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Remote ControlRemote control allows a remote user on a client computer to control another computer, called the host, across a LAN or WAN connection.. Another example of remote control so

Remote Control

Remote control allows a remote user on a client computer to control another computer, called

the host, across a LAN or WAN connection This type of remote access first requires a nection between the client and host The connection could be a dedicated WAN line (such as

con-a T1), con-an Internet connection, or even con-a dicon-al-up connection estcon-ablished directly between theclient’s modem and the host’s modem Also, the host must be configured to allow access fromthe client by setting user name or computer name and password credentials A host may allowclients a variety of privileges, from merely viewing the screen to running programs and modi-fying data files on the host’s hard disk After connecting, if the remote user has sufficient priv-ileges, she can send keystrokes and mouse clicks to the host and receive screen output inreturn In other words, to the remote user, it appears as if she is working on the LAN- or WAN-connected host Remote control software is specially designed to require little bandwidth, whichmakes it suitable for use over dial-up connections

One example of such remote control software is Symantec’s pcAnywhere Another example of remote control software is the Remote Desktop feature that comes with the Windows 95, 98,

NT, 2000, XP, and Server 2003 operating systems Remote Desktop relies on the RDP (Remote Desktop Protocol), which is an Application layer protocol that uses TCP/IP to trans-

mit graphics and text quickly RDP also carries session, licensing, and encryption information

To enable your Windows XP Professional computer as a Remote Desktop host:

1. First log on to the computer as Administrator or another user name with tor-level privileges

administra-2. Click Start, and then click Control Panel If necessary, click Switch to Category View The Control Panel window opens in Category view.

FIGURE 7-24 Protocols used in a remote access Internet connection

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3. Click Performance and Maintenance, and then click the System icon The System

Properties dialog box opens

4. Click the Remote tab Options for remote connections to your computer appear, as

shown in Figure 7-25

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FIGURE 7-25 Remote tab in the Windows XP System Properties window

5. Check the Allow remote users to connect remotely to this computer option.

6. If this is the first time you’ve enabled remote services, the Remote Sessions windowopens, alerting you that accounts used for remote access must have passwords to con-

nect to your computer Click OK.

7. Click Select Remote Users to choose from a list of users who you will allow to nect to your computer The Remote Desktop Users dialog box opens.

con-8. Click Add to add a user to the list The Select Users dialog box opens If you have

created multiple user accounts on your computer, these accounts will be listed under

“Enter object names to select (examples):”

9. Check the user names that will have access to your computer, and then click OK.

10. Click OK again to close the Remote Desktop Users dialog box.

11. Click OK once more to close the System Properties dialog box and save your changes.

The previous steps describe how to establish your computer as a host To start a remote top session from a Windows XP client:

desk-1. Make sure the remote desktop client software has been installed on the computer

Also make sure that the host and remote computers are connected to networks thatcan exchange data (for example, the host might be a desktop on a company’s officeWAN and the remote client might be a home computer that can connect to thatWAN over the Internet)

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2. Click Start, point to All Programs, point to Accessories, point to Communications, and then click Remote Desktop Connection The Remote Desktop Connection win-

dow opens, as shown in Figure 7-26

FIGURE 7-26 Remote Desktop Connection window

3. In the Computer: text box, enter the name of the host computer to which you want toconnect The host computer must be running the Remote Desktop software and youmust have permission to log on to it

4. Click Connect.

5. In the Log On to Windows dialog box, type your user name, password, and domain (if necessary), and then click OK to log on to this host.

