to the next router.


local-area network. High-speed, low-error data network covering a relatively small geographic area (up to a few thousand meters). LANs connect workstations, peripherals, terminals, and other devices in a single building or other geographically limited area. LAN standards specify cabling and signaling at the physical and data link layers of the OSI model. Ethernet, FDDI, and Token Ring are widely used LAN technologies. Compare with MAN and WAN.


LAN emulation. Technology that allows an ATM network to function as a LAN backbone. The ATM network must provide multicast and broadcast support, address mapping (MAC-to-ATM), SVC management, and a usable packet format. LANE also defines Ethernet and Token Ring ELANs. See also ELAN.

LAN emulation


LAN Emulation Client

See LEC.

LAN Emulation Configuration Server


LAN Emulation Server

See LES.

LAN Extender

Any of the products in the Cisco 1000 series. Cisco LAN Extenders provide a transparent connection between a central site and a remote site, logically extending the central network to include the remote LAN. LAN Extender products support all standard network protocols and are configured and managed through a host router at the central site, requiring no technical expertise at the remote end. See also Cisco 1000.

LAN Manager

Distributed NOS, developed by Microsoft, that supports a variety of protocols and platforms.

LAN Manager for UNIX

See LM/X.

LAN Network Manager

See LNM.

LAN Server

Server-based NOS developed by IBM and derived from LNM. See also LNM.

LAN switch

High-speed switch that forwards packets between data-link segments. Most LAN switches forward traffic based on MAC addresses. This variety of LAN switch is sometimes called a frame switch. LAN switches are often categorized according to the method they use to forward traffic: cut-through packet switching or store-and-forward packet switching. Multilayer switches are an intelligent subset of LAN switches. An example of a LAN switch is the Cisco Catalyst 5000. Compare with multilayer switch. See also cut-through packet switching and store and forward packet switching.


Link Access Procedure, Balanced. Data link layer protocol in the X.25 protocol stack. LAPB is a bit-oriented protocol derived from HDLC. See also HDLC and X.25.


Link Access Procedure on the D channel. ISDN data link layer protocol for the D channel. LAPD was derived from the LAPB protocol and is designed primarily to satisfy the signaling requirements of ISDN basic access. Defined by ITU-T Recommendations Q.920 and Q.921.


Link Access Procedure for Modems. ARQ used by modems implementing the V.42 protocol for error correction. See also ARQ and V.42.


light amplification by stimulated emission of radiation. Analog transmission device in which a suitable active material is excited by an external stimulus to produce a narrow beam of coherent light that can be modulated into pulses to carry data. Networks based on laser technology are sometimes run over SONET.


local-area transport. A network virtual terminal protocol developed by Digital Equipment Corporation.


local access and transport area. Geographic telephone dialing area serviced by a single local telephone company. Calls within LATAs are called "local calls." There are well over 100 LATAs in the United States.


1. Delay between the time a device requests access to a network and the time it is granted permission to transmit.
2. Delay between the time when a device receives a frame and the time that frame is forwarded out the destination port.


line card control. Process that runs on the NP for each CLC, LSC, and MSC of a LightStream 2020 ATM switch. LCC establishes VCCs, maintains the link management protocol for the line card, continually monitors line quality on each trunk using TUD, and performs other functions. See also ECC.


logical channel identifier. See VCN.


logical channel number. See VCN.

leaf internetwork

In a star topology, an internetwork whose sole access to other internetworks in the star is through a core router.

learning bridge

Bridge that performs MAC address learning to reduce traffic on the network. Learning bridges manage a database of MAC addresses and the interfaces associated with each address. See also MAC address learning.

leased line

Transmission line reserved by a communications carrier for the private use of a customer. A leased line is a type of dedicated line. See also dedicated line.


