Chapter 3: Physical Layer

Chapter Outline

INTRODUCTION

CIRCUITS Circuit Configuration Data Flow Communication Media
Multiplexing
How DSL Transmits Data COMMUNICATION MEDIA
Guided Media
Wireless Media
Media Selection

DIGITAL TRANSMISSION OF DIGITAL DATA Coding Transmission Modes Digital Transmission How Ethernet Transmits Data

ANALOG TRANSMISSION OF DIGITAL DATA Modulation Capacity of a Voice Circuit How Modems Transmit Data

DIGITAL TRANSMISSION OF ANALOG DATA Translating from Analog to Digital How Telephones Transmit Voice Data How Instant Messenger Transmits Voice Data VoIP

IMPLICATIONS FOR MANAGEMENT

SUMMARY

Teaching Notes

Generally preconceptions about what analog and digital transmission are vary greatly among undergraduate students. For this reason it is necessary to spend at least about 3-4 hours covering this chapter.

Make sure at first that the students understand the dual characteristic state of what constitutes a circuit- the concepts of logical and physical. Circuits are probably the most important aspect of comprehending the physical layer. A circuit can be viewed and generally is viewed as a wire or a cable. This is the physical aspect of the circuit. There is also a logical aspect of a circuit. It is not just how long, wide or what type of cable; it is also what it is that propagates through the cable.

When I discuss circuits and media, I try to remember to hand around twisted pair cable, coax cable and fiber optic cable. I also try to link the media to common media with which the student are familiar: twisted pair to telephone cable, coax to cable TV cable, radio to cell phones, and infrared to TV controllers. If you can illustrate circuits in this or similar ways, coverage of the physical characteristics of circuits will not be as complex.

On the other hand, covering the logical aspects of circuits will form a greater part of this chapter. So allow yourself ample time to define, explore and discuss analog and digital and how the two are mixed to optimize different forms of transmission.

Coverage on MODEMs, CODECs and Multiplexers rounds out the chapter and illustrates both the physical and logical aspects of circuits.

War Stories

Every network, including those using wireless connectivity must have electrical power to function. The physical layer of the network is highly dependent on electrical power. During Desert Storm the network supporting COMNAVSURFLANT in Norfolk, Virginia was completely down for about 5 hours one day at the height of the first Persian Gulf war. This network supported 290 network users that were housed in 7 different buildings on the Naval Station at Norfolk. These users were responsible for expediting the shipment of replacement parts in support of the Atlantic Fleet Operations including those Navy and Marine Units on the ground in the Gulf region.

SURFLANT as they called it, was a campus-like network environment. Inside the buildings they used twisted-pair cabling to connect the client PCs to multi-port HUBs located in the floor closet spaces. The HUBs were interconnected within the building closets using coaxial cable. The HUB stacks in each building were all connected across the naval station via multi-mode fiber-optic cable.

As the War progressed in the Gulf, a terrible storm hit the east coast of the U.S. on this day in January of 1991. Lightening struck the main power station at Naval Station Norfolk. All electricity was out across the base. Needless to say none of the network hardware, except those devices running on backup (battery) power was operating. To complicate matters further, when the power did come back on, power surges occurred at several locations around the network. These surges were responsible for burning out the circuit boards on 4 network interface cards (NICs) and 1 multi-port HUB. This resulted in all of the users connected via that HUB to be down until the bad HUB could be replaced. In addition the client PCs housing the 4 bad NICs, which were connected via other HUBs still operating were down until the NICs could be replaced. All of the circuits were fine but as you can see how we terminate and power circuits becomes a critical network management factor at the physical layer.

Answers to End-of-Chapter Questions

1. How does a multipoint circuit differ from a point-to-point circuit?

A point-to-point configuration is so named because it goes from one point to another (e.g., one computer to another computer). These circuits sometimes are called dedicated circuits because they are dedicated to the use of these two computers. In a multipoint configuration (also called a shared circuit), many computers are connected on the same circuit. This means that each must share the circuit with the others, much like a party line in telephone communications. The disadvantage is that only one computer can use the circuit at a time. Multipoint configurations are cheaper than point-to-point configurations.

2. Describe the three types of data flows.

The three types of data flows are simplex, half-duplex and full duplex. Simplex is one-way transmission, such as that in radio or TV transmission. Half duplex is two-way transmission, but you can transmit in only one direction at a time. A half duplex communication link is similar to a walkie-talkie link; only one computer can transmit at a time. With full duplex transmission, you can transmit in both directions simultaneously, with no turnaround time.

