Chapter 25
`
`All local traffic
`does not have to
`pass through a
`traditional
`switching office.
`
`Cables from homes run directly to the switching office or to an intermediate node. At
`nodes, individual phone lines may be multiplexed together to generate a single digital sig-
`nal for transmission from the node to the switching office over fiber or over copper.
`Typically homes near the switching office are served directly from there, but homes farther
`away— particularly clustered in new developments— may be served from a local node or
`concentrator that distributes signals from a remote point.
`The small businesses in the business district receive their service through a local distri-
`bution node. The insurance office needs the capacity of a T l line but most other busi-
`nesses, like rhe shoe-repair shop, need only one or two ordinary phone lines. The Town
`Hall has its own T l line direct from the central office. The biggest business in our little
`town has its own T3 fiber-optic connection to the switching office.
`All these services are processed through the central office. If the town manager calls the
`shoe repair shop to see if her shoes are finished, the call goes through the central office, where
`a switch connects one of the town phone lines to another phone line going to the shoe shop.
`If someone wants to call overseas, the switching office directs the call to the long-distance
`network, which makes the overseas connection. Incoming calls from the regional phone
`network go through the switching office, which makes the connections needed to direct
`them to the proper destination.
`Telephone switching offices do not have one outgoing line for every customer because
`normally all phone lines are not in use simultaneously. Telephone companies decide how
`many output lines to allocate to a switch, based on statistical averages of usage.
`
`"Access" Customers
`Changes in the telecommunications network mean that local traffic need not pass through
`a traditional telephone switching office. Figure 25.1 showed a number of other organiza-
`tions on the edge of the network, such as large corporations and universities. These organi-
`zations could have their own internal switches and be hooked directly to a metro network,
`as seen in Figure 24.3. Their telephone traffic could go directly to long-distance carriers, to
`regional carriers, or over lines that they leased from regional carriers for corporate use. For
`example, a state university might have leased lines from administration offices in the state
`capitol to campuses around the state.
`Internet Service Providers are also on the edge of the network. Instead of sending circuit-
`switched signals to telephone switches, they generate packet-switched signals, which are routed
`over the Internet Protocol network. As you learned in Chapter 23, some Internet traffic winds
`up on leased phone lines.
`Large organizations typically generate both Internet and telephone traffic, which they
`may transmit over metro networks to their Internet and telephone connections.
`
`jhone and
`htive phone
`anies have
`;ir own
`ing offices.
`
`Other Carriers
`Back in the days of telephone monopolies, every community had one central office, oper-
`ated by the telephone company. In urban and suburban parts of the United States, that was
`generally AT&T, bur in small towns it was often a small local company that served j ust that
`community and maintained links to A T& T’s regional network.
`
`MASIMO 2014
`PART 10
`Apple v. Masimo
`IPR2020-01526
`
`

`

`Local Telephone or "Access" Networks
`
`FIGURE 25.3
`Cell-phone
`network.
`
`Two trends have changed that pattern: the spread of mobile or cellular telephones, and
`the introduction of competition into the local fixed telephone network.
`Cellular or mobile phones connect to the telephone network using radio waves. The service
`area is divided into cells, as shown in Figure 25.3, each with an antenna and a base station that
`serve phones located within the cell’s area. As a user moves across the service area, the call is
`handed off from cell to cell. The antennas and base stations function like distribution nodes
`in a wire-line phone system, receiving signals from the switch and relaying them to individual
`phones. Cables carry the signals from the base transceiver back to a base station controller,
`which links to the switching office that serves the cellular phone network.
`Because this book is about fiber optics, we won’t look closely at the cellular network.
`FFowever, the cellular network isn’t always divided up into the neat hexagons shown in
`Figure 25.3. Terrain, buildings, vegetation, and changes in weather can affect transmission.
`Telecommunications regulations encouraged development of competitive local exchange
`carriers (CLECs), which provide local telephone service over cables. Many CLECs folded
`when the telecommunications bubble collapsed, but four basic types remain:
`
`9 Companies that lease space on the dominant phone company’s network of cables,
`paying a rate set by regulators. The cables they lease connect to the competitive
`carrier’s telephone switches, which make the required connections in the area and
`around the world.
`
`CLECs compete
`with incumbent
`wire-line phone
`companies.
`
`