6. The Remote Desktop window opens, showing you the desktop of the host computer

At this point, your keystrokes and mouse clicks will act on the host computer, not onyour client computer

Although remote control is used less often than other forms of remote access, some situationscall for it For example, suppose a traveling salesperson must submit weekly sales figures to herhome office every Friday afternoon While out of town, she discovers a problem with herspreadsheet program, which should automatically calculate her sales figures (for example, thepercentage of a monthly quota she’s reached for any given product) after she enters the rawdata She calls the home office, and a support technician attempts to resolve her issue on thephone When this doesn’t work, the technician may decide to run a remote control programand “take over” the salesperson’s PC (over a WAN link) to troubleshoot the spreadsheet prob-lem Every keystroke and mouse click the technician enters on his PC is then issued to thesalesperson’s PC After the problem is resolved, the technician can disconnect from the sales-person’s PC

Advantages to using the remote control access method are that it is simple to configure andcan run over any type of connection This benefits telecommuters who must use dial-up con-nections and who need to work with processing-intensive applications such as databases Inthis scenario, the data processing occurs on the host without the data having to traverse theslower modem connection to the remote workstation Another advantage to remote controlconnections is that a single host can accept simultaneous connections from multiple clients Apresenter can use this feature to establish a virtual conference, for example, in which several

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attendees log on to the host and watch the presenter manipulate the host computer’s screen andkeyboard However, network managers don’t favor remote control connections because theyoffer minimal security Although remote control software requires a user to log on with an IDand password, the connection does not go through the network backbone, where stricter secu-rity controls are apt to be in place If frequent remote access to processing-intensive applica-tions is necessary, a better solution would be to use terminal services, as described in thefollowing section.

Terminal Services

A popular method for gaining remote access to LANs is by using terminal services In nal services, multiple remote computers can connect to a terminal server on the LAN A ter- minal server is a computer that runs specialized software that allows it to act as a host and

supply applications and resource sharing to remote clients As with remote control, in nal services remote users send only keystrokes and mouse clicks and receive screen updates fromthe host To the remote user, connecting to a LAN from afar appears no different from being

termi-a directly connected LAN user However, termintermi-al services differ from remote control in termi-a fewkey ways First, a terminal server allows multiple simultaneous connections Second, a termi-nal server is optimized for fast processing and application handling, offering better performancefor remote users than could a LAN-connected workstation Third, implementing terminal ser-vices requires more sophisticated software and significant configuration For example, it allowsusers to connect via any type of media (not only a modem and phone line) Also, a terminalserver can be situated on the network such that remote user connections must pass throughfirewalls, switches, and routers and be subject to security, addressing, resource access, andVLAN controls, if applicable As a result, this option offers much greater flexibility and secu-rity than remote control

Many companies have created software to supply terminal services In fact, the Microsoft

ver-sion of this solution is called Terminal Services (Windows XP clients connecting to a

Microsoft terminal server use the Remote Desktop software described previously.) Another

popular option is Citrix System, Inc.’s Metaframe With the Citrix option, remote tions rely on software known as an ICA (Independent Computing Architecture) client to

worksta-connect with a remote access server and exchange keystrokes, mouse clicks, and screen updates.Citrix’s ICA client can work with virtually any operating system or application Its ease of useand broad compatibility have made the ICA client one of the most popular methods for sup-plying widespread remote access across an organization Potential drawbacks to this methodinclude the relatively high cost of Citrix’s products and the complex nature of its server soft-ware configuration

A workstation that uses terminal services to access a LAN is often called a thin client, because

very little hard disk space or processing power is required of the workstation In fact, the termthin client can apply to any end-user workstation that relies on another networked computer

to bear primary processing and disk access responsibilities, including clients that connectthrough Web portals, as discussed next

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Web Portals

Another remote access option that’s growing in popularity is running LAN applications from

a Web portal A Web portal is simply a secure, Web-based interface to an application This

option is attractive because it places few requirements on the client Users merely need an net connection, Web browser software, and the proper credentials to log on to the application.Any type of Internet connection is sufficient for using Web portals, though of course, a DSL

Inter-or broadband cable connection perfInter-orms better than a PSTN connection

On the host side, a Web server supplies the application to multiple users upon request ever, first an application must be designed for Web-based access Making applications Web-ready typically requires significant programming However, more and more applications arebeing designed this way from the start In addition, managers must carefully configure theaccess properties for the Web server hosting the application to make sure only authorized userscan access the application In fact, a company may decide to outsource its Web portal services

