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Broadband local access technology

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1 Broadband local access technology
(B.A.Forouzan, Business Data Communications A.S.Tanenbaum, Computer Networks 4/e) Forouzan, Business Data Communications

2 A telephone system Figure 8-1
The telephone network had its beginnings in the late 1800s. The entire network, which is referred to as the plain old telephone system (POTS), was originally an analog system using analog signals to transmit voice. With the advent of the computer era, the network, in the 1980s, began to carry data in addition to voice. During the last decade, the telephone network has undergone many technical changes. The network is now digital as well as analog. Forouzan, Business Data Communications

3 Major Components of the Telephone System
Local loops Analog twisted pairs going to houses and businesses Trunks Digital fiber optics connecting the switching offices Switching offices Where calls are moved from one trunk to another Major components: Local Loops One component of the telephone network is the local loop, a twisted-pair cable that connects the subscriber telephone to the nearest end office or local central office. The local loop, when used for voice, has a bandwidth of 4000 Hz (4 kHz). It is interesting to note that each local loop is associated with a telephone number. The first three digits of a local telephone number define the office and the next four digits define the local loop number. Trunks Trunks are transmission media that handle the communication between offices. A trunk normally handles hundreds or thousands of connections through multiplexing. Transmission is usually through optical fibers or satellite links. Switching Offices To avoid having a permanent physical link between any two subscribers, the telephone company has switches located in a switching office. A switch connects several local loops or trunks and allows a connection between different subscribers. Forouzan, Business Data Communications

4 Internet access via the local loop
Figure 8-5 Internet access via the local loop With the advent of the Internet, people wanted home access to the Internet to send and receive , to surf the web, and so on. This access requires a physical connection to an ISP. Instead of laying new cable to connect a residential user to the local ISP, the existing cables (local loops) that connect each residence to the telephone switching office can be used. Now the problem is how to connect the switching offices to the local ISPs. This was accomplished using high-bandwidth trunks. Figure 8.5 shows how residential users can be connected to the Internet using the local loop, which was formerly used only for voice communication. However, there is still another problem: A local loop is an analog line, but the data to be sent and received from the Internet is digital. The line, designed in the past for an analog signal, has a bandwidth of 4 kHz, which cannot handle a digital signal. Somehow, digital data must be sent through these analog lines. During the past few decades, several solutions were proposed for this problem: conventional modems, 56K modems, and DSL modems. The term modem is a composite word that refers to the two functional entities that make up the device: a signal modulator and a signal demodulator. A modulator converts a digital signal into an analog signal using ASK, FSK, PSK, or a combination. A demodulator converts an analog signal into a digital signal. Modems are used to transmit digital signal via analog local loop. Forouzan, Business Data Communications

5 Figure 8-6 Modem concept Figure 8.6 shows the relationship of modems to a communication link. The computer creates a digital signal and relays it to the modem. The modulated signal is received by the demodulation function of the second modem. The demodulator takes the signal and decodes it into whatever format its computer can accept. It then relays the resulting digital signal to the receiving computer. Traditional telephone lines can effectively carry frequencies between 300 Hz and 3300 Hz, resulting in a bandwidth of 3000 Hz. All of this range is used for transmitting voice, where a great deal of interference and distortion can be accepted without loss of intelligibility. However, data signals require a higher degree of accuracy to ensure integrity. For safety's sake, therefore, the boundaries of this range are not used for data communication. Forouzan, Business Data Communications

