AAA
Short for authentication, authorization and accounting, a system in IP-based networking to control what computer resources users have access to and to keep track of the activity of users over a network.

AAA services often require a server that is dedicated to providing the three services. RADIUS is an example of an AAA service.

Accounting
In information technology, accounting is the process of keeping track of a user's activity while accessing a network's resources, including the amount of time spent in the network, the services accessed while there and the amount of data transferred during the session. Accounting data is used for trend analysis, capacity planning, billing, auditing and cost allocation.

Application Service Provider
Application Service Providers are third-party entities that manage and distribute software-based services and solutions to customers across a wide area network from a central data center.

In essence, ASPs are a way for companies to outsource some or almost all aspects of their information technology needs.

ASPs may be commercial ventures that cater to customers, or not-for-profit or government organizations, providing service and support to end users.

Attenuation
Reduction of signal strength during transmission. Attenuation is the opposite of amplification, and is normal when a signal is sent from one point to another. If the signal attenuates too much, it becomes unintelligible, which is why most networks require repeaters at regular intervals. Attenuation is measured in decibels.

Attenuation Crosstalk Ratio
Also known as headroom, attenuation crosstalk ratio (ACR) is the difference between attenuation and crosstalk at a given frequency along a cable. Measured in decibels, ACR is a calculation used in networking transmission to assure that a signal transmitted across a twisted-pair cable is stronger at the receiving end than any interference signals imposed on that same pair by crosstalk from adjacent pairs.

AUI
Short for Attachment Unit Interface, the portion of the Ethernet standard that specifies how a cable is to be connected to an Ethernet card. AUI specifies a coaxial cable connected to a transceiver that plugs into a 15-pin socket on the network interface card (NIC).

Bastion Host
A bastion host is a gateway between an inside network and an outside network. Used as a security measure, the bastion host is designed to defend against attacks aimed at the inside network.

Depending on a network's complexity and configuration, a single bastion host may stand guard by itself, or be part of a larger security system with different layers of protection.

BECN
Short for backward explicit congestion notification. A Frame Relay message that notifies the sending device that there is congestion in the network. A BECN bit is sent in the opposite direction in which the frame is traveling, toward its transmission source.

Broadband Transmission
A type of data transmission in which a single medium (wire) can carry several channels at once. Cable TV, for example, uses broadband transmission. In contrast, baseband transmission allows only one signal at a time.

Most communications between computers, including the majority of local-area networks, use baseband communications. An exception is B-ISDN networks, which employ broadband transmission.

Broadcast
To simultaneously send the same message to multiple recipients. Broadcasting is a useful feature in e-mail systems. It is also supported by some fax systems.

In networking, a distinction is made between broadcasting and multicasting. Broadcasting sends a message to everyone on the network whereas multicasting sends a message to a select list of recipients.

Broadcast Storm
A state in which a message that has been broadcast across a network results in even more responses, and each response results in still more responses in a snowball effect. A severe broadcast storm can block all other network traffic, resulting in a network meltdown. Broadcast storms can usually be prevented by carefully configuring a network to block illegal broadcast messages.

Burst
(n) A set of bits, bytes or characters grouped together for transmission.

(v) An intermittent asynchronous transmission of a specific amount of data. Contrast with streaming.

CAM
(1) Short for channel access method, a protocol for how data is transmitted in the bottom two layers of the OSI model. CAMs describe how networking systems put data on the network media, how low-level errors are dealt with, and how the network polices itself. Polling, contention and token passing are three examples of CAMs.

(2) Acronym for computer-aided manufacturing, a type of computer application that helps automate a factory.

CAN
Acronym for campus-area network. An interconnection of local-area networks within a limited geographical space, such as a school campus or a military base.

Cell
(1) In spreadsheet applications, a cell is a box in which you can enter a single piece of data. The data is usually text, a numeric value, or a formula. The entire spreadsheet is composed of rows and columns of cells. A spreadsheet cell is analogous to a field in database management systems.

Individual cells are usually identified by a column letter and a row number. For example, D12 specifies the cell in column D and row 12.

(2) In communications and networking, a fixed-size packet of data.

3) In cellular telephone systems, a geographic area.

Cell Relay
A data transmission technology based on transmitting data in relatively small, fixed-size packets or cells. Each cell contains only basic path information that allows switching devices to route the cell quickly. Cell relay systems can reliably carry live video and audio because cells of fixed size arrive in a more predictable way than systems with packets or frames of varying size.

Asynchronous Transfer Mode (ATM) is the cell relay standard set by the CCITT organization. ATM uses a cell of 53 bytes.

Cells in Frames
A specification that enables ATM cells to be carried in Ethernet packets. This makes it possible to implement the ATM protocol while using existing Ethernet equipment, especially network interface cards (NICs). CIF provides the advantages of ATM, such as Quality of Service (QoS), without the usual hardware expense.

Choke Packet
A specialized packet that is used for flow control along a network. A router detects congestion by measuring the percentage of buffers in use, line utilization and average queue lengths. When it detects congestion, it sends choke packets across the network to all the data sources associated with the congestion. The sources respond by reducing the amount of data they are sending.

CIR
Short for committed information rate, a specified amount of guaranteed bandwidth (measured in bits per second) on a Frame Relay service. Typically, when purchasing a Frame Relay service, a company can specify the CIR level they wish. The Frame Relay network vendor guarantees that frames not exceeding this level will be delivered. It's possible that additional traffic may also be delivered, but it's not guaranteed.

Some Frame Relay vendors offer inexpensive services with a CIR equal to zero. This essentially means that the network will deliver as many frames as it can, but it doesn't guarantee any bandwidth level.

Circuit Switching
A type of communications in which a dedicated channel (or circuit) is established for the duration of a transmission. The most ubiquitous circuit-switching network is the telephone system, which links together wire segments to create a single unbroken line for each telephone call.

The other common communications method is packet switching, which divides messages into packets and sends each packet individually. The packets may take different routes and may arrive out of order. The Internet is based on a packet-switching protocol, TCP/IP.

Circuit-switching systems are ideal for communications that require data to be transmitted in real-time. Packet-switching networks are more efficient if some amount of delay is acceptable.

