1 Voice and Delivery Data Networks Chapter 11 Learning Objectives After reading this chapter, you should be able to: Identify the basic elements of a telephone system Describe the composition of the telephone industry before and after the 1984 Modified Final Judgment and explain the differences Describe the difference between a local exchange carrier and an interexchange carrier Differentiate between the roles of the local telephone company before and after the Telecommunications Act of 1996 Describe the basic characteristics of a 56k modem List the types of leased lines that are available and their basic characteristics Identify the main characteristics of digital subscriber line, and recognize the difference between a symmetric system and an asymmetric system Identify the main characteristics of a cable modem List the basic characteristics of frame relay, such as permanent virtual circuits and committed information rate Identify the main characteristics of Asynchronous Transfer Mode, including the roles of the virtual path connection and the virtual channel connection, the importance of the classes of service available, and ATM s advantages and disadvantages Describe the concept of convergence, and identify several examples of it in the networking industry Identify the impact that MPLS and Metro Ethernet are having on frame relay and ATM services Chapter Outline 1. Introduction 2. The Basic Telephone System a. Telephone lines and trunks b. The telephone network before and after 1984 c. Telephone networks after 1996 d. Limitations of telephone signals e. The 56k dial-up modem
2 3. Digital Subscriber Line a. DSL basics b. DSL formats 4. Cable Modems 5. T-1 Leased Line Service 6. Frame Relay a. Frame relay setup b. Committed Information Rate (CIR) 7. Asynchronous Transfer Mode (ATM) a. ATM classes of service b. Advantages and disadvantages of ATM 8. Comparison of DSL, cable modems, frame relay, and ATM 9. Convergence a. Computer-Telephony Integration (CTI) 10. Telecommunications Systems In Action: A Company Makes a Service Choice a. Prices b. Making the choice 11. Summary Lecture Notes Introduction Today, computers are starting to take over the voice portion of communications, and the line between data networks and telecommunication systems is blurring at an alarming rate. Just knowing data networks is no longer sufficient. For example, many businesses are installing single systems that can support both voice and data over the same circuits. One such system is computer-telephony integration (CTI), a combination of local area networks and the telephone system that is capable of providing telephone services, such as voice mail, and typical computer applications, such as database systems and electronic mail over a single set of wires. In addition, systems that were designed to transfer data, such as frame relay and the Internet, are now transmitting voice with various degrees of success. Asynchronous Transfer Mode, which was originally designed for both voice and data, is another example of a system that is adept at transferring both forms of signals.
3 Basic Telephone Systems The basic telephone system, or POTS, has been in existence since the early 1900s. During most of those years, POTS was an analog system capable of supporting a voice conversation. It wasn t until the 1980s that POTS began carrying computer data signals as well as voice signals. The growth of data became so large that near the end of the 20 th century, POTS carried more data than voice. Even though POTS has seen a number of technological changes, such as the increasing use of digital signals in place of analog signals, POTS is still basically a voicecarrying medium. 56k Modems Approximately two years after the 33,600 bps modem was announced, the 56,000 bps modem was announced. Did something change to allow the faster transmission speed, or were the industry experts wrong (that 33,600 was the fastest a modem would transmit)? The experts were correct. Two important facts changed with the 56,000 bps modems: digital signaling was introduced and the signal power level was increased. Alternatives to Dial-up Modems: DSL and Cable Modems There are five alternative transmission technologies available, other than the traditional dial-up modem, that can be used to connect a computer into a remote network system: T-1 digital telephone lines, cable television networks, Integrated Services Digital Network (ISDN), Digital Subscriber Line (DSL), and cable modems. Each of these transmission technologies requires a particular kind of device that converts the digital data of a computer to the proper form for transmission. T-1 Leased Line Services Businesses require permanent high-speed connections to connect themselves to the Internet and access other businesses, remote databases, and high-speed data transfer services such as public data networks. These users desire a faster circuit, perhaps one that does not need to be dialed at all but is constantly connected. One possible solution is to lease a dedicated line from the telephone company. One leased line service is the 56K leased line. A second option for companies requiring a data rate faster than 56,000 bits per second is the T1 (or T-1) service. Frame Relay Frame relay is a packet-switched network that was designed for transmitting data over fixed lines (not dial-up lines). Most long-distance telephone companies, such as AT&T, Sprint, and MCI, offer frame relay service over most of the country. Once the service is established, the customer only needs to transmit his or her data over a local link to a nearby frame relay station. The frame relay network is responsible for transmitting the user s data across the network and delivering it to the intended destination site.
