Network Plus 2005 In Depth

342Chapter 7 WANS, INTERNET ACCESS, AND REMOTE CONNECTIVITY

To exchange data, remote access servers and clients must communicate through special Data Link layer protocols, such as PPP or SLIP, that encapsulate higher-layer protocols, such as TCP and IP. PPP is the preferred protocol. When PPP is used on an Ethernet network, as is the case with most modern broadband Internet connections, it is called PPP over Ethernet, or PPPoE.

Remote control uses specialized client and host software to allow a remote user to connect via modem to a LAN-attached workstation and control that host. After connecting, the remote user can perform functions just as if she were directly connected to the LAN. Remote Desktop is a remote control client and server package that comes with Windows 95, 98, NT, 2000, XP, and Server 2003 operating systems.

In terminal services, a special terminal server allows simultaneous LAN access for multiple remote users. It requires specialized client and server software. Terminal servers are optimized for fast processing and application handling. They are often connected 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 access through any type of Internet connection. This option requires applications to be designed 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 existing public transmission systems. A VPN offers connectivity only to an organization’s users, while keeping the data secure and isolated from other (public) traffic. To accomplish this, VPNs may be softwareor hardware-based. Either way, they depend on secure protocols and transmission methods to keep data private.

To make sure a VPN can carry all types of data in a private manner over any kind of connection, 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 antennas can cover 50 kilometers (or approximately 30 miles) and connections can achieve a maximum throughput 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 between them 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 bandwidth in one direction (either from the customer to the carrier, or vice versa) than in the other direction.

asymmetrical DSL—A variation of DSL that offers more throughput when data travels downstream, downloading from a local carrier’s switching facility to the customer, than when it travels 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 can transmit 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 credentials 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 effective throughput 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 common form of ISDN employed by home users.

broadband cable—A method of connecting to the Internet over a cable network. In broadband cable, computers are connected to a cable modem that modulates and demodulates signals to and from the cable company’s head-end.

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 customer’s house.

cable modem—A device that modulates and demodulates signals for transmission and reception via cable wiring.

cable modem access—See broadband cable.

central office—The location where a local or long-distance telephone service provider terminates 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 user name and password.

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CSU (channel service unit)—A device used with T-carrier technology that provides termination for the digital signal and ensures connection integrity through error correction and line monitoring. 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 modern CSU/DSUs also contain a multiplexer. A CSU/DSU may be a separate device or an expansion card 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 calling signals.

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, whereas upstream throughputs are practically limited to 53 Kbps and are usually lower. Therefore, dial return 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 customer. In asymmetrical communications, downstream throughput is usually much higher than upstream throughput. In symmetrical communications, downstream and upstream throughputs are equal.

DS0 (digital signal, level 0)—The equivalent of one data or voice channel in T-carrier technology, as defined by ANSI physical layer standards. All other signal levels are multiples of DS0.

DSL (digital subscriber line)—A dedicated WAN technology that uses advanced data modulation techniques at the Physical layer to achieve extraordinary throughput over regular phone lines. DSL comes in several different varieties, the most common of which is asymmetric DSL (ADSL).

DSL access multiplexer—See DSLAM.

DSL modem—A device that demodulates an incoming DSL signal, extracting the information 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 larger carrier or to the Internet backbone.

DSU (data service unit)—A device used in T-carrier technology that converts the digital signal used by bridges, routers, and multiplexers into the digital signal 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 2.048-Mbps throughput.

E3—A digital carrier standard used in Europe that offers 480 channels and a maximum of 34.368-Mbps throughput.

fractional T1—An arrangement that allows a customer to lease only some of the channels on a T1 line.

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, which are then relayed from one node to another without any verification or processing. Frame Relay offers throughputs between 64 Kbps and 45 Mbps. A Frame Relay customer chooses the amount 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 connected 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. Geosynchronous satellites are the type used to provide satellite Internet access.

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 customer’s house. HFC upgrades to existing cable wiring are required before current TV cable systems can provide Internet access.

hot spot—An area covered by a wireless access point that provides visitors with wireless services, 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 and exchange keystrokes, mouse clicks, and screen updates. Citrix’s ICA client can work with virtually 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 Transport layers of the OSI Model. ISDN lines may carry voice and data signals simultaneously. Two types 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 data channels (D channels).

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

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 distinguished by its compatibility among different manufacturers’ equipment, its ability to connect between clients, routers, and servers alike, and also by the fact that it can connect nodes belonging to different Layer 3 networks.

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 MEO satellites. LEO satellites cover a smaller geographical range than GEO satellites and require less power.

local loop—The part of a phone system that connects a customer site with a telecommunications 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 a larger area of the earth’s surface than LEO satellites while using less power and causing less signal 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 follow between any two points.

Metaframe—A software package from Citrix Systems, Inc. that supplies terminal services to remote clients.

network service provider—See NSP.

Network Termination 1—See NT1.

Network Termination 2—See NT2.

NSP (network service provider)—A carrier that provides long-distance (and often global) connectivity between major data-switching centers across the Internet. AT&T, PSINet, Sprintlink, and UUNET (MCI Worldcom) are all examples of network service providers. Customers, including ISPs, can lease dedicated private or public Internet connections from an NSP.