Circuit-switching networks are sometimes called connection-oriented networks. Note, however, that although packet switching is essentially connectionless, a packet switching network can be made connection-oriented by using a higher-level protocol. TCP, for example, makes IP networks connection-oriented.

Class C Network
In a 32-bit IP address, the number of bits used to identify the network and the host vary according to the network class of the address. In a Class C network, the first 3 bits, or the high-order bits, are always "110." The next 21 bits are used to define the Class C network, and the final eight bits are used to identify the host. The IP address is represented in dotted decimal notation of four 8-bit fields, or octets, that have been converted from binary to decimal numbers.

Client/Server Architecture
A network architecture in which each computer or process on the network is either a client or a server. Servers are powerful computers or processes dedicated to managing disk drives (file servers), printers (print servers), or network traffic (network servers ). Clients are PCs or workstations on which users run applications. Clients rely on servers for resources, such as files, devices, and even processing power.

Another type of network architecture is known as a peer-to-peer architecture because each node has equivalent responsibilities. Both client/server and peer-to-peer architectures are widely used, and each has unique advantages and disadvantages.

Cloud
Also referred to as a network cloud. In telecommunications, a cloud refers to a public or semi-public space on transmission lines (such as T1 or T3) that exists between the end points of a transmission. Data that is transmitted across a WAN enters the network from one end point using a standard protocol suite such as Frame Relay and then enters the network cloud where it shares space with other data transmissions. The data emerges from the cloud -- where it may be encapsulated, translated and transported in myriad ways -- in the same format as when it entered the cloud. A network cloud exists because when data is transmitted across a packet-switched network in a packet, no two packets will necessarily follow the same physical path. The unpredictable area that the data enters before it is received is the cloud.

Clustering
Connecting two or more computers together in such a way that they behave like a single computer. Clustering is used for parallel processing, for load balancing and for fault tolerance.

Clustering is a popular strategy for implementing parallel processing applications because it enables companies to leverage the investment already made in PCs and workstations. In addition, it's relatively easy to add new CPUs simply by adding a new PC to the network.

Collapsed Backbone
Network backbone that consists of the backplane of a single switch, rather than multiple switches connected together.

From the ports of the single switch, cables connect to the hubs of individual LAN segments.

Collapsed backbones are typically used for mid-sized LAN networks. The architecture is easier to manage and easier to keep secure, not to mention less costly.

Collision
The situation that occurs when two or more devices attempt to send a signal along the same channel at the same time. The result of a collision is generally a garbled message. All computer networks require some sort of mechanism to either prevent collisions altogether or to recover from collisions when they do occur.

Connectionless
Refers to network protocols in which a host can send a message without establishing a connection with the recipient. That is, the host simply puts the message onto the network with the destination address and hopes that it arrives. Examples of connectionless protocols include Ethernet, IPX, and UDP.

In contrast, connection-oriented protocols require a channel to be established between the sender and receiver before any messages are transmitted. Examples of connection-oriented protocols include the telephone, TCP, and HTTP.

Connectivity
A computer buzzword that refers to a program or device's ability to link with other programs and devices. For example, a program that can import data from a wide variety of other programs and can export data in many different formats is said to have good connectivity. On the other hand, computers that have difficulty linking into a network (many laptop computers, for example) have poor connectivity.

Contention
(1) Competition for resources. The term is used especially in networks to describe the situation where two or more nodes attempt to transmit a message across the same wire at the same time.

(2) A type of network protocol that allows nodes to contend for network access. That is, two or more nodes may try to send messages across the network simultaneously. The contention protocol defines what happens when this occurs. The most widely used contention protocol is CSMA/CD, used by Ethernet.

Crosstalk
A disturbance, caused by electromagnetic interference, along a circuit or a cable pair. A telecommunication signal disrupts a signal in an adjacent circuit and can cause the signals to become confused and cross over each other.

CSMA/CD
Short for Carrier Sense Multiple Access / Collision Detection, a set of rules determining how network devices respond when two devices attempt to use a data channel simultaneously (called a collision). Standard Ethernet networks use CSMA/CD. This standard enables devices to detect a collision. After detecting a collision, a device waits a random delay time and then attempts to re-transmit the message. If the device detects a collision again, it waits twice as long to try to re-transmit the message. This is known as exponential back off.

Daisy Chain
(n) A hardware configuration in which devices are connected one to another in a series. The SCSI interface , for example, supports a daisy chain of up to 7 devices.

(v) To connect devices in a daisy chain pattern.

Data Mirroring
The act of copying data from one location to a storage device in real time. Because the data is copied in real time, the information stored from the original location is always an exact copy of the data from the production device. Data mirroring is useful in the speedy recovery of critical data after a disaster. Data mirroring can be implemented locally or offsite at a completely different location.

DDR
Short for Dial-on-Demand Routing. DDR is a routing technique developed by Cisco that allows a user to utilize existing telephone lines, or public circuit-switched networks, to form a WAN instead of lines that are dedicated specifically to the WAN. DDR is typically implemented by users that do not need permanent, continuous links between sites on the WAN because the volume of traffic over the WAN is low and the transmissions are periodic as opposed to continuous. The connection only becomes active when data is sent to the remote site. When no data has been sent over the link for a specified amount of time, the link is disconnected.

Using DDR, a connection between sites is only established when a specific type of traffic initiates the call or when you a backup link is needed for redundancy or load sharing.

DDR is used in order to save on the costs of a dedicated WAN line for organizations that do not need permanent continuous connection and as a back-up by organizations that use the dedicated line for critical applications.

Dial-up Access
Refers to connecting a device to a network via a modem and a public telephone network. Dial-up access is really just like a phone connection, except that the parties at the two ends are computer devices rather than people. Because dial-up access uses normal telephone lines, the quality of the connection is not always good and data rates are limited. In the past, the maximum data rate with dial-up access was 56 Kbps (56,000 bits per second), but new technologies such as ISDN are providing faster rates.

An alternative way to connect two computers is through a leased line, which is a permanent connection between two devices. Leased lines provide faster throughput and better quality connections, but they are also more expensive.

