FOURTH EDITION
`
`om uter etwor s~
`
`ANDREW S. TANENBAUM
`
`
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`p
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`Computer Networks
`Fourth Edition
`
`Andrew S. Tanenbaum
`Vrije Universiteit
`Amsterdam, The Netherlands
`
`■
`
`Prentice Hall PTR
`Upper Saddle River, NJ 07458
`www.phptr.com
`
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`Library of Congress Cataloglng-in-Puhllcation IJata
`
`Tanenbaum, Andrew S.
`Computer networks/ Andrew S. Tanenbaum.--4th ed.
`p.cm.
`Includes bibliographical references.
`ISBN 0-13-066102-3
`1. Computer networks. I. Title.
`TK5105.5 .T36 2002
`004.6--dc2 l
`
`2002029263
`
`Editorial/production supervision: Patti Guerrieri
`Cover design director: Jerry Votta
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`Cover design: Andrew S. Tanenbaum
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`Typesetting: Andrew S. Tanenbaum
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`■ © 2003, 1996 Pearson Education, Inc.
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`Printed in the United States of America
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`ISBN 0-13-066102-3
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`152
`
`THE Pl IYSICJ\L LA YER
`
`2.6 THE MOBILE TELEPHONE SYSTEM
`
`CHAP, 2
`
`The traditional telephone system (e~en if it so~e day gets multigigabit end.
`to-end fiber) will still not be able to sat1sf y a grow mg group of users: people
`. 1
`on
`f
`the go. People now expect to make phone calls rom a1rp anes, cars, swimrn·
`pools, and while jogging in the park. Within a few years they will also expec:~g
`send e-mail and surf the Web from all these locations and more. Consequent) 0
`there is a tremendous amount of interest in wireless telephony. In the fo11owi:,
`g
`sections we will study this topic in some detail.
`Wireless telephones come in two basic varieties: cordless phones and mobile
`phones (sometimes called cell phones). Cordless pho_ne~ are devices consisting
`of a base station and a handset sold as a set for use w1thm the home. These are
`never used for networking, so we will not examine them further. Instead we wil)
`concentrate on the mobile system, which is used for wide area voice and data
`communication.
`Mobile phones have gone through three distinct generations, with different
`technologies:
`
`1. Analog voice.
`
`2. Digital voice.
`
`3. Digital voice and data (Internet, e-mail, etc.).
`
`Although most of our discussion will be about the technology of these systems, it
`is interesting to note how political and tiny marketing decisions can have a huge
`impact. The first mobile system was devised in the U.S. by AT&T and mandated
`for the whole country by the FCC. As a result, the entire U.S. had a single (ana(cid:173)
`log) system and a mobile phone purchased in California also worked in New
`York. In contrast, when mobile came to Europe, every country devised its own
`system, which resulted in a fiasco.
`Europe learned from its mistake and when digital came around, the gov(cid:173)
`ernment-run PTTs got together and standardized on a single system (GSM), so
`any European mobile phone will work anywhere in Europe. By then, the U.S. had
`decided that government should not be in the standardization business, so it left
`digital to the marketplace. This decision resulted in different equipment manufac·
`turers producing different kinds of mobile phones. As a consequence, the U.S.
`now has two major incompatible digital mobile phone systems in operation (plus
`one minor one).
`Despite an initial lead by the U.S., mobile phone ownership and usage in
`Europe is now far greater than in the U.S. Having a single system for all of Eur·
`ope is part of the reason, but there is more. A second area where the U.S. a~d
`Europe differed is in the humble matter of phone numbers. In the U.S. mobile
`phones are mixed in with regular (fixed) telephones. Thus, there is no way for a
`
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`THE MOBILE TELEPHONE SYSTEM
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`153
`
`cnlk_r to sec if. say, (~ 12) 234-5678 is a fixe<l telephone (cheap or free call) or a
`mohtlc phone (expensive call). To keep people from getting nervous about using
`the tck~1honc,_ the telephone companies decided to make the mobile phone owner
`pay for rnc~mmg calls •. As a consequence, many people hesitated to buy a mobile
`phone for fear of •~nmng up a big bill by just receiving calls. In Europe, mobile
`phones have a s~ecial area code (analogous to 800 and 900 numbers) so they are
`instantl_y recogniz~ble. Consequently, the usual rule of "caller pays" also applies
`to 1nobile phones in Europe (except for international calls where costs are split).
