must navigate between the sender and receiver all reduce the actual throughput. The number of central offices, switches, and modems through which your phone call travels also affect throughput. Each time the signal passes through a switch or is converted from analog to dig- ital or digital to analog, it loses a little throughput. If you’re surfing the Web, for example, by the time a Web page returns to you, the connection may have lost from 5 to 30 Kbps, and your effective throughput might have been reduced to 30 Kbps or less. In addition, the FCC (Fed- eral Communications Commission), the regulatory agency that sets standards and policy for telecommunications transmission and equipment in the United States, limits the use of PSTN lines to 53 Kbps to reduce the effects of crosstalk. Thus, you will never actually achieve full 56- Kbps throughput using a modem over the PSTN. Nor can the PSTN provide the quality required by many network applications. The quality of a WAN connection is largely determined by how many data packets that it loses or that become corrupt during transmission, how quickly it can transmit and receive data, and whether it drops the connection altogether. To improve this quality, most protocols employ error check- ing techniques. For example, TCP/IP depends on acknowledgments of the data it receives. In addition, many (though not all) PSTN links are now digital, and digital lines are more reliable than the older analog lines. Such digital lines reduce the quality problems that once plagued purely analog PSTN connections. Although nearly all central offices in the PSTN handle digitized data, most still use circuit switching rather than the more efficient packet switching. Recall that in circuit switching, data travels over a point-to-point connection that is reserved by a transmission until all of its data has been transferred. You might think that circuit switching makes the PSTN more secure than other types of WAN connections; in fact, the PSTN offers only marginal security. Because it is a public network, PSTN presents many points at which communications can be intercepted and interpreted on their way from sender to receiver. For example, an eavesdropper could eas- ily tap into the connection where your local telephone company’s line enters your house. The PSTN is not limited to servicing workstation dial-up WAN connections. Following sec- tions describe other, more sophisticated WAN technologies that also rely on the public tele- phone network. X.25 and Frame Relay X.25 is an analog, packet-switched technology designed for long-distance data transmission and standardized by the ITU in the mid-1970s. The original standard for X.25 specified a max- imum of 64-Kbps throughput, but by 1992 the standard was updated to include maximum throughput of 2.048 Mbps. It was originally developed as a more reliable alternative to the voice telephone system for connecting mainframe computers and remote terminals. Later it was adopted as a method of connecting clients and servers over WANs. 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. Unfortunately, this verification also renders X.25 302 Chapter 7 WANS, INTERNET ACCESS, AND REMOTE CONNECTIVITY NET+ 2.15 NET+ 2.14 comparatively slow and unsuitable for time-sensitive applications, such as audio or video. On the other hand, X.25 benefits from being a long-established, well-known, and low-cost tech- nology. X.25 was never widely adopted in the United States, but was accepted by other coun- tries and was for a long time the dominant packet-switching technology used on WANs around the world. Chapter 7 303 X.25 AND FRAME RELAY Recall that, in packet switching, packets belonging to the same data stream may fol- low different, optimal paths to their destination. As a result, packet switching uses bandwidth more efficiently and allows for faster transmission than if each packet in the data stream had to follow the same path, as in circuit switching. Packet switching is also more flexible than circuit switching, because packet sizes may vary. NOTE Frame Relay is an updated, digital version of X.25 that also relies on packet switching. ITU and ANSI standardized Frame Relay in 1984. However, because of a lack of compatibility with other WAN technologies at the time, Frame Relay did not become popular in the United States and Canada until the late 1980s. Frame Relay protocols operate at the Data Link layer of the OSI Model and can support multiple different Network and Transport layer protocols (for example, TCP/IP and IPX/SPX). The name is derived from the fact that data is sepa- rated into frames, which are then relayed from one node to another without any verification or processing. An important difference between Frame Relay and X.25 is that Frame Relay does not guar- antee reliable delivery of data. X.25 checks for errors and, in the case of an error, either cor- rects the damaged data or retransmits the original