Diskless Workstation
A workstation or PC on a local-area network (LAN) that does not have its own disk. Instead, it stores files on a network file server. Diskless workstations can reduce the overall cost of a LAN because one large-capacity disk drive is usually less expensive than several low-capacity drives. In addition, diskless workstations can simplify backups and security because all files are in one place -- on the file server. Also, accessing data from a large remote file server is often faster than accessing data from a small local storage device.

One disadvantage of diskless workstations, however, is that they are useless if the network fails.

When the workstation is a PC, it is often called a diskless PC or a Net PC.

DLCI
Short for data link connection identifier. A number of a private or switched virtual circuit in a Frame Relay network that tells the Frame Relay how to route the data. The DLCI field identifies which logical circuit the data travels over.

Domain
A group of computers and devices on a network that are administered as a unit with common rules and procedures. Within the Internet, domains are defined by the IP address. All devices sharing a common part of the IP address are said to be in the same domain.

Domain Name
A name that identifies one or more IP addresses. For example, the domain name microsoft.com represents about a dozen IP addresses. Domain names are used in URLs to identify particular Web pages. For example, in the URL http://www.jargon.com/index.html, the domain name is jargon.com

Because the Internet is based on IP addresses, not domain names, every Web server requires a Domain Name System (DNS) server to translate domain names into IP addresses.

DSS
Short for digital satellite system, a network of satellites that broadcast digital data. An example of a DSS is DirecTV, which broadcasts digital television signals. DSS's are expected to become more important as the TV and computer converge into a single medium for information and entertainment.

DWDM
Short for Dense Wavelength Division Multiplexing, an optical technology used to increase bandwidth over existing fiber optic backbones.

DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber. In effect, one fiber is transformed into multiple virtual fibers. So, if you were to multiplex eight OC -48 signals into one fiber, you would increase the carrying capacity of that fiber from 2.5 Gb/s to 20 Gb/s. Currently, because of DWDM, single fibers have been able to transmit data at speeds up to 400Gb/s. And, as vendors add more channels to each fiber, terabit capacity is on its way.

A key advantage to DWDM is that it's protocol and bit-rate independent. DWDM-based networks can transmit data in IP, ATM, SONET /SDH, and Ethernet, and handle bit-rates between 100 Mb/s and 2.5 Gb/s. Therefore, DWDM-based networks can carry different types of traffic at different speeds over an optical channel.

Enterprise
Literally, a business organization. In the computer industry, the term is often used to describe any large organization that utilizes computers. An intranet, for example, is a good example of an enterprise computing system.

Etherloop
Created by Nortel spin-off Elastic Networks, EtherLoop (a.k.a next generation DSL) is Ethernet over standard twisted pair (POTS ) technology that allows for simultaneous voice and high-speed data communications. Speeds range from 125 Kbps to 6 Mbps over distances of up to 21,000 feet.

Similar to Ethernet, Etherloop transmits data packets in bursts. Between bursts, EtherLoop looks for problems and interference in the lines and knows to steer clear and find an alternate path. Also, EtherLoop is Ethernet compliant, so it is easily adaptable to existing Ethernet systems.

EtherLoop also borrows from the best of DSL to offer point-to-point security and a non-shared medium (more users don't slow down connection speed), But unlike DSL, EtherLoop avoids impacting other services such as ISDN, T-1, ADSL, HDSL and SDSL, which are in the same cable binder group.

EtherLoop is not proprietary. Elastic Networks plans on licensing the technology to manufacturers who can design and create their own EtherLoop solutions.

Extranet
A new buzzword that refers to an intranet that is partially accessible to authorized outsiders. Whereas an intranet resides behind a firewall and is accessible only to people who are members of the same company or organization, an extranet provides various levels of accessibility to outsiders. You can access an extranet only if you have a valid username and password, and your identity determines which parts of the extranet you can view.

Extranets are becoming a very popular means for business partners to exchange information.

Failover
A backup operation that automatically switches to a standby database, server or network if the primary system fails or is temporarily shut down for servicing. Failover is an important fault tolerance function of mission-critical systems that rely on constant accessibility. Failover automatically and transparently to the user redirects requests from the failed or down system to the backup system that mimics the operations of the primary system.

FECN
Short for forward explicit congestion notification. A Frame Relay message that notifies the receiving device that there is congestion in the network. A FECN bit is sent in the same direction in which the frame was traveling, toward its destination.

FQDN
A fully qualified domain name consists of a host and domain name, including top-level domain. For example, www.webopedia.com is a fully qualified domain name. www is the host, webopedia is the second-level domain, and.com is the top level domain.

A FQDN always starts with a host name and continues all the way up to the top-level domain name, so www.parc.xerox.com is also a FQDN.

Frame Relay
A packet-switching protocol for connecting devices on a Wide Area Network (WAN). Frame Relay networks in the U.S. support data transfer rates at T-1 (1.544 Mbps) and T-3 (45 Mbps) speeds. In fact, you can think of Frame Relay as a way of utilizing existing T-1 and T-3 lines owned by a service provider. Most telephone companies now provide Frame Relay service for customers who want connections at 56 Kbps to T-1 speeds. (In Europe, Frame Relay speeds vary from 64 Kbps to 2 Mbps.

In the U.S., Frame Relay is quite popular because it is relatively inexpensive. However, it is being replaced in some areas by faster technologies, such as ATM.

Gateway
In networking, a combination of hardware and software that links two different types of networks. Gateways between e-mail systems, for example, allow users on different e-mail systems to exchange messages.

HAN
Short for home area network. A HAN is a network contained within a user's home that connects a person's digital devices, from multiple computers and their peripheral devices to telephones, VCRs, televisions, video games, home security systems, "smart" appliances, fax machines and other digital devices that are wired into the network.

HBA
Short for host bus adapter. An HBA is an I/O adapter that sits between the host computer's bus and the Fibre Channel loop and manages the transfer of information between the two channels. In order to minimize the impact on host processor performance, the host bus adapter performs many low-level interface functions automatically or with minimal processor involvement.

Heterogeneous Network
A network that includes computers and other devices from different manufacturers. For example, local-area networks (LANs) that connect PCs with Apple Macintosh computers are heterogeneous.