3. Describe three types of guided media.

Guided media are those in which the message flows through a physical media such as a twisted pair wire, coaxial cable, or fiber optic cable; the media “guides” the signal.

One of the most commonly used types of guided media is twisted pair wire, insulated or unshielded twisted pairs (UTP) of wires that can be packed quite close together. Bundles of several thousand wire pairs are placed under city streets and in large buildings. Twisted pair wire is usually twisted to minimize the electromagnetic interference between one pair and any other pair in the bundle.

Coaxial cable is another type of commonly used guided media. Coaxial cable has a copper core (the inner conductor) with an outer cylindrical shell for insulation. The outer shield, just under the shell, is the second conductor. Coaxial cables have very little distortion and are less prone to interference, they tend to have low error rates.

Fiber optics, is becoming much more widely used for many applications, and its use is continuing to expand. Instead of carrying telecommunication signals in the traditional electrical form, this technology utilizes high-speed streams of light pulses from lasers or LEDs (light emitting diodes) that carry information inside hair-thin strands of glass or plastic called optical fibers.

4. Describe four types of radiated media.

Radiated media are those in which the message is broadcast through the air, such as radio, infrared, microwave, or satellite.

One of the most commonly used forms of radiated media is radio (usually just called wireless). Radio data transmission uses the same basic principles as standard radio transmission. Each device or computer on the network has a radio receiver/transmitter that uses a specific frequency range that does not interfere with commercial radio stations. The transmitters are very low power, designed to transmit a signal up to 500 feet and are often built into portable or handheld computers.

Infrared transmission uses low frequency light waves (below the visible spectrum) to carry the data through the air on a direct line-of-sight path between two points. This technology is similar to the technology used in infrared TV remote controls. It is prone to interference, particularly from heavy rain, smoke, and fog that obscure the light transmission. Infrared is not very common, but it is sometimes used to transmit data from building to building.

A microwave is an extremely high frequency radio communication beam that is transmitted over a direct line-of-sight path between any two points. As its name implies, a microwave signal is an extremely short wavelength. Microwave radio transmissions perform the same functions as cables. Similar to visible light waves, microwave signals can be focused into narrow, powerful beams that can be projected over long distances.

Transmission via satellite is similar to transmission via microwave except, instead of transmitting to another nearby microwave dish antenna, it transmits to a satellite 22,300 miles in space.

5. How does analog data differ from digital data?

Computers produce digital data that are binary, either on or off. In contrast, telephones produce analog data whose electrical signals are shaped like the sound waves they transfer. Analog data are signals that vary continuously within a range of values (e.g., temperature is analog).

6. Clearly explain the differences among analog data, analog transmission, digital data, and digital transmission.

Data can be transmitted through a circuit in the same form they are produced. Most computers, for example, transmit their data through digital circuits to printers and other attached devices. Likewise, analog voice data can be transmitted through telephone networks in analog form. In general, networks designed primarily to transmit digital computer data tend to use digital transmission, and networks designed primarily to transmit analog voice data tend to use analog transmission (at least for some parts of the transmission).

7. Explain why most telephone company circuits are now digital.

Most telephone company circuits are now digital because digital transmission is “better” than analog transmission. Specifically, digital transmission offers several key benefits over analog transmission:

• Digital transmission produces fewer errors than analog transmission. Because the transmitted data is binary (only two distinct values), it is easier to detect and correct errors.
• Digital transmission is more efficient. Time division multiplexing (TDM) is more efficient than frequency division multiplexing (FDM) because TDM requires no guardbands. TDM is commonly used for digital transmission, while FDM is used for analog transmission.
• Digital transmission permits higher maximum transmission rates. Fiber optic cable, for example, is designed for digital transmission.
• Digital transmission is more secure because it is easier to encrypt.
• Digital transmission is less expensive than analog in many instances.

Finally, and most importantly, integrating voice, video and data on the same circuit is far simpler with digital transmission.

8. What is coding?

Coding is the representation of one set of symbols by another set of symbols. In data communications, this coding is a specific arrangement of binary 0s and 1s used to represent letters, numbers, and other symbols that have meaning.

