m Chapter 19
`
`FIGURE 19.13
`
`Incoming Data Packets
`
`D a ta T ra n sm issio n
`
`directly from data bundled in packets, as shown in Figure 19.13. The packets do not have
`to arrive at a particular rate or contain a particular number of bits, although they must have
`headers that specify their destination. Figure 19.7 compares this to TD M circuit switching.
`The data rate is set by the clock speed of the transmitter, which generates a specified
`number of pulses per second. Data packets are lined up for transmission in the sequence of
`their arrival. The transmitter sends one packet at a time, starting with the header, which
`contains address information, and proceeds through the rest of the bits in sequence. If an-
`other packet is waiting, the transmitter starts sending it; otherwise it sends blank intervals.
`It’s like lining people up to get onto a moving staircase where each step holds one per-
`son. The people line up at the base, and one gets onto each passing step. As long as people
`are in line, every step is filled. Steps only go empty if no one is left in line. To make the
`analogy better, you could imagine tour groups lining up, with a single leader acting as the
`“header” of each group.
`Multiple packet streams are combined by a technique called statistical multiplexing,
`rather than by interleaving bits. Incoming data packets on each channel accumulate in sep-
`arate storage buffers. The multiplexer takes packets from each buffer in turn, keeping track
`of the traffic on each channel, then allocates more time to the busiest channels.
`Statistical multiplexing is used for transmitting Internet Protocol (IP) traffic. Like time-
`division multiplexing, it combines data streams from multiple sources into a single faster
`signal. However, it does not require a rigid hierarchy of incoming data rates. Because sta-
`tistical multiplexers average bursty traffic over many channels, the total capacity of the in-
`put channels may be higher than their output capacity. That is, a statistical multiplexer
`might have 10 inputs delivering up to 100 Mbit/s, but one output able to transmit only
`600 Mbit/s. That design can work as long as the average input is below the peak capacity.
`For example, if each input channel averages only 20 Mbit/s, the combined inputs should
`be safely below 600 Mbit/s most of the time. (This sort of averaging is common in tele-
`phone networks, which don’t have output connections for every input line because most
`lines are used only a small fraction of the time. Problems only arise when everyone makes
`long-distance calls on Mother’s Day.)
`
`Statistical
`multiplexing
`combines multiple
`packet streams
`that may have
`different data
`rates.
`
`MASIMO 2014
`PART 8
`Apple v. Masimo
`IPR2020-01526
`
`

`

`System and Optical Networking Concepts
`
`Statistical multiplexing requires a set of priorities because too many packets may queue
`up while waiting to be transmitted. One approach is to allow packets to “time out” after a
`certain interval, like a person who gives up after waiting on hold for customer service. This
`method is used in the traditional version of the Internet Protocol (IPv4). A refinement is
`to set higher priority for delay-sensitive traffic, such as telephone conversations or stream-
`ing video, so it goes to the head of the transmission queue, while lower-priority data packets
`wait; this is used in IPv6, which is not yet implemented across the entire Internet.
`Wavelength-Division Multiplexing
`Wavelength-division multiplexing (WDM) is the transmission of different signals on multiple
`wavelengths through the same fiber. It closely resembles electronic frequency-division multi-
`plexing, but is done at the much higher frequencies of light waves, as shown in Figure 19.14.
`The optical channels at 193.1, 193.2, 193.3, and 193.4 THz are the optical counterparts of
`the radio-frequency channels at 174, 180, 186, and 192 MHz shown in Figure 19.13.
`Channel spacing may be defined in terms of frequency or wavelength, but if the car-
`riers are in the optical or infrared part of the spectrum, the process is generally called
`wavelength-division multiplexing. Standards have been set for two types: dense-W DM
`(DWDM) and coarse-WDM (CWDM).
`Dense-WDM packs signals closely together, usually for long-distance transmission, and
`usually in the erbium -am plifier band between about 1530 and 1610 nm. Standards specify
`DWDM spacings in frequency units as 50, 100, and 200 GHz, which correspond to about
`0.4, 0.8, and 1.6 nm in wavelength units near 1550 nm. The same standards also allow for
`spacing at 400 and 1000 GHz, which usually are not considered DWDM. The packing
`density of DWDM systems is limited by the modulation bandwidth of the signals, so
`2.5-Gbit/s signals can be packed more tightly together than can 10-Gbit/s signals.
`
`W DM transmits
`signals at multiple
`wavelengths
`through one fiber.
`
`FIGURE 19.14
`Wavelength-division multiplexing.
`
`

