L2TP (Layer 2 Tunneling Protocol) 

L2TP is a tunneling protocol that delivers PPP (point-to-point protocol) sessions across an ATM network, frame relay network, or the Internet.                                                           

L2TP can help reduce the cost of remote dial-up networking for users who normally dial into a corporate network over a long-distance connection. L2TP is often called a “virtual dial-up protocol” because it extends a dial-up PPP session across the Internet. Consider the traditional dial-up session without tunneling: a remote user in Los Angeles who needs to connect with the corporate network in New York dials the home office remote access telephone  number. A dedicated circuit is created across the PSTN from L.A. to New York. Obviously, this is not a cost-effective way to access the corporate network. In addition, the long-distance call does not meet the digital requirements for V.90 modems, so the data rate is usually around 33 Kbits/sec or worse, not the 56 Kbits/sec rate that is possible with V.90 modems.

Label Switching

Label switching is technique for overcoming the inefficiency of traditional layer 3 hop-by-hop routing. Labels are assigned to packets that allows network devices to forward packets in layer2 at high speed. The label points to an entry in a forwarding table that specifies where the packet should be forwarded. This label switching technique is much faster than the traditional routing method where each packet is examined before a forwarding decision is made.     

LAN (Local Area Network)   

A LAN is a shared communication system to which many computers are attached. A LAN, as its name implies, is limited to a local area. This has to do more with the physical characteristics of the medium than the fact that many early LANs were designed for departments, although the latter accurately describes a LAN as well.          

Latency

According to RFC 1242 (Benchmarking Terminology for Network Interconnection Devices, July1991), latency for store-and-forward devices is the time interval starting when the last bit of the input frame reaches the input port, and ending when the first bit of the output frame is seen on the output port. For bit-forwarding devices, latency is the time interval starting when the end of the first bit of the input frame reaches the input port, and ending when the start of the first bit of the output frame is seen on the output port. Variability of latency is called “jitter.”

Layer 2/Layer 3/Layer 4 Switching  

See Multilayer Switching; IP over ATM; and Label Switching.

Layered Architecture

See Network Architecture. 

LDP (Label Distribution Protocol) 

 LDP (Label Distribution Protocol) is a protocol used in MPLS (Multi

protocol Label Switching) environments by LSR (Label Switching Routers) to inform other LSRs of the label assignment they have made. LDP allows LSRs to agree with one another on the meaning of labels so that one device knows which label to use in order to forward traffic through another device.    

Leased Line

A leased line is a permanent or switched communication circuit between a customer site and a service provider’s wide area network, or between two sites owned by a customer (a private line). Leased lines almost always refer to carrier-based TDM circuits, although, in general, any communication link with monthly fee could be called a leased line.

Link-State Routing 

 Routing protocols discover routes on interconnected networks and build routing tables that provide routers with packet-forwarding information. Dynamic routing protocols(as opposed to manually configured static routing)automatically discover routes and create routing tables without operator intervention .Since network topologies are subject to change at any time(a link may fail),dynamic protocols are essential for routing around failed links in large internetworks. Routing protocols may use distance-vector routing or link-state algorithms(also called shortest path first or SPF a lgorithms).

LLC (Logical Link Control)     

The LLC is part of the data link layer in a protocol stack. The data link layer controls access to the network medium and defines how upper-layer data in the form of packets or datagram is inserted in to frames for delivery on a particular network. The underlying physical layer then transmits the framed data as a stream of bits on the network medium.

LMDS (Local Multipoint Distribution Service)

LMDS is a radio system developed by Bellcore as a wireless local loop system that can be used in areas where installing physical cable is prohibitive, such as rural areas. While LMDS will never provide data rates comparable to fiber, it can provide the most economical way to provide high-speed last-mile connections for business in metro areas. Alcatel calls LMDS“ wireless IP.” The service is categorized as “fixed wireless,” which really means that roaming between cells or other LMDS systems is usually not allowed. It also implies that subscribers are normally stationary, although antennas and equipment may be moved but the antenna must be aligned afterward. LMDS uses a point-to-multipoint radio topology where a base station broadcasts to subscriber antennas. The subscriber to base station link is point to point. Point-to-multipoint and related topologies are illustrated and discussed further under the topic “Wireless Broadband Access Technologies.  

Load Balancing

Load balancing is the process of distributing some load across multiple links, servers, processors, or other devices in order to improve performance and overcome deficiencies in existing equipment. This topic is primarily about server load balancing, which is the process of distributing requests from clients across a group of servers.          Another type of load balancing takes place inaggre gated communication links, where network traffic is distributed across two or more links that appear as a single link with high bandwidth (and redundancy).

Local Loop

The local loop is a pair of wires that connect telephone subscribers to the telephone company’s CO (central office). The CO is where telephone company switching equipment is located. Some neighborhood buildings are remote switching offices that are connected to a main office via atrunk cable (coaxial or fiber). The two copper wires are individually insulated and twisted around one another to reduce crosstalk.