9. Briefly describe the two most important coding schemes. There are two predominant coding schemes in use today. United States of America Standard Code for Information Interchange (USASCII), or more commonly ASCII, is the most popular code for data communications and is the standard code on most terminals and microcomputers. There are two types of ASCII; one is a 7-bit code that has 128 valid character combinations, and the other is an 8-bit code that has 256 combinations. Extended Binary Coded Decimal Interchange Code (EBCDIC) is IBM's standard information code. This code has 8 bits, giving 256 valid character combinations.

10. How is data transmitted in parallel?

Parallel mode is the way the internal transfer of binary data takes place inside a computer. If the internal structure of the computer is 8-bit, then all eight bits of the data element are transferred between main memory and the central processing unit simultaneously on 8 separate connections. The same is true of computers that use a 32-bit structure; all 32 bits are transferred simultaneously on 32 connections.

11. What feature distinguishes serial mode from parallel mode?

Serial mode is distinguished from parallel mode by the time cycle in which the bits are transmitted. Parallel implies that all bits of a character are transmitted, followed by a time delay, and then all bits of the next character are transmitted, followed by a time delay. Serial implies that characters are sent one bit at a time, with each bit followed by a time delay. Put another way, parallel is character-by-character and serial is bit-by-bit.

12. How does bipolar signaling differ from unipolar signaling? Why is Manchester encoding more popular than either?

With unipolar signaling, the voltage is always positive or negative (like a dc current). In bipolar signaling, the 1’s and 0’s vary from a plus voltage to a minus voltage (like an ac current). In general, bipolar signaling experiences fewer errors than unipolar signaling because the signals are more distinct. Manchester encoding is a special type of unipolar signaling in which the signal is changed from a high to low or from low to high in the middle of the signal. A change from high to low is used to represent a 1 (or a 0), while the opposite (a change low to high) is used to represent a 0 (or a 1). Manchester encoding is less susceptible to having errors go undetected, because if there is no transition in mid-signal the receive knows that an error must have occurred.

13. What are three important characteristics of a sound wave?

Sounds waves have three important characteristics. The first is the height of the wave, called amplitude. Our ears detect amplitude as the loudness or volume of sound. The second characteristic is the length of the wave, usually expressed as the number of waves per second or frequency. Frequency is expressed in hertz (Hz). Our ears detect frequency as the pitch of the sound. Human hearing ranges from about 20 hertz to about 14,000 hertz, although some people can hear up to 20,000 hertz. The third characteristic is the phase, refers to the direction in which the wave begins.

14. What is bandwidth? What is the bandwidth in a traditional North American telephone circuit?

Bandwidth refers to a range of frequencies. It is the difference between the highest and the lowest frequencies in a band; thus the bandwidth of human voice is from 20 Hz to 14,000 Hz or 13,880 Hz. The bandwidth of a voice grade telephone circuit is from 0 to 4000 Hz, or 4000 Hz; however, not all of this is available for use by telephone or data communications equipment. To start, there is a 300-hertz guardband at the bottom of the bandwidth and a 700-hertz guardband at the top. These prevent data transmissions from interfering with other transmissions when these circuits are multiplexed using frequency division multiplexing. This leaves the bandwidth from 300 to 3300 hertz or a total of 3000 Hz for voice or data transmission. 15. Describe how data could be transmitted using amplitude modulation.

With amplitude modulation (AM) (also called amplitude shift keying (ASK)), the amplitude or height of the wave is changed. One amplitude is defined to be zero, and another amplitude is defined to be a one. 16. Describe how data could be transmitted using frequency modulation.

Frequency modulation (FM) (also called frequency shift keying (FSK)), is a modulation technique whereby each 0 or 1 is represented by a number of waves per second (i.e., a different frequency). In this case, the amplitude does not vary. One frequency (i.e., a certain number of waves per second) is defined to be a one, and a different frequency (a different number of waves per second) is defined to be a one. 17. Describe how data could be transmitted using phase modulation.

Phase modulation (PM) (also called phase shift keying (PSK)), is the most difficult to understand. Phase refers to the direction in which the wave begins. Until now, the waves we have shown start by moving up and to the right (this is called a 0º phase wave). Waves can also start down and to the right. This is called a phase of 180º. With phase modulation, one phase is defined to be a zero and the other phase is defined to be a one. 18. Describe how data could be transmitted using a combination of modulation techniques.