`A third issue that has had a large impact on adoption is the widespread use of
`prepaid mobile phones in Europe (up to 75% in some areas). These can be pur(cid:173)
`chased in n1any stores with no more formality than buying a radio. You pay and
`you go. They are preloaded with, for example, 20 or 50 euro and can be re-(cid:173)
`charged ( using a secret PIN code) when the balance drops to zero. As a conse(cid:173)
`quence, practically every teenager and many small children in Europe have (usu(cid:173)
`ally prepaid) mobile phones so their parents can locate them, without the danger
`of the child running up a huge bill. If the mobile phone is used only occasionally,
`its use is essentially free since there is no monthly charge or charge for incoming
`calls.
`
`2.6.1 First-Generation Mobile Phones: Analog Voice
`
`Enough about the politics and marketing aspects of mobile phones. Now let
`us look at the technology, starting with the earliest system. Mobile radiotele(cid:173)
`phones were used sporadically for maritime and military communication during
`the early decades of the 20th century. In 1946, the first system for car-based tele(cid:173)
`phones was set up in St. Louis. This system used a single large transmitter on top
`of a tall building and had a single channel, used for both sending and receiving.
`To talk, the user had to push a button that enabled the transmitter and disabled the
`receiver. Such systems, known as push-to-talk systems, were installed in several
`cities beginning in the late 1950s. CB-radio, taxis, and police cars on television
`programs often use this technology.
`In the 1960s, IMTS (Improved Mobile Telephone System) was installed.
`It, too, used a high-powered (200-watt) transmitter, on top of a hill, but now had
`two frequencies, one for sending and one for receiving, so the push-to-talk button
`was no longer needed. Since all communication from the mobile telephones went
`inbound on a different channel than the outbound signals, the mobile users could
`not hear each other (unlike the push-to-talk system used in taxis).
`IMTS supported 23 channels spread out from 150 MHz to 450 MHz. Due to
`the small number of channels, users often had to wait a long titne before getting a
`dial tone. Also, due to the large power of the hilltop transmitter, adjacent systems
`had to be several hundred kilometers apart to avoid interference. All in all, the
`limited capacity made the system impractical.
`
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`154
`
`n n~ l'llYSI( J\L LA)' ER
`
`CHAP. 2
`
`.
`Adlanrt--,11\tohilc l'hmw Sy~trn1
`All that changed with AMPS (Advanced Mobile Phone System), invented
`by Bell Labs and first installed in the United States in 1982. It was also used in
`England, where it was called TACS, and in Japan, where it was cal1ed MCS-LJ.
`Although no longer state of the art, we will look ~t it i? some ?eta~! ?ecause many
`of its fundan1ental prope1ties have been directly 1nhented by its d1g1tal successor,
`D-AMPS, in order to achieve backward compatibility.
`In all mobile phone syste1ns, a geographic region is divided up into cells,
`which is why the devices are sometimes called cell phones. In AMPS, the cells
`are typically 10 to 20 km across; in digital systems, the cells are smaller. Each
`cell uses some set of frequencies not used by any of its neighbors. The key idea
`that gives cellular systems far more capacity than previous systems is the use of
`relatively small cells and the reuse of transmission frequencies in nearby (but not
`adjacent) cells. Whereas an IMTS system 100 km across can have one call on
`each frequency, an AMPS system might have 100 10-km cells in the same area
`and be able to have 10 to 15 calls on each frequency, in widely separated cells.
`Thus, the cellular design increases the system capacity by at least an order of
`magnitude, more as the cells get smaller. Furthermore, smaller cells mean that
`less power is needed, which leads to smaller and cheaper transmitters and hand(cid:173)
`sets. Hand-held telephones put out 0.6 watts; transmitters in cars are 3 watts, the
`maximum allowed by the FCC.
`The idea of frequency reuse is illustrated in Fig. 2-41(a). The cells are nor(cid:173)
`mally roughly circular, but they are easier to model as hexagons. In Fig. 2-4l(a),
`the cells are all the same size. They are grouped in units of seven cells. Each
`letter indicates a group of frequencies. Notice that for each frequency set, there is
`a buff er about two cells wide where that frequency is not reused, providing for
`good separation and low interference.