`

`Chapter 25
`
`Cables distribute
`signals from the
`central office to
`subscribers.
`
`Twisted pairs of
`copper wires
`connect to
`telephones.
`
`FIGURE 25.4
`Subscriber loop
`distribution.
`
`• Facilities-based competitive carriers, which build networks of cables that overlay an
`area served by the dominant telephone carrier. Few of these companies exist today.
`• Cable television companies that offer telephone service over their coaxial cable
`networks (see Chapter 27).
`• Voice over Internet Protocol (VoIP) systems, which digitize voice signals at homes
`or offices and transmit the resulting data signals over the Internet or over digital lines
`using Internet Protocol. Many cable television systems use this technology.
`
`The rest of this chapter concentrates on standard local wire-line telephone networks.
`You will learn more about VoIP systems later. Chapter 27 covers voice service on cable
`networks.
`
`Distribution, Concentrators, and the Subscriber Loop
`A network of cables distributes signals from the central office to subscribers. The structure
`of this network varies from place to place.
`Figure 25.4 shows a typical network structure. Homes near the switching office are
`served by copper wire pairs running directly to the switch, as shown at top. Thick cables
`with hundreds of wire pairs run along main streets to service boxes or manholes, where the
`cables are split into smaller cables with dozens of wire pairs, as shown at bottom left. The
`smaller cables run down side streets and distribute service to individual homes. The wires
`running from these cables to homes are called drops.
`The copper wiring used in these cables, called twisted pairs, traditionally is a pair of wires
`twisted together to reduce the noise they pick up from other sources. Flat ribbon cables
`
`S in g le C o pper
`P a irs (m a y be in
`o n e cable)
`
`Fiber cable to remote
`distribution node
`(transmits multiplexed
`signal to distant parts
`of town).
`
`M ain S treet
`C a ble
`(h u n d re d s of
`w ire pairs)
`
`Copper pairs
`grouped in
`cable
`running
`along street.
`
`\
`
`C o p p e r Pairs
`
`D e m u ltip le xe r
`a n d D istrib u tio n
`N o de
`
`D e m u ltip le xe r
`a nd D istrib u tio n
`N o de
`
`

`

`Local Telephone or "Access" Networks
`
`Copper wires can
`carry voice, but
`not DSL, for
`several miles.
`
`with two or four lines are used over short distances, such as in household telephone wiring,
`but long runs of untwisted wiring can act as antennas and pick up noise like radio signals.
`Each wire pair carries traffic for a single telephone circuit or phone line.
`Distant homes are served by multiplexing signals at the central office and sending the
`multiplexed signals through fiber-optic cables to remote distribution nodes, as shown at the
`right in Figure 25.4. Although the scale of the figure doesn’t show it, typically these fiber
`cables run a few miles. Copper cables run from the remote distribution node to homes,
`with fat cables subdividing into smaller cables with lower wire counts on side streets, as
`shown near the central office in the figure.
`Copper wires can carry voice telephone signals several miles without serious degradation.
`However, the maximum transmission distance is shorter for other services, particularly
`Digital Subscriber Line (DSL), which I will cover later in this chapter. That has led to some
`refinements in design of subscriber loops.
`In the 1980s and early 1990s, telephone companies installed systems called digital loop
`carriers that multiplexed together many voice channels and distributed them at remote lo-
`cations, as shown at the right of Figure 25.4. This seemed like a good idea at the time, be-
`cause a single fiber transmitting at the modest 6.3 Mbit/s T2 rate could carry 96 telephone
`circuits. The fiber cables were much smaller, cheaper, and sturdier than copper cables with
`equivalent capacity. A single 12-fiber cable could easily carry 500 phone lines at 6.3 Mbit/s
`per fiber, so these systems were installed to new developments or during upgrading of ex-
`isting phone systems.
`However, old digital loop carriers posed a problem when telephone companies started
`thinking about DSL. Digital loop carriers transmit only the voice frequencies carried by
`phone lines, but DSL relies on transmitting higher frequencies that are lost in multiplex-
`ing. Thus the old digital loop carriers can’t deliver DSL to phone lines they serve. It’s hardly
`the only case where yesterday’s bright idea becomes tomorrow’s bottleneck.
`To get around this bottleneck, telephone carriers must install modern fiber systems that
`transmit signals differently. Data to be delivered over DSL can be separated from digitized
`voice signals at the central office, then multiplexed in separate data streams. At the remote
`demultiplexing node, the two signals can be combined into an analog voice/DSL line, as
`they would be at the central office if the DSL subscriber were closer. The important dif-
`ference is that the modern systems can move more functions of the switching office to the
`remote node; the older systems can only multiplex voice signals.
`Business subscribers are served the same way as home telephone subscribers. The larger
`the business, the bigger the information pipelines needed to serve them. You can see this if
`you look back to Figure 25.2. The smaller businesses like the shoe-repair shop need just a
`phone line or two. Businesses that need more phone lines or high-speed Internet access
`may lease blocks of lines from the phone company. The insurance office and town hall both
`have T1 lines. The “pretty big business” has a 45-Mbit/s T3 line. The higher-capacity ser-
`vices may be delivered partly or entirely over fiber. A T3 fiber-optic cable runs from the
`switching office to the node serving the business district, and a short copper cable delivers
`T l service from that node to the insurance company.
`The structure of the local phone network is changing as technology develops, companies
`offer new services, and subscribers respond to the new technology and services. We will re-
`turn to these evolving trends later, after describing conventional and emerging services.
`
`