It is possible to use amplitude modulation, frequency modulation, and phase modulation techniques on the same circuit. For example, we could combine amplitude modulation with four defined amplitudes (capable of sending two bits) with frequency modulation with four defined frequencies (capable of sending two bits) to enable us to send four bits on the same symbol. 19. Is the bit rate the same as the symbol rate? Explain.

The terms bit rate (i.e., the number bits per second transmitted) and baud rate are used incorrectly much of the time. They often are used interchangeably, but they are not the same. In reality, the network designer or network user is interested in bits per second because it is the bits that are assembled into characters, characters into words and, thus, business information. Because of the confusion over the term baud rate among the general public, ITU-T now recommends the term baud rate be replaced by the term symbol rate. The bit rate and the symbol rate (or baud rate) are the same only when one bit is sent on each symbol. For example, if we use amplitude modulation with two amplitudes, we send one bit on one symbol. Here the bit rate equals the symbol rate. However, if we use QAM, we can send four bits on every symbol; the bit rate would be four times the symbol rate.

20. What is a modem?

Modem is an acronym for MOdulator/DEModulator. A modem takes the digital electrical pulses received from a computer, terminal, or microcomputer and converts them into a continuous analog signal that is needed for transmission over an analog voice grade circuit. Modems are either internal (i.e., inside the computer) or external (i.e., connected to the computer by a cable).

21. What is quadrature amplitude modulation (QAM)?

One popular technique is quadrature amplitude modulation (QAM). QAM involves splitting the symbol into eight different phases (three bits) and two different amplitudes (one bit), for a total of 16 different possible values. Thus, one symbol in QAM can represent four bits.

22. What is 64- QAM?

If we use 64-QAM, the bit rate is six times the symbol rate. A circuit with a 10 MHz bandwidth using 64-QAM can provide up to 60 Mbps.

23. What factors affect transmission speed?

The factors that affect the transmission speed are the number of bits per signal sample and the number of samples per second.

24. What is oversampling?

For voice digitization, one typically samples at twice the highest frequency transmitted, or a minimum of 8,000 times a second. Sampling more frequently than this will improve signal quality. For example, CDs sample at 44,100 times a second and use 16 bits per sample to produce almost error-free music. 25. Why is data compression so useful?

Data compression can increase throughput of data over a communication link literally by compressing the data. A 2:1 compression ratio means that for every two characters in the original signal, only one is needed in the compressed signal (e.g., if the original signal contained 1000 bytes, only 500 would needed in the compressed signal). In 1996, ITU-T revised the V.34 standard to include a higher data rate 33.6 Kbps. This revision is popularly known as V.34+. The faster data rate is accomplished by using a new form of TCM that averages 9.8 bits per symbol (symbol rate remains at 3429).

26. What data compression standard uses Lempel-Ziv encoding? Describe how it works.

V.42bis, the ISO standard for data compression, uses Lempel-Ziv encoding. As a message is being transmitted, Lempel-Ziv encoding builds a dictionary of two, three, and four character combinations that occur in the message. Any time the same character pattern reoccurs in the message, the index to the dictionary entry is transmitted rather than sending the actual data. V.42bis compression can be added to almost any modem standard; thus a V.32 modem providing a data rate of 14,400 bps, could provide a data rate of 57,600 bps when upgraded to use V.42bis.

27. Explain how pulse code modulation (PCM) works.

Analog voice data must be translated into a series of binary digits before they can be transmitted. With PAM-based methods, the amplitude of the sound wave is sampled at regular intervals, and translated into a binary number. The most commonly used type of PAM is Pulse Code Modulation (PCM). With PCM, the input voice signal is sampled 8000 times per second. Each time the input voice signal is sampled, eight bits are generated. Therefore, the transmission speed on the digital circuit must be 64,000 bits per second (8 bits per sample x 8,000 samples per second) in order to transmit a voice signal when it is in digital form.

28. What is quantizing error?

Quantizing error is the difference between the replicated analog signal and its original form, shown with jagged “steps” rather than the original, smooth flow. Voice transmissions using digitized signals that have a great deal of quantizing error will sound metallic or machinelike to the human ear.

29. What is the term used to describe the placing of two or more signals on a single circuit?

Multiplexing is the term used to describe the placing of two or more signals on a single circuit.