`Finding locations high in the air to place base station antennas is a major is(cid:173)
`sue. This problem has led some telecommunication carriers to forge alliances
`with the Roman Catholic Church, since the latter owns a substantial number of
`exalted potential antenna sites worldwide, all conveniently under a single manage(cid:173)
`ment.
`In an area where the number of users has grown to the point that the system is
`overloaded, the power is reduced, and the overloaded cells are split into smaller
`microcells to permit more frequency reuse, as shown in Fig. 2-41 (b ). Telephone
`companies sometimes create temporary microcells, using portable towers with
`satellite links at sporting events, rock concerts, and other places where large num(cid:173)
`bers of mobile users congregate for a few hours. How big the cells should be is a
`complex matter, which is treated in (Hae, 1995).
`At the center of each cell is a base station to which all the telephones in the
`cell transmit. The base station consists of a computer and transmitter/receiver
`connected to an antenna. In a small system, all the base stations are connected to
`
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`sec. 2.6
`
`THE M081LE TELEPI IONE SYSTEM
`
`155
`
`(a)
`
`(b)
`
`Figure 2-41. (a) Frequencies are not reused in adjacent cells. (b) To add more
`users, smaller cells can be used.
`
`a single device called an MTSO (Mobile Telephone Switching Office) or MSC
`(Mobile Switching Center). In a larger one, several MTS Os may be needed, all
`of which are connected to a second-level MTSO, and so on. The MTSOs are
`essentially end offices as in the telephone system, and are, in fact, connected to at
`least one telephone system end office. The MTSOs communicate with the base
`stations, each other, and the PSTN using a packet-switching network.
`At any instant, each mobile telephone is logically in one specific cell and
`under the control of that cell's base station. When a mobile telephone physically
`leaves a cell, its base station notices the telephone's signal fading away and asks
`all the surrounding base stations how much power they are getting from it. The
`base station then transfers ownership to the cell getting the strongest signal, that
`is, the cell where the telephone is now located. The telephone is then informed of
`its new boss, and if a call is in progress, it will be asked to switch to a new chan(cid:173)
`nel (because the old one is not reused in any of the adjacent cells). This process,
`called handoff, takes about 300 msec. Channel assignment is done by the MTSO,
`the nerve center of the system. The base stations are really just radio relays.
`Handoffs can be done in two ways. In a soft handoff, the telephone is ac(cid:173)
`quired by the new base station before the previous one signs off. In this way there
`is no loss of continuity. The downside here is that the telephone needs to be able
`to tune to two frequencies at the same time (the old one and the new one). Nei(cid:173)
`ther first nor second generation devices can do this.
`In a hard handoff, the old base station drops the telephone before the new
`one acquires it. If the new one is unable to acquire it (e.g., because there is no
`available frequency), the call is disconnected abruptly. Users tend to notice this,
`but it is inevitable occasionally with the current design.
`
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`156
`
`Channels
`
`THE PIIYSfC/\L LAYER
`
`CHAP. 2
`
`The AMPS system uses 832 full-duplex channels, each consisting of a pair of
`simplex channels. There are 832 simplex transmission channels from 824 to 849
`MHz and 832 simplex receive channels from 869 to 894 MHz. Each of these sim(cid:173)
`plex channels is 30 kHz wide. Thus, AMPS uses FDM to separate the channels.
`In the 800-MHz band, radio waves are about 40 cm long and travel in straight
`lines. They are absorbed by trees and plants and bounce off the ground and build(cid:173)
`ings. It is possible that a signal sent by a mobile telephone will reach the base sta(cid:173)
`tion by the direct path, but also slightly later after bouncing off the ground or a
`building. This may lead to an echo or signal distortion (multipath fading). Some(cid:173)
`times, it is even possible to hear a distant conversation that has bounced several
`times.
`The 832 channels are divided into four categories:
`
`I. Control (base to mobile) to manage the system.
`2. Paging (base to mobile) to alert mobile users to calls for them.
`3. Access (bidirectional) for call setup and channel assignment.
`4. Data (bidirectional) for voice, fax, or data.
`Twenty-one of the channels are reserved for control, and these are wired into a
`PROM in each telephone. Since the same frequencies cannot be reused in nearby
`cells, the actual number of voice channels available per cell is much smaller than
`832, typically about 45.