4 Asynchronous Transfer Mode Asynchronous Transfer Mode (ATM), similar to frame relay, is a relatively new high-speed, packet-switched service that is offered by the telephone companies. A business that wants to send data between two points (either within a building or across the country) at very high transfer rates might consider using ATM. The transfer rates are as fast as 622 Mbps with even faster speeds possible. Other unique features of ATM include a small cell size (53 bytes), support for a wide range of traffic types, and quality of service options. Convergence Convergence in the telecommunications industry is constant. One form of convergence is computer-telephony Integration (CTI). CTI is an exciting field that combines more traditional voice networks with modern computer networks. CTI integrates the PBX phone switch with computer services, thus creating modern voice and data applications that run on computer systems. CTI combines the power of computer systems with the services of a telephone network. Running the appropriate telephony operations on a workstation computer, a user can perform typical telephone operations by clicking in a window of a program. Telecommunications Systems In Action: A Company Makes a Service Choice The In Action example for this chapter chronicles a company and how they decide to select a frame relay service over other types of long-distance data communication services. Quick Quiz 1. What is the difference between a POTS line and a leased line? POTS line is voice-grade dial-up, while leased line is better quality and always on. 2. What is frame relay used for? Frame relay is a high-speed data packet transfer service for local and long distance. Frame relay can also carry voice now. 3. What is Asynchronous Transfer Mode used for? Asynchronous Transfer Mode is also a high-speed data packet transfer service but can support a wide variety of traffic types at very high transfer speeds. 4. What are the primary uses of digital subscriber line and cable modems? They both are used to provide home and business users with high-speed connections into the Internet.
5 Discussion Topics 1. Will the telephone companies ever replace all copper telephone lines with digital, fiber-optic lines? 2. Will the distinction between local telephone calls and long-distance telephone calls ever disappear? What may cause this to happen? 3. Will frame relay someday pass into obscurity along with token ring LANs? 4. What do you envision as some possible future applications of computer telephony integration? Teaching Tips 1. Emphasize that the local telephone loop normally carries two channels of voice for a regular telephone line (one channel for each direction of traffic). But using modern technology, this loop can be extended to carry many more channels (such as used in DSL). 2. Emphasize the importance of the Modified Final Judgment of 1984 and its effect on longdistance service and the Telecommunications Act of 1996 and its effect on local telephone service. 3. Be sure to demonstrate an example of a business with at least three sites scattered across the country and how they can use either T1 lines or frame relay service to interconnect. Emphasize the three costs involved with a frame relay service: the PVC, the port charge, and the telecommunications line that connects the office to the frame relay network. 4. Using the Web, pull up some local advertisements for digital subscriber line service, cable modem service, or any others. Have the students go to Web sites such as or to determine local availability. Solutions to Review Questions 1. What is the plain old telephone system (POTS)? The standard telephone system that enters most homes and businesses 2. What is the typical frequency range for the human voice? 200 Hz to 3500 Hz (a range of 3300 Hz)
6 3. The local loop connects what to what? A home or business to the nearest central office 4. What is a LATA? Local Area Transport Area regions of the country that define local telephone calls and longdistance telephone calls 5. How does a trunk differ from a telephone line? A trunk carries multiple telephone signals, is usually digital, is most commonly found between telephone central offices, and is not associated with a single telephone number. 6. List the most important results of the Modified Final Judgment of AT&T. Breakup of AT&T; opening up of long-distance lines; definition of LATAs 7. What is the difference between a local exchange carrier and an interexchange carrier? A local exchange carrier provides a local telephone service, while an interexchange carrier provides a long-distance telephone service. 8. List the most important results of the Telecommunications Act of Incumbent local telephone companies must open up their local telephone lines to new, competitive local telephone companies; long-distance providers as well as anyone else can now offer local telephone service. 9. Why are the 56 kbps modems faster than the older 33,600 bps modems? They use digital signaling techniques. 10. Why don t the 56 kbps modems transmit at 56 kbps? Too much noise, lowered power levels, and analog/digital conversions 11. What are the basic features and transfer speeds of DSL? Always on connection, speeds up to 1 Mbps for residential users, several Mbps for commercial users 12. What are the basic features and transfer speeds of cable modems? Always on connection, speeds up to a few Mbps
7 13. How does a leased line service differ from POTS? POTS line is voice-grade dial-up, while leased line is better quality and always on. 14. What are the basic features of a T-1 line? All digital, synchronous time division multiplexing, Mbps total data rate, can support up to 24 separate telephone lines 15. What are the basic services of a T-3 line? All digital, synchronous time division multiplexing, 45 Mbps total data rate 16. What are the basic characteristics of frame relay? Digital, packet-switched, high-speed, long-distance, data (primarily) and voice transmission service 17. What features make frame relay so attractive? All the answers in Question 14 are advantages. 18. How do you create a frame relay permanent virtual circuit? Ask the frame relay carrier to create the circuit 19. How does a frame relay switched virtual circuit differ from a permanent virtual circuit? The switched virtual circuit can be created by the user. 20. Is it possible to have more than one PVC over a physical line? Explain. Yes. Since PVCs are virtual circuits, the physical circuit is capable of supporting multiple virtual circuits. 21. What is agreed on when a customer and a frame relay service agree on a committed information rate? If the user keeps the data transfer rate under a certain value, the carrier will guarantee a certain percentage rate of packets delivered. 22. What happens when a user transmits data faster than the agreed upon committed information rate? The user s frames will be marked and may be discarded if the network becomes congested.