`

`m Chapter 25
`
`Traditional phone
`lines were
`designed to carry
`only analog voice
`signals.
`
`Businesses lease
`phone lines for
`high-volume
`
`Subscriber and Access Services
`
`The traditional subscriber loop was designed to carry voice telephone signals, called Plain
`Old Telephone Service or POTS in the industry. The local telephone network now provides
`many other services. When facsimile machines came into use, it was much easier to add
`them to the existing telephone network than to build a separate fax network. All you did
`was install a new phone line, plug in the fax machine, and tell people to send faxes to the
`new phone number. Likewise, dial-up modems take advantage of existing phone networks
`between your home or office and your Internet Service Provider instead of requiring a com-
`plete new network. Digital Subscriber Line technology expands on this trend by enhanc-
`ing the transmission capacity of voice phone lines.
`A broader range of services is offered to business and access customers, whose demand
`has steadily grown from multiple voice phone lines to large numbers of phone lines and
`high-speed data links. We’ll look briefly at the most important services.
`Leased Lines
`Telecommunication users who need a large volume of service often lease lines, renting
`transmission capacity in bulk from a carrier. When dealing with telephone companies, they
`generally lease bulk capacity in a standard telephone-industry format, such as a T l or T3
`line, or an OC-3 carrier. The lines may run between user facilities, such as between build-
`ings used by a large company or between a city’s data-processing center and city hall. They
`also may run from the user’s facility to another point, such as between a university and a
`regional Internet node or a long-distance carrier.
`Although the service is called a leased line, the signals may not be carried over a physically sep-
`arate wire, fiber, or cable along the entire route. If a company leases a T3 line between a down-
`town office building and a suburban factory, it is buying a guaranteed capacity of 45 Mbit/s on
`that route. The telephone company may time-division multiplex that 45-Mbit/s signal into a
`2.5-Gbit/s OC-48 signal that a fiber transmits from downtown to the suburban central office,
`then run a separate fiber pair to the plant. The user sees no difference between that service and
`a separate pair of fibers— but the phone company can lease the capacity more cheaply.
`Access lines generally serve the same purpose, but may be arranged differently. For ex-
`ample, the company may rent one optical channel on a fiber in a metro network that runs
`from downtown to near its suburban plant. The carrier may transport the signal in OC-1
`or OC-3 form through the fiber, using its own equipment. Alternatively, the company
`could supply its own transmission equipment and send the signal in whatever form it
`wanted. If the line is intended to link the local-area networks in the factory and company
`headquarters, the signal might be in Gigabit Ethernet form.
`Access lines also can go beyond the region. If a large magazine publisher has offices in
`Boston and Chicago, it might lease an OC-12 circuit between the two cities so it can trans-
`mit signals at up to 622 Mbit/s. The publisher might also lease OC-3 lines between both
`magazine offices and its printer in Mississippi. The company’s signals can be combined
`with other signals and multiplexed to higher speeds for parr of the route, but the user
`would not see any difference. Alternatively, users could lease wavelengths or dark fibers and
`have all the capacity they could use for themselves between two points.
`
`