HomeRF
Short for home radio frequency. Designed specifically for wireless networks in homes - in contrast to 802.11, which was created for use in businesses -- HomeRF networks are designed to be more affordable to home users than other wireless technologies. Based on frequency hopping and using radio frequency waves for the transmission of voice and data, HomeRF has a range of up to 150 feet.

Hop
An intermediate connection in a string of connections linking two network devices. On the Internet, for example, most data packets need to go through several routers before they reach their final destination. Each time the packet is forwarded to the next router, a hop occurs. The more hops, the longer it takes for data to go from source to destination. You can see how many hops it takes to get to another Internet host by using the PING or traceroute utilities.

Some Internet Service Providers (ISPs) advertise how many hops away from Internet backbone they are. Theoretically, the fewer hops it takes to get your data onto the backbone, the faster your access will be.

Hot Potato Routing
In hot potato routing , or deflection routing, the nodes of a network have no buffer to store packets in before they are moved on to their final predetermined destination. In normal routing situations, when multiple packets contend for a single outgoing channel, packets that are not buffered are dropped to avoid congestion. But in hot potato routing, each packet that is routed is constantly transferred until it reaches its final destination because the individual communication links can not support more than one packet at a time. The packet is bounced around like a "hot potato," sometimes moving further away from its destination because it has to keep moving through the network. This technique allows multiple packets to reach their destinations without being dropped. This is in contrast to "store and forward" routing where the network allows temporary storage at intermediate locations. Hot potato routing has applications in optical networks where messages made from light can not be stored in any medium.

HPNA
Also referred to as HomePNA. A de facto home networking standard developed by the Home Phoneline Networking Alliance. This technology, building on Ethernets, allows all the components of a home network to interact over the home's existing telephone wiring without disturbing the existing voice or fax services. In the same way a LAN operates, home networking processes, manages, transports and stores information, which enables the disparate devices in a home network such as telephones, fax machines, desktops, laptops, printers, scanners and Web cameras to connect and integrate over a home's unpredictable wiring topology.

HPNA technology must coexist with telephone service and comply with FCC Part 68.

HSSI
Short for High-Speed Serial Interface. HSSI is a serial interface that supports transmission rates up to 52 mbps. It is used to connect routers on local area networks with wide area networks over a 680x0 or similar high-speed line. HSSI can also be used to provide high-speed connectivity between LANs, such as token ring and Ethernet.

HSSI is a DTE /DCE interface developed by Cisco Systems and T3plus Networking to address the need for high-speed communication over WAN links.

I/O
Short for input/output (pronounced "eye-oh"). The term I/O is used to describe any program, operation or device that transfers data to or from a computer and to or from a peripheral device. Every transfer is an output from one device and an input into another. Devices such as keyboards and mouses are input-only devices while devices such as printers are output-only. A writable CD-ROM is both an input and an output device.

I/O Adapter
An adapter that converts between the timing and protocol requirements of an intelligent device's memory bus and those of an I/O bus or network.

Internetworking
The art and science of connecting individual local-area networks (LANs) to create wide-area networks (WANs) , and connecting WANs to form even larger WANs. Internetworking can be extremely complex because it generally involves connecting networks that use different protocols. Internetworking is accomplished with routers, bridges, and gateways.

Intranet
A network based on TCP/IP protocols (an internet) belonging to an organization, usually a corporation, accessible only by the organization's members, employees, or others with authorization. An intranet's Web sites look and act just like any other Web sites, but the firewall surrounding an intranet fends off unauthorized access.

Like the Internet itself, intranets are used to share information. Secure intranets are now the fastest-growing segment of the Internet because they are much less expensive to build and manage than private networks based on proprietary protocols.

IP Switching
A new type of IP routing developed by Ipsilon Networks, Inc. Unlike conventional routers, IP switching routers use ATM hardware to speed packets through networks. Although the technology is new, it appears to be considerably faster than older router techniques.

Jabber
(1) an error in which a faulty device (usually a NIC ) continuously transmits corrupted or meaningless data onto a network. This may halt the entire network from transmitting data beacuse other devices will perceive the network as busy.

(2) a sent data packet greater than the maximum 1518 bytes specified in IEEE 802.3. To prevent this, jabber control should be added to the hardware to make the circuitry incapable of sending information for more than 150 milliseconds ( approximately 1500 bytes).

LANE
Short for LAN emulation. LANE is a protocol that allows existing networked applications and protocols to run over an ATM backbone. LANs are connectionless while ATM is a connection-oriented technology. LANE provides a means for ATM hardware and networks to communicate with existing Ethernet and Token Ring networks. LANE works at the MAC (layer 2) networking layer of the OSI model and therefore supports both bridging and routing.

The main objective of LAN emulation is to allow existing applications to access the ATM network by way of MAC drivers as if they were running over traditional LAN's. Standard interfaces for MAC device drivers include NDIS and ODI. These interfaces specify how access to a MAC driver is performed. LANE provides these interfaces and services to the upper layers of the OSI model.

LAT
Short for Local Area Transport, a DEC proprietary Ethernet protocol for connecting terminals to a LAN. Connections are typically between a DEC terminal server and a VAX. LAT operates at the transport layer. LAT is not routable because it lacks a network layer and therefore must be bridged in an enterprise network instead of routed.

Latency
(1) In general, the period of time that one component in a system is spinning its wheels waiting for another component. Latency, therefore, is wasted time. For example, in accessing data on a disk, latency is defined as the time it takes to position the proper sector under the read/write head.

(2) In networking, the amount of time it takes a packet to travel from source to destination. Together, latency and bandwidth define the speed and capacity of a network.

Leased Line
A permanent telephone connection between two points set up by a telecommunications common carrier. Typically, leased lines are used by businesses to connect geographically distant offices. Unlike normal dial-up connections, a leased line is always active. The fee for the connection is a fixed monthly rate. The primary factors affecting the monthly fee are distance between end points and the speed of the circuit. Because the connection doesn't carry anybody else's communications, the carrier can assure a given level of quality.

For example, a T-1 channel is a type of leased line that provides a maximum transmission speed of 1.544 Mbps. You can divide the connection into different lines for data and voice communication or use the channel for one high speed data circuit. Dividing the connection is called multiplexing.