`

`Chapter 19
`
`Coarse-WDM is
`used in metro
`networks, which
`don't require
`amplification.
`
`Most WDM
`systems have few
`populated
`channels.
`
`Multiplexing is
`involved in signal
`management.
`
`Granularity
`measures how
`finely a signal
`can be broken up
`into its component
`parts.
`
`Coarse-WDM spaces channels more loosely to reduce equipment costs. Current specifi-
`cations space 18 channels at 20-nm intervals between 1270 and 1610 nm. The primary ap-
`plications of coarse-WDM are in metro networks, which can carry heavy traffic but don’t
`require optical amplification.
`Total fiber capacity is limited by the range over which W DM is possible. In practice,
`amplifier bandwidth limits the range in amplified systems. The range is larger in unampli-
`fied systems, where the limits come from fiber attenuation. Potential capacities can reach
`staggering levels. The entire erbium band can accommodate more than 100 channels at
`10 Gbit/s, allowing commercial systems to offer total capacity of more than 1 Tbit/s.
`Experimental systems have transmitted more than 10 Tbit/s a few hundred kilometers
`through a single fiber.
`Today’s DWDM systems generally populate only a few optical channels because actual
`transmission requirements fall far short of total fiber capacity. However, carriers reserve the
`extra channel capacity for inexpensive future expansion.
`
`Signal Management and Optical Networking
`Multiplexing is closely related to the management of signals transmitted through a
`telecommunications network. Multiplexing combines signals so they can be transmitted
`through fiber or other media. Multiplexed signals may be demultiplexed or otherwise bro-
`ken up when they have to be switched, redirected, or otherwise managed. Electronically
`multiplexed signals are broken up into their component bit streams (or analog channels).
`W DM signals are broken up into their component wavelengths.
`Much publicized during the telecommunications bubble, optical networking is signal
`management in the optical domain based on wavelength. An important advantage of opti-
`cal networking is that W DM channels are entirely independent of each other, unlike TDM
`signals that must be in the same family of formats. A single fiber can carry several wave-
`lengths, each transmitting in different formats. One wavelength can carry Gigabit Ethernet,
`another a 2.5-Gbit/s stream of Internet data, a third a single digitized high-definition video
`signal, and a fourth an analog cable-television signal. All you need are transmitters, receivers,
`fibers, optical amplifiers, and WDM optics that can handle the required data rates.
`Signal management in the optical domain also requires optical switches that can select
`one or more optical channels in a WDM system. Ideally, optical networks also should be
`able to convert signals from one wavelength to another for transmission in other parts of
`the network.
`The ability to switch individual optical channels can aid signal management by easing
`access to individual data streams. Suppose 40 streams of 2.5-Gbit/s data are multiplexed in
`a single fiber. Optical networking could easily isolate one data stream if the signals were
`40 WDM optical channels. Isolating one data stream from a 100-Gbit/s TDM signal is much
`harder. The ease of breaking up multiplexed signals into their components is called granu-
`larity. The more granular the signal, the better carriers can meet customer needs and the
`more efficiently they can manage their networks.
`Optical networking is still evolving, and it is too early to predict its final shape. Issues to
`be resolved include the technologies for wavelength conversion and other functions, what
`services should be offered, and what customers require.
`
`