`

`Local Telephone or "Access" Networks
`
`Telephone Lines
`Traditional phone lines carry analog signals at 300 to 3000 Hz for voice telephony. This
`isn’t high fidelity, but it’s adequate for intelligible speech. The bandwidth is limited by
`the attenuation of copper wire pairs, which increases with frequency and distance. Over
`short distances, copper wires can have surprisingly high bandwidths, so suitable copper
`cables can carry 1 Gbit/s on the desktop, but telephone companies long ignored these
`capabilities.
`Fax machines and dial-up modems encode digital data as audio tones in the 300 to 3000 Hz
`audio range transmitted by analog phone lines. These signals must be clear and strong
`enough to survive conversion to digital format for regional and long-distance transmission.
`Standard level 3 fax signals can transmit up to 14,400 bits per second, and dial-up modems
`have nominal data rates to 56,000 bits per second, although in practice they are limited to
`about 53,000 bits/s.
`The traditional telephone system converts these analog signals to digital form at the
`switching office or an intermediate node between the subscriber and the switch. Newer
`services transmit signals in digital format direct from the subscriber.
`The original digital telephone service was Integrated Services D igital Network (ISDN),
`transmitting 144 kbit/s over twisted wire pairs. As envisioned in the 1980s, that capacity
`would be divided between two 64-kbit/s digitized voice lines and one 16-kbit/s data line.
`More recent versions dedicate all the transmission capacity to digital data. However, ISDN
`required expensive special telephone equipment, phone companies were very slow to offer
`it, and few customers wanted it, especially in the United States. The service is still available
`in some locations, but is often called IDSL so that it sounds similar to DSL, although the
`two use different technologies.
`
`DIGITAL SUBSCRIBER LINE (DSL)
`D igital Subscriber Line (DSL) transmits digital data over copper wires at frequencies higher
`than those used for analog voice transmission. Ideally, DSL signals can be transmitted over
`the same twisted-pair lines used for voice conversations, with a standard phone responding
`only to the voice signals and the DSL modem responding only to the data. In practice, it
`often isn’t that simple, and may require a device, called a splitter, that separates the low ana-
`log voice frequencies from the higher frequencies carrying the digital data.
`There are several versions of DSL. The data rates they can transmit depend on the qual-
`ity of the phone lines and the length of wire separating the subscriber from the switching
`office, as well as on the design. Load coils used to improve the quality of analog voice trans-
`mission also attenuate the high frequencies that carry DSL signals, so they can’t be used
`with DSL. Some phone lines can’t carry high frequencies well enough. Even in the best
`phone lines, attenuation increases with frequency, so the data rate possible decreases with
`transmission distance, as shown in Figure 25-5 for two types of DSL.
`Table 25.1 lists several DSL-related technologies and their nominal data rates and max-
`imum transmission distances. The actual data rate achieved at the maximum distance typ-
`ically is well below the maximum rate, unless noted. The limiting distance is measured
`along the cable route, which is not a straight line, so many homes fall outside the limit even
`
`Fax machines and
`dial-up modems
`encode digital
`data as audio
`tones.
`
`DSL transmits data
`on phone lines at
`frequencies above
`the normal voice
`range.
`
`