How-to an ISP In that case, the company pays the ISP How-to provide connectivity, house and maintainthe Web server, make sure the application is operating correctly, and prevent unauthorizedaccess to the application

As you can imagine, making an application accessible via the Web also makes it vulnerable touse by unauthorized individuals Thus, the use of Web portals calls for secure transmissionprotocols Secure transmission protocols are also integral to creating virtual private networks,which are discussed in the following section

VPNs (Virtual Private Networks)

VPNs (virtual private networks) are wide area networks logically defined over public

trans-mission systems To allow access to only authorized users, traffic on a VPN is isolated fromother traffic on the same public lines For example, a national insurance provider could estab-lish a private WAN that uses Internet connections but serves only its agent offices across thecountry By relying on the public transmission networks already in place, VPNs provide a way

of constructing a convenient and relatively inexpensive WAN In the example of a nationalinsurance provider, the company gains significant savings by having each office connect to theInternet separately rather than leasing point-to-point connections between each office and thenational headquarters

The software required to establish VPNs is usually inexpensive, and in some cases is beingincluded with other widely used software For example, the Windows Server 2003 RRASallows you to create a simple VPN by turning a Windows server into a remote access serverand allowing clients to dial into it Alternately, clients could dial into an ISP’s remote accessserver, then connect with the VPN managed by RRAS For Novell-based networks, you canuse BorderManager, a NetWare add-on product, to connect nodes and form a VPN Third-party software companies also provide VPN programs that work with NetWare, Windows,UNIX, Linux, and Macintosh OS X Server network operating systems Or VPNs can be

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created simply by configuring special protocols on the routers or firewalls that connect each site

in the VPN This is the most common implementation of VPNs on UNIX-based networks.Figure 7-27 depicts one possible VPN layout The beauty of VPNs is that they are tailored to

a customer’s distance and bandwidth needs, so, of course, every one is different

PPTP (Point-to-Point Tunneling Protocol) is a protocol developed by Microsoft that expands

on PPP by encapsulating it so that any type of PPP data can traverse the Internet masked as

an IP or IPX transmission PPTP supports the encryption, authentication, and access servicesprovided by the Windows Server 2003 RRAS (and previous versions of this remote access soft-ware) Users can either dial directly into an RRAS access server that’s part of the VPN, or theycan dial into their ISP’s remote access server first, then connect to a VPN Either way, data istransmitted from the client to the VPN using PPTP Windows, UNIX, Linux, and Macintosh

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clients are all capable of connecting to a VPN using PPTP PPTP is easy to install, and isavailable at no extra cost with Microsoft networking services However, it provides less strin-gent security than other tunneling protocols.

Another VPN tunneling protocol is L2TP (Layer 2 Tunneling Protocol), based on

technol-ogy developed by Cisco and standardized by the IETF It encapsulates PPP data in a similarmanner to PPTP, but differs in a few key ways Unlike PPTP, L2TP is a standard acceptedand used by multiple different vendors, so it can connect a VPN that uses a mix of equipmenttypes—for example, a 3Com router, a Cisco router, and a NetGear router Also, L2TP canconnect two routers, a router and a remote access server, or a client and a remote access server.Another important advantage to L2TP is that tunnel endpoints do not have to reside on thesame packet-switched network In other words, an L2TP client could connect to a router run-ning L2TP on an ISP’s network The ISP could then forward the L2TP frames to anotherVPN router, without interpreting the frames This L2TP tunnel, although not direct from node

to node, remains isolated from other traffic Because of its many advantages, L2TP is morecommonly used than PPTP

