Glossary M

LAN: local area network: Broadly, any network linking two or more computers and related devices within a limited geographical area (up to a few kilometers). LANs are typically high- speed, low-error networks within a company. Cabling and signaling at the physical and Data Link layers of the OSI are dictated by LAN standards. Ethernet, FDDI, and Token Ring are among the most popular LAN technologies. Compare to: MAN.
LANE: LAN emulation: The technology that enables an ATM network to operate as a LAN backbone. To do so, the ATM network is required to provide multicast and broadcast support, address mapping (MAC-to-ATM), and SVC management, in addition to an operable packet format. Additionally, LANE defines Ethernet and Token Ring ELANs. See also: ELAN.
LAN switch: A high-speed, multiple-interface transparent bridging mechanism, transmitting packets between segments of data links, usually referred to specifically as an Ethernet switch. LAN switches transfer traffic based on MAC addresses. See also: multi-layer switch and store- and-forward packet switching.
LAPB: Link Accessed Procedure, Balanced: A bit-oriented Data Link layer protocol that is part of the X.25 stack and has its origin in SDLC. See also: SDLC and X.25.
LAPD: Link Access Procedure on the D channel. The ISDN Data Link layer protocol used specifically for the D channel and defined by ITU-T Recommendations Q.920 and Q.921. LAPD evolved from LAPB and is created to comply with the signaling requirements of ISDN basic access.
latency: Broadly, the time it takes a data packet to get from one location to another. In specific networking contexts, it can mean either (1) the time elapsed (delay) between the execution of a request for access to a network by a device and the time the mechanism actually is permitted transmission, or (2) the time elapsed between when a mechanism receives a frame and the time that frame is forwarded out of the destination port.
laws of physics delay: Delays that cannot be altered, such as the speed of electrical current traveling in a wire, or light in a fiber.
layer 2 switching: Layer 2 switching is hardware based, which means it uses the MAC address from the hosts' NIC cards to filter the network. Switches use Application-Specific Integrated Circuits (ASICs) to build and maintain filter tables. It is OK to think of a layer 2 switch as a multiport bridge.
layer 3 switch: See: multi-layer switch.
layer 3 switching: A switching decision made with a layer 3 address as opposed to a MAC address.
layer 3 switching module: A card or interface module performing layer 3 switching.
layer 4 switching: A switching decision made with port and protocol or IPX socket information in addition to a layer 3 address.
layered architecture: Industry standard way of creating applications to work on a network. Layered architecture allows the application developer to make changes in only one layer instead of the whole program.
LCP: Link Control Protocol: The protocol designed to establish, configure, and test Data Link connections for use by PPP. See also: PPP.
leaf router: The name given to a router occupying a specific topological location in a multicast network. A leaf router has no downstream PIM neighbors. Leaf routers always send PIM Dense Mode 'prune' messages.
leaky bucket: An analogy for the basic (generic) cell rate algorithm (GCRA) used in ATM networks for checking the conformance of cell flows from a user or network. The bucket's 'hole' is understood to be the prolonged rate at which cells can be accommodated, and the 'depth' is the tolerance for cell bursts over a certain time period.
learning bridge: A bridge that transparently builds a dynamic database of MAC addresses and the interfaces associated with each address. Transparent bridges help to reduce traffic congestion on the network.
LE ARP: LAN Emulation Address Resolution Protocol: The protocol providing the ATM address that corresponds to a MAC address.
leased lines: Permanent connections between two points leased from the telephone companies.
LEC: LAN Emulation Client: Software providing the emulation of the Link layer interface that allows the operation and communication of all higher-level protocols and applications to continue. The LEC client runs in all ATM devices, which include hosts, servers, bridges, and routers. The LANE client is responsible for address resolution, data transfer, address caching, interfacing to the emulated LAN, and driver support for higher-level services. See also: ELAN and LES.
LECS: LAN Emulation Configuration Server: An important part of emulated LAN services, providing the configuration data that is furnished upon request from the LES. These services include address registration for Integrated Local Management Interface (ILMI) support, configuration support for the LES addresses and their corresponding emulated LAN identifiers, and an interface to the emulated LAN. See also: LES and ELAN.
LES: LAN Emulation Server: The central LANE component that provides the initial configuration data for each connecting LEC. The LES typically is located on either an ATM-integrated router or a switch. Responsibilities of the LES include configuration and support for the LEC, address registration for the LEC, database storage and response concerning ATM addresses, and interfacing to the emulated LAN. See also: ELAN, LEC, and LECS.
line loss: Losses associated with a cable. These could be because of poor quality installation or local interference.
link compression: See: compression.
link-state routing algorithm: A routing algorithm that enables each router to broadcast or multicast information regarding the cost of reaching all its neighbors to every node in the internetwork. Link-state algorithms provide a consistent view of the network and are therefore not vulnerable to routing loops. However, this is achieved at the cost of somewhat greater difficulty in computation and more widespread traffic (compared with distance-vector routing algorithms). See also: distance-vector routing algorithm.
LLC: Logical Link Control: Defined by the IEEE, the higher of two Data Link layer sublayers. LLC is responsible for error detection (but not correction), flow control, framing, and software- sublayer addressing. The predominant LLC protocol, IEEE 802.2, defines both connectionless and connection-oriented operations. See also: Data Link layer and MAC.
LMI: An enhancement to the original Frame Relay specification. Among the features it provides are a keep-alive mechanism, a multicast mechanism, global addressing, and a status mechanism.
LNNI: LAN Emulation Network-to-Network Interface: In the Phase 2 LANE specification, an interface that supports communication between the server components within one ELAN.
local explorer packet: In a Token Ring SRB network, a packet generated by an end system to find a host linked to the local ring. If no local host can be found, the end system will produce one of two solutions: a spanning explorer packet or an all-routes explorer packet.
local loop: Connection from a demarcation point to the closest switching office.
local services: Users trying to get to network services that are located on the same subnet or network are defined as local services. Users do not cross layer 3 devices, and the network services are in the same broadcast domain as the users. This type of traffic never crosses the backbone.
local VLAN: A VLAN configured by geographic location; this location can be a building or just a closet in a building, depending on switch size. Geographically configured VLANs are designed around the fact that the business or corporation is using centralized resources, such as a server farm.
loop avoidance: If multiple connections between switches are created for redundancy, network loops can occur. STP is used to stop network loops and allow redundancy.
LSA: Link State Advertisement: Contained inside of link-state packets (LSPs), these advertisements are usually multicast packets, containing information about neighbors and path costs, that are employed by link-state protocols. Receiving routers use LSAs to maintain their link- state databases and, ultimately, routing tables.
LUNI: LAN Emulation User-to-Network Interface: Defining the interface between the LAN Emulation Client (LEC) and the LAN Emulation Server, LUNI is the ATM Forum's standard for LAN emulation on ATM networks. See also: LES and LECS.
LZW algorithm: A data compression process named for its inventors, Lempel, Ziv, and Welch. The algorithm works by finding longer and longer strings of data to compress with shorter representations.

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