`

`Chapter 25
`
`FIGURE 25.5
`DSL data rates vs.
`wire length.
`
`DSL rates depend
`on length and
`quality of lines
`and marketing
`decisions.
`
`4000
`
`12000
`8000
`W ire Length (ft)
`
`16000
`
`20000
`
`in cities or inner suburbs. Because data rate decreases with distance, it generally is impos-
`sible to achieve the maximum speed over the maximum possible distance. For example, the
`Asymmetric D igital Subscriber Line (ADSL) format is rated to send 8.448 Mbit/s to a ter-
`minal 9000 ft (2.7 km) away, but only 1.5 Mbit/s to a terminal at 18,000 ft (5.5 km).
`As Table 25.1 shows, higher-speed DSL formats, notably VDSL, can’t carry signals far.
`That’s an intentional design choice; VDSL is intended for use in areas where a short length
`of copper runs between a home and a fiber-optic node. That trade-off is inevitable because
`of the bandwidth limitations of copper.
`Different sources may quote different maximum data rates for DSL services, and some
`subscribers may not be able to receive DSL service at all. This reflects wide differences in
`the quality and length of phone lines, evolution of the technology, and marketing decisions
`by companies offering DSL services. Some phone lines are not suitable for DSL. Twisted
`pair attenuation is higher on the high end of the DSL spectrum, so long lines can’t handle
`DSL signals. The nominal limit used to be 18,000 feet, but has been edging higher. Other
`phone lines may have incompatible equipment attached, or may not meet quality require-
`ments. The reasons aren’t always obvious. For unknown reasons, my phone company says
`two of its three phone lines into my home are suitable for DSL, but the third is not.
`After some early problems, DSL has become popular for broadband Internet connec-
`tions. DSL lags behind cable modems in the United States, but is more common in most
`other countries.
`
`Emerging Services and
`Competing Technologies
`
`In telecommunications some technologies that are “just around the corner” stay that way for
`a long time before quietly evaporating. One example is the video-telephone, which started
`as the stuff of pulp science fiction in the 1920s (see Figure 25.6). AT&T introduced the first
`
`

`

`Local Telephone or "Access" Networks
`
`Table 25.1 Typ es o f dig ital subscriber line services
`
`Technology
`
`Standards
`
`Nominal Data Rate
`
`ISDN
`
`ANSI/ITU
`
`128 or 144 kbit/s
`both ways
`
`G.Lite
`(“Splitterless”
`DSL)
`
`ITU
`
`ADSL (Asymmetric
`Digital Subscriber
`Line)
`
`ANSI
`
`ADSL
`
`SHDSL
`(Symmetric
`High-rate DSL)
`
`ANSI
`
`ITU
`6.991
`
`Digital
`Telephone
`Hierarchy
`
`T l
`
`RADSL
`(Rate Adaptive
`DSL)
`
`VDSL (Very high
`DSL)
`
`ANSI
`
`1.5 Mbit/s
`downstream,
`384 kbit/s
`upstream
`8 Mbit/s
`640 kbit/s
`downstream,
`upstream
`1.5 Mbit/s
`downstream
`2.3 Mbit/s
`both ways
`on one pair
`192 kbit/s
`both ways
`1.5 Mbit/s
`both ways
`
`Adaptive—
`to 9 Mbit/s
`downstream,
`1 Mbit/s
`upstream
`13 to 52 Mbit/s
`down, 1.5 to
`2.3 Mbit/s
`upstream
`
`Specified
`Maximum
`Distance
`
`18,000 ft (5.5 km)
`(longer distances
`possible)
`18,000 ft (5.5 km)
`
`9,000 ft (2.7 km)
`(at 8 Mbit/s)
`
`18,000 ft (5.5 km)
`(at 1.5 Mbit/s)
`10,000 ft (3 km)
`(at 2.3 Mbit/s)
`
`20,000 ft (6 km)
`(at 192 kbit/s)
`3,000 ft (900 m)
`
`12,000 ft
`(3.6 km)
`
`4,500 ft (1.4 km)
`at 13 Mbit/s;
`1000 ft (300 m)
`at 52 Mbit/s
`
`commercial video-telephone service, called Picturephone, in 1970, but it quietly faded away.
`Today webcams are cheap additions to personal computers, and their pictures appear on
`thousands of Web sites, but few people bother using them for videoconferencing.
`Nonetheless, telecommunications is changing rapidly, so now we’ll look at future trends
`both in the network and in the services it offers.
`
`