6 Figure 8-7 56K modem Traditional modems have a limitation on the data rate (maximum of 33.6 kbps), as determined by the noisiness of the telephone line (two analogue local loops). Modems with a bit rate of 56,000 bps, called 56K modems (or V.90), are now popular. These modems may be used only if one party is using digital signaling (such as an Internet provider) – one end of the connection is purely digital, as it is with the most ISP now. They are asymmetrical since the downloading (flow of data from the Internet provider to the resident) is a maximum of 56 kbps, while uploading (flow of data from the residence to the Internet provider) is a maximum of 33.6 kbps. These modems use PCM and inverse PCM to accomplish the theoretical data rate of 33.6 in the downloading direction. Figure 8.7 shows the concept. Uploading: In the figure, transmission of data from the subscriber to the Internet provider (uploading) follows these steps: 1. Digital data are modulated by the modem at site A. 2. Analog data are sent from the modem to the switching station on the local loop. 3. At the switching station, data are converted to digital using PCM. 4. Digital data travel through the digital network of the telephone company and are received by the Internet provider computer. The limiting factor in these steps is step 3. However, the user does not often need such a high data rate in this direction since only small blocks of data (such as or a small file) are sent. Downloading Transmission of data from the Internet provider to the modem at site A (downloading) follows these steps: 1. Digital data are sent by the computer of the Internet provider through the digital telephone network. 2. At the switching station, digital data are converted to analog using inverse PCM. 3. Analog data are sent from the switching station at site A to the modem on the local loop. 4. Analog data are demodulated by the modem at site A. Note that, in this direction, there is no quantization of data using PCM. The limitation when uploading is not an issue here; data can be sent at 56 kbps. This is what the user is looking for, since large files are typically downloaded from the Internet. The maximum data rate in the uploading direction is 33.6 kbps, while the data rate in the downloading direction is 56 kbps. Forouzan, Business Data Communications

7 Digital Subscriber Lines
We have repeatedly stated that the bandwidth of the basic POTS analog loop is limiled, with a -3-dB value of just several kHz. Although the loop can carry voiceband signals that are understandable and recognizable, the overall audio quality is far less than what a good music system provides. Despite this limited bandwidth, engineers have managed to use sophisticated coding, modulation, and error-correction techniques in modems to achieve bit rates as high as 56 kbits/s at under 4000 baud. Yet even this speed is not enough for today's wired world. People want rate on the order of megabits/s for fast Internet access, for example. The most direct way to do this is to run new coax cable or fiber optic links from each modem to a high-speed node at the phone company or another service provider. Unfortunately, this is very expensive and would take years. The real challenge is to get the maximum possible data rate through the existing, in-place local analog loop. Digital subscriber line (DSL). DSL is a technology that uses the existing telecommunication networks such as the local loop telephone lines to accomplish high-speed delivery of data, voice, video, and multimedia. POTS: plain old telephone services. The reason that modems are so slow is that telephones were invented for carrying the human voice and the entire system has been carefully optimized for this purpose. Data have always been stepchildren. At the point where each local loop terminates in the end office, the wire runs through a filter that attenuates all frequencies below 300 Hz and above 3400 Hz. The cutoff is not sharp-300 Hz and 3400 Hz are the 3 dB points-so the bandwidth is usually quoted as 4000 Hz even though the distance between the 3 dB points is 3100 Hz. Data are thus also restricted to this narrow band. The trick that makes xDSL work is that when a customer subscribes to it, the incoming line is connected to a different kind of switch, one that does not have this filter, thus making the entire capacity of the local loop available. The limiting factor then becomes the physics of the local loop, not the artificial 3100 Hz bandwidth created by the filter. Unfortunately, the capacity of the local loop depends on several factors, including its length, thickness, and general quality. A plot of the potential bandwidth as a function of distance is given in Fig The lower the chosen speed, the larger the radius and the more customers covered. But the lower the speed, the less attractive the service and the fewer the people who will be willing to pay for it. The xDSL services have all been designed with certain goals in mind. First, the services must work over the existing category 3 twisted pair local loops. Second, they must not affect customers' existing telephones and fax machines. Third, they must be much faster than 56 kbps. Fourth, they should be always on, with just a monthly charge but no per-minute charge. Bandwidth versus distanced over category 3 UTP for DSL. How does the xDSL concept extend the date rate to the Mbps? Forouzan, Business Data Communications