`

`System and Optical Networking Concepts
`
`Transmission Distance
`The telecommunications network is global in extent, but transmission distance remains an
`important element of system performance. Two types of transmission span are important
`in fiber optics— the distance between individual optical amplifiers, and the distance be-
`tween regenerators or repeaters, which are separated by a series of amplifiers.
`The span between a pair of amplifiers is limited by the input power and the fiber loss.
`When signal power diminishes to an unacceptable level, you need an amplifier. Amplifiers
`aren’t needed in short systems, where the required power reaches the receiver without
`amplification.
`The end-to-end distance is limited by the degradation the signal experiences as it passes
`through a series of amplifiers. Once noise and dispersion have degraded the signal suffi-
`ciently, it must be regenerated. Typically regenerators are built into terminal nodes of a sys-
`tem, so the space between regeneration points is the length of a system, such as that of a
`submarine cable link from New York to London, or a terrestrial cable from Chicago to
`Saint Louis.
`As you will learn, trade-offs are involved in the two distances. Increasing the spacing be-
`tween amplifiers also increases the noise added to the signal and reduces the end-to-end
`distance. That’s why repeaters have to be spaced much closer together in cables that cross
`the Atlantic ocean than in cables that run a few hundred kilometers between large cities.
`
`Cost and Reliability
`Network cost and reliability are critical concerns for carriers that provide telecommunica-
`tion services. A cutting-edge system that sets transmission speed records is fine in the lab-
`oratory, but no carrier will want to install it if it is out of service half the time or requires
`a small army of top-level engineers to maintain.
`Costs fall into two broad categories: capital expense (capex) and operating expense (opex).
`Capital expense is the cost of buying new equipment. Operating expense is the cost of run-
`ning the network, including power, normal maintenance, required adjustments, and re-
`pairs. Often they are charged to separate departments, but choices are made based on
`trade-offs between the two. For example, an optical add-drop multiplexer that can be re-
`configured from a central facility will incur more capital expense than one that requires
`manual adjustment. But the manually operated model will have higher operating expense
`because technicians have to travel to the site to make adjustments. Engineers decide be-
`tween the two types of systems based on their expectations of operating requirements.
`Typically spending an extra $50,000 on the latest, greatest, automated system isn’t reason-
`able if the system is only 50 miles from the control center and will require only one ad-
`justment every year. But it’s another matter if the site is 500 miles from the control center
`and requires adjustment twice a week.
`Construction and installation costs typically count as capital expense and are another
`important factor. Normally carriers plan capacity for future expansion, so their network de-
`signs include spare fibers and extra wavelength slots in fibers to provide that capacity.
`Adding an extra pair of fibers to a cable being installed now is much cheaper than installing
`an additional cable later.
`
`Signal
`degradation limits
`end-to-end
`distance.
`
`Costs are divided
`into capital
`expense and
`operating
`expense.
`
`

`

`Chapter 19
`
`R ing N e tw o rk
`
`Break here does not stop
`service because there is
`an alternative path.
`
`Ring and mesh
`networks can
`better withstand
`cable failures than
`branching
`networks.
`
`Reliability depends on factors such as network topology and operating environment as
`well as the choice of equipment. Traditional networks branched out from a few central
`points, as shown in Figure 19.15. This reduced cable and installation costs, but left the net-
`work vulnerable to interruption by a single cable break. Modern systems that carry heavy
`traffic are arranged in rings or meshes, so an alternative path remains between any two
`points in the network if a cable breaks.
`The operating environment also is important for reliability. Much equipment is outdoors,
`where it must withstand moisture and extreme temperatures. Remote optics and electronics
`are installed in shielded cases, or “repeater huts,” which provide some protection. Sensitive
`equipment usually is installed in temperature-controlled office-like environments.
`
`What Have You Learned?
`
`1. The global telecommunications network is evolving rapidly in response to
`changes in business and technology.
`2. The network includes copper, wireless (radio), and fiber links.
`3. Optical networking manages light signals, usually in the heart of the network.
`4. Telecommunications networks are made of links between nodes. They carry
`voice, video, and data signals, which have different transmission requirements.
`5. The telephone system is circuit-switched. The Internet is packet-switched.
`6. Standards or protocols allow different equipment to exchange signals in a
`common format.
`7. Point-to-point transmission links pairs of terminals permanently. Point-to-point
`multipoint or broadcast transmission sends signals from one transmitter to many
`terminals.
`8. Networks link many terminals with each other in various configurations.
`
`