PPTP and L2TP are not the only protocols that can be used to carry VPN traffic For works where security is critical, it is advisable to use protocols that can provide both tunnelingand data encryption Such protocols are discussed in detail in Chapter 14, which focuses onnetwork security

net-Chapter Summary

◆ WANs are distinguished from LANs by the fact that WANs traverse a wider graphical area They usually employ point-to-point, dedicated communicationsrather than point-to-multipoint communications They also use different connectiv-ity devices, depending on the WAN technology in use

geo-◆ A WAN in which each site is connected in a serial fashion to no more than twoother sites is known as a bus topology WAN This topology often provides the bestsolution for organizations with only a few sites and access to dedicated circuits

◆ In a ring topology WAN, each site is connected to two other sites so that the entireWAN forms a ring pattern This architecture is similar to the LAN ring topology,except that most ring topology WANs have the capability to reverse the directiondata travels to avoid a failed site

◆ In the star topology WAN, a single site acts as the central connection point for eral other points This arrangement allows one connection to fail without affectingother connections Therefore, star topology WANs are more fault-tolerant than bus

sev-or ring WANs

◆ A mesh topology WAN consists of many directly interconnected sites In partialmesh WANs, only some of the WAN sites are directly interconnected In full meshWANs, every site is directly connected to every other site The full mesh topology isthe most fault-tolerant and also the most expensive WAN topology to implement

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◆ A tiered topology WAN is one in which sites that are connected in star or ring mations are interconnected at different levels, with the interconnection points beingorganized into layers to form hierarchical groupings.

for-◆ The PSTN (Public Switched Telephone Network) is the network of lines and

switching centers that provides traditional telephone service It was originally posed of analog lines alone, but now also uses digital transmission over fiber-opticand copper twisted-pair cable, microwave, and satellite connections The local loopportion of the PSTN is still primarily UTP; it is this portion that limits throughput

com-on the PSTN

◆ A remote user can use the PSTN to access a remote server via a dial-up connection

In a dial-up connection, the user’s modem converts the computer’s digital pulses intoanalog signals These signals travel through PSTN to the receiving computer’s

modem, which then converts the analog signals back into digital pulses Unlike othertypes of WAN connections, dial-up connections provide a fixed period of access tothe network Throughput is limited to a maximum of 53 Kbps

◆ X.25 is an analog, packet-switched technology optimized for reliable, long-distancedata transmission It can support 2-Mbps throughput X.25 was originally developedand used for communications between mainframe computers and remote terminals.Though less common in North America, it remains a WAN standard around the

world

◆ Frame Relay, like X.25, relies on packet switching, but carries digital signals It is

digital, and it does not analyze frames to check for errors, but simply relays them

from node to node, so Frame Relay supports higher bandwidth than X.25, offering amaximum of 45-Mbps throughput

◆ Both X.25 and Frame Relay are configured as PVCs (permanent virtual circuits), orpoint-to-point connections over which data may follow different paths When leas-ing an X.25 or Frame Relay circuit from a telecommunications carrier, a customerspecifies endpoints and the amount of bandwidth required between them

◆ ISDN (Integrated Services Digital Network) is an international standard for cols at the Physical, Data Link, and Transport layers that allows the PSTN to carrydigital signals ISDN lines may carry voice and data signals simultaneously, but

proto-require an ISDN phone to carry voice traffic and an ISDN router and ISDN nal adapter to carry data

termi-◆ Two types of ISDN connections are commonly used by consumers in North ica: BRI (Basic Rate Interface) and PRI (Primary Rate Interface) Both use a combi-nation of bearer channels (B channels) and data channels (D channels) B channelstransmit and receive data or voice from point to point The D channel carries infor-mation about the call, such as session initiation and termination signals, caller iden-tity, call forwarding, and conference calling signals

Amer-◆ BRI uses two 64-Kbps circuit-switched B channels and a 16-Kbps D channel Themaximum throughput for a BRI connection is 128 Kbps PRI uses 23 B channels andone 64-Kbps D channel The maximum potential throughput for a PRI connection is

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1.544 Mbps Individual subscribers rarely use PRI, preferring BRI instead, but PRImay be used by businesses and other organizations that need more throughput.