data. Frame Relay, on the other hand, simply checks for errors. It leaves the error correction up to higher-layer protocols. Partly because it doesn’t perform the same level of error correction that X.25 performs (and thus has less overhead), Frame Relay supports higher throughput than X.25. It 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. Both X.25 and Frame Relay may be configured as SVCs (switched virtual circuits) or PVCs (permanent virtual circuits). SVCs are connections that are established when parties need to transmit, then terminated after the transmission is complete. PVCs are connections that are established before data needs to be transmitted and maintained after the transmission is com- plete. Note that in a PVC, the connection is established only between the two points (the sender and receiver); the connection does not specify the exact route the data will travel. Thus, in a PVC, data may follow any number of paths from point A to point B. For example, a trans- mission traveling over a PVC from Baltimore to Phoenix might go from Baltimore to Wash- ington, D.C., to Chicago, then to Phoenix; the next transmission over that PVC, however, might go from Baltimore to Boston to St. Louis to Denver to Phoenix. NET+ 2.14 PVCs are not dedicated, individual links. When you lease an X.25 or Frame Relay circuit from your local carrier, your contract reflects the endpoints you specify and the amount of bandwidth you require between those endpoints. The service provider guarantees a minimum amount of bandwidth, called the CIR (committed information rate). Provisions usually account for bursts of traffic that occasionally exceed the CIR. When you lease a PVC, you share bandwidth with the other X.25 and Frame Relay users on the backbone. PVC links are best suited to frequent and consistent data transmission. On networking diagrams, packet-switched networks such as X.25 and Frame Relay are depicted as clouds, as shown in Figure 7-9, because of the indeterminate nature of their traf- fic patterns. 304 Chapter 7 WANS, INTERNET ACCESS, AND REMOTE CONNECTIVITY FIGURE 7-9 A WAN using frame relay You may have seen the Internet depicted as a cloud on networking diagrams, similar to the Frame Relay cloud in Figure 7-9. In its early days, the Internet relied largely on X.25 and Frame Relay transmission—hence the similar illustration. NOTE The advantage to leasing a Frame Relay circuit over leasing a dedicated service is that you pay for only the amount of bandwidth required. Another advantage is that Frame Relay is much less expensive than some newer WAN technologies offered today. Also, Frame Relay is a long- established worldwide standard. On the other hand, because Frame Relay and X.25 use shared lines, their throughput remains at the mercy of variable traffic patterns. In the middle of the night, data over your Frame Relay NET+ 2.14 network may zip along at 1.544 Mbps; during midday, when everyone is surfing the Web, it may slow down to less than your CIR. In addition, Frame Relay circuits are not as private (and potentially not as secure) as dedicated circuits. Nevertheless, because they use the same con- nectivity equipment as T-carriers, they can easily be upgraded to T-carrier dedicated lines. ISDN ISDN (Integrated Services Digital Network) is an international standard, originally estab- lished by the ITU in 1984, for transmitting digital data over the PSTN. In North America, a standard ISDN implementation wasn’t finalized until 1992, because telephone switch manu- facturers couldn’t agree on compatible technology for supporting ISDN. The technology’s uncertain start initially made telephone companies reluctant to invest in it, and ISDN didn’t catch on as quickly as predicted. However, in the 1990s ISDN finally became a popular method of connecting WAN locations to exchange both data and voice signals. ISDN specifies protocols at the Physical, Data Link, and Transport layers of the OSI Model. These protocols handle signaling, framing, connection setup and termination, routing, flow control, and error detection and correction. ISDN relies on the PSTN for its transmission medium. Connections can be either dial-up or dedicated. Dial-up ISDN is distinguished from the workstation dial-up connections discussed previously because it relies exclusively on digi- tal transmission. In other words, it does not convert a computer’s digital signals to analog before transmitting them over the PSTN. Also, ISDN is distinguished because it can simultaneously carry as many as two voice calls and one data connection on a single line. Therefore, ISDN can eliminate the need to pay for separate phone lines to support faxes, modems, and voice calls at one location. All ISDN connections are based on two types of channels: B channels and D channels. The B channel is the “bearer” channel, employing circuit-switching techniques to carry voice, video, audio, and other types of data over the