8 Data Rate Downstream; Upstream
xDSL Summary Table DSL Type Description Data Rate Downstream; Upstream Distance Limit Application DSL Lite (same as G.Lite) "Splitterless" DSL without the "truck roll" From Mbps to 6 Mbps downstream, depending on the subscribed service 18,000 feet on 24 gauge wire The standard ADSL; sacrifices speed for not having to install a splitter at the user's home or business HDSL High bit-rate Digital Subscriber Line 1.544 Mbps duplex on two twisted-pair lines; Mbps duplex on three twisted-pair lines 12,000 feet on 24 gauge wire T1/E1 service between server and phone company or within a company; WAN, LAN, server access SDSL Symmetric DSL 1.544 Mbps duplex (U.S. and Canada); Mbps (Europe) on a single duplex line downstream and upstream Same as for HDSL but requiring only one line of twisted-pair ADSL Asymmetric Digital Subscriber Line 1.544 to 6.1 Mbps downstream; 16 to 640 Kbps upstream 1.544 Mbps at 18,000 feet; Mbps at 16,000 feet; Mpbs at 12,000 feet; Mbps at 9,000 feet Used for Internet and Web access, motion video, video on demand, remote LAN access RADSL Rate-Adaptive DSL from Westell Adapted to the line, 640 Kbps to 2.2 Mbps downstream; 272 Kbps to Mbps upstream Not provided Similar to ADSL VDSL Very high Digital Subscriber Line 12.9 to 52.8 Mbps downstream; 1.5 to 2.3 Mbps upstream; 1.6 Mbps to 2.3 Mbps downstream 4,500 feet at Mbps; 3,000 feet at Mbps; 1,000 feet at Mbps ATM networks; Fiber to the Neighborhood The various xDSL techniques are defined by different standards groups, with different data rate, modes, and distance. Forouzan, Business Data Communications

9 ADSL Design Asymmetric Frequency division multiplexing Range 5.5km
Greater capacity downstream than upstream Frequency division multiplexing Lowest 25kHz for voice Plain old telephone service (POTS) Use echo cancellation or FDM to give two bands Use FDM within bands Range 5.5km ADSL: Asymmetric Digital Subscriber Line. ADSL is the most widely publicised of a family of new modem technologies. Asymmetric refers to the fact that ADSL provides more capacity downstream than upstream. Originally, it is design for Video On Demand (VOD). Forouzan, Business Data Communications

10 Figure 8-8 Bands for ADSL ADSL worked by dividing the spectrum available on the local loop, which is about 1 MHz, into three frequency bands: lowest 25 kHz for voice, know as POTS (Plain Old Telephone Service), upstream (user to end office) and downstream (end office to user). The technique of having multiple frequency bands is called frequency division multiplexing; we will study it in detail in a later section. Subsequent offerings from other providers have taken a different approach, and it appears this one is likely to win out, so we will describe it below. Forouzan, Business Data Communications

11 Digital Subscriber Lines (2)

12 Discrete Multitone DMT: Discrete Multitone
Multiple carrier signals at different frequencies Some bits on each channel 4kHz subchannels Send test signal and use subchannels with better signal to noise ratio On initialization, the DMT modem sends out test signals on each subchannel to determine the signal-to-noise ratio. The modem then assigns more bits to channels with better signal transmission qualities and less bits to channels with poorer signal transmission qualities. Figure 8.19 illustrates this process, Each subchannel can carry a data rate of from 0 to 60 kbps. The figure shows a typical situation in which there is increasing attenuation and hence decreasing signal-to-noise ratio at higher frequencies. As a result, the higher-frequency subchannels carry less of the load. Forouzan, Business Data Communications

13 DMT Transmitter After initialization, the bit stream to be transmitted is divided into a number of substreams, one for each subchannel that will carry data. The sum of the data rates of the substrearns is equal to the total data rate. Each substrearn is then converted to an analogue signal using Quadrature Amplitude Modulation (QAM). Each QAM signal occupies a distinct frequency band, so these signals can be combined by simple addition to produce the composite signal for transmission. Forouzan, Business Data Communications