`

`System and Optical Networking Concepts
`
`9. Packet switching uses routers to deliver signals in a network; the routers read
`data headers on the packets to determine their destination. The Internet uses
`packet switching.
`10. Circuit switching establishes temporary connections between pairs of terminals; it
`is used for telephone systems.
`11. A signal modulates a carrier wave at a higher frequency. It may modulate
`amplitude, frequency, or phase of the carrier. Amplitude modulation is most
`common in fiber optics.
`12. No return to zero (NRZ) coding is the most common way to code digital
`signals for fiber-optic transmission. Signals in NRZ form do not return to zero
`after a 1 bit is transmitted.
`13. Error-correction codes operate on blocks of data, correcting errors that arise in
`transmission. The number of corrections depends on the coding scheme.
`14. Transmission capacity is the bandwidth or data rate of a communication system.
`15. Time-division multiplexing interleaves data in several incoming bit streams to
`generate a composite bit stream containing all the data. All input is at the same
`rate, and the output rate is a multiple of the input rate.
`16. Statistical multiplexing combines multiple streams of data packets, which may arrive at
`different rates, to generate a single stream of data packets. It is used on the Internet.
`17. Wavelength-division multiplexing and frequency-division multiplexing combine
`signals at different wavelengths or frequencies. W DM applies to optical
`transmission through a fiber; FDM applies to radio or microwave transmission
`through air or coaxial cable.
`18. Most W DM systems have few populated channels.
`19. Granularity measures how finely signals can be broken up into their component
`parts. It is important for signal management.
`20. Transmission distance may be measured as spacing between amplifiers or
`regenerators. Amplifier spacing depends on power and attenuation; regenerator
`spacing depends on noise and dispersion.
`21. Costs are divided into capital expense and operating expense.
`What's Next?
`
`In Chapter 20, you will learn about the standards developed for fiber-optic systems.
`
`Further Reading
`
`Roger L. Freeman, Fundamentals o f Telecommunications (Wiley InterScience, 1999)
`Gary M. Miller, Modern Electronic Communications, 6th ed. (Prentice Hall, 1999)
`Jean Walrand and Pravin Varaiya, High-Performance Computer Networks, 2nd ed. (Morgan
`Kaufmann, 2000)
`
`

`

`Questions to Think About
`
`1. Internet routers read headers on data packets, and use that information to direct
`the packets toward the proper destination. What type of network architecture
`would you expect to be used to connect routers in the Internet backbone? Why?
`2. Can you use headers to control the flow of data packets around a ring network
`in which signals pass through all the terminals anyway?
`3. An electronic time-division multiplexer generates signals at 1 Gbit/s. If it has
`eight inputs, what are their data rates?
`4. How would an optical time-division multiplexer work?
`5. How is wavelength-division multiplexing analogous to frequency-division
`multiplexing?
`6. What are the prime limitations on amplifier spacing and regenerator spacing?
`How do the two affect each other?
`
`Chapter Quiz
`
`1 . What type of telecommunications is packet-switched?
`a. cable television
`b. standard telephones
`c. the Internet
`d. all
`e. none
`2. What type of telecommunications is circuit-switched?
`a. cable television
`b. standard telephones
`c. the Internet
`d. all
`e. none
`3. What protocol covers the arrangement of data for packet switching?
`a. the Internet Protocol
`b. SONET
`c. Asynchronous Transfer Mode
`d. Plesiochronous Digital Hierarchy
`e. NRZ coding
`4. A router does which of the following?
`a. It makes circuit-switched connections between terminals.
`b. It broadcasts signals to many points.
`c. It optically directs light signals to their destinations.
`
`