1. LAN Emulation Client. Entity in an end system that performs data forwarding, address resolution, and other control functions for a single ES within a single ELAN. A LEC also provides a standard LAN service interface to any higher-layer entity that interfaces to the LEC. Each LEC is identified by a unique ATM address, and is associated with one or more MAC addresses reachable through that ATM address. See also ELAN and LES.
2. local exchange carrier. Local or regional telephone company that owns and operates a telephone network and the customer lines that connect to it.


LAN Emulation Configuration Server. Entity that assigns individual LANE clients to particular ELANs by directing them to the LES that corresponds to the ELAN. There is logically one LECS per administrative domain, and this serves all ELANs within that domain. See also ELAN.


light emitting diode. Semiconductor device that emits light produced by converting electrical energy. Status lights on hardware devices are typically LEDs.

LEN node

low-entry networking node. In SNA, a PU 2.1 that supports LU protocols, but whose CP cannot communicate with other nodes. Because there is no CP-to-CP session between a LEN node and its NN, the LEN node must have a statically defined image of the APPN network.


LAN Emulation Server. Entity that implements the control function for a particular ELAN. There is only one logical LES per ELAN, and it is identified by a unique ATM address. See also ELAN.

Level 1 router

Device that routes traffic within a single DECnet or OSI area.

Level 2 router

Device that routes traffic between DECnet or OSI areas. All Level 2 routers must form a contiguous network.

light amplification by stimulated emission of radiation

See laser.

light emitting diode

See LED.

limited resource link

Resource defined by a device operator to remain active only when being used.

limited-route explorer packet

See spanning explorer packet.


1. In SNA, a connection to the network.
2. See link.

line card

Card on a LightStream 2020 ATM switch that, together with its access card, provides I/O services for the switch. There are four types of line cards: CLC, LSC, MSC, and PLC.

line card control

See LCC.

line code type

One of a number of coding schemes used on serial lines to maintain data integrity and reliability. The line code type used is determined by the carrier service provider. See also AMI, B8ZS, and HBD3.

line conditioning

Use of equipment on leased voice-grade channels to improve analog characteristics, thereby allowing higher transmission rates.

line driver

Inexpensive amplifier and signal converter that conditions digital signals to ensure reliable transmissions over extended distances.

Line Interface


line of sight

Characteristic of certain transmission systems such as laser, microwave, and infrared systems in which no obstructions in a direct path between transmitter and receiver can exist.

line printer daemon

See LPD.

line turnaround

Time required to change data transmission direction on a telephone line.


Line Interface. Interface card used on the LightStream 100 ATM switch. The LINF receives cells sent over a line, checks them for errors, and forwards them toward their destination.


Network communications channel consisting of a circuit or transmission path and all related equipment between a sender and a receiver. Most often used to refer to a WAN connection. Sometimes referred to as a line or a transmission link.

Link Access Procedure, Balanced


Link Access Procedure for Modems


Link Access Procedure on the D channel


link layer

See data link layer.

link-layer address

See MAC address.

link-state advertisement

See LSA.

link-state packet

See LSA.

link state routing algorithm

Routing algorithm in which each router broadcasts or multicasts information regarding the cost of reaching each of its neighbors to all nodes in the internetwork. Link state algorithms create a consistent view of the network and are therefore not prone to routing loops, but they achieve this at the cost of relatively greater computational difficulty and more widespread traffic (compared with distance vector routing algorithms). Compare with distance vector routing algorithm. See also Dijkstra's algorithm.


Method of storing or transmitting data in which the least significant bit or byte is presented first. Compare with big-endian.


Logical Link Control. Higher of the two data link layer sublayers defined by the IEEE. The LLC sublayer handles error control, flow control, framing, and MAC-sublayer addressing. The most prevalent LLC protocol is IEEE 802.2, which includes both connectionless and connection-oriented variants. See also data link layer and MAC.


Logical Link Control, type 2. Connection-oriented OSI LLC-sublayer protocol. See also LLC.