NT1 (Network Termination 1)—A device used on ISDN networks that connects the incoming twisted-pair wiring with the customer’s ISDN terminal equipment.

NT2 (Network Termination 2)—An additional connection device required on PRI to handle the multiple ISDN lines between the customer’s network termination connection and the local phone company’s wires.

OC (Optical Carrier)—An internationally recognized rating that indicates throughput rates for SONET connections.

Optical Carrier—See OC.

partial mesh WAN—A version of a mesh topology WAN in which only critical sites are directly interconnected and secondary sites are connected through star or ring topologies. Partial mesh WANs are less expensive to implement than full mesh WANs.

permanent virtual circuit—See PVC.

plain old telephone service (POTS)—See PSTN.

Point-to-Point Protocol—See PPP.

Point-to-Point Protocol over Ethernet—See PPPoE.

Point-to-Point Tunneling Protocol—See PPTP.

POTS—See PSTN.

PPP (Point-to-Point Protocol)—A communications protocol that enables a workstation to connect to a server using a serial connection. PPP can support multiple Network layer protocols and can use both asynchronous and synchronous communications. It performs compression and error correction and requires little configuration on the client workstation.

PPPoE (Point-to-Point Protocol over Ethernet)—PPP running over an Ethernet network.

PPTP (Point-to-Point Tunneling Protocol)—A Layer 2 protocol developed by Microsoft that encapsulates PPP data for transmission over VPN connections. PPTP operates with Windows RRAS access services and can accept connections from multiple different clients. It is simple, but less secure than other modern tunneling protocols.

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PRI (Primary Rate Interface)—A type of ISDN that uses 23 bearer channels and one 64Kbps data channel, represented by the notation 23B+D. PRI is less commonly used by individual subscribers than BRI, but it may be used by businesses and other organizations needing more throughput.

PSTN (Public Switched Telephone Network)—The traditional telephone network, from the lines that connect homes and businesses to the network centers that connect different regions of a country. Now, except for the local loop, nearly all of the PSTN uses digital transmission. Its traffic is carried by fiber-optic and copper twisted-pair cable, microwave, and satellite connections.

Public Switched Telephone Network—See PSTN.

PVC (permanent virtual circuit)—A point-to-point connection over which data may follow any number of different paths, as opposed to a dedicated line that follows a predefined path. X.25, Frame Relay, and some forms of ATM use PVCs.

RAS (Remote Access Service)—The dial-up networking software provided with Microsoft Windows 95, 98, NT, and 2000 client operating systems and Windows NT and 2000 network operating systems. RAS requires software installed on both the client and server, a server configured to accept incoming clients, and a client with sufficient privileges (including user name and password) on the server to access its resources. In more recent versions of Windows, RAS has been incorporated into the RRAS (Routing and Remote Access Service).

RDP (Remote Desktop Protocol)—An Application layer protocol that uses TCP/IP to transmit graphics and text quickly over a remote client-host connection. RDP also carries session, licensing, and encryption information.

remote access—A method for connecting and logging on to a LAN from a workstation that is remote, or not physically connected, to the LAN. Remote access can be accomplished by one of many ways, including dial-up connections, terminal services, remote control, or Web portals.

Remote Access Service—See RAS.

Remote Desktop—An optional feature in Windows XP operating systems that allows a Windows XP computer to be remotely controlled from a client running the Windows 95, 98, Me, NT, XP, 2000, or Server 2003 operating system. Remote Desktop is also the program Windows XP clients use to connect with computers using Windows Terminal Server.

Remote Desktop Protocol—See RDP.

ring topology WAN—A type of WAN in which each site is connected to two other sites so that the entire WAN forms a ring pattern.

Routing and Remote Access service (RRAS)—The software included with Windows NT, Windows 2000 Server, and Windows Server 2003 that enables a server to act as a router, firewall, and remote access server. Using RRAS, a server can provide network access to multiple remote clients.

remote control—A remote access method in which the remote user dials into a workstation that is directly attached to a LAN. Software running on both the remote user’s computer and the LAN computer allows the remote user to “take over” the LAN workstation. Only keystrokes, mouse clicks, and screen updates are exchanged between the two computers.

RRAS—See Routing and Remote Access Service.

satellite return—A type of satellite Internet access service in which a subscriber sends and receives data to and from the Internet over the satellite link. This is a symmetrical technology, in which both upstream and downstream throughputs are advertised to reach 400–500 Kbps; in reality, throughput is often higher.

SDH (Synchronous Digital Hierarchy)—The international equivalent of SONET.

self-healing—A characteristic of dual-ring topologies that allows them to automatically reroute traffic along the backup ring if the primary ring becomes severed.

Serial Line Internet Protocol—See SLIP.

signal level—An ANSI standard for T-carrier technology that refers to its Physical layer electrical signaling characteristics. DS0 is the equivalent of one data or voice channel. All other signal levels are multiples of DS0.