30. What is the purpose of multiplexing?

A multiplexer puts two or more simultaneous transmissions on a single communication circuit. Multiplexing a voice telephone call means that two or more separate conversations are sent simultaneously over one communication circuit between two different cities. Multiplexing a data communication network means that two or more messages are sent simultaneously over one communication circuit. In general, no person or device is aware of the multiplexer; it is “transparent.”

31. How does DSL (digital subscriber line) work?
DSL services are quite new, and not all common carriers offer them. In general, DSL services have advanced more quickly in the Canada (and Europe, Australia and Asia) than in the United States due to their newer telephone networks from the end offices to the customer.
Unlike other services that operate through the telephone network end-to-end from the sender to the receiver, DSL only operates in the local loop from the carrier's end office to the customer's telephone. DSL uses the existing local loop cable, but places one DSL network interface device (called customer premises equipment (CPE)) in the home or business and another one in the common carrier's end office. The end office DSL device is then connected to a high speed digital line from the end office to elsewhere in the carrier's network (often an Internet service provider) using some other service (e.g., T carrier, SMDS).
32. Of the different types of multiplexing, what distinguishes

a. Frequency division multiplexing (FDM)?

Frequency division multiplexing can be described as dividing the circuit “horizontally” so that many signals can travel a single communication circuit simultaneously. The circuit is divided into a series of separate channels, each transmitting on a different frequency, much like series of different radio or TV stations. All signals exist in the media at the same time, but because they are on different frequencies, they do not interfere with each other.

b. Time division multiplexing (TDM)?

Time division multiplexing shares a communication circuit among two or more terminals by having them take turns, dividing the circuit “vertically.” In TDM, one character is taken from each terminal in turn, transmitted down the circuit, and delivered to the appropriate device at the far end. Time on the circuit is allocated even when data are not be transmitted, so that some capacity is wasted when terminals are idle.

c. Statistical time division multiplexing (STDM)?

Statistical time division multiplexing is the exception to the rule that the capacity of the multiplexed circuit must equal the sum of the circuits it combines. STDM allows more terminals or computers to be connected to a circuit than FDM or TDM. STDM is called statistical because selecting the transmission speed for the multiplexed circuit is based on a statistical analysis of the usage requirements of the circuits to be multiplexed. STDM is like TDM, except that each frame has a terminal address and no blanks are sent.

d. Wavelength division multiplexing (WDM)?

Wavelength division multiplexing is a version of FDM used in fiber optic cables. WDM works by using lasers to transmit different frequencies of light (i.e., colors) through the same fiber optic cable; each channel is assigned a different frequency so that the light generated by one laser does not interfere with the light produced by another. WDM permits up to 40 simultaneous circuits each transmitting up to 10 Gbps, giving a total network capacity in one fiber optic cable of 400 Gbps (i.e., 400 billion bits per second).

33. What is the function of inverse multiplexing (IMUX)?

Inverse multiplexing (IMUX) combines several low speed circuits to make them appear as one high-speed circuit to the user.

34. If you were buying a multiplexer, why would you choose either TDM or FDM? Why?

If buying a multiplexer, you would choose TDM over FDM. In general, TDM is preferred to FDM, because it provides higher data transmission speeds and because TDM multiplexers are cheaper. Time division multiplexing generally is more efficient than frequency division multiplexing, because it does not need guardbands. Guardbands use “space” on the circuit that otherwise could be used to transmit data. It is not uncommon to have time division multiplexers that share a line among 32 different low speed terminals. It is easy to change the number of channels in a time division multiplexer. Time division multiplexers generally are less costly to maintain.

35. Some experts argue that MODEMs may soon become obsolete. Do you agree? Why or why not?

The traditional context of MODEM no doubt has become obsolete. We no longer can consider a MODEM a device that connects two computers by merely modulating and demodulating transmission signals over the Public Switched Network at speeds ranging up to 56Kbps. This type of MODEM now must be evaluated in light of newer technologies such as xDSL MODEMs and Cable MODEMs each of which generates, propagates and transmits signals at far greater speeds. In addition we must evaluate the traditional MODEMs as well in light of newer protocols and compression techniques which have greatly improved overall bandwidth and throughput with the traditional types of MODEMs. In short, though many new forms of MODEMs and MODEM supported technology have come in to play, the traditional MODEM continues to be a cost-effective and flexible means of networking if your bandwidth requirements remain under the 56Kbps threshold.