◆ T-carrier technology uses TDM (time division multiplexing) to divide a single nel into multiple channels for carrying voice, data, video, or other signals Devices atthe sending end arrange the data streams (multiplex), then devices at the receivingend filter them back into separate signals (demultiplex)

chan-◆ The most common T-carrier implementations are T1 and T3 A T1 circuit can carrythe equivalent of 24 voice channels, giving a maximum data throughput of 1.544Mbps A T3 circuit can carry the equivalent of 672 voice channels, giving a maxi-mum data throughput of 44.736 Mbps

◆ The signal level of a T-carrier refers to its Physical layer electrical signaling teristics, as defined by ANSI standards DS0 is the equivalent of one data or voicechannel All other signal levels are multiples of DS0

charac-◆ T1 technology can use UTP or STP However, twisted-pair wiring cannot quately carry the high throughput of multiple T1s or T3 transmissions For T3transmissions, fiber-optic cable or microwave connections are necessary

ade-◆ The CSU/DSU is the connection point for a T1 line at the customer’s site TheCSU/DSU provides termination for the digital signal, ensures connection integritythrough error correction and line monitoring, and converts the T-carrier frames intoframes the LAN can interpret, and vice versa It also connects T-carrier lines withterminating equipment A CSU/DSU often includes a multiplexer

◆ DSL uses advanced phase or amplitude modulation in the higher (inaudible) quencies on a phone line to achieve throughputs of up to 51.8 Mbps DSL comes ineight different varieties, each of which is either asymmetrical or symmetrical Inasymmetrical transmission, more data can be sent in one direction than in the otherdirection In symmetrical transmission, throughput is equal in both directions Themost popular form of DSL is ADSL

fre-◆ DSL technology creates a dedicated circuit At the consumer end, a DSL modemconnects computers and telephones to the DSL line At the carrier end, a DSLAM(DSL access multiplexer) aggregates multiple incoming DSL lines before connectingthem to the Internet or to larger carriers

◆ Broadband cable is a dedicated service that relies on the cable wiring used for TVsignals The service can theoretically provide as much as 36-Mbps downstream and10-Mbps upstream throughput, though actual throughput is much lower The asym-metry of cable technology makes it a logical choice for users who want to surf theWeb or download data from a network

◆ Broadband cable connections require that the customer use a special cable modem totransmit and receive signals over coaxial cable wiring In addition, cable companiesmust have replaced their coaxial cable plant with hybrid fiber-coax cable to supportbidirectional, digital communications

◆ SONET is a high-bandwidth WAN signaling technique that specifies framing andmultiplexing techniques at the Physical layer of the OSI Model Its four key

strengths are that it can integrate many other WAN technologies (for example, carriers, ISDN, and ATM technology), it offers fast data transfer rates, it allows forsimple link additions and removals, and it provides a high degree of fault tolerance.Internationally, SONET is known as SDH

T-◆ SONET depends on fiber-optic transmission media and uses multiplexers to nect to network devices (such as routers or telephone switches) at the customer’s

con-end A typical SONET network takes the form of a dual-ring topology If one ringbreaks, SONET technology automatically reroutes traffic along a backup ring Thischaracteristic, known as self-healing, makes SONET very reliable

◆ Wireless Internet access can be achieved through one of several technologies

Libraries, universities, coffee shops, and airports might offer access by allowing thepublic to connect with their IEEE 802.11 (a, b, or g) access points These organiza-tions, in turn, connect their access points to dedicated, high-speed Internet connec-tions such as T1 links

◆ IEEE 802.16a (WiMAX) is a wireless Internet access technology designed for

MANs It relies on antennas that do not require line-of-sight paths to exchange dataand have ranges up to 20 miles WiMAX can achieve throughputs of up to 70 Mbpsusing the 2–10GHz frequency range

◆ Geosynchronous satellites are used to provide Internet access This type of setup

requires a stationary antenna at the customer’s premises, which is connected to a

modem connected to the customer’s computer Downstream throughput for satelliteInternet access is advertised at throughputs of 400 Kbps, but is often higher In thecase of a dial return arrangement, upstream throughputs are limited by the analogtelephone line’s 53-Kbps maximum throughput