`

`System and Optical Networking Concepts
`
`It reads packet headers and directs signals to their destinations.
`d.
`It is equivalent to a switch.
`e.
`5. A switch does which of the following?
`a. It makes circuit-switched connections among terminals.
`b. It broadcasts signals to many points.
`c. It converts packet headers to circuit-switching directions.
`d.
`It reads packet headers and directs signals to their destinations.
`e.
`It is equivalent to a router.
`6. Amplitude modulation is used for
`a. digital transmission of a single 2.5-Gbit/s optical channel over fiber.
`b. analog transmission of cable-television signals.
`c. AM radio broadcasting.
`d. optical transmission of Internet data.
`e. all of the above
`7. What is the proper name for digital coding in which a strong signal means a 1
`and a low or zero signal means a 0?
`a. no return to zero (NRZ)
`b. return to zero (RZ)
`c. Manchester coding
`d. frequency-division multiplexing
`e. phase modulation
`
`8. Interleaving incoming bit streams to produce a faster output signal is called
`
`a. packet switching.
`b. frequency-division multiplexing.
`c. time-division multiplexing.
`d. statistical multiplexing.
`e. wavelength-division multiplexing.
`9. Simultaneously transmitting separate signals through an optical fiber at different
`wavelengths is called
`a. packet switching.
`b. frequency-division multiplexing.
`c. time-division multiplexing.
`d. statistical multiplexing.
`e. wavelength-division multiplexing.
`1 0 . What type of multiplexing requires all incoming signals to be at the same data rate?
`a. packet switching
`b. frequency-division multiplexing
`c. time-division multiplexing
`
`

`

`Chapter 19
`
`d. statistical multiplexing
`e. wavelength-division multiplexing
`1 1 . Transmission capacity of an optical fiber is the
`a. total amount of information the fiber can transmit.
`b. distance between amplifiers.
`c. number of wavelengths the fiber can transmit, regardless of data rate.
`d. distance from end to end.
`e. data rate that can be transmitted at 1550 nm.
`1 2. A fiber-optic system can transmit 2.5 Gbit/s on each of 40 optical channels,
`with an amplifier spacing of 100 km. The company that operates the system has
`installed transmitters and receivers at only 4 wavelengths. What is the data rate
`of the installed system?
`a. 2.5 Gbit/s
`b. 10 Gbit/s
`c. 40 Gbit/s
`d. 100 Gbit/s
`
`

`

`Fiber System
`Standards
`
`About This Chapter
`
`For two people to communicate, they must speak the same language. Communication
`systems likewise work only if transmitters and receivers attached to them speak the same
`language. Communication engineers have devised standards to assure that equipment
`from different companies will be able to interface properly.
`This chapter will introduce you to the system-level standards most important for
`fiber-optic systems. Some are specific to fiber optics; others also cover other communi-
`cations technologies. The topic of standards is complex and continuously evolving, so I
`will not go into much depth, especially for standards with little direct impact on fiber-
`optic systems. However, you should at least learn to recognize the most important stan-
`dards and their functions.
`
`Why Standards Are Needed
`
`As you learned earlier, signals can be transmitted in a variety of ways, with different
`types of digital or analog coding. However, those differences only scratch the surface of
`the immense potential for variations. You can think of those physical differences as
`being similar to the distinctions among the media you use to communicate with other
`people— speech, the written word, sign language, pictures, and so on.
`There are many other levels of variations in signal formats, just as people speak many
`different languages or computer programs store data in different formats. Unlike human
`languages, signal formats are designed by engineers to transmit signals efficiently and
`economically. Their choices depend on the types of signals being carried, the distances
`
`