`

`Chapter 25
`
`FIGURE 25.6
`Videophones were
`part o f the
`background that
`Hugo Gernsback,
`publisher o f the
`first science-fiction
`magazines in the
`1920s, used fo r his
`first science fiction
`novel. However,
`the cover artist’s
`vision in this
`paperback still
`included a dial.
`(Courtesy o f
`Fantasy Books)
`
`Digitized voice
`can be sent as
`packets over the
`Internet.
`
`Packet delays can
`degrade voice
`transmission.
`
`Voice over Internet Protocol (VoIP)
`Voice over Internet Protocol (VoIP) has become a favorite technology of market pundits and
`is offered by a number of companies. VoIP digitizes voice signals at the receiver and trans-
`mits them over the Internet as packets. One advantage is that sending Internet packets is
`much cheaper than using voice circuits. Another advantage is the potential of harnessing
`computers to process phone calls. These two features are pushing VoIP in opposite direc-
`tions. One extreme is making cheap phone calls around the world from a computer, an ap-
`proach that appeals to people with friends and family dispersed around the world. The
`other is feature-laden phones that turn voice messages into digital files that can be
`processed electronically. It remains to be seen how well the technology will live up to the
`considerable expectations of the pundits.
`Quality of service is a serious issue in VoIP. Voice conversations are sensitive to delay, and
`packets transmitted over separate routes are subject to delay. The public Internet has only
`begun to adapt IPv6, which can assign special priorities to packets carrying voice signals;
`most nodes use the older IPv4, which treats voice packets like any other packets. The re-
`sulting transmission can be unintelligible. This problem can be avoided if telecommunica-
`tions companies build their own IP networks using IPv6, and some companies have begun
`to do that. In this case, the IP lines simply replace SONET lines or other circuit-switched
`connections, and users should not hear much difference in their calls.
`
`

`

`Local Telephone or "Access" Networks
`
`Another issue is compatibility with existing phones. Some VoIP systems require expen-
`sive special terminal equipment to use their advanced features. Others provide adapters that
`convert VoIP signals to the format required by standard analog phones. Extra features may
`be offered through computers, such as e-mailing digital voice-mail accounts.
`Still other potential issues include the loss of features peculiar to the analog voice phone
`system. Because the phone network has its own power source, you can phone the power
`company to report an outage on an analog wire-line phone, but not on a cordless phone
`or a VoIP phone. Emergency 911 services require special equipment present on standard
`subscriber loops but not on VoIP networks.
`Telephone service provided over cable-television networks faces many of the same
`issues as VoIP, and many cable networks use VoIP technology for their voice service.
`Like any new technology, VoIP will have to convince customers that it’s better than ex-
`isting phone service. It may succeed in some niches but not others. Stay tuned.
`Cellular versus Wire-Line Phones
`Recently people have started to drop wire-line phones and use their cell phones as their main
`line. Typical examples are young, highly mobile people who are rarely at home to receive phone
`calls. It’s not clear how far this trend will go. The convenience of cell phones is offset by their
`poorer sound quality and problems in finding a “sweet spot” to make a good connection to the
`cellular network. Cutting the cord to go all-cellular may not seem like such a good idea if your
`elderly rich uncle can’t understand a word you say on your cell phone. On the other hand, cell
`phones are good backups for emergency calls or during power failures.
`Video and "Triple-Play" Services
`Convergence in telecommunications has led telephone and cable-television companies to
`talk about “triple-play” packages— combinations of voice, video, and broadband Internet
`connections. Cable companies have exploited this trend successfully to offer voice and data
`services, but phone companies have had trouble offering video service because their wires
`have limited bandwidth. Phone companies now offer video services in three ways:
`
`9 Partnerships with satellite television companies, which compete with cable
`companies and can’t offer voice service efficiently.
`• Video-on-demand services offered over VDSL, which can switch video channels to
`individual subscribers. Video-on-demand could be packaged with satellite broadcasts so
`subscribers can request individualized programming as well as broadcast channels.
`• Fiber-to-the-home (or premises) systems, which have dedicated bandwidth to carry
`video signals. Fiber can carry the full bandwidth of a cable network, putting phone
`companies on equal footing with cable companies, which have long had the lead in
`bandwidth. We’ll look at this technology in the final section of this chapter.
`
`Fixed Wireless Broadband Service
`Fixed wireless broadband is a potentially competitive service that has been “just around the
`corner” for several years. This service installs fixed wireless transmitters in each neighborhood
`
`•
`"Triple-play"
`services offer
`voice, ^ a' anc^
`
`