ISDN connection. A single B channel has a maximum throughput of 64 Kbps (although it is sometimes limited to 56 Kbps by the ISDN provider). The number of B channels in a single ISDN connection may vary. The D channel is the “data” channel, employing packet-switching techniques to carry information about the call, such as session initiation and termination signals, caller identity, call forwarding, and conference call- ing signals. A single D channel has a maximum throughput of 16 or 64 Kbps, depending on the type of ISDN connection. Each ISDN connection uses only one D channel. In North America, two types of ISDN connections are commonly used: BRI (Basic Rate Inter- face) and PRI (Primary Rate Interface). BRI (Basic Rate Interface) uses two B channels and one D channel, as indicated by the notation 2B+D. The two B channels are treated as separate connections by the network and can carry voice and data or two data streams simultaneously and separate from each other. In a process called bonding, these two 64-Kbps B channels can be combined to achieve an effective throughput of 128 Kbps—the maximum amount of data traffic that a BRI connection can accommodate. Most consumers who subscribe to ISDN from home use BRI, which is the most economical type of ISDN connection. Chapter 7 305 ISDN NET+ 2.14 NET+ 2.14 Figure 7-10 illustrates how a typical BRI link supplies a home consumer with an ISDN link. From the telephone company’s lines, the ISDN channels connect to a Network Termination 1 device at the customer’s site. The NT1 (Network Termination 1) device connects the twisted- pair wiring at the customer’s building with the ISDN terminal equipment via RJ-11 (stan- dard telephone) or RJ-45 data jacks. The ISDN TE (terminal equipment) may include cards or standalone devices used to connect computers to the ISDN line (similar to a network adapter used on Ethernet or Token Ring networks). 306 Chapter 7 WANS, INTERNET ACCESS, AND REMOTE CONNECTIVITY FIGURE 7-10 A BRI link So that the ISDN line can connect to analog equipment, the signal must first pass through a terminal adapter. A TA (terminal adapter) converts digital signals into analog signals for use with ISDN phones and other analog devices. (Terminal adapters are sometimes called ISDN modems, though they are not, technically, modems.) Typically, telecommuters who want more throughput than their analog phone line can offer choose BRI as their ISDN connection. For a home user, the terminal adapter would most likely be an ISDN router, whereas the terminal equipment could be an Ethernet card in the user’s workstation plus, perhaps, a phone. The BRI configuration depicted in Figure 7-10 applies to installations in North Amer- ica only. Because transmission standards differ in Europe and Asia, different numbers of B channels are used in ISDN connections in those regions. NOTE PRI (Primary Rate Interface) uses 23 B channels and one 64-Kbps D channel, as represented by the notation 23B+D. PRI is less commonly used by individual subscribers than BRI is, but it may be selected by businesses and other organizations that need more throughput. As with BRI, the separate B channels in a PRI link can carry voice and data, independently of each other or bonded together. The maximum potential throughput for a PRI connection is 1.544 Mbps. PRI and BRI connections may be interconnected on a single network. PRI links use the same kind of equipment as BRI links, but require the services of an extra network termination device, NET+ 2.14 called a NT2 (Network Termination 2), to handle the multiple ISDN lines. Figure 7-11 depicts a typical PRI link as it would be installed in North America. Individual customers who need to transmit more data than a typical modem can handle or who want to use a single line for both data and voice may use ISDN lines. ISDN, although not available in every location of the United States, can be purchased from most local telephone companies. Costs vary depending on the customer’s location. PRI and B-ISDN are signifi- cantly more expensive than BRI. Dial-up ISDN service is less expensive than dedicated ISDN service. In some areas, ISDN providers charge customers additional usage fees based on the total length of time they remain connected. One disadvantage of ISDN is that it can span a distance of only 18,000 linear feet before repeater equipment is needed to boost the signal. For this reason, it is only feasible to use for the local loop portion of the WAN link. Chapter 7 307 T-CARRIERS FIGURE 7-11 A PRI link T-Carriers Another WAN transmission method that grew from a need to transmit digital data at high speeds over the PSTN is T-carrier technology, which includes T1s, fractional T1s, and T3s. T- carrier standards specify a method of signaling, which means they belong to the Physical layer of the OSI Model. A T-carrier uses TDM (time division multiplexing) over two wire pairs (one for transmitting and one for receiving) to