`
`Call Management
`
`Each mobile telephone in AMPS has a 32-bit serial number and a IO-digit
`telephone number in its PROM. The telephone number is represented as a 3-digit
`area code in 10 bits, and a 7-digit subscriber number in 24 bits. When a phone is
`switched on, it scans a preprogrammed list of 21 control channels to find the most
`powerful signal.
`The phone then broadcasts its 32-bit serial number and 34-bit telephone nwn(cid:173)
`ber. Like all the control information in AMPS, this packet is sent in digital fonn,
`multiple times, and with an error-correcting code, even though the voice channels
`themselves are analog.
`When the base station hears the announcement, it tells the MTSO, which
`records the existence of its new customer and also informs the customer,s home
`MTSO of his current location. During normal operation, the mobile telephone re(cid:173)
`registers about once every 15 minutes.
`To make a call, a mobile user switches on the phone, enters the number to be
`called on the keypad, and hits the SEND button. The phone then transmits the
`
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`··---------------
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`SEC. 2.6
`
`THE MOBILE TELEPIIONE SYSTEM
`
`157
`
`number to b~ ca!lcd an_d its own identity on the access channel. If a collision
`occurs there, 1~ tnes agam ~ater. When the base station gets the request, it informs
`the !\1TSO. lf the caller is a customer of the MTSO's company (or one of its
`partners), the M~SO looks for an idle channel for the call. If one is found, the
`channel. number !s sent back on the control channel. The mobile phone then
`automatically switches to the selected voice channel and waits until the ca1led
`party picks up the phone.
`Incoming ca~ls work differently. To start with, all idle phones continuously
`listen to the pag!ng channel to detect messages directed at them. When a call is
`placed ~o a mobile phone ~ either from a fixed phone or another mobile ph?ne ), a
`packet 1s sent to the callee s home MTSO to find out where it is. A packet 1s then
`sent to the base station in its current cell, which then sends a broadcast on the pag(cid:173)
`ing channel of the form "Unit 14, are you there?" The called phone then responds
`with "Yes" on the access channel. The base then says something like: "Unit 14,
`call for you on channel 3." At this point, the called phone switches to channel 3
`and starts making ringing sounds ( or playing some melody the owner was given as
`a birthday present).
`
`2.6.2 Second-Generation Mobile Phones: Digital Voice
`
`The first generation of mobile phones was analog; the second generation was
`digital. Just as there was no worldwide standardization during the first generation,
`there was also no standardization during the second, either. Four systems are in
`use now: D-AMPS, GSM, CDMA, and PDC. Below we will discuss the first
`three. PDC is used only in Japan and is basically D-AMPS modified for back(cid:173)
`ward compatibility with the first-generation Japanese analog system. The name
`PCS (Personal Communications Services) is sometimes used in the marketing
`literature to indicate a second-generation (i.e., digital) system. Originally it meant
`a mobile phone using the 1900 MHz band, but that distinction is rarely made now.
`
`D-AMPS-The Digital Advanced Mobile Phone System
`
`The second generation of the AMPS systems is D-AMPS and is fully digital.
`It is described in International Standard IS-54 and its successor IS-136. D-AMPS
`was carefully designed to co-exist with AMPS so that both first- and second(cid:173)
`generation mobile phones could operate simultaneously in the same cell. In par(cid:173)
`ticular, D-AMPS uses the same 30 kHz channels as AMPS and at the same fre(cid:173)
`quencies so that one channel can be analog and the adjacent ones can be digital.
`Depending on the mix of phones in a cell, the cell's MTSO determines which
`channels are analog and which are digital, and it can change channel types
`dynamically as the mix of phones in a cell changes.
`When D-AMPS was introduced as a service, a new frequency band was n1ade
`available to handle the expected increased load. The upstream channels were in
`
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`158
`
`THE PIIYSICt\L Lt\ YLR
`
`CHAP. 2
`
`the 1850-1910 MHz range, and the corresponding downstream channels were in
`the 1930-1990 MHz range, again in pairs, as in AMPS. In this band, the waves
`are 16 cm long, so a standard ¼-wave antenna is only 4 cm long, leading to
`smaller phones. However, many D-AMPS phones can use both the 850-MHz and
`1900-MHz bands to get a wider range of available channels.