8 23. How does frame relay compare to sending data over the Internet? Frame relay is more secure and faster, but it costs more. 24. What are the basic features of ATM? Fast, complex, expensive, WAN (mostly) and LAN, wide range of traffic types 25. What is the relationship between an ATM virtual channel connection and a virtual path connection? The carrier creates the virtual path and the customer can create one or more virtual channels over the virtual path. 26. What is meant by the ATM classes of service? ATM can support a wide range of traffic types, such as video, music, high- and low-speed data, voice, and more. 27. Describe some applications that incorporate computer-telephony integration. Corporate systems that require people to use the phone extensively, such as retail order taking and customer service Suggested Solutions to Exercises 1. The telephone line that connects your house or business to the central office (the local loop) carries your conversation and the conversation of the person to whom you are talking. What do you estimate is the bandwidth of a local loop? Since the human voice has a bandwidth of approximately 3300 Hz, the telephone system allocates two 4000 Hz channels, or 8000 Hz altogether. 2. If you play a CD for a friend over the telephone, will the friend hear high-quality music? If not, why not? No, because the telephone is designed to transmit signals with a relatively small bandwidth (4000 Hz). Music CDs require a bandwidth approaching 20,000 Hz. 3. If you place a telephone call and it leaves your LATA and enters another LATA, what kind of telephone call have you placed? What kind of telephone company handles this telephone call? Long-distance telephone call; Interexchange carrier (IEC)
9 4. For each of the following scenarios, state whether a telephone line or a trunk should be used: The connection from your home to the local telephone company The connection between a large company s PBX and the telephone company The connection between two central offices Line, trunk, trunk 5. Many within the telephone industry feel we will eventually run out of area codes and telephone numbers. How many different area codes are currently available? What would you suggest to increase the number of telephone numbers? First digit can be 2-9 (eight combinations), and second and third digits can be 0-9 (10 combinations each), so 8 * 10 * 10 = 800 possible area codes. Go to a longer telephone number (more digits). 6. State whether each of the following was a result of the Modified Final Judgment of 1984, the Telecommunications Act of 1996, or neither: a. The FCC was created to watch over interstate telephone systems b. AT&T had to sell off its local telephone companies c. The LATA was created d. Cable television companies could provide local telephone service e. Customers could choose between different long-distance telephone providers f. AT&T split off its technology division, which became Lucent Neither, 1984, 1984, 1996, 1984, neither 7. You want to start your own local telephone company. Do you have to install your own telephone lines to each house and business? Explain. No. The Telecommunications Act of 1996 says we can lease equipment and lines from existing local telephone companies (ILECs). 8. What type of telephone service do you use to connect a PBX to a central office if you want to allow 40 users to dial out simultaneously? You most likely would use two T-1s, or two Primary ISDNs. 9. If you install a 56 kbps modem into your computer and dial into a remote network that only has 33,600 bps modems, is your modem useless? No, the 56k modem will fallback to the appropriate speed.