divide a single channel into multiple channels. For example, multiplexing enables a single T1 circuit to carry 24 channels, each capable of 64- Kbps throughput; thus a T1 has a maximum capacity of 24 × 64 Kbps, or 1.544 Mbps. Each channel may carry data, voice, or video signals. The medium used for T-carrier signaling can be ordinary telephone wire, fiber-optic cable, or wireless links. AT&T developed T-carrier technology in 1957 in an effort to digitize voice signals and thereby enable such signals to travel longer distances over the PSTN. Before that time, voice signals, which were purely analog, were expensive to transmit over long distances because of the num- ber of connectivity devices needed to keep the signal intelligible. In the 1970s, many busi- nesses installed T1s to obtain more voice throughput per line. In the 1990s, with increased NET+ 2.14 NET+ 2.14 data communication demands, such as Internet access and geographically dispersed offices, T1s became a popular way to connect WAN sites. The next section describes the various types of T-carriers, then the chapter moves on to T-car- rier connectivity devices. Types of T-Carriers A number of T-carrier varieties are available to businesses today, as shown in Table 7-1. The most common T-carrier implementations are T1 and, for higher bandwidth needs, T3. A T1 circuit can carry the equivalent of 24 voice or data channels, giving a maximum data through- put of 1.544 Mbps. A T3 circuit can carry the equivalent of 672 voice or data channels, giving a maximum data throughput of 44.736 Mbps (its throughput is typically rounded up to 45 Mbps for the purposes of discussion). Table 7-1 Carrier specifications Number Number Throughput Signal Level Carrier of T1s of Channels (Mbps) DS0 — 1/24 1 .064 DS1 T1 1 24 1.544 DS1C T1C 2 24 3.152 DS2 T2 4 96 6.312 DS3 T3 28 672 44.736 DS4 T4 168 4032 274.176 308 Chapter 7 WANS, INTERNET ACCESS, AND REMOTE CONNECTIVITY You may hear signal level and carrier terms used interchangeably—for example, DS1 and T1. In fact, T1 is the implementation of the DS1 standard used in North America and most of Asia. In Europe, the standard high-speed carrier connections are E1 and E3. Like T1s and T3s, E1s and E3s use time division multiplexing. However, an E1 allows for 30 channels and offers 2.048-Mbps throughput. An E3 allows for 480 chan- nels and offers 34.368-Mbps throughput. In Japan, the equivalent carrier standards are J1 and J3. Like a T1, a J1 connection allows for 24 channels and offers 1.544- Mbps throughput. A J3 connection allows for 480 channels and offers 32.064-Mbps throughput. Using special hardware, T1s can interconnect with E1s or J1s and T3s with E3s or J3s for international communications. NOTE NET+ 2.14 The speed of a T-carrier depends on its signal level. The signal level refers to the T-carrier’s Physical layer electrical signaling characteristics as defined by ANSI standards in the early 1980s. DS0 (digital signal, level 0) is the equivalent of one data or voice channel. All other signal levels are multiples of DS0. As a networking professional, you are most likely to work with T1 or T3 lines. In addition to knowing their capacity, you should be familiar with their costs and uses. T1s are commonly used by businesses to connect branch offices or to connect to a carrier, such as an ISP. Tele- phone companies also use T1s to connect their smaller central offices. ISPs may use one or more T1s or T3s, depending on the provider’s size, to connect to their Internet carriers. Because a T3 provides 28 times more throughput than a T1, many organizations may find that multiple T1s—rather than a single T3—can accommodate their throughput needs. For exam- ple, suppose a university research laboratory needs to transmit molecular images over the Internet to another university, and its peak throughput need (at any given time) is 10 Mbps. The laboratory would require seven T1s (10 Mbps divided by 1.544 Mbps equals 6.48 T1s). Leasing seven T1s would prove much less expensive for the university than leasing a single T3. The cost of T1s varies from region to region. On average, leasing a full T1 might cost between $500 and $1500 to install, plus an additional $300 to $1000 per month in access fees. The longer the distance between the provider (such as an ISP or a telephone company) and the subscriber, the higher a T1’s monthly charge. For example, a T1 between Houston and New York will cost more than a T1 between Washington, D.C., and New York. Similarly, a T1 from a suburb of New York to the city center will cost more than a T1 from the city center to a busi- ness three blocks away. For organizations that do not need as much as 1.544-Mbps throughput, a fractional T1 might be a better option. A fractional T1 lease allows organizations to use only some of the channels on a T1 line and be charged according to the number of channels they use. Thus, fractional T1 bandwidth can be leased in multiples