◆ As a remote user, you can connect to a LAN or WAN in one of several ways:

dial-up networking, connecting to a remote access server, remote control, terminal vices, Web portals, or through a VPN (virtual private network)

ser-◆ Dial-up networking involves a remote client dialing into a remote access server andconnecting via a PSTN, X.25, or ISDN connection The client must run dial-up

software to initiate the connection and the server runs specialized remote access

software to accept and interpret the incoming signals The Microsoft RAS softwareprovides dial-up connectivity on Windows 95, 98, NT, and 2000 client operating

systems and its Windows NT and 2000 network operating systems

◆ Remote access servers accept incoming connections from remote clients, cate users, allow them to log on to a LAN or WAN, and exchange data by encapsu-lating higher-layer protocols, such as TCP and IP in specialized protocols such asPPP The Microsoft RRAS (Routing and Remote Access Service) is the remote

authenti-access software that comes with the Windows XP and Server 2003 operating

systems

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◆ To exchange data, remote access servers and clients must communicate through cial Data Link layer protocols, such as PPP or SLIP, that encapsulate higher-layerprotocols, such as TCP and IP PPP is the preferred protocol When PPP is used on

spe-an Ethernet network, as is the case with most modern broadbspe-and Internet tions, it is called PPP over Ethernet, or PPPoE

connec-◆ Remote control uses specialized client and host software to allow a remote user toconnect via modem to a LAN-attached workstation and control that host Afterconnecting, the remote user can perform functions just as if she were directly con-nected to the LAN Remote Desktop is a remote control client and server packagethat comes with Windows 95, 98, NT, 2000, XP, and Server 2003 operating systems

◆ In terminal services, a special terminal server allows simultaneous LAN access formultiple remote users It requires specialized client and server software Terminalservers are optimized for fast processing and application handling They are oftenconnected to the network in such a way as to subject remote users to typical router,firewall, and other access controls

◆ A Web portal supplies Web-based applications to remote users who gain accessthrough any type of Internet connection This option requires applications to bedesigned for Web use and also requires stringent security controls on the Web server

◆ VPNs (virtual private networks) represent one way to construct a WAN from ing public transmission systems A VPN offers connectivity only to an organization’susers, while keeping the data secure and isolated from other (public) traffic Toaccomplish this, VPNs may be software- or hardware-based Either way, theydepend on secure protocols and transmission methods to keep data private

exist-◆ To make sure a VPN can carry all types of data in a private manner over any kind ofconnection, special VPN protocols encapsulate higher-layer protocols via tunneling.Common tunneling protocols include PPTP and L2TP

Key Terms

802.16—An IEEE standard for wireless MANs that specifies the use of frequency ranges

between 10 and 66 GHz and requires line-of-sight paths between antennas 802.16 antennascan cover 50 kilometers (or approximately 30 miles) and connections can achieve a maximumthroughput of 70 Mbps

802.16a—An IEEE standard for wireless MANs that specifies the use of the frequency ranges

between 2 and 11 GHz In IEEE 802.16a, antennas do not require a line-of-sight path betweenthem and can exchange signals with multiple stations at once 802.16a is capable of achieving

up to 70-Mbps throughput and its range is 50 kilometers (or approximately 30 miles)

asymmetrical—The characteristic of a transmission technology that affords greater

band-width in one direction (either from the customer to the carrier, or vice versa) than in the otherdirection

asymmetrical DSL—A variation of DSL that offers more throughput when data travels

down-stream, downloading from a local carrier’s switching facility to the customer, than when ittravels upstream, uploading from the customer to the local carrier’s switching facility

asynchronous—A transmission method in which data being transmitted and received by nodes

does not have to conform to any timing scheme In asynchronous communications, a node cantransmit at any time and the destination node must accept the transmission as it comes

authentication—The process of comparing and matching a client’s credentials with the

cre-dentials in the NOS user database to enable the client to log on to the network