14 Typical ADSL configuration
256 subchannels at 4kHz (60kbps), 1 channel for POTS, 5 channels gap to separate voice and data. Of the remaining 250 channels, The ANSI standard defines a rate of 60 Kbps for each 4-KHz channel, which means a QAM modulation with 15 bits per baud. • The upstream channel usually occupies 25 or more channels, which means a bit rate of 25 x 60 Kbps, or 1.5 Mbps. Normally, however, the bit rate in this direction ranges from 64 Kbps to 1 Mbps due to noise. • The downstream channel usually occupies 200 or more channels, which means a bit rate of 200 x 60 Kbps, or 12 Mbps. Normally, however, the bit rate in this direction ranges from 500 Kbps to 8 Mbps due to noise. Sometimes splitter is combined with the network interface device (NID). an analog filter that separates the Hz band used by POTS from the data. The POTS signal is routed to the existing telephone or fax machine, and the data signal is routed to an ADSL modem. The ADSL modem is actually a digital signal processor that has been set up to act as 250 QAM modems operating in parallel at different frequencies. Since most current ADSL modems are external, the computer must be connected to it at high speed. Usually, this is done by putting an Ethernet card in the computer and operating a very short two-node Ethernet containing only the computer and ADSL modem. Occasionally the USB port is used instead of Ethernet. In the future, internal ADSL modem cards will no doubt become available. At the other end of the wire, on the end office side, a corresponding splitter is installed. Here the voice portion of the signal is filtered out and sent to the normal voice switch. The signal above 26 kHz is routed to a new kind of device called a DSLAM (Digital Subscriber Line Access Multiplexer), which contains the same kind of digital signal processor as the ADSL modem. Once the digital signal has been recovered into a bit stream, packets are formed and sent off to the ISP. This complete separation between the voice system and ADSL makes it relatively easy for a telephone company to deploy ADSL. All that is needed is buying a DSLAM and splitter and attaching the ADSL subscribers to the splitter. Other high-bandwidth services (e.g., ISDN) require much greater changes to the existing switching equipment. Forouzan, Business Data Communications

15 Community Antenna Television
Early cable TV system. Was Called Community Antenna Television. Head end: amplifier. One way transmission. A system with fibre for the long-haul runs and coaxial cable to the houses is called an HFC(hybrid Fibre Coax) system. Fibre nodes are used ad the interface between optical and electrical parts of the system. Most cable TV companies have changed their cable media to include both fiber-optic and coaxial cables. Due to these changes and other improvements, the bandwidth has increased. The bandwidth between 5 to 54 MHz is used for upstream communication, 54 to 550 MHz for downstream communication. The bandwidth between 550 to 750 MHz is used by some cable companies to deliver digital TV Digital TV uses compression and this reduces the previous 6 MHz bandwidth requirement. One digital channel now needs less than I MHz. More than 200 TV channels can be accommodated. If there comes a time when the entire band is digital, cable TV could deliver more than 750 channels. If a cable TV system is used to internet access and telephony business, all one-way amplifiers in the system have to be replaced by two-way amplifiers. Forouzan, Business Data Communications

16 Internet over Cable (2) INTERNET ACCESS VIA CABLE TV
The data rate limitation of traditional modems is mostly due to the narrow bandwidth of the local loop telephone line (up to 4 kHz). If higher bandwidths are available, one can design a modem that can handle much higher data rates. Fortunately, cable TV provides residential premises with a coaxial cable that has a bandwidth up to 750 MHz and sometimes even more. This bandwidth is normally divided into 6 MHz bands using frequency division multiplexing. Most cable TV companies have changed their cable media to include both fiber-optic and coaxial cables. Due to these changes and other improvements, the bandwidth has increased. The bandwidth between 5 to 54 MHz is used for upstream communication, 54 to 550 MHz for downstream communication. The bandwidth between 550 to 750 MHz is used by some cable companies to deliver digital TV Digital TV uses compression and this reduces the previous 6 MHz bandwidth requirement. One digital channel now needs less than I MHz. More than 200 TV channels can be accommodated. The difference between HFC (hybrid fibre coax – cable modem) telephone system (ADSL) A single cable is shared by many house every house has its own private local loop Suited for TV broadcast suited for private conversation More users on one cable, every ADSL user has his fixed bandwidth the more competition for bandwidth The bandwidth of coax >> that of the twist pair Forouzan, Business Data Communications