SLIP (Serial Line Internet Protocol)—A communications protocol that enables a workstation to connect to a server using a serial connection. SLIP can support only asynchronous communications and IP traffic, and requires some configuration on the client workstation. SLIP has been made obsolete by PPP.

SONET (Synchronous Optical Network)—A high-bandwidth WAN signaling technique that specifies framing and multiplexing techniques at the Physical layer of the OSI Model. It can integrate many other WAN technologies (for example, T-carriers, ISDN, and ATM technology) and allows for simple link additions and removals. SONET’s topology includes a double ring of fiber-optic cable, which results in very high fault tolerance.

star topology WAN—A type of WAN in which a single site acts as the central connection point for several other points. This arrangement provides separate routes for data between any two sites; however, if the central connection point fails, the entire WAN fails.

SVC (switched virtual circuit)—A logical, point-to-point connections that relies on switches to determine the optimal path between sender and receiver. ATM technology uses SVCs.

switched virtual circuit—See SVC.

symmetrical—A characteristic of transmission technology that provides equal throughput for data traveling both upstream and downstream and is suited to users who both upload and download significant amounts of data.

symmetrical DSL—A variation of DSL that provides equal throughput both upstream and downstream between the customer and the carrier.

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synchronous—A transmission method in which data being transmitted and received by nodes must conform to a timing scheme.

Synchronous Digital Hierarchy—See SDH.

Synchronous Optical Network—See SONET.

T1—A digital carrier standard used in North America and most of Asia that provides 1.544Mbps throughput and 24 channels for voice, data, video, or audio signals. T1s rely on time division multiplexing and may use shielded or unshielded twisted-pair, coaxial cable, fiber-optic, or microwave links.

T3—A digital carrier standard used in North America and most of Asia that can carry the equivalent of 672 channels for voice, data, video, or audio, with a maximum data throughput of 44.736 Mbps (typically rounded up to 45 Mbps for purposes of discussion). T3s rely on time division multiplexing and require either fiber-optic or microwave transmission media.

T-carrier—The term for any kind of leased line that follows the standards for T1s, fractional T1s, T1Cs, T2s, T3s, or T4s.

TA (terminal adapter)—A device used to convert digital signals into analog signals for use with ISDN phones and other analog devices. TAs are sometimes called ISDN modems.

TE (terminal equipment)—The end nodes (such as computers and printers) served by the same connection (such as an ISDN, DSL, or T1 link).

terminal adapter—See TA.

terminal equipment—See TE.

terminal server—A computer that runs specialized software to act as a host and supply applications and resource sharing to remote clients.

terminal services—A remote access method in which a terminal server acts as a host for multiple remote clients. Terminal services requires specialized software on both the client and server. After connecting and authenticating, a client can access applications and data just as if it were directly attached to the LAN.

Terminal Services—The Microsoft software that enables a server to supply centralized and secure network connectivity to remote clients.

thin client—A client that relies on another host for the majority of processing and hard disk resources necessary to run applications and share files over the network.

tiered topology WAN—A type of WAN in which sites that are connected in star or ring formations are interconnected at different levels, with the interconnection points being organized into layers to form hierarchical groupings.

transponder—The equipment on a satellite that receives an uplinked signal from earth, amplifies the signal, modifies its frequency, then retransmits it (in a downlink) to an antenna on earth.

tunnel—A secured, virtual connection between two nodes on a VPN.

tunneling—The process of encapsulating one type of protocol in another. Tunneling is the way in which higher-layer data is transported over VPNs by Layer 2 protocols.

uplink—A connection from an earth-based transmitter to an orbiting satellite.

upstream—A term used to describe data traffic that flows from a customer’s site to a carrier’s facility. In asymmetrical communications, upstream throughput is usually much lower than downstream throughput. In symmetrical communications, upstream and downstream throughputs are equal.

virtual private network—See VPN.

VPN (virtual private network)—A logically constructed WAN that uses existing public transmission systems. VPNs can be created through the use of software or combined software and hardware solutions. This type of network allows an organization to carve out a private WAN through the Internet that serves only its offices, while keeping the data secure and isolated from other (public) traffic.

WAN link—A point-to-point connection between two nodes on a WAN.

Web portal—A secure, Web-based interface to an application or group of applications.

WiMAX—See 802.16a.

wireless broadband—The term used to describe the recently released standards for highthroughput, long-distance digital data exchange over wireless connections. WiMAX (IEEE 802.16a) is one example of a wireless broadband technology.

Worldwide Interoperability for Microwave Access (WiMAX)—See 802.16a.

X.25—An analog, packet-switched WAN technology optimized for reliable, long-distance data transmission and standardized by the ITU in the mid-1970s. The X.25 standard specifies protocols at the Physical, Data Link, and Network layers of the OSI Model. It provides excellent flow control and ensures data reliability over long distances by verifying the transmission at every node. X.25 can support a maximum of only 2-Mbps throughput.

xDSL—The term used to refer to all varieties of DSL.

Review Questions

1.A WAN in which each site is directly connected to no more than two other sites in a serial fashion is known as a _________________________.

a.bus topology WAN

b.star topology WAN

c.ring topology WAN

d.logical topology WAN