B channel—In ISDN, the “bearer” channel, so named because it bears traffic from point to

point

Basic Rate Interface—See BRI.

bonding—The process of combining more than one bearer channel of an ISDN line to increase

throughput For example, BRI’s two 64-Kbps B channels are bonded to create an effectivethroughput of 128 Kbps

BRI (Basic Rate Interface)—A variety of ISDN that uses two 64-Kbps bearer channels and

one 16-Kbps data channel, as summarized by the notation 2B+D BRI is the most commonform of ISDN employed by home users

broadband cable—A method of connecting to the Internet over a cable network In

broad-band cable, computers are connected to a cable modem that modulates and demodulates nals to and from the cable company’s head-end

sig-bus topology WAN—A WAN in which each location is connected to no more than two other

locations in a serial fashion

cable drop—A fiber-optic or coaxial cable that connects a neighborhood cable node to a

cus-tomer’s house

cable modem—A device that modulates and demodulates signals for transmission and

recep-tion via cable wiring

cable modem access—See broadband cable.

central office—The location where a local or long-distance telephone service provider

termi-nates and interconnects customer lines

channel service unit—See CSU.

CIR (committed information rate)—The guaranteed minimum amount of bandwidth

selected when leasing a Frame Relay circuit Frame Relay costs are partially based on CIR

committed information rate—See CIR.

credentials—A user’s unique identifying characteristics that enable him to authenticate with

a server and gain access to network resources The most common type of credentials are a username and password

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CSU (channel service unit)—A device used with T-carrier technology that provides

termina-tion for the digital signal and ensures connectermina-tion integrity through error correctermina-tion and linemonitoring Typically, a CSU is combined with a DSU in a single device, a CSU/DSU

CSU/DSU—A combination of a CSU (channel service unit) and a DSU (data service unit)

that serves as the connection point for a T1 line at the customer’s site Most modernCSU/DSUs also contain a multiplexer A CSU/DSU may be a separate device or an expansioncard in another device, such as a router

D channel—In ISDN, the “data” channel is used to carry information about the call, such as

session initiation and termination signals, caller identity, call forwarding, and conference ing signals

call-data service unit—See DSU.

dedicated—A continuously available link or service that is leased through another carrier.

Examples of dedicated lines include ADSL, T1, and T3

dial return—A satellite Internet access connection in which a subscriber receives data from

the Internet via the satellite link, but sends data to the satellite via an analog modem (dial-up)connection With dial return, downstream throughputs are rated for 400–500 Kbps, whereasupstream throughputs are practically limited to 53 Kbps and are usually lower Therefore, dialreturn satellite Internet access is an asymmetrical technology

dial-up—A type of connection in which a user connects to a distant network from a computer

and stays connected for a finite period of time

dial-up networking—The process of dialing into a remote access server to connect with a

network, be it private or public

digital subscriber line—See DSL.

downlink—A connection from an orbiting satellite to an earth-based receiver.

downstream—A term used to describe data traffic that flows from a carrier’s facility to the

cus-tomer In asymmetrical communications, downstream throughput is usually much higher thanupstream throughput In symmetrical communications, downstream and upstream throughputsare equal

DS0 (digital signal, level 0)—The equivalent of one data or voice channel in T-carrier

tech-nology, as defined by ANSI physical layer standards All other signal levels are multiples ofDS0

DSL (digital subscriber line)—A dedicated WAN technology that uses advanced data

mod-ulation techniques at the Physical layer to achieve extraordinary throughput over regularphone lines DSL comes in several different varieties, the most common of which is asymmetricDSL (ADSL)

DSL access multiplexer—See DSLAM.

