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.
services often require a server that is dedicated to providing the three services.
RADIUS is an example of an AAA service.
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 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.
essence, ASPs are a way for companies to outsource some or almost all aspects
of their information technology needs.
may be commercial ventures that cater to customers, or not-for-profit or government
organizations, providing service and support to end users.
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.
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.
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 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.
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.
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
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.
communications between computers, including the majority of local-area networks,
use baseband communications. An exception is B-ISDN networks, which employ broadband
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.
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.
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.
(n) A set of bits, bytes or characters grouped together for transmission.
An intermittent asynchronous transmission of a specific amount of data. Contrast
(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.
Acronym for computer-aided manufacturing, a type of computer application that
helps automate a factory.
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.
(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.
cells are usually identified by a column letter and a row number. For example,
D12 specifies the cell in column D and row 12.
In communications and networking, a fixed-size packet of data.
In cellular telephone systems, a geographic area.
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.
Transfer Mode (ATM) is the cell relay standard set by the CCITT organization.
ATM uses a cell of 53 bytes.
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
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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
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.
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
Network backbone that consists of the backplane of a single switch,
rather than multiple switches connected together.
the ports of the single switch, cables connect to the hubs of individual LAN segments.
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.
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.
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.
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.
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.
(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.
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.
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.
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.
(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.
To connect devices in a daisy chain pattern.
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
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.
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
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.
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.
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.
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.
disadvantage of diskless workstations, however, is that they are useless if the
the workstation is a PC, it is often called a diskless PC or a Net PC.
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.
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.
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
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.
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.
Short for Dense Wavelength Division Multiplexing, an optical technology used to
increase bandwidth over existing fiber optic backbones.
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.
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.
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.
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.
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.
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.
is not proprietary. Elastic Networks plans on licensing the technology to manufacturers
who can design and create their own EtherLoop solutions.
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.
are becoming a very popular means for business partners to exchange information.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
technology must coexist with telephone service and comply with FCC Part 68.
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.
is a DTE /DCE interface developed by Cisco Systems and T3plus Networking to address
the need for high-speed communication over WAN links.
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.
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.
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,
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
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.
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.
(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.
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
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
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.
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
(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.
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.
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.
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.
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.
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.
In networks, local refers to files, devices, and other resources at your workstation.
Resources located at other nodes on the networks are remote.
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).
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
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.
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.
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.
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 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.
loopback interface allows IT professionals to test IP software without worrying
about broken or corrupted drivers or hardware.
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.
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.
its development stage, MSCS was code-named Wolfpack.
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.
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.
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.
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
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
was originally designed by IBM for their Lan Manager server and later extended
by Microsoft and Novell.
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.
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.
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.
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.
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.
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.
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.
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.
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.
(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.
In tree structures, a point where two or more lines meet.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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
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.
(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
Making continuous requests for data from another device. For example, modems that
support polling can call another system and request data.
Passive Optical Network is a high bandwidth point to multipoint optical fibre
network based on the asynchronous transfer mode protocol (ATM), Ethernet or TDM.
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.
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.
rely on lightwaves for data transfer.
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.
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.
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.
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.
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.
(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.
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.
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.
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.
play a central role in Frame Relay networks. They're also supported in some other
types of networks, such as X.25.
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.
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.
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.
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.
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.
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
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
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.
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
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.
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.
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.
Wireles Access Protocol
Developed by the HomeRF Working Group, SWAP is
a specification for wireless voice and data networking in the home.
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.
operates in the 2400 MHz band at 50 hops per second. Data travels at a rate between
1 Mbps and 2 Mbps.
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.
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.
(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.
A receptacle into which a plug can be inserted.
A receptacle for a microprocessor or other hardware component.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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
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.
works with wired or wireless networks and can be supported on any operating system.
UPnP boasts device-driver independence and zero-configuration networking.
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.
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.
circuits can be either permanent (called PVCs) or temporary (called SVCs).
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.
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.
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.
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.
A computer network that spans a relatively large geographical
area. Typically, a WAN consists of two or more local-area networks (LANs).
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.
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).