Increasingly, leased lines are being used by companies, and even individuals, for Internet access because they afford faster data transfer rates and are cost-effective if the Internet is used heavily.

Load Balancing
Distributing processing and communications activity evenly across a computer network so that no single device is overwhelmed. Load balancing is especially important for networks where it's difficult to predict the number of requests that will be issued to a server. Busy Web sites typically employ two or more Web servers in a load balancing scheme. If one server starts to get swamped, requests are forwarded to another server with more capacity. Load balancing can also refer to the communications channels themselves.

Local
In networks, local refers to files, devices, and other resources at your workstation. Resources located at other nodes on the networks are remote.

Local-Area Network
A computer network that spans a relatively small area. Most LANs are confined to a single building or group of buildings. However, one LAN can be connected to other LANs over any distance via telephone lines and radio waves. A system of LANs connected in this way is called a wide-area network (WAN).

Most LANs connect workstations and personal computers. Each node (individual computer ) in a LAN has its own CPU with which it executes programs, but it is also able to access data and devices anywhere on the LAN. This means that many users can share expensive devices, such as laser printers, as well as data. Users can also use the LAN to communicate with each other, by sending e-mail or engaging in chat sessions.

There are many different types of LANs Ethernets being the most common for PCs. Most Apple Macintosh networks are based on Apple's AppleTalk network system, which is built into Macintosh computers.

LANs are capable of transmitting data at very fast rates, much faster than data can be transmitted over a telephone line; but the distances are limited, and there is also a limit on the number of computers that can be attached to a single LAN.

Log On
To make a computer system or network recognize you so that you can begin a computer session. Most personal computers have no log-on procedure -- you just turn the machine on and begin working. For larger systems and networks, however, you usually need to enter a username and password before the computer system will allow you to execute programs.

Log Out
To end a session at the computer. For personal computers, you can log out simply by exiting applications and turning the machine off. On larger computers and networks, where you share computer resources with other users, there is generally an operating system command that lets you log off.

Loopback Address
Loopback address is a special IP number (127.0.0.1) that is designated for the software loopback interface of a machine. The loopback interface has no hardware associated with it, and it is not physically connected to a network.

The loopback interface allows IT professionals to test IP software without worrying about broken or corrupted drivers or hardware.

MAN
Short for Metropolitan Area Network, a data network designed for a town or city. In terms of geographic breadth, MANs are larger than local-area networks (LANs), but smaller than wide-area networks (WANs). MANs are usually characterized by very high-speed connections using fiber optical cable or other digital media.

MSCS
Short for Microsoft Cluster Server, a clustering technology built into Windows NT 4.0 and later versions. MSCS supports clustering of two NT servers to provide a single fault-tolerant server.

During its development stage, MSCS was code-named Wolfpack.

Multihomed
An adjective used to describe a host that is connected to two or more networks or has two or more network addresses. For example, a network server may be connected to a serial line and a LAN or to multiple LANs.

Named Pipes
An interprocess control (IPC) protocol for exchanging information between two applications, possibly running on different computers in a network. Named Pipes are supported by a number of network operating systems (NOSs), including Netware and LAN Manager.

Net PC
A type of network computer designed cooperatively by Microsoft and Intel. In some respects, the Net PC is really just a scaled-down PC since it is able to execute Windows applications locally. However, it also includes features to simplify connecting it to a network and to administer it remotely.

Net PCs are based on the Wintel platform, but are configured to be as inexpensive as possible and to discourage users from configuring the machines themselves. Consequently, they have no floppy disk drive or CD-ROM drive. They do have a hard disk though it's meant to be used as a temporary cache to improve performance rather than for permanently storing data. Configuration and management of a Net PC is performed through a network server and Microsoft's Zero Administration Windows (ZAW) system.

Netbeui
Pronounced net-booey, Netbeui is short for NetBios Enhanced User Interface. It is an enhanced version of the NetBIOS protocol used by network operating systems such as LAN Manager, LAN Server, Windows for Workgroups, Windows 95 and Windows NT.

Netbeui was originally designed by IBM for their Lan Manager server and later extended by Microsoft and Novell.

Network Computer
A computer with minimal memory, disk storage and processor power designed to connect to a network, especially the Internet. The idea behind network computers is that many users who are connected to a network don't need all the computer power they get from a typical personal computer. Instead, they can rely on the power of the network servers.

This is really a variation on an old idea -- diskless workstations -- which are computers that contain memory and a processor but no disk storage. Instead, they rely on a server to store data. Network computers take this idea one step further by also minimizing the amount of memory and processor power required by the workstation. Network computers designed to connect to the Internet are sometimes called Internet boxes, Net PCs, and Internet appliances.

One of the strongest arguments behind network computers is that they reduce the total cost of ownership (TCO) -- not only because the machines themselves are less expensive than PCs, but also because network computers can be administered and updated from a central network server.

Network Meltdown
A state in which a network grinds to a halt due to excessive traffic. A network meltdown generally starts as a broadcast storm that gets out of control, but even legitimate network messages can cause a meltdown if the network hasn't been designed to accommodate that level of traffic.

Network Transparency
A condition in which an operating system or other service allows the user access to a remote resource through a network without needing to know if the resource is remote or local. For example, Sun Microsystem's NFS, which has become a de facto industry standard, provides access to shared files through an interface called the Virtual File System (VFS) that runs on top of TCP/IP. Users can manipulate shared files as if they were stored locally on the user's own hard disk.

Network-Attached Storage
A network-attached storage (NAS) device is a server that is dedicated to nothing more than file sharing. NAS does not provide any of the activities that a server in a server-centric system typically provides, such as e-mail, authentication or file management. NAS allows more hard disk storage space to be added to a network that already utilizes servers without shutting them down for maintenance and upgrades. With a NAS device, storage is not an integral part of the server. Instead, in this storage-centric design, the server still handles all of the processing of data but a NAS device delivers the data to the user. A NAS device does not need to be located within the server but can exist anywhere in a LAN and can be made up of multiple networked NAS devices.