17 Spectrum Allocation Cable television channels in North America normally occupy the MHz region (except for FM radio from 88 to 108 MHz). These channels are 6 MHz wide, including guard bands. In Europe the low end is usually 65 MHz and the channels are 6-8 MHz wide for the higher resolution required by PAL and SECAM but otherwise the allocation scheme is similar. The low part of the band is not used. Modem cables can also operate well above 550 MHz, often to 750 MHz or more. The solution chosen was to introduce upstream channels in the 5-42 MHz band (slightly higher in Europe) and use the frequencies at the high end for the downstream. Asymmetric NTL Service Download Speed Upload Speed 150K Broadband Up to 150Kbps Up to 64Kbps 600K Broadband Up to 600Kbps Up to 128Kbps 1Mb Broadband Up to 1024Kbps Up to 256Kbps Forouzan, Business Data Communications

18 Modulation and data rate
Downstream Upstream Bandwidth =200MHz 42-5=37M channels 200/6=33 37/6=6 Modulation 64-QAM or 256-QAM QPSK Theoretical Data rate 56M = 30 Mbps (1bit for error con) 26M = 12 Mbps Limited data rate 10 Mbps as 10base-T interface to PC Less than 12 Mbps Sharing broadcast timesharing Downstream Data Band The downstream data (from the Internet to the subscriber premises) occupies the upper band, from 550 to 750 MHz. This hand is also divided into 6-MHz channels. Modulation Downstream data are modulated using the 64-QAM (or possibly 256-QAM) modulation technique. Downstream data are modulated using the 64-QAM modulation technique. Data Rate There are 6 bits for each baud in 64-QAM. One bit is used for forward error correction; this leaves 5 hits of data per baud. The standard specifies 1 Hz for each baud; this means that, theoretically, downstream data can be received at 30 Mbps (5 bits/Hz x 6 MHz). The standard specifies only 27 Mbps. However, since the cable modem is connected to the computer through a 10base-T cable (see Chapter 14), this limits the data rate to 10 Mbps. The theoretical downstream data rate is 30 Mbps. Upstream Data Band The upstream data (from the subscriber premises to the Internet) occupies the lower band, from 5 to 42 MHz. This hand is also divided into 6-MHz channels. Modulation The upstream data band uses lower frequencies that are more suscepti­ble to noise and interference. For this reason, the QAM technique is not suitable for this hand. A better solution is QPSK.  Upstream data are modulated using the QPSK modulation technique.  Data Rate There are 2 hits for each baud in QPSK. The standard specifics 1 Hz for each baud: this means that, theoretically, downstream data can he sent at 12 Mbps (2 hits/Hz x 6 MHz). However, the data rate is usually less than 12 Mbps. The theoretical upstream data rate is 12 Mbps. Sharing Both upstream and downstream bands are shared by the subscribers. Upstream Sharing The upstream data bandwidth is only 37 MHz. This means that there are only six 6-MHz channels available in the upstream direction. A subscriber needs to use one channel to send data in the upstream direction. The question is, how can six channels be shared in an area with 1000, or even 100,000 subscribers? The solution is timesharing. The hand is divided into channels using FDM; these channels must be shared between sub-scribers in the same neighbourhoods. The cable provider allocates one channel, statically or dynamically, for a group of subscribers. If one subscriber wants to send data, she or he contends for the channel with others who want access; the subscriber must wait until the channel is available. Downstream Sharing We have a similar situation in the downstream direction. The downstream hand has 33 channels of 6 MHz. A cable provider probably has more than 33 subscribers; therefore, each channel must be shared between a group of subscribers. However, the situation is different for the downstream direction; here we have a multicasting situation. If there are data for any of the subscribers in the group, the data are sent to that channel. Each subscriber is sent the data. But since each subscriber also has an address registered with the provider, the cable modem for the group matches the address carried with the data to the address assigned by the provider. If the address matches, the data are kept; otherwise. they are discarded. Forouzan, Business Data Communications