10 10. Why would you want to use the newer V.92 modem rather than the older V.90? Why might an online Internet service not want you to use the V.92 modem? The V.92 modem can pause a data connection if an incoming voice call arrives. Internet service providers may not want you to use this because then you can stay connected for long periods of time and not worry about missing incoming voice calls. 11. You dial in to your Internet service provider using your 56k modem. Once connected, your data rate is shown as 42,000 bps. Why not 56k bps? List all possible reasons. Noise, analog/digital conversions, FCC requiring a lower power level 12. The next day, you dial in to an Internet service provider different from the one used in the previous question. This time you get connected at 24,000 bps. Could there be different reasons for this slower connection? Explain your response. The ISP might have an older modem; there is an additional analog/digital conversion somewhere in the circuit. 13. Cable modems, like 56k modems, create asymmetric connections. What does this mean, and why doesn t this asymmetric connection affect the average Web user? An asymmetric connection is one in which the downstream data transfer speed is higher than the upstream speed. This is common, since the average Web user is usually downloading lots of data (Web pages), not uploading. 14. What are the basic functions of a cable modem? Provides high-speed access to the Internet; provides cable television; splits the signal between computer and television 15. Give an example in which someone would want to use a symmetric DSL service. You would want to use a symmetric DSL service if the upstream load is the same as the downstream load. For example, if you have a Web server on site, you would be uploading as much data as, if not more than, you would be downloading. 16. What is the main advantage of asymmetric DSL over symmetric DSL? It is very often cheaper, and the downstream in asymmetric is sometimes bigger than the downstream in symmetric. 17. In a DSL service, what is the function of the DSLAM? Combines regular phone signals with Internet data
11 18. You live in a house that is 10 miles from the nearest city. Will you be able to get DSL service to your home? If the answer is no, what could the DSL service provider do to provide you with DSL? More than likely not. Two to three miles is the maximum distance for DSL. But a service provider could run fiber-optic cable to your neighborhood or maybe offer a wireless solution. 19. If you have a frame relay service installed, do you just pick up the telephone and dial the number of the party you want to connect to? Explain. No. Assuming the frame relay service is providing a PVC, the carrier establishes the PVC for the customer. Once established, it s always there. 20. Suppose you want to have a frame relay connection between your Chicago office and your New York office. Itemize the different charges that you will have to pay for this connection. PVC to connect between Chicago and New York Telephone lines (two) to connect Chicago and New York offices to frame relay network Port charges (two), one in Chicago and one in New York 21. You have established a frame relay connection with a committed information rate of 256 kbps and a burst rate of 128 kbps. Several times a day, your computer systems transmit in excess of 512 kbps. What will happen to your data? Probably nothing. If the frame relay network experiences congestion, your frames that exceed the CIR of 256k + burst rate of 128k may be discarded, but not likely. 22. What exactly does frame relay do if a frame is garbled and produces a checksum error? Drops the frame 23. For each of the following activities, state which is the better transmission medium, frame relay or the Internet: sending sending high-speed data interactive voice communications receiving a live video stream participating in a chat room or newsgroup Internet, frame relay, frame relay, frame relay, Internet
12 24. One of the disadvantages of ATM is the 5-byte cell tax. Explain what the 5-byte cell tax means. All data is transmitted in 53-byte cells, and each cell has 48-bytes of data with a 5-byte header. The 5-byte header is the tax. 25. You have an ATM connection that goes from your location to a network entity, on to a second network entity, and then to your desired destination. Draw a simple sketch that shows each virtual channel connection. 26. State which ATM class of service would best support each of the following applications: with image attachments interactive video simple text voice conversation VBR or ABR; CBR or VBR; ABR or UBR; CBR or VBR 27. Using CTI, how can a local area network support telephone operations? Telephone functions such as answering calls, forwarding calls, conference calls, fax retrieval, voice mail services, and providing user with a menu of options can all be performed on a workstation connected to a LAN. 28. Describe a business or school application that would benefit from CTI. Telemarketing, online database service, help desks, information centers Thinking Outside the Box 1. A company wants to connect two offices located in Memphis, Tennessee and Laramie, Wyoming. The offices need to transfer data at 512 kbps. Which is the less expensive solution More than likely, in both cases a frame relay solution is the most economical. Long-distance T- 1s are very pricey. 2. You are consulting for a hospital that wants to send three-dimensional, high-resolution, color ultrasound images between the main hospital and an outpatient clinic It sounds like high-capacity data transfers in real-time, best handled by ATM. If you do this continuously, you might need a CBR service; otherwise, you can probably get by with a RT- VBR.
13 3. Your company wants to create an application that allows employees to dial in from a remote location and, using a single connection, access their voice mail, , and data files. What kind of system would allow this? Describe the necessary hardware and software components. You will want to consider some form of CTI. You will need a system that is capable of supporting both voice calls and data transfers.
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