NFS
Abbreviation of Network File System, a client/server application designed by Sun Microsystems that allows all network users to access shared files stored on computers of different types. NFS provides access to shared files through an interface called the Virtual File System (VFS) that runs on top of TCP/IP. Users can manipulate shared files as if they were stored locally on the user's own hard disk.

With NFS, computers connected to a network operate as clients while accessing remote files, and as servers while providing remote users access to local shared files. The NFS standards are publicly available and widely used.

NOC
Short for network operations center, the physical space from which a typically large telecommunications network is managed, monitored and supervised. The NOC coordinates network troubles, provides problem management and router configuration services, manages network changes, allocates and manages domain names and IP addresses, monitors routers, switches, hubs and UPS systems that keep the network operating smoothly, manages the distribution and updating of software and coordinates with affiliated networks. NOCs also provide network accessibility to users connecting to the network from outside of the physical office space or campus.

Node
(1) In networks, a processing location. A node can be a computer or some other device, such as a printer. Every node has a unique network address, sometimes called a Data Link Control (DLC) address or Media Access Control (MAC) address.

(2) In tree structures, a point where two or more lines meet.

Packet
A piece of a message transmitted over a packet-switching network. See under packet switching. One of the key features of a packet is that it contains the destination address in addition to the data. In IP networks, packets are often called datagrams.

Packet Collision
In a network, when two or more stations attempt to transmit a packet across the network at the same time, a packet collision occurs. This is not uncommon in a shared medium such as an Ethernet that has many computers in the same network segment. When a packet collision occurs, the packets are either discarded or sent back to their originating stations and then retransmitted in a timed sequence to avoid further collision. Packet collisions can result in the loss of packet integrity or can impede the performance of a network.

Packet Collision Rate
The amount of packet collisions that occur in a network in a specified time period, usually one minute. The packet collision rate is typically monitored by the router.

Packet Switching
Refers to protocols in which messages are divided into packets before they are sent. Each packet is then transmitted individually and can even follow different routes to its destination. Once all the packets forming a message arrive at the destination, they are recompiled into the original message.

Most modern Wide Area Network (WAN) protocols, including TCP/IP, X.25, and Frame Relay, are based on packet-switching technologies. In contrast, normal telephone service is based on a circuit-switching technology, in which a dedicated line is allocated for transmission between two parties. Circuit-switching is ideal when data must be transmitted quickly and must arrive in the same order in which it's sent. This is the case with most real-time data, such as live audio and video. Packet switching is more efficient and robust for data that can withstand some delays in transmission, such as e-mail messages and Web pages.

A new technology, ATM, attempts to combine the best of both worlds -- the guaranteed delivery of circuit-switched networks and the robustness and efficiency of packet-switching networks.

PAN
Based on the electric-field transmission medium, Personal Area Network is an IBM technology that allows individuals to exchange data with a simple touch or grasp, such as a handshake.

A PAN user is equipped with a receiver and a transmitter, which constantly sends infinitesimal data-carrying currents -- in the 0.1-1 MHz band -- through the body and picks up currents when in very close range with another device or individual carrying a transmitter.

Patch Cord
Also known as a patch cable, a patch cord is a piece of copper wire or fiber optic cable that connects circuits on a patch panel.

Patch Panel
A panel of network ports contained together, usually within a telecommunications closet, that connects incoming and outgoing lines of a LAN or other communication, electronic or electrical system. In a LAN, the patch panel connects the network's computers to each other and to the outside lines that enable the LAN to connect to the Internet or another WAN. Connections are made with patch cords. The patch panel allows circuits to be arranged and rearranged by plugging and unplugging the patch cords.

Peer-To-Peer Architecture
A type of network in which each workstation has equivalent capabilities and responsibilities. This differs from client/server architectures, in which some computers are dedicated to serving the others. Peer-to-peer networks are generally simpler, but they usually do not offer the same performance under heavy loads.

Pervasive Computing
The idea that technology is moving beyond the personal computer to everyday devices with embedded technology and connectivity as computing devices become progressively smaller and more powerful. Also called ubiquitous computing, pervasive computing is the result of computer technology advancing at exponential speeds -- a trend toward all man-made and some natural products having hardware and software. Pervasive computing goes beyond the realm of personal computers: it is the idea that almost any device, from clothing to tools to appliances to cars to homes to the human body to your coffee mug, can be imbedded with chips to connect the device to an infinite network of other devices. The goal of pervasive computing, which combines current network technologies with wireless computing, voice recognition, Internet capability and artificial intelligence, is to create an environment where the connectivity of devices is embedded in such a way that the connectivity is unobtrusive and always available.

Polling
(1) Polling is a CAM. In a master/slave scenario, the master queries each slave device in turn as to whether it has any data to transmit. If the slave answers yes then the device is permitted to transmit its data. If the slave answers no then the master moves on and polls the next slave device. The process is repeated continuously.

(2) Making continuous requests for data from another device. For example, modems that support polling can call another system and request data.

PON
Passive Optical Network is a high bandwidth point to multipoint optical fibre network based on the asynchronous transfer mode protocol (ATM), Ethernet or TDM.

PONs generally consist of an OLT (Optical Line Termination), which is connected to ONUs (Optical Network Units), aka subscriber terminals, using only fibre cables, optical splitters and other passive components (do not transmit signals using electricity). Up to 32 ONUs can be connected to an OLT.

The OLT is located at a local exchange, and the ONU is located either on the street, in a building, or even in a user's home.

PONs rely on lightwaves for data transfer.

In a PON, signals are routed over the local link with all signals along that link going to all interim transfer points. Optical splitters route signals through the network; optical receivers at intermediate points and subscriber terminals tuned for specific wavelengths of light direct signals intended for their groups of subscribers. At the final destination, a specific residence or business can detect its specified signal.

PONs are capable of delivering high volumes of upstream and downstream bandwidth (up to 622 Mbps downstream and 155 Mbps upstream), which can be changed "on-the-fly" depending on an individual user's needs.

Proxy Server
A server that sits between a client application, such as a Web browser, and a real server. It intercepts all requests to the real server to see if it can fulfill the requests itself. If not, it forwards the request to the real server.