DSL modem—A device that demodulates an incoming DSL signal, extracting the

informa-tion and passing it to the data equipment (such as telephones and computers) and modulates

an outgoing DSL signal

DSLAM (DSL access multiplexer)—A connectivity device located at a telecommunications

carrier’s office that aggregates multiple DSL subscriber lines and connects them to a largercarrier or to the Internet backbone

DSU (data service unit)—A device used in T-carrier technology that converts the digital

sig-nal used by bridges, routers, and multiplexers into the digital sigsig-nal used on cabling Typically,

a DSU is combined with a CSU in a single device, a CSU/DSU

E1—A digital carrier standard used in Europe that offers 30 channels and a maximum of

Frame Relay—A digital, packet-switched WAN technology whose protocols operate at the

Data Link layer The name is derived from the fact that data is separated into frames, whichare then relayed from one node to another without any verification or processing Frame Relayoffers throughputs between 64 Kbps and 45 Mbps A Frame Relay customer chooses theamount of bandwidth he requires and pays for only that amount

full mesh WAN—A version of the mesh topology WAN in which every site is directly

con-nected to every other site Full mesh WANs are the most fault-tolerant type of WAN

GEO (geosynchronous orbit or geostationary orbit)—The term used to refer to a satellite

that maintains a constant distance from a point on the equator at every point in its orbit synchronous satellites are the type used to provide satellite Internet access

Geo-geostationary orbit—See GEO.

geosynchronous—See GEO.

head-end—A cable company’s central office, which connects cable wiring to many nodes

before it reaches customers’ sites

HFC (hybrid fiber-coax)—A link that consists of fiber cable connecting the cable company’s

offices to a node location near the customer and coaxial cable connecting the node to the tomer’s house HFC upgrades to existing cable wiring are required before current TV cablesystems can provide Internet access

cus-hot spot—An area covered by a wireless access point that provides visitors with wireless

ser-vices, including Internet access

hybrid fiber-coax—See HFC.

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ICA (Independent Computing Architecture) client—The software from Citrix Systems, Inc.

that, when installed on a client, enables the client to connect with a remote access server andexchange keystrokes, mouse clicks, and screen updates Citrix’s ICA client can work with vir-tually any operating system or application

Integrated Services Digital Network—See ISDN.

ISDN (Integrated Services Digital Network)—An international standard that uses PSTN

lines to carry digital signals It specifies protocols at the Physical, Data Link, and Transportlayers of the OSI Model ISDN lines may carry voice and data signals simultaneously Twotypes of ISDN connections are used in North America: BRI (Basic Rate Interface) and PRI(Primary Rate Interface) Both use a combination of bearer channels (B channels) and datachannels (D channels)

J1—A digital carrier standard used in Japan that offers 24 channels and 1.544-Mbps

through-put

J3—A digital carrier standard used in Japan that offers 480 channels and 32.064-Mbps

throughput

L2TP (Layer 2 Tunneling Protocol)—A protocol that encapsulates PPP data, for use on

VPNs L2TP is based on Cisco technology and is standardized by the IETF It is guished by its compatibility among different manufacturers’ equipment, its ability to connectbetween clients, routers, and servers alike, and also by the fact that it can connect nodes belong-ing to different Layer 3 networks

distin-last mile—See local loop.

Layer 2 Tunneling Protocol—See L2TP.

LEO (low earth orbiting)—A type of satellite that orbits the earth with an altitude between

700 and 1400 kilometers, closer to the earth’s poles than the orbits of either GEO or MEOsatellites LEO satellites cover a smaller geographical range than GEO satellites and requireless power

local loop—The part of a phone system that connects a customer site with a

telecommunica-tions carrier’s switching facility

low earth orbiting—See LEO.

medium earth orbiting–See MEO.

MEO (medium earth orbiting)—A type of satellite that orbits the earth 10,390 kilometers

above its surface, positioned between the equator and the poles MEO satellites can cover alarger area of the earth’s surface than LEO satellites while using less power and causing lesssignal delay than GEO satellites

mesh topology WAN—A type of WAN in which several sites are directly interconnected.

Mesh WANs are highly fault-tolerant because they provide multiple routes for data to followbetween any two points