of 64 Kbps. A fractional T1 is best suited to businesses that expect their traffic to grow and that may require a full T1 eventually, but can’t currently justify leasing a full T1. T3s are very expensive and are used by the most data-intensive businesses—for example, com- puter consulting firms that provide online data backups and warehousing for a number of other businesses or large long-distance carriers. A T3 is much more expensive than even mul- tiple T1s. It may cost as much as $3000 to install, plus monthly service fees based on usage. If a customer uses the full T3 bandwidth of 45 Mbps, for example, the monthly charges might be as high as $18,000. Of course, T3 costs will vary depending on the carrier, your location, and the distance covered by the T3. In any event, however, this type of connection is signifi- cantly more expensive than a T1. Therefore, only businesses with extraordinary bandwidth requirements should consider using T3s. T-Carrier Connectivity The approximate costs mentioned previously include monthly access and installation, but not connectivity hardware. Every T-carrier line requires connectivity hardware at both the customer Chapter 7 309 T-CARRIERS NET+ 2.14 site and the local telecommunications provider’s switching facility. Connectivity hardware may be purchased or leased. If your organization uses an ISP to establish and service your T-carrier line, you will most likely lease the connectivity equipment. If you lease the line directly from the local carrier and you anticipate little change in your connectivity requirements over time, however, you may want to purchase the hardware. T-carrier lines require specialized connectivity hardware that cannot be used with other WAN transmission methods. In addition, T-carrier lines require different media, depending on their throughput. In the following sections, you will learn about the physical components of a T- carrier connection between a customer site and a local carrier. Wiring As mentioned earlier, the T-carrier system is based on AT&T’s original attempt to digitize existing long-distance PSTN lines. T1 technology can use UTP or STP (unshielded or shielded twisted-pair) copper wiring—in other words, plain telephone wire—coaxial cable, microwave, or fiber-optic cable as its transmission media. Because the digital signals require a clean con- nection (that is, one less susceptible to noise and attenuation), STP is preferable to UTP. For T1s using STP, repeaters must regenerate the signal approximately every 6000 feet. Twisted- pair wiring cannot adequately carry the high throughput of multiple T1s or T3 transmissions. Thus, for multiple T1s, coaxial cable, microwave, or fiber-optic cabling may be used. For T3s, microwave or fiber-optic cabling is necessary. CSU/DSU (Channel Service Unit/Data Service Unit) Although CSUs (channel service units) and DSUs (data service units) are actually two sepa- rate devices, they are typically combined into a single standalone device or an interface card called a CSU/DSU. The CSU/DSU is the connection point for a T1 line at the customer’s site. The CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring. The DSU converts the T-carrier frames into frames the LAN can interpret and vice versa. It also connects T-carrier lines with terminating equipment. Finally, a DSU usually incorporates a multiplexer. (In some T-carrier installations, the multiplexer can be a separate device connected to the DSU.) For an incoming T-carrier line, the multiplexer separates its combined channels into individual signals that can be inter- preted on the LAN. For an outgoing T-carrier line, the multiplexer combines multiple signals from a LAN for transport over the T-carrier. After being demultiplexed, an incoming T-car- rier signal passes on to devices collectively known as terminal equipment. Examples of termi- nal equipment include switches, routers, or telephone exchange devices that accept only voice transmissions (such as a telephone switch). Figure 7-12 depicts a typical use of a CSU/DSU with a point-to-point T1-connected WAN. In the following sections, you will learn how routers and switches integrate with CSU/DSUs and multiplexers to connect T-carriers to a LAN. 310 Chapter 7 WANS, INTERNET ACCESS, AND REMOTE CONNECTIVITY NET+ 2.14 NET+ 1.6 2.14 Terminal Equipment On a typical T1-connected data network, the terminal equipment will consist of switches, routers, or bridges. Usually, a router or Layer 3 or higher switch is the best option, because these devices can translate between different Layer 3 protocols that might be used on the WAN and LAN. The router or switch accepts incoming signals from a CSU/DSU and, if necessary, trans- lates Network layer protocols, then directs data to its destination exactly as it does on any LAN. On some implementations, the CSU/DSU is not a separate device, but is integrated with the router or switch as an expansion card. Compared to a standalone CSU/DSU, which must connect to the terminal equipment via a cable, an integrated CSU/DSU offers faster signal processing and better network performance. In most cases, it is also a less expensive and lower-maintenance solution than using a separate CSU/DSU device. Figure 7-13 illustrates Chapter 7 311 T-CARRIERS FIGURE 7-12 A point-to-point T-carrier connection FIGURE 7-13 A T-carrier connection to a LAN through a router NET+ 1.6 2.14 [...]