Public Carrier
A government-regulated organization that provides telecommunications services to the public. This includes AT&T, MCI, and Western Union. Most public carriers provide electronic- mail services that enable you to send messages and documents over a telephone line to other computer users.

Pull
To request data from another program or computer. The opposite of pull is push, where data is sent without a request being made. The terms push and pull are used frequently to describe data sent over the Internet. The World Wide Web is based on pull technologies, where a page isn't delivered until a browser requests it. Increasingly, however, Information services are harnessing the Internet to broadcast information using push technologies. A prime example is the PointCast Network.

Push
(1) In client/server applications, to send data to a client without the client requesting it. The World Wide Web is based on a pull technology where the client browser must request a Web page before it is sent. Broadcast media, on the other hand, are push technologies because they send information out regardless of whether anyone is tuned in.

Increasingly, companies are using the Internet to deliver information push-style. Probably the oldest and most widely used push technology is e-mail. This is a push technology because you receive mail whether you ask for it or not -- that is, the sender pushes the message to the receiver.

(2) In programming, to place a data item onto a stack. The opposite of push is pop, which means to remove an object from a stack.

PVC
Short for permanent virtual circuit, a virtual circuit that is permanently available. The only difference between a PVC and a switched virtual circuit (SVC) is that an SVC must be reestablished each time data is to be sent. Once the data has been sent, the SVC disappears. PVCs are more efficient for connections between hosts that communicate frequently.

PVCs play a central role in Frame Relay networks. They're also supported in some other types of networks, such as X.25.

QoS
Short for Quality of Service, a networking term that specifies a guaranteed throughput level. One of the biggest advantages of ATM over competing technologies such as Frame Relay and Fast Ethernet, is that it supports QoS levels. This allows ATM providers to guarantee to their customers that end-to-end latency will not exceed a specified level.

Remote Access
The ability to log onto a network from a distant location. Generally, this implies a computer, a modem, and some remote access software to connect to the network. Whereas remote control refers to taking control of another computer, remote access means that the remote computer actually becomes a full-fledged host on the network. The remote access software dials in directly to the network server. The only difference between a remote host and workstations connected directly to the network is slower data transfer speeds.

Remote Access Server
A server that is dedicated to handling users that are not on a LAN but need remote access to it. The remote access server allows users to gain access to files and print services on the LAN from a remote location. For example, a user who dials into a network from home using an analog modem or an ISDN connection will dial into a remote access server. Once the user is authenticated he can access shared drives and printers as if he were physically connected to the office LAN.

Routing
In internetworking, the process of moving a packet of data from source to destination. Routing is usually performed by a dedicated device called a router. Routing is a key feature of the Internet because it enables messages to pass from one computer to another and eventually reach the target machine. Each intermediary computer performs routing by passing along the message to the next computer. Part of this process involves analyzing a routing table to determine the best path.

Routing is often confused with bridging, which performs a similar function. The principal difference between the two is that bridging occurs at a lower level and is therefore more of a hardware function whereas routing occurs at a higher level where the software component is more important. And because routing occurs at a higher level, it can perform more complex analysis to determine the optimal path for the packet.

RTU
Short for remote terminal unit. In SCADA systems, an RTU is a device installed at a remote location that collects data, codes the data into a format that is transmittable and transmits the data back to a central station, or master. An RTU also collects information from the master device and implements processes that are directed by the master. RTUs are equipped with input channels for sensing or metering, output channels for control, indication or alarms and a communications port.

SAN
Storage Area Network (SAN) is a high-speed subnetwork of shared storage devices. A storage device is a machine that contains nothing but a disk or disks for storing data.

A SAN's architecture works in a way that makes all storage devices available to all servers on a LAN or WAN. As more storage devices are added to a SAN, they too will be accessible from any server in the larger network. In this case, the server merely acts as a pathway between the end user and the stored data.

Because stored data does not reside directly on any of a network's servers, server power is utilized for business applications, and network capacity is released to the end user.

Server Farm
Also referred to as server cluster, computer farm or ranch. A server farm is a group of networked servers that are housed in one location. A server farm streamlines internal processes by distributing the workload between the individual components of the farm and expedites computing processes by harnessing the power of multiple servers. The farms rely on load-balancing software that accomplishes such tasks as tracking demand for processing power from different machines, prioritizing the tasks and scheduling and rescheduling them depending on priority and demand that users put on the network. When one server in the farm fails, another can step in as a backup.

Combining servers and processing power into a single entity has been relatively common for many years in research and academic institutions. Today, more and more companies are utilizing server farms as a way of handling the enormous amount of computerization of tasks and services that they require.

A Web server farm, or Web farm, refers to either a Web site that runs off of more than one server or an ISP that provides Web hosting services using multiple servers.

Shared Wireles Access Protocol
Developed by the HomeRF Working Group, SWAP is a specification for wireless voice and data networking in the home.

SWAP works together with the PSTN network and the Internet through existing cordless telephone and wireless LAN technologies. It supports TDMA for interactive data transfer and CSMA/CA for high-speed packet transfer.

SWAP operates in the 2400 MHz band at 50 hops per second. Data travels at a rate between 1 Mbps and 2 Mbps.

On a SWAP network via cordless handheld devices, users will be able to voice activate home electronic systems; access the Internet from anywhere in the home, and forward fax, voice and e-mail messages.

Sneakernet
Refers to the channel by which electronic information is transmitted from one computer to another by physically carrying it stored on a floppy disk, CD or other removable medium. This play on words stems from the idea that a person is using their feet, i.e., sneakers, to transfer data instead of through the Internet or an organization's intranet.

Socket
(1) In UNIX and some other operating systems, a software object that connects an application to a network protocol. In UNIX, for example, a program can send and receive TCP/IP messages by opening a socket and reading and writing data to and from the socket. This simplifies program development because the programmer need only worry about manipulating the socket and can rely on the operating system to actually transport messages across the network correctly. Note that a socket in this sense is completely soft - it's a software object, not a physical component.

(2) A receptacle into which a plug can be inserted.

(3) A receptacle for a microprocessor or other hardware component.