36. What is the maximum capacity of an analog circuit with a bandwidth of 4,000 Hz using QAM?

Under perfect circumstances, the maximum symbol rate is about 4,000 symbols per second. If we were to use QAM (4 bits per symbol), the maximum data rate would be 4 bits per symbol X 4,000 symbols per second = 16,000 bps.

37. What is the maximum data rate of an analog circuit with a 10 MHz bandwidth using 64-QAM and V.44?

A circuit with a 10 MHz bandwidth using 64-QAM can provide up to 60 Mbps. A V.44 modem can provide as much as 6:1 compression ratio, depending on the type of data sent. Thus, the maximum data rate of 64-QAM with compression is 360 Mbps.

38. What is the capacity of a digital circuit with a symbol rate of 10 MHz using Manchester encoding?

B = s * n
B = (10 MHz) * 1 bit per symbol
Capacity is 10 Mbps

39. What is the symbol rate of a digital circuit providing 100 Mbps if it uses bipolar NRZ signaling?

B = s * n
100 Mbps = s * 1 bit per symbol
Symbol rate is 100 Mhz

40. What is VoIP?
Voice over IP is commonly used to transmit phone conversations over the digital networks. VoIP uses digital phones with built-in codecs to convert analog to digital.

Mini-cases
I. Eureka! (Part 1)

Eureka staff answers the phone and respond to requests entered on the Eureka! Web site. Much of their work is spent on the phone and on computers searching on the Internet. They have just leased a new office building and are about to wire it. What media would you suggest they install in their office and why?

If you are comfortable with prefacing a discussion on “wiring” a new office building with a talk about IEEE 802.11 wireless Local Area Networking, it may add value to the class, clarify some questions which many students may be hearing about companies not needing to wire building anymore and sharpen your focus on the main topic which is wiring the building to support networking.

Certainly today all if not most companies in this scenario will pre-wire their new workspaces to support modular, multimedia connectivity. That translates into CATEGORY 5 (CAT-5) or better (i.e. 5E, 6) with RJ-45 faceplates supporting connections for data and voice.

II. Eureka! (Part 2)

What type of connections should they consider from their offices to the outside world, in terms of phone and Internet? Outline the pros and cons of each alternative below and make a recommendation. They have four alternatives:

1. Should they use traditional analog services, with standard voice lines and use modems to dial into their Internet Service Provider ($40 per month for each voice line plus $20 per month for each Internet access line)?

Option 1 requires an individual trunk line, MODEM, connection cable(s) and emulation software on each and every computer to network data this way. Also when the line is used for data it cannot support voice. Slow speed data.

2. Should they use standard voice lines, but use DSL for their data ($60 per month per line for both services)?

Option 2 requires a individual DSL line, DSL MODEM, connection cable(s) and DSL software on each and every computer to network data this way. Higher cost. Each telephone requires an individual trunk line. Higher bandwidth speeds would be an advantage with each DSL line.

3. Should they separate their voice and data needs using standard analog services for voice but finding some advanced digital transmission services for data ($40 per month for each voice line and $300 per month for a circuit with 1.5 Mbps of data)?

Option 3 requires an individual dedicated circuit, Channel Service Unit (CSU), Router, connection cables and software configuration. But if the company has a Local Area Network in place, they can share much higher bandwidth speeds with any computer connected to this LAN. Each telephone requires an individual trunk line. Unit costs would be proportional, though voice line costs would be the same the company could leverage the data line costs to show that after 5-6 computer connection, the company would save money on the data side and achieve much higher bandwidth thresholds with a large degree of scalability.

4. Should the company search for all digital services for both voice and data ($60 per month for an all digital circuit that provides two PCM phone lines that can be used for two voice calls, one voice call and one data call at 64 Kbps, or one data call at 128 Kbps)?

The context here outlines the use of a BRI (i.e. ISDN) connection. Specifically, a BRI service with 2B channels, each channel providing 64Kbps. With the correct terminating hardware in place, the BRI could support both data and voice up to 128Kbps.

III. Amalgamated Stores

Would you recommend buying the STDM multiplexer? Why or why not? Would you recommend purchasing it if Amalgamated Stores was planning to change its credit card authorization system (used to verify customer’s credit cards as they pay for merchandise) to use this network? Why or why not?