19 Cable Modems Cable modems are devices that allow high-speed access to the Internet via a cable television network. a cable modem is significantly more powerful, capable of delivering data approximately 500 times faster. a cable modem is a 64/256 QAM RF receiver capable of delivering up to 30 to 40 Mbps of data in one 6-MHz cable channel. This is approximately 500 times faster than a 56.kbps modem. Data from a user to the network is sent in a flexible and programmable system under control of the headend. The data is modulated using a QPSK/16 QAM transmitter with data rates from 320 kbps up to 10 Mbps. The upstream and downstream data rates may be flexibly configured using cable modems to match subscriber needs. For instance, a business service can be programmed to receive as well as transmit higher bandwidth. A residential user, however, may be configured to receive higher bandwidth access to the Internet while limited to low bandwidth transmission to the network. When a telephone line is used in conjunction with a one-way broadcast network, the cable data system is referred to as a telephony return interface (TRI) system. Forouzan, Business Data Communications

20 Cable vs. ADSL effective capacity unpredictable Seriously affect
If you have a cable Encryption Specific bandwidth for every one user Increase numbers of user has little affect Max coverage 5.5km More secure, more reliable In practice, it is hard to generalize about effective capacity. ADSL providers give specific statements about the bandwidth (e.g., I Mbps downstream, 256 kbps upstream) and generally achieve about 80% of it consistently. Cable providers do not make any claims because the effective capacity depends on how many people are currently active on the user's cable segment. Sometimes it may be better than ADSL and sometimes it may be worse. What can be annoying, though, is the unpredictability. Having great service one minute does not guarantee great service the next minute since the biggest bandwidth hog in town may have just turned on his computer. As an ADSL system acquires more users, their increasing numbers have little effect on existing users, since each user has a dedicated connection. With cable, as more subscribers sign up for Internet service, performance for existing users will drop. The only cure is for the cable operator to split busy cables and connect each one to a fiber node directly. Doing so costs time and money, so their are business pressures to avoid it. As an aside, we have already studied another system with a shared channel like cable: the mobile telephone system. Here, too, a group of users, we could call them cellmates, share a fixed amount of bandwidth. Normally, it is rigidly divided in fixed chunks among the active users by FDM and TDM because voice traffic is fairly smooth. But for data traffic, this rigid division is very inefficient because data users are frequently idle, in which case their reserved bandwidth is wasted. Nevertheless, in this respect, cable access is more like the mobile phone system than it is like the fixed system. Availability is an issue on which ADSL and cable differ. Everyone has a telephone, but not all users are close enough to their end office to get ADSL. On the other hand, not everyone has cable, but if you do have cable and the company provides Internet access, you can get it. Distance to the fiber node or headend is not an issue. It is also worth noting that since cable started out as a television distribution medium, few businesses have it. Being a point-to-point medium, ADSL is inherently more secure than cable. Any cable user can easily read all the packets going down the cable. For this reason, any decent cable provider will encrypt all traffic in both directions. Nevertheless, having your neighbor get your encrypted messages is still less secure than having him not get anything at all. The telephone system is generally more reliable than cable. For example, it has backup power and continues to work normally even during a power outage. With cable, if the power to any amplifier along the chain fails, all downstream users are cut off instantly. Finally, most ADSL providers offer a choice of ISPs. Sometimes they are even required to do so by law. This is not always the case with cable operators. The conclusion is that ADSL and cable are much more alike than they are different. They offer comparable service and, as competition between them heats up, probably comparable prices. DSL vs. Cable Modem One basic difference between DSL and cable modem technology is that DSL is a circuit oriented network, where individual connections are independent of each other, whereas cable modem is broadcast oriented, meaning that all subscribers in the same area receive the same signals and share the same trunk line. Cable modems operate over hybrid fiber coaxial (HFC) networks, which is much more expensive to install as compared to copper telephone wires that are already in place. Also, in areas where traditional coaxial cables are used, great expenses are incurred to replace the signal repeaters to be bi-directional, to allow signals to travel both to and from the subscribers. Although coaxial cable has a greater capacity than copper wires, because of the shared nature of the cable medium, the throughput experienced by any one subscriber is determined by how many other subscribers are using cable modems in the area at one time. The greater the number of cable modem users in the area, the lower the experienced data transfer rate by any one subscriber. Cable Modems are able to trans i data at rates up to 4 Mbps or 10 Mbps. However, this is not a promised (or guaranteed) rate. Instead, the bandwidth is shared among all the customers served by a single channel. Therefore, the real bandwidth varies while customers pay a fixed price. Cable modems have become available in many residential areas within the past few years. If the configuration is done properly, cable has the ability and capacity to transmit data at speeds similar to DSL. Most of the time Cable and DSL do not compete in the business market due to the fact that Cable lines are not available in the vast majority of commercial districts. Forouzan, Business Data Communications