`On a D-AMPS mobile phone, the voice signal picked up by the microphone is
`digitized and compressed using a model that is more sophisticated than the delta
`modulation and predictive encoding schemes we studied earlier. Compression
`takes into account detailed properties of the human vocal system to get the band(cid:173)
`width from the standard 56-kbps PCM encoding to 8 kbps or less. The compres(cid:173)
`sion is done by a circuit called a vocoder (Bellamy, 2000). The compression is
`done in the telephone, rather than in the base station or end office, to reduce the
`number of bits sent over the air link. With fixed telephony, there is no benefit to
`having compression done in the telephone, since reducing the traffic over the local
`loop does not increase system capacity at all.
`With mobile telephony there is a huge gain from doing digitization and com(cid:173)
`pression in the handset, so much so that in D-AMPS, three users can share a sin(cid:173)
`gle frequency pair using time division multiplexing. Each frequency pair supports
`25 frames/sec of 40 msec each. Each frame is divided into six time slots of 6.67
`msec each, as illustrated in Fig. 2-42(a) for the lowest frequency pair.
`
`TOM frame
`40 msec
`
`TOM frame
`40 msec
`
`Upstream j 1 I 2 I 3 I 1 I 2 I 31 ~
`Downstream I 3 I 1 I 2 I 3 I 1 I 2 I ~;;~·~5 ~~~e
`
`8;
`
`~j~~o~~!e
`
`y
`324 bit slot:
`64 bits of control
`101 bits of error correction
`159 bits of speech data
`(a)
`
`(b)
`
`Figure 2-42. (a) AD-AMPS channel with three users. (b) AD-AMPS channel
`with six users.
`
`Each frame holds three users who take turns using the upstream and down(cid:173)
`stream Jinks. During slot 1 of Fig. 2-42(a), for example, user 1 may transmit to
`the base station and user 3 is receiving from the base station. Each slot is 324 bits
`long, of which 64 bits are used for guard times, synchronization, and control pur(cid:173)
`poses, leaving 260 bits for the user payload. Of the payload bits, 101 are used for
`error correction over the noisy air link, so ultimately only 159 bits are left for
`compressed speech. With 50 slots/sec, the bandwidth available for compressed
`speech is just under 8 kbps, 1 /7 of the standard PCM bandwidth.
`
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`SEC. 2.6
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`THE MOBILE TELEPHONE SYSTEM
`
`159
`
`Using b~tter com~ression algorithms, it is possible to get the speech down to 4
`kbps, in winch case six users can be stuffed into a frame, as illustrated in Fig. 2-
`42(b). From the oper~tor's perspective, being able to squeeze three to six times as
`many D-AMPS users mto the same spectrum as one AMPS user is a huge win and
`explains much of the popularity of PCS. Of course, the quality of speech at 4
`kbps is not c~m~ar~ble to what can be achieved at 56 kbps, but few PCS operators
`advertise their hi-f1 sound quality. It should also be clear that for data, an 8 kbps
`channel is not even as good as an ancient 9600-bps modem.
`The control structure of D-AMPS is fairly complicated. Briefly summarized,
`groups of 16 frames form a superframe, with certain control information present
`in each superframe a limited number of times. Six main control channels are
`used: system configuration, real-time and nonreal-time control, paging~ access re(cid:173)
`sponse, and short messages. But conceptually, it works like AMPS. When a mo(cid:173)
`bile is switched on, it makes contact with the base station to announce itself and
`then listens on a control channel for incoming calls. Having picked up a new
`mobile, the MTSO informs the user's home base where he is, so calls can be
`routed correctly.
`One difference between AMPS and D-AMPS is how handoff is handled. In
`AMPS, the MTSO manages it completely without help from the mobile devices.
`As can be seen from Fig. 2-42, in D-AMPS, 1/3 of the time a mobile is neither
`sending nor receiving. It uses these idle slots to measure the line quality. When it
`discovers that the signal is waning, it complains to the MTSO, which can then
`break the connection, at which time the mobile can try to tune to a stronger signal
`from another base station. As in AMPS, it still takes about 300 msec to do the
`handoff. This technique is called MAHO (Mobile Assisted HandOfT).