Yes. I would recommend using STDM in the first instance. STDM allows more computers to be connected to a circuit than does FDM or TDM. The key benefit of STDM is that it provides more efficient use of the circuit and saves money. Amalgamated can purchase a lower-speed, less expensive circuit than it could using either FMD or TDM.

If Amalgamated is going to change its credit card authorization system, it may want to consider another alternative as one of the challenges of STDM is the potential for delays if all devices are transmitting or receiving at the same time. This will not be acceptable to a customer attempting to pay for products.

IV. Speedy Package

1.) Assuming that each label is 1000 bytes long, how long does it take to transmit one label over the cell network, assuming that the cell phone network operates at 14 kbps (14,000 bits per second and that there are 8 bits in a byte)? 2.) If Speedy were to upgrade to the new, faster digital phone network that transmits data at 114 Kbps (114,000 bits per second) how long would it take to transmit?

14,000/8 = 1,750 bytes/second
1,000/1,750 = .57 seconds

114,000/8 = 14,250 bytes/second
1,000 /14,250 = .07 seconds

V. Networking Australia

This Mini-Case is based on Management Focus 3-1 “Australia Plugs Into Fiber” (p. 81). What other alternatives do you think Southern Cross considered?

Southern Cross possibly also considered a wireless alternative, including microwave and satellite. They considered these alternatives based on the challenges involved in constructing a trans-oceanic fiber network, and the expense of a reliable system.

VI. Boyle Transportation

This Mini-Case is based on Management Focus 3-3 “Satellite Communications Improves Performance” (p. 84). What other alternatives do you think Boyle Transportation considered? Why do you think they did what they did?

Boyle probably also considered a microwave system. Such a system presents similar functionality as a satellite system. It differs in requiring long-distance antennae and repeater stations to be placed approximately 25-50 miles apart throughout the service area.

VII. NASA’s Ground Network

This Mini-Case is based on Management Focus 3-5 “NASA’s Ground Communications Network” (p. 104). What other alternatives do you think NASA considered? Why do you think they did what they did?

NASA uses a combination of satellite and fiber-optic circuits to communicate among the various DSCC’s around the world. It might have considered using fiber only or satellite only circuits. They chose a mixed approach, using satellites for the overseas DSCCs, to facilitate the rapid deployment of the international sites.

Next Day Air Service Case Study

1. President Coone is baffled about how digital information from a computer can be sent over a telephone line. Prepare a brief position paper for management explaining the way information is transferred from one computer to another over telephone lines. Keep it simple. Be sure to describe the types of modems used in data transmission over telephone circuits. Include comments on the role of data compression in increasing transmission rates. Justify the observation that as a general rule, it is best to purchase the fastest modem your communications lines can support.
Student answers will vary but the key points are:
(1) A basic idea is that digital data is transmitted as a series of 0s and 1s. This binary information is encoded into sets of characters. The two basic codes are United States of America Standard Code for Information Exchange (USASCII or ASCII), a 7-bit data code with 128 valid character combinations and Extended Binary Coded Decimal Interchange Code (EBCDIC), IBM’s 8-bit data code with 256 valid character combinations.
(2) The data is sent over the telephone line in serial fashion, one bit at a time, with each bit followed by a time delay.
(3) Modems are used to convert the digital (pulse) signals from the sending computer to analog signals for transmission as tones over the telephone lines. The modem also converts analog signals back to the digital signal at the receiving computer.
(4) The four common types of modems are: a. V.22 modems - They have been the standard for several years. They send at a 1200 or 2400 bps data rate. b. V.32 modems - They send at rate of 9600 bps. c. V.34 modems - They are the newest modems. They send at 28,800 bps over dial-up telephone lines. d. V.42bis modems - These modems use a data compression technique in which the modem strips out unnecessary bits for transmission. The gain provided by the data compression usually ranges form 3.5:1 to 4:1. This means that a 14,400 bps modem can send at the effective rate of up to 4x14,400 bps or 57,600 bps.
(5) The more throughput you can get, the lower the cost per transmission. The determining factor in throughput is the modem speed. The cost of the modem is a one-time cost, with only a relatively small difference in cost between the faster and slower modems. The line costs are ongoing and relatively stable. Therefore, it is more cost-effective to obtain the fastest modem the line can support.
2) As you begin thinking about the redesign of the network, you realize there are various types of media to choose from. Which media will you use in the network and where will you use these types?
There are two main types of media to choose from: wired and wireless. Within each LAN, twisted pair is probably used to connect individual PCs to switches and the switches back to the “computer room.” Fiber optic cable might be used to connect servers to the switches and/or routers. Wireless is often used within a LAN to connect laptops.
Between locations across the country, leased fiber optic lines are probably used due to the speed and distance they offer.