Local Management Interface. Set of enhancements to the basic Frame Relay specification. LMI includes support for a keepalive mechanism, which verifies that data is flowing; a multicast mechanism, which provides the network server with its local DLCI and the multicast DLCI; global addressing, which gives DLCIs global rather than local significance in Frame Relay networks; and a status mechanism, which provides an on-going status report on the DLCIs known to the switch. Known as LMT in ANSI terminology.


See LMI.


LAN Manager for UNIX. Monitors LAN devices in UNIX environments.


LAN Network Manager. SRB and Token Ring management package provided by IBM. Typically running on a PC, it monitors SRB and Token Ring devices, and can pass alerts up to NetView.

load balancing

In routing, the ability of a router to distribute traffic over all its network ports that are the same distance from the destination address. Good load-balancing algorithms use both line speed and reliability information. Load balancing increases the utilization of network segments, thus increasing effective network bandwidth.

local access and transport area


local acknowledgment

Method whereby an intermediate network node, such as a router, responds to acknowledgments for a remote end host. Use of local acknowledgments reduces network overhead and, therefore, the risk of time-outs. Also known as local termination.

local-area network

See LAN.

local-area transport

See LAT.

local bridge

Bridge that directly interconnects networks in the same geographic area.

local database

See configuration database.

local exchange carrier

See LEC.

local explorer packet

Generated by an end system in an SRB network to find a host connected to the local ring. If the local explorer packet fails to find a local host, the end system produces either a spanning explorer packet or an all-routes explorer packet. See also all-routes explorer packet, explorer packet, and spanning explorer packet.

local loop

Line from the premises of a telephone subscriber to the telephone company CO.

Local Management Interface

See LMI.


Apple proprietary baseband protocol that operates at the data link and physical layers of the OSI reference model. LocalTalk uses CSMA/CD media access scheme and supports transmissions at speeds of 230 Kbps.

local termination

See local acknowledgment.

local traffic filtering

Process by which a bridge filters out (drops) frames whose source and destination MAC addresses are located on the same interface on the bridge, thus preventing unnecessary traffic from being forwarded across the bridge. Defined in the IEEE 802.1 standard. See also IEEE 802.1.

logical address

See network address.

logical channel

Nondedicated, packet-switched communications path between two or more network nodes. Packet switching allows many logical channels to exist simultaneously on a single physical channel.

logical channel identifier

See LCI.

logical channel number

See LCN.

Logical Link Control

See LLC.

Logical Link Control, type 2

See LLC2.

logical unit

See LU.

Logical Unit 6.2

See LU 6.2.


Route where packets never reach their destination, but simply cycle repeatedly through a constant series of network nodes.

loopback test

Test in which signals are sent and then directed back toward their source from some point along the communications path. Loopback tests are often used to test network interface usability.


Characteristic of a network that is prone to lose packets when it becomes highly loaded.

low-entry networking node

See LEN node.

low-speed line card

See LSC.


line printer daemon. Protocol used to send print jobs between UNIX systems.


link-state advertisement. Broadcast packet used by link-state protocols that contains information about neighbors and path costs. LSAs are used by the receiving routers to maintain their routing tables. Sometimes called a link-state packet (LSP).


low-speed line card. Card on the LightStream 2020 ATM switch that can be configured as an edge or a trunk card. An LSC, in conjunction with an access card, supports eight trunk or edge ports (Frame Relay or frame forwarding) at individual port speeds up to 3.584 Mbps, or an aggregate rate of 6 Mbps per line card. See also edge card, MSC, and trunk card.


link-state packet. See LSA.


logical unit. Primary component of SNA, an LU is an NAU that enables end users to communicate with each other and gain access to SNA network resources.

LU 6.2

Logical Unit 6.2. IN SNA, an LU that provides peer-to-peer communication between programs in a distributed computing environment. APPC runs on LU 6.2 devices. See also APPC.


Real-time, UNIX-like operating system that runs on the NP of a LightStream 2020 ATM switch.