Subnet
A portion of a network that shares a common address component. On TCP/IP networks, subnets are defined as all devices whose IP addresses have the same prefix. For example, all devices with IP addresses that start with 100.100.100. would be part of the same subnet. Dividing a network into subnets is useful for both security and performance reasons. IP networks are divided using a subnet mask.

Subnet Mask
A mask used to determine what subnet an IP address belongs to. An IP address has two components, the network address and the host address. For example, consider the IP address 150.215.017.009. Assuming this is part of a Class B network, the first two numbers (150.215) represent the Class B network address, and the second two numbers (017.009) identify a particular host on this network.

SVC
Short for switched virtual circuit, a temporary virtual circuit that is set up and used only as long as data is being transmitted. Once the communication between the two hosts is complete, the SVC disappears. In contrast, a permanent virtual circuit (PVC) remains available at all times.

TC
Short for Telecommunications Closet, an area, typically a room or closet, that houses all the equipment associated with telecommunications wiring systems. The TC also serves as a termination point for the horizontal cabling system of a network, the point of circuit administration and contains the network's distribution panels, cross-connects and backbone. All telecommunications wiring is channeled through the TC. The TC may also house auxiliary power supplies for workstation equipment. The larger the network, the more TCs are needed since the end workstations can only be a certain distance away from the TC because of constraints in the type of wiring used. Networks that span multi-level buildings, such as hospitals, typically have a TC on each floor.

Telematics
Refers to the broad industry related to using computers in concert with telecommunications systems. This includes dial-up service to the Internet as well as all types of networks that rely on a telecommunications system to transport data.

Telenet
One of the largest public data networks (PDNs) in the United States. Telenet is owned by U.S. Sprint Communications Corporation. A competing network, called Tymnet, is owned by McDonnell Douglas. Telenet serves as the communications backbone for many online services.

Telnet
A terminal emulation program for TCP/IP networks such as the Internet. The Telnet program runs on your computer and connects your PC to a server on the network. You can then enter commands through the Telnet program and they will be executed as if you were entering them directly on the server console. This enables you to control the server and communicate with other servers on the network. To start a Telnet session, you must log in to a server by entering a valid username and password. Telnet is a common way to remotely control Web servers.

Throughput
The amount of data transferred from one place to another or processed in a specified amount of time. Data transfer rates for disk drives and networks are measured in terms of throughput. Typically, throughputs are measured in kbps, Mbps and Gbps.

Token Passing
A type of CAM. Token passing uses a token, or series of bits, to grant a device permission to transmit over the network. Whichever device has the token can put data into the network. When its transmission is complete, the device passes the token along to the next device in the topology. System rules in the protocol specifications mandate how long a device may keep the token, how long it can transmit for and how to generate a new token if there isn't one circulating.

Traffic
The load on a communications device or system. One of the principal jobs of a system administrator is to monitor traffic levels and take appropriate actions when traffic becomes heavy.

Tunneling
A technology that enables one network to send its data via another network's connections. Tunneling works by encapsulating a network protocol within packets carried by the second network. For example, Microsoft's PPTP technology enables organizations to use the Internet to transmit data across a virtual private network (VPN). It does this by embedding its own network protocol within the TCP/IP packets carried by the Internet.

Tymnet
One of the largest public data networks (PDNs) in the United States. Tymnet is owned by MCI. A competing network, called Telenet, is owned by U.S. Sprint Communications Corporation.

UPnP
Short for Universal Plug and Play, a networking architecture that provides compatibility among networking equipment, software and peripherals of the 400+ vendors that are part of the Universal Plug and Play Forum.

UPnP works with wired or wireless networks and can be supported on any operating system. UPnP boasts device-driver independence and zero-configuration networking.

vBNS
Short for very high-speed Backbone Network Service, an experimental wide-area network backbone sponsored by the National Science Foundation (NSF) and implemented by MCI. vNBS has replaced NSFnet and is designed to serve as a platform for testing new, high-speed Internet technologies and protocols. It currently links several Supercomputer Centers (SCCs) and Network Access Points (NAPs) at OC-12 speeds (622 Mbps). By 1998, it will support data, voice and video traffic at 2.5 Gbps.

Virtual Circuit
A connection between two devices that acts as though it's a direct connection even though it may physically be circuitous. The term is used most frequently to describe connections between two hosts in a packet-switching network. In this case, the two hosts can communicate as though they have a dedicated connection even though the packets might actually travel very different routes before arriving at their destination. An X.25 connection is an example of a virtual circuit.

Virtual circuits can be either permanent (called PVCs) or temporary (called SVCs).

Virtual Router
An abstract object managed by VRRP that acts as a default router for hosts on a shared LAN. It consists of a Virtual Router Identifier and a set of associated IP addresses across a common LAN.

VLAN
Short for virtual LAN, a network of computers that behave as if they are connected to the same wire even though they may actually be physically located on different segments of a LAN. VLANs are configured through software rather than hardware, which makes them extremely flexible. One of the biggest advantages of VLANs is that when a computer is physically moved to another location, it can stay on the same VLAN without any hardware reconfiguration.

VPN
Short for virtual private network, a network that is constructed by using public wires to connect nodes. For example, there are a number of systems that enable you to create networks using the Internet as the medium for transporting data. These systems use encryption and other security mechanisms to ensure that only authorized users can access the network and that the data cannot be intercepted.

VRRP
Short for Virtual Router Redundancy Protocol. An election protocol that dynamically assigns responsibility for one or more virtual router(s) to the VRRP router(s) on a LAN, allowing several routers on a multiaccess link to utilize the same virtual IP address. A VRRP router is configured to run the VRRP protocol in conjunction with one or more other routers attached to a LAN. In a VRRP setup, one router is elected as the master router with the other routers acting as backups in case of the failure of the master router.

Wide-Area Network
A computer network that spans a relatively large geographical area. Typically, a WAN consists of two or more local-area networks (LANs).

Computers connected to a wide-area network are often connected through public networks, such as the telephone system. They can also be connected through leased lines or satellites. The largest WAN in existence is the Internet.

Wolfpack
The codename for Microsoft's clustering solution. Wolfpack was released in September, 1997 as part of Windows NT 4.0, enterprise Edition. Its official name is Microsoft Cluster Server (MSCS).