21 Comparison With Other Broadband Technologies
DSL vs. T1 DSL vs. ISDN DSL vs. 56K Modem DSL vs. T1 From a technical point of view, DSL offers services comparable to TI. However, TI needs to be offered over "clean" lines, whereas DSL is more flexible in adjusting to ill-conditioned lines. When providing TI services, the telephone company must make sure that the local loop that is used is in good condition, and sometimes must install repeaters every mile to boost signal strength. Also, any bridge taps and multiple gauge wiring must be removed from the local loop in order for TI to work properly. With DSL technology, the telephone company does not have to mess with the local loop infrastructure, resulting in a great cost savings over T1 as you will see in the next section. T I provides point-to-point connectivity and is very common in all kinds of telephony transport. A TI connection provides a high data rate of up to Mbps, sufficient for small to medium businesses. Tl is a technology that bundles several phone lines together to provide downstream access speeds of 1.54 Mbps. The decision between DSL and TI access when DSL is readily available is what some might call a no-brainer. Generally speaking, for the same high bandwidth access, a business will pay at least twice as much and sometimes 3 or 4 times as much every month for a TI line as it would for DSL. In other words, for the cost of a single TI 1.54 Mbps connection, you could install three 1. 1 Mbps DSL connections. Typically, installation costs for a T I line are 3 -4 times as much as installation and setup of DSL services. This is due in large part to the fact that DSL uses ordinary, preinstalled telephone lines as opposed to T 1, which requires installation of special lines. TI also usually takes longer to install than DSL and makes private networking between numerous regions prohibitively expensive and extremely complicated. DSL makes interconnecting multiple regions for private networking much more cost-effective. When you look at these facts alone, it is easy to see why many have termed DSL the 71killer". Forouzan, Business Data Communications

22 An ADSL system using DMT allocates 3/4 of the available data channels to the downstream link. It uses QAM-64 modulation on each channel. What is the capacity of the downstream link? (Assuming ADSL modulate at 4000 baud). There are 256 channels in all, minus 6 for POTS and 2 for control, leaving 248 for data. If 3/4 of these are for downstream, that gives 186 channels for downstream. ADSL modulation is at 4000 baud, so with QAM-64 (6 bits/baud) we have 24,000 bps in each of the 186 channels. The total bandwidth is then 186*24kbps=4.464 Mbps downstream.

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