`
`GSM-The Global System for Mobile Communications
`
`D-AMPS is widely used in the U.S. and (in modified form) in Japan. Virtu(cid:173)
`ally everywhere else in the world, a system called GSM (Global System for
`Mobile communications) is used, and it is even starting to be used in the U.S. on
`a limited scale. To a first approximation, GSM is similar to D-AMPS, Both are
`cellular systems. In both systems, frequency division multiplexing is used, with
`each mobile transmitting on one frequency and receiving on a higher frequency
`(80 MHz higher for D-AMPS, 55 MHz higher for GSM). Also in both systems, a
`single frequency pair is split by time-division multiplexing into time slots shared
`by multiple mobiles. However, the GSM channels are much wider than the
`AMPS channels (200 kHz versus 30 kHz) and hold relatively few additional users
`(8 versus 3), giving GSM a much higher data rate pe~ user tha~ D-AMPS.
`Below we will briefly discuss some of the mam properties of GSM. How(cid:173)
`ever, the printed GSM standard is over 5000 [sic] pages long. A large fraction of
`this material relates to engineering aspects of the system, especially the design of
`
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`160
`
`THE PHYSICAL LAYER
`
`CHAP. 2
`receivers to handle multipath signa1 propaga~ion, and synchronizing transmitters
`and receivers. Nl1nc of this will be even mentioned below.
`Each frequency hand is 200 kHz wide, as shown in Fig. ~-43. A GSM systern
`has 124 pairs of simplex channels. Each simplex channel 1s 200 kHz wide and
`supports eight separate connections on it, using time division multiplexing. Each
`currently active station is assigned one time slot on one channel pair. Theoreti(cid:173)
`cally, 992 channels can be supported in each cell, but many of them are not avail(cid:173)
`able, to avoid frequency conflicts with neighboring cells. In Fig. 2-43, the eight
`shaded time slots all belong to the same connection, four of them in each direc(cid:173)
`tion. Transmitting and receiving does not happen in the same time slot because
`the GSM radios cannot transmit and receive at the same time and it takes time to
`switch from one to the other. If the mobile station assigned to 890.4/935.4 MHz
`and time slot 2 wanted to transmit to the base station, it would use the lower four
`shaded slots ( and the ones fallowing them in time), putting some data in each slot
`until all the data had been sent.
`
`TOM frame
`
`Channel
`
`959•8 MHz I I I I I I I I
`
`I I I I I I
`
`I
`
`I I I I I I I
`
`I
`
`I I I I I I I 1
`
`>,
`0
`C:
`
`I I I I I I I
`
`I
`
`I I I I I I I
`
`I
`
`I I I I I I I
`
`I
`
`I I I I I I I 1
`
`124
`
`Base
`
`124
`
`2
`1
`
`Mobile
`to base
`
`• • •
`I ::::: ~:: I : ~ : : : : I : 8 : : : : I : ~ : : : : I : ~ : : : : I ~ to mobile
`Cl) g 914.8 MHz I
`...
`• • •
`u..
`:::: ~:~ I : : : : : c~ I : : : : : 3 I : : : : : ~ I : : : : : 3 I
`
`Time - - -
`
`Figure 2-43. GSM uses 124 frequency channels, each of which uses an eight(cid:173)
`slot TDM system.
`The TDM slots shown in Fig. 2-43 are part of a complex framing hierarchy.
`Each TDM slot has a specific structure, and groups of TDM slots form multi(cid:173)
`frames, also with a specific structure. A simplified version of this hierarchy is
`shown in Fig. 2-44. Here we can see that each TDM slot consists of a 148-bit
`data frame that occupies the channel for 577 µsec (including a 30-µsec guard time
`after each slot). Each data frame starts and ends with three Obits, for frame de(cid:173)
`lineation purposes. It also contains two 57-bit Information fields, each one having
`a control bit that indicates whether the f o11owing Information field is for voice or
`data. Between the Information fields is a 26-bit Sync (training) field that is used
`by the receiver to synchronize to the sender's frame boundaries.
`
`
`Ex.1011 / Page 12 of 47Ex.1011 / Page 12 of 47
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`

`

`sEC. 2.6
`
`THE MOBILE TELEPIIONE SYSTEM
`
`161
`
`--
`0 1 2 3 4 5 6
`
`-
`
`--
`
`---
`
`--
`
`--
`
`I
`
`__________ 32,500-Bit multiframe sent In 120 msec _________ ..,
`~
`C
`7 8 9 10 11 T 13 14 15 16 17 18 19
`--
`L
`---
`--
`---
`
`20 21 22 23 24 ~-I
`---------------------- --
`6 I
`
`---
`
`Reserved
`for future
`use
`
`- - -
`11
`
`8.25-bit
`(30 µsec)
`guard time
`
`... ...