... at a combined access point and router behind the counter That device can connect the coffee shop with its ISP while allowing patrons within the access point’s range to log on to the Internet, as shown in Figure 7-20 At T-Mobile hot spots, access points are connected (via routers) to T1 links FIGURE 7-20 A hot spot providing wireless Internet access In general, to access the Internet from an 802.11... networking services, 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 the Windows XP and Server 2003 operating systems, RAS has been incorporated into a more comprehensive remote access package called the RRAS (Routing and... the earth turns Consequently, at every point in their orbit, the satellites maintain a constant distance from a specific point on the earth’s equator Because satellites are generally used to relay information from one point on earth to another, information sent to earth from a satellite first has to be transmitted to the satellite from earth in an uplink An uplink is the creation of a communications... access point to accept a user’s connection based on his computer’s MAC address, in addition to the user’s logon id and password Wireless security measures are discussed in detail in Chapter 14 At each hot spot, the access point available for public use is connected to the Internet using technology other than 802.11 For example, a local coffee shop might lease a DSL line that terminates at a combined access... find the last game’s score As you know, the first step in this process is establishing a TCP connection with the team’s Web server Your TCP request message leaves your computer’s NIC and travels over your home network to a DSL modem A DSL modem is a device that modulates outgoing signals and demodulates incoming DSL signals Thus, it contains receptacles to connect both to your incoming telephone line... properties to use DHCP (In Windows XP, for example, check the “Obtain an IP address automatically” option in the Internet Protocol TCP/IP Properties dialog box.) ◆ Make sure your computer is not configured to automatically use a dial-up connection WIRELESS WANS AND INTERNET ACCESS NET+ 2.15 Chapter 7 323 ◆ Choose infrastructure mode rather than ad hoc mode (In Windows XP, for example, in the Wireless Connection... its use of a double-ring topology (similar to FDDI) over fiber-optic cable In this type of layout, one ring acts as the primary route for data, transmitting in a clockwise direction The second ring acts as a backup, transmitting data counterclockwise around the ring If, for example, a backhoe operator severs the primary ring, SONET would automatically reroute traffic to the backup ring without any loss... to an orbiting satellite Often, the uplink signal information is scrambled (in other words, its signal is encoded) before transmission to prevent unauthorized interception At the satellite, a transponder receives the uplink signal, then transmits it to an earth-based receiver in a downlink A typical satellite contains 24 to 32 transponders Each satellite uses unique frequencies for its downlink These... that signals traveling in one direction (for example from a satellite to the earth) do not interfere with signals traveling in the other direction (for example, signals from the earth to a satellite) Satellite Internet access providers typically use frequencies in the C- or Ku-bands Newer satellite Internet access technologies are currently being developed for the Ka-band Satellite Internet Services... from the Internet using a satellite uplink and downlink This is a symmetrical technology, in which both upstream and downstream throughputs are advertised to reach 400–500 Kbps In reality, throughputs are often higher To establish a satellite Internet connection, each subscriber must have a dish antenna, which is approximately two feet high by three feet wide, installed in a fixed position In North . signal intelligible. In the 1970s, many busi- nesses installed T1s to obtain more voice throughput per line. In the 1990s, with increased NET+ 2.14 NET+ 2.14 data communication demands, such as Internet. integrity through error correction and line monitoring. The DSU converts the T-carrier frames into frames the LAN can interpret and vice versa. It also connects T-carrier lines with terminating equipment division multiplexing) over two wire pairs (one for transmitting and one for receiving) to divide a single channel into multiple channels. For example, multiplexing enables a single T1 circuit