`

`B Chapter 20
`
`and types of terminals involved, and the hardware and software they have available. The
`results can vary widely with factors such as time and network scale.
`These differences become a problem when you want networks to connect to each other
`or when you want to combine two or more generations of equipment, such as existing tele-
`phones with new digital switches and transmission lines. Then you need common lan-
`guages and ways of translating signals between formats. That’s when you need standards.
`Standards have evolved considerably over the years, changing with both the marketplace
`and the technology. In the 1970s, AT&T was effectively America’s telephone monopoly, so
`it set the standards for telecommunication systems. Since the 1984 breakup of AT&T,
`industry groups have come to set the standards. Many standards have become interna-
`tional, so you can make phone calls to Brazil, send faxes to India, and dispatch e-mail to
`Indonesia.
`Changing standards have accommodated changes in industry practice. In the 1960s,
`telephone lines carried only analog voice telephone conversations. By the 1970s, the tele-
`phone network started to convert to digital voice transmission between switching centers.
`In the 1980s, the telephone network started to handle more computer data and video
`transmission, plus fax signals. Today, the high-speed lines operated by long-distance carriers
`are digital data highways that transmit a wide variety of signals, all digitized into a common
`form that can be reconverted to other formats at the receiving end. Standards make this
`multipurpose system possible.
`Standards serve a variety of functions. They establish common physical features as well
`•
`as comm0n languages, so devices can fit together as well as understand each other. They
`Standards serve
`diverse functions. may ajSQ assure that equipment meets the requirements of specific customers such as tele-
`phone companies or the military.
`This chapter covers standards that apply to fiber-optic systems. Some important examples
`include:
`
`• Physical standards for connectors, to make sure they mate optically and
`mechanically.
`• Optical, electronic, and mechanical interface standards for packaged
`devices, so they connect properly to each other.
`• Standard formats for data transmission.
`• Telephone industry standards for data transmission and device
`performance.
`
`Other standards also affect fiber-optic equipment. Fire-safety standards affect the choice
`of materials for cables. Component standards assure that connectors, transmitters, and laser
`modules are interchangeable. We won’t talk about these standards here because they affect
`system design only indirectly.
`Standards may be industry-wide, proprietary, or some combination of the two. For
`example, industry committees set standard data formats for Ethernet transmission, but soft-
`ware usually stores data it generates in a proprietary format. In some cases proprietary formats
`become de facto standards, like Microsoft Word format for word processing files. Usually pro-
`prietary standards are optimized for a particular company’s equipment, while industry-wide
`standards are a compromise that works reasonably well on everyone’s equipment.
`
`

`

`Fiber System Standards
`
`Standards are
`crucial in an
`open, deregulated
`market.
`
`Families of
`standards exist for
`voice, data, and
`video systems.
`
`Most standards
`specify interfaces
`rather than
`internal
`operations.
`
`Companies often battle over standards as they try to gain a competitive edge. Sometimes
`two or more groups create competing standards. However, agreed-upon standards are cru-
`cial to the function of an open, deregulated market involving many vendors and equipment
`that must interconnect.
`You need to be sure you can plug any phone you buy today into the telephone jack in
`your wall and use it with any local or long-distance carrier. Most standards take into
`account the existence of old equipment and can accommodate much of it. You can use
`your digital PCS cell phone to call your grandmother on the heavy black dial phone she
`has used since 1952. Neither you nor your grandmother should notice the automatic elec-
`tronic conversion between the two formats. You should remember, however, that some
`new standards do not accommodate old equipment, such as standards for digital televi-
`sion transmission, which make no effort to talk with the “old” analog set you bought
`brand new in 2004.
`
`Families of Standards
`
`Families of standards have been developed to meet specific transmission requirements.
`Typically this means sending voice, video, or data signals in different environments. For
`example, there are standards for how to digitize voice telephone calls and how to inter-
`leave the data streams from individual conversations to give higher and higher data rates.
`Other standards specify formats for data transmission over certain types of computer
`networks.
`Standards for voice, video, and data signals evolved separately and remain somewhat dis-
`tinct. Many standards are administered by different organizations, or by different sub-
`groups within large organizations such as the International Telecommunications Union.
`Table 17.2 lists many of the organizations involved in fiber-optic system standards.
`Telecommunications standards evolved to meet specific industry requirements. In gen-
`eral, they specify interfaces rather than internal operations. In other words, what matters
`is not what goes on inside the box, but what goes into and comes out of the box. For
`example, it doesn’t matter if your home telephone looks like a beer can, Mickey Mouse, or
`an antique pay phone as long as it sends and receives standard voice signals. This provides
`industry the flexibility to improve the technology as long as it stays within performance
`standards.
`Traditional circuit-switched telephone transmission has well-established standards that
`range from requirements for wire-line telephones to high-speed data-transmission rates.
`The telephone industry designed this family of standards to work together, and many of these
`standards are used in long-distance transmission.
`Standards for transmitting computer data now center on the packet-switching formats
`used on the Internet. These standards include Ethernet and the Internet Protocol, and can
`handle data flow at uneven rates. Packet-switching standards continue to evolve to handle
`other services. An example is Voice over Internet Protocol (VoIP), which converts voice sig-
`nals into packets for Internet transmission.
`Video transmission standards are largely industry-specific, developed by television broad-
`casters and cable companies. Broadcast signal formats require approval by government
`
`