`
`000
`
`Information Sync
`
`Information 000
`
`Bits
`
`3
`
`57
`
`26
`Voice/data bit
`
`57
`
`3
`
`Figure 2-44. A portion of the GSM framing structure.
`A data frame is transmitted in 547 µsec, but a transmitter is only allowed to
`send one data frame every 4.615 msec, since it is sharing the channel with seven
`other stations. The gross rate of each channel is 270,833 bps, divided among eight
`users. This gives 33.854 kbps gross, more than double D-AMPS' 324 bits 50
`times per second for 16.2 kbps. However, as with AMPS, the overhead eats up a
`large fraction of the bandwidth, ultimately leaving 24.7 kbps worth of payload per
`user before error correction. After error correction, 13 kbps is left for speech, giv(cid:173)
`ing substantially better voice quality than D-AMPS (at the cost of using corres(cid:173)
`pondingly more bandwidth).
`As can be seen from Fig. 2-44, eight data frames make up a TDM frame and
`26 TDM frames make up a 120-msec multiframe. Of the 26 TDM frames in a
`multiframe, slot 12 is used for control and slot 25 is reserved for future use, so
`only 24 are available for user traffic.
`However, in addition to the 26-slot multiframe shown in Fig. 2-44, a 51-slot
`multiframe (not shown) is also used. Some of these slots are used to hold several
`control channels used to manage the system. The broadcast control channel is a
`continuous stream of output from the base station containing the base station's
`identity and the channel status. All mobile stations monitor their signal strength
`to see when they have moved into a new cell.
`The dedicated control channel is used for location updating, registration,
`and call setup. In particular, each base station maintains a database of mobile sta(cid:173)
`tions currently under its jurisdiction. Information needed to 1naintain this data(cid:173)
`base is sent on the dedicated control channel.
`Finally, there is the common control channel, which is split up into three
`logical subchannels. The first of these subchannels is the paging channel, which
`
`
`Ex.1011 / Page 13 of 47Ex.1011 / Page 13 of 47
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`

`162
`
`THE PHYSIC AL LA YER
`
`Cf-IAp ·2
`the base station uses to announce ~ncoming ~al~s. ,'.~a~I~ mo.bi~e Sla~ion monitors i
`continuou~ly to watch for calls it should lmswct. I he second JS_ the rand
`t
`access channel. ,vhkh allows users to rcqucSl a slot on the dedicated con~O't
`ch~nncl. If two requests collide, they arc garbk~ ~nd have to be retried lat:~I
`Usmg the dedicated control cham~cl slot, the statton can set up a call. l'he
`assigned slot is announced on the tlnrd subchannel, the access grant channel.
`
`CDl\tA-Code Dh1ision Multiple Access
`
`D-AMPS and GSM are fairly conventional systems. Th~y us~ both FDM and
`TDM to divide the spectnnn into channels and the channels into time slots. How(cid:173)
`eve_r, there is a third kid on the block, CDMA (Code Div~sion Multiple Access),
`which works completely differently. When CDMA was fust proposed, the indus(cid:173)
`try gave it approximately the same reaction that Columbus first got from Queen
`Isabella when he proposed reaching India by sailing in the wrong direction. How(cid:173)
`ever, through the persistence of a single company, Qualcomm, CDMA has mat(cid:173)
`ured to the point where it is not only acceptable, it is now viewed as the best tech(cid:173)
`nical solution around and the basis for the third-generation mobile systems. It is
`also widely used in the U.S. in second-generation mobile systems, competing
`head-on with D-AMPS. For example, Sprint PCS uses CDMA, whereas AT&T
`Wireless uses D-AMPS. CDMA is described in International Standard IS-95 and
`is sometimes ref erred to by that name. The brand name cdmaOne is also used.
`CDMA is completely different from AMPS, D-AMPS, and GSM. Instead of
`dividing the allowed frequency range into a few hundred narrow channels, CDMA
`allows each station to transmit over the entire frequency spectrum all the time.
`Multiple simultaneous transmissions are separated using coding theory. CDMA
`also relaxes the assumption that colliding frames are totally garbled. Instead, it
`assumes that multiple signals add linearly.
`Before getting into the algorithm, let us consider an analogy: an airport lounge
`with many pairs of people conversing. TDM is comparable to all the people being
`in the middle of the room but taking turns speaking. FDM