L

Latency

The delay that occurs when a packet or signal is transmitted from one part of a network to another. A network with high latency can experience unpredictable delays. These delays usually do not affect data transmission appreciably since network protocols such as Internet Protocol (IP) are connectionless, but they have a serious impact on transmissions such as streaming audio and video because the human ear and eye can easily detect latency in these forms of transmission. The term "latency" can also refer to the delay in forming a connection, such as the 15 to 30 seconds required to establish a modem connection.

Intrinsic latency in a transmission is caused by the finite transmission speed of the electrical signals through the wires (or the light signals through the fiber-optic cabling). Intrinsic latency cannot be eliminated but is usually quite small. Much greater latency is usually introduced into a network by gateway devices such as routers and bridges, which process packets and perform protocol conversion. The latency for a bridge is thus the time delay between the moment when the packet enters one port of the bridge and the moment when it leaves another port—usually a fraction of a millisecond.

Layer 2 Forwarding (L2F)

A media-independent tunneling protocol developed by Cisco Systems. The Layer 2 Forwarding (L2F) protocol tunnels data-link layer frames in such protocols as Point-to-Point Protocol (PPP) or Serial Line Internet Protocol (SLIP), making it possible to create virtual private networks (VPNs) over a public network such as the Internet. On the server side, L2F can be used with such features as user authentication through Remote Authentication Dial-In User Service (RADIUS), dynamic allocation of addresses, and quality of service (QoS). L2F is implemented in Cisco routers through Cisco’s Internetwork Operating System (IOS).

Layer 2 switch

A form of Ethernet switch that switches packets by looking at their physical addresses (MAC addresses). These switches operate at the data-link layer (or layer 2) of the Open Systems Interconnection (OSI) reference model. They essentially perform a bridging function between LAN segments because they forward frames based on their destination address without any concern for the network protocol being used. Thus, Layer 2 switches are essentially multiport bridges that operate near wire speed and have extremely low latency.

Layer 2 Tunneling Protocol (L2TP)

An Internet Engineering Task Force (IETF) standard tunneling protocol that is used to encapsulate Point-to-Point Protocol (PPP) frames for transmission over TCP/IP, X.25, frame relay, or Asynchronous Transfer Mode (ATM) networks. You can use Layer 2 Tunneling Protocol (L2TP) to create virtual private networks (VPNs) over public networks such as the Internet. Because L2TP is an IETF standard, it provides the interoperability between different VPN vendors that protocols such as Microsoft’s Point-to-Point Tunneling Protocol (PPTP) and Cisco’s Layer 2 Forwarding (L2F) protocol do not, although L2TP essentially combines the best features of these two protocols and is an extension of them.

Layer 3 switch

An Ethernet switch that switches packets by looking at both their network address (for example, their IP or IPX address) and their physical address (for example, their MAC address). This type of switch operates at both the network layer (layer 3) and the data-link layer (layer 2) of the Open Systems Interconnection (OSI) reference model. A Layer 3 switch combines the speed of an Ethernet switch with some of the capabilities of a router for building advanced, high-speed Ethernet networks.

Layer 4 switch

Essentially, a Layer 3 switch that is capable of examining layer 4 of each packet that it switches. In TCP/IP networking, this is equivalent to examining the Transmission Control Protocol (TCP) layer information in the packet. Vendors tout Layer 4 switches as being able to use TCP information for prioritizing traffic by application. For example, to prioritize Hypertext Transfer Protocol (HTTP) traffic, a Layer 4 switch would give priority to packets whose layer 4 (TCP) information includes TCP port number 80, the standard port number for HTTP communication.

Some vendors foresee higher-layer switches that examine layer 5, 6, or 7 information to provide more control over prioritizing application traffic, but this might be just vendor hype.

Line coding

A method of placing digital signals on a wire. Line coding specifies the relationship between the binary information in a data bitstream and the square-wave voltage variations on the wire that represent this information electrically.

For example, Integrated Services Digital Network (ISDN) technologies use several different line coding schemes. The U interface, which is located at the ISDN line termination point at the customer premises where a two-wire metallic cable terminates with an RJ-11 jack, uses the 2 binary, 1 quaternary (2B1Q) line coding scheme for Basic Rate Interface ISDN (BRI-ISDN) and the Bipolar with 8 Zero Substitution (B8ZS) scheme for Primary Rate Interface ISDN (PRI-ISDN) in the United States. European ISDN uses 4 binary, 3 ternary (4B3T) for BRI-ISDN and High Density Bipolar 3 (HDB3) for PRI-ISDN.

In the 2B1Q line coding scheme, a block of two binary bits can represent four different values: 00, 01, 10, and 11. These four values are mapped to one quaternary value, which is encoded using four different voltages. The first bit represents a positive or negative voltage, and the second bit represents either 1-volt or 3-volt line potential. The following table shows the four possible combinations.

Line conditioner

Any device that is used to prevent undesirable electrical signals from damaging computer, networking, or telecommunication equipment and to guard against data loss due to electrical noise, sags, and surges. Sometimes called a line shaper, a line conditioner can also ensure that the parameters of the signal remain within specifications for the medium or interface being used, even over excessively long or noisy transmission lines. By maintaining signal integrity, line conditioners can allow communication devices to function at higher throughput rates.

Line driver

A device that can use installed twisted-pair phone lines or leased lines to connect terminals to servers in different parts of a building or in different buildings. A line driver is essentially a combination of a signal converter and an amplifier for digital signals. The signal converter performs line conditioning, and the amplifier increases the signal strength. Also called a "short-haul" device, a line driver allows a signal produced by a serial transmission device using an interface such as RS-232 to be carried over a longer distance than the interface standard allows, which for RS-232 is only 15 meters.

local area network (LAN)

A group of computers located in the same room, on the same floor, or in the same building that are connected to form a single network. Local area networks (LANs) allow users to share storage devices, printers, applications, data, and other network resources. They are limited to a specific geographical area, usually less than 2 kilometers in diameter. They might use a dedicated backbone to connect multiple subnetworks, but they do not use any telecommunication carrier circuits or leased lines except to connect with other LANs to form a wide area network (WAN).

Local loop

The portion of the telephone system that connects your home or office to the nearest central office (CO) of your local telco. The wiring used in the local loop is usually unshielded twisted-pair (UTP) cabling, the transmission method is analog transmission, and the maximum distance from the telco’s CO to the subscriber’s customer premises is about 5 kilometers.

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