3)
a) Compute each office's number of bits sent per day (origin to destination) based on the data provided in Figure 4-16. Use ten bits per character to keep computations simple, and assume all transmissions are error free. Hint: Bits per day = Packages x 750 x 10 x 2.

City
Bits per day Transmission Time
(Minutes/Day)
Atlanta, GA 63,150,000 36.55
Chicago, IL 37,800,000 21.88
Dallas, TX 73,500,000 42.53
Denver, CO 17,250,000 9.98
Houston, TX 30,150,000 17.45
Jackson, MS 21,900,000 12.67
Jacksonville, FL 21,150,000 12.24
Los Angeles, CA 11,550,000 6.68
Memphis, TN 28,050,000 16.23
Miami, FL 42,150,000 24.39
Montgomery, AL 6,750,000 3.91
New Orleans, LA 31,800,000 18.40
Orlando, FL 41,250,000 23.87
St. Louis, MO 14,850,000 8.59
Tampa, FL 47,850,000 27.69
Washington, DC 7,350,000 4.25 286.15 or 286

b. How many minutes will it take for each city's modem to transmit its invoices? Use the bits per day calculated in Question 2, and assume that the V.34 modems transmit at an average rate of 28,800 bits per second.
See the third column in the table of the previous question.
The 36.55 minutes for Atlanta was calculated in the following way:
(63,150,000 bits per day)/(28800 bps x 60 second/minute) = 36.55 minutes per day
c. Previously, you calculated the transmission time in minutes per day, based on a 28,800 bits-per-second modem and ten bits per character. Now calculate the “file transfer time” for Atlanta to transmit all of its invoices to Tampa at the end of the workday. Why is this answer different from the time calculated for Atlanta previously?
The 17.69 minutes is one half the time shown in the top right column of the answer to Question 3 (see the table in the answer for question 2). The reason is that Question 3 included transmitting the invoices twice, once to the destination city and once to Tampa.
Put another way, file transfer time is (4210 packages x 750 characters/invoice x 10 bits/character)/ (28800 bits per second transmission speed) or 1096.354 seconds / 60 seconds per minute = 18.27 minutes.
Modem Packages Bits Bits/Bps Minutes Hours
28800 29910 224325000 7789.0625 129.8177 2.163628
57600 29910 224325000 3894.53125 64.90885 1.081814

d. Could all the NDAS offices transmit their invoices to Tampa between 5:00 P.M. and 6:00 P.M. each evening? How would they achieve this goal?
Not with the current 28800 modem. Look at the answer to Question 3 (the right column in the table in the answer for question 2). The sum of all the transmission times in minutes per day (other than those from Tampa = 33100-3190 packages) is 259.6354 minutes. That figure includes two transmissions for cities other than Tampa, one to a destination city and one to Tampa. Therefore, 259.6354/2 = 129.8177 minutes to transmit to Tampa. Then, 129.8177/60 minute per hour = 2.163628 hours. Assuming there are no errors, it would take little over 2¼ hours for all the offices to transmit their invoices to Tampa with the current modem. However, using a 56 Kbps modem would decrease the time to (28800/57600)*2.16 hours or 0.5*2.16 = 1.08 hours or 64.90885 minutes. This would nearly meet the goal. Therefore, you should recommend obtaining 56 Kbps modems for all the offices.

4. Would it be better to transmit all invoice information from the origin to Tampa and for Tampa to send the information to the destination? Why?
With the speed at which data travels today, the transmittal of invoice data to Tampa and then back out to the destination isn’t an issue with the volume discussed in the case. The network could easily handle that volume.
Would it be better to transmit from the origin to Tampa and from Tampa to the destination, probably not. Due to the capacity though, it would be possible without much problem.
5. With the additions of a network that can handle voice, video, and data, what other uses and benefits can NDAS derive from the new network?
Student answers may vary considerably, but may include some of the following:
• Videoconferencing
• Teleconferencing
• Live training
• VoIP