`

`Chapter 25
`
`Verizon began
`fiber-to-the-home
`installations in
`2004.
`
`Fiber installation
`will cost $1000 to
`$1500 per home.
`
`to transmit broadband signals— usually video and computer data— to subscribers. The goal
`is to avoid the high cost of stringing fiber or cable to each home.
`The potential savings have attracted many companies from time to time, but the prac-
`tical drawbacks have stalled deployment of the technology. The microwave frequencies that
`carry the signals are attenuated by rain and can be blocked by foliage, terrain, or buildings.
`In short, fixed wireless broadband may work well if you can see the transmitter from where
`you put your antenna, but you can’t count on that.
`
`Fiber to the Home or Premises
`
`The narrow bandwidth of copper wires reaching individual homes has long limited the
`telephone network’s ability to deliver telecommunications services to homes. DSL is part
`of a long-term effort to increase that bandwidth so phone companies can offer new services.
`Plans also include running fiber closer to individual subscribers.
`Exactly how close the fiber should come to homes has been controversial. Although it
`seems logical to bring fiber all the way to the home, many analysts have been skeptical be-
`cause of the potentially tremendous costs of overhauling the entire local telephone net-
`work. However, in the early 2000s rural phone companies and developers of large
`subdivisions began installing fiber-to-the-home systems. The large regional telephone com-
`panies have followed suit. Verizon began constructing its first system in Texas in 2004, with
`plans to run fiber past a million homes by the end of the year. Two other regional phone
`companies, SBC and BellSouth, have announced similar plans. Because this is a book on
`fiber optics, we will devote the rest of this chapter to fiber to the home.
`The industry has developed a family of designs grouped as “fiber to the X ” (FTTX), with
`Vbeing a particular point in the network. Important variations are:
`
`9 FTTB: Fiber to the Business (or sometimes, Fiber to the Building)
`9 FTTC: Fiber to the Curb (near homes, but not all the way to them)
`9 FTTD : Fiber to the Desk
`4 * FTTFI: Fiber to the Flome
`9 FTTN : Fiber to the Neighborhood (or Fiber to the Node)
`9 FTTP: Fiber to the Premises (equivalent to Fiber to the Home)
`Spreading Fiber into the Local Phone System
`There is wide agreement that the local phone system needs more bandwidth if it is to sur-
`vive. The big questions are how much bandwidth, how best to provide it, and how to de-
`velop a future broadband network from todays limited telephone network. The central
`problem is the expense of replacing the existing network.
`The existing telephone network is both an asset and a problem. It’s an asset in that the
`phone companies have already built it and paid for it— but a problem in that its capacity
`is limited, and parts of it are aging. It’s like an old computer that doesn’t support the latest
`Web browsers and other newer applications. But the real problem is that replacing the
`existing network is very costly.
`
`

`

`Local Telephone or "Access" Networks
`
`FIGURE 25.7
`Fiber to the
`new
`
`Phone companies estimate that replacing existing local phone networks costs $1000 to
`$ 1500 per home. Fiber-optic equipment costs slightly more than copper wire, but most of
`the expense is the labor of installing new cables and equipment. Time is needed to run new
`cables along overhead poles and drop new lines to each home. Costly equipment and more
`time are required to replace existing buried utilities with new underground lines to each
`home. (It’s relatively cheap to install fiber along with other utilities in new developments
`because the holes are already in the ground.)
`The sheer scale of the job makes it a budget-buster for phone companies, so they are
`phasing in fiber. Fiber networks have gradually spread out from switching offices to serve
`neighborhood nodes. Separately, phone companies plan to replace existing distribution
`networks gradually with fiber, one neighborhood or community at a time.
`Fiber to the Neighborhood
`Today’s telephone networks use fibers to connect remote neighborhoods to the local dis-
`tribution system, as shown in Figure 25.4. The next logical step is fib er to the neighborhood
`(or node), or FTTN, shown in Figure 25.7. High-speed fiber distributes signals to neigh-
`borhood nodes, which transfer the signals to copper wires that run along local streets and
`distribute signals to individual homes. An FTTN node might serve a few hundred tele-
`phone subscribers, including small local businesses. Similar fiber nodes service business and
`government, like the City Hall at the lower right in Figure 25.7. Copper cables run from
`the switching office to serve its immediate neighborhood.
`
`FTTN nodes serve
`hundreds of
`subscribers.
`
`

`

`Chapter 25
`
`FIGURE 25.8
`Fiber to the curb.
`
`Fiber to the curb
`is used with high-
`speed VDSL.
`
`Demand for
`Internet bandwidth
`pushed home
`fiber development.
`
`Fiber Node
`
`S id e w a lk
`
`D rive w a y 1X1
`
`/
`B u ried
`■ C o p p e r -----