`

`J p Chapter 20
`
`Modern standards
`are structured in
`layers that serve
`distinct functions.
`
`The services most
`users see are the
`top of a stack of
`layers.
`
`agencies that regulate broadcasting, such as the Federal Communications Commission in
`the United States. Video transmission is evolving slowly from old analog formats to new
`digital formats, producing completely new standards that are incompatible with old
`equipment.
`We will explore specific standards later, but first you should learn how modern standards
`are structured.
`
`Layers of Standards
`
`Modern standards are developed as a series of layers, each of which serves a distinct func-
`tion. Essentially, each layer provides a set of interfaces for users of that layer, which effec-
`tively covers over deeper layers that the users don’t need to worry about. The layered
`structure comes from the Open System Interconnection (OSI) model developed by the
`International Organization for Standardization. Many older standards, like those for digital
`telephone transmission, have been modified to fit the OSI layered model. Analog services
`appear only as inputs to the top layer of the stack.
`You can see how a layered standard operates by considering the voice layer of the modern
`telephone network, shown in Figure 20.1. When you make a telephone call, you dial a
`phone number and hear the phone ring and a person answer, just as if the call were traveling
`over standard telephone wires, whether you’re calling across town or across the country. You
`shouldn’t notice that your voice is chopped into bits that are interleaved with bits from
`other telephone calls. All these operations are in the “cloud” of the telecommunications
`system (shown at the bottom of the figure). The system acts like you’re calling one of your
`neighbors over dedicated local phone lines (shown at the top of the figure). The network
`reaches both cell phones and regular phones, and you can tell them apart only by their
`sounds. Faxes also travel over voice telephone lines, and they respond the same way whether
`you call a fax machine or a computer with fax software.
`A layer in a layered standard can be viewed as providing interfaces to black boxes that
`represent other layers. The engineer who designs a phone, for example, works only on the
`voice layer of the telephone network, with no need to know about the other layers. The
`engineer is concerned only with the standard interfaces the phone has with the user and
`the network, which are other layers in the OSI model.
`
`Layers in the Network
`Figure 20.2 shows the overall layered standard structure in a somewhat simplified form that
`neglects data-transmission details normally handled by software. It shows the layers used in
`the telephone network and the Internet. There has been serious discussion about simplifying
`this layered structure to reduce equipment costs, but change has been slow. Note the layers
`were numbered before the emergence of wavelength-division multiplexing.
`Each service shown in the top layer has its own standard format. Some of these services
`actually perform multiple functions for the user. Standard analog voice telephone lines can
`transmit signals from faxes and dial-up modems as well as voice. The faxes and modems
`send digital data as audio tones that the phone network processes like ordinary voice phone
`
`

`

`Fiber System Standards
`
`FIGURE 20.1
`Voice layer o f the
`telephone system.
`
`r -
`
`ı n
`
`&
`
`&
`
`— ■.
`Y o u r c o m p u te r
`he a rs a n o th e r
`fax.
`
`\
`
`T h in k s it’s
`he a rin g a
`fa x
`/
`
`H e ars
`a n o th e r
`pho n e
`
`j
`This is what
`you hear—a phone j
`connection.
`!
`
`1i
`
`C ell Phone
`
`This is the real
`transmission route.
`
`You don’t see what’s really out there, digitizing your calls, interleaving
`them with other calls, switching, and otherwise manipulating them.
`
`“T h e C lo u d ”— T h e L a rg e r T e le c o m m u n ic a tio n s N e tw o rk
`
`calls. Broadband Internet connections provide service directly in digital form. This top
`layer is called the