Cellular Communications

Ce llu la r Com m u n ica t ion s D e finit ion A cellular mobile communications system uses a large number of low-power wireless transmitters to create cells—the basic geographic service area of a wireless communications system. Variable power levels allow cells to be sized according to the subscriber density and demand within a particular region. As mobile users travel from cell to cell, their conversations are "handed off" between cells in order to maintain seamless service. Channels (frequencies) used in one cell can be reused in another cell some distance away. Cells can be added to accommodate growth, creating new cells in unserved areas or overlaying cells in existing areas. Ove r vie w This tutorial discusses the basics of radio telephony systems, including both analog and digital systems. Upon completion of this tutorial, you should be able to accomplish the following: 1. describe the basic components of a cellular system 2. identify and describe digital wireless technologies Topics 1. Mobile Communicatio ns Principles 2. Mobile Telephone System Using the Cellular Concept 3. Cellular System Architecture 4. North American Analog Cellular Systems 5. Cellular System Components 6. Digital Systems Self- Test Correct Answers Acronym Guide 1 . M obile Com m u n ica t ion s Pr in ciple s Each m obile uses a separate, tem porary radio channel to talk to the cell site. The cell site talks to many mobiles at once, using one channel per mobile. Channels use a pair of frequencies for com m unication—one frequency, the forward link, for transm itting from the cell site, and one frequency, the reverse link, for the cell site to receive calls from the users. Radio energy dissipates over distance, so m obiles m ust stay near the base station to m aintain com m unications. The basic structure of m obile networks include telephone system s and radio services. Where m obile radio service operates in a closed network and has no access to the telephone system , m obile telephone service allows interconnection to the telephone network (see Figure 1). Fig u r e 1 : Ba sic M ob ile Te le p h on e Se r v ice N e t w or k Ea r ly M obile Te le ph on e Sy st e m Ar ch it e ct u r e Traditional m obile service was structured sim ilar to television broadcasting: One very powerful transm itter located at the highest spot in an area would broadcast in a radius of up to fifty kilometers. The cellular concept" structured the mobile telephone network in a different way. Instead of using one powerful transm itter, m any low-power transm itters were placed throughout a coverage area. For exam ple, by dividing a m etropolitan region into one hundred different areas (cells) with low-power transm itters using twelve conversations (channels) each, Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 2/ 23 the system capacity theoretically could be increased from twelve conversations— or voice channels using one powerful transm itter—to twelve hundred conversations (channels) using one hundred low-power transm itters. Figure 2 shows a m etropolitan area configured as a traditional m obile telephone network with one high-power transm itter. Fig u r e 2 : Ea r ly M ob ile Te le p h on e Sy st e m Ar ch it e ct u r e 2 . M obile Te le phone Syst e m Using t he Ce llula r Conce pt Interference problems caused by mobile units using the same channel in adjacent areas proved that all channels could not be reused in every cell. Areas had to be skipped before the sam e channel could be reused. Even though this affected the efficiency of the original concept, frequency reuse was still a viable solution to the problem s of m obile telephony system s. Engineers discovered that the interference effects were not due to the distance between areas, but to the ratio of the distance between areas to the transm itter power (radius) of the areas. By reducing the radius of an area by fifty percent, service providers could increase the num ber of potential custom ers in an area fourfold. System s based on areas with a one-kilom eter radius would have one hundred tim es m ore channels than system s with areas ten kilom eters in radius. Speculation led to the conclusion that by reducing the radius of areas to a few hundred m eters, m illions of calls could be served. The cellular concept em ploys variable low-power levels, which allows cells to be sized according to the subscriber density and dem and of a given area. As the population grows, cells can be added to accom m odate that growth. Frequencies used in one cell cluster can be reused in other cells. Conversations can be handed Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 3/ 23 off from cell to cell to m aintain constant phone service as the user m oves between cells (see Figure 3). Fig u r e 3 : M ob ile Te le p h on e Sy st e m U sin g a Ce llu la r Ar ch it e ct u r e The cellular radio equipm ent (base station) can com m unicate with m obiles as long as they are within range. Radio energy dissipates over distance, so the m obiles m ust be within the operating range of the base station. Like the early m obile radio system , the base station com m unicates with m obiles via a channel. The channel is made of two frequencies, one for transmitting to the base station and one to receive inform ation from the base station. 3 . Ce llu la r Syst e m Ar ch it e ct u r e Increases in dem and and the poor quality of existing service led m obile service providers to research ways to im prove the quality of service and to support m ore users in their system s. Because the am ount of frequency spectrum available for m obile cellular use was lim ited, efficient use of the required frequencies was needed for m obile cellular coverage. In m odern cellular telephony, rural and urban regions are divided into areas according to specific provisioning guidelines. Deploym ent param eters, such as am ount of cell-splitting and cell sizes, are determ ined by engineers experienced in cellular system architecture. Provisioning for each region is planned according to an engineering plan that includes cells, clusters, frequency reuse, and handovers. Ce lls A cell is the basic geographic unit of a cellular system . Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 4/ 23 Th e term cellular com es from the honeycom b shape of the areas into which a coverage region is divided. Cells are base stations transmitting over small geographic areas that are represented as hexagons. Each cell size varies depending on the landscape. Because of constraints im posed by natural terrain and m an-m ade structures, the true shape of cells is not a perfect hexagon. Clu st e r s A cluster is a group of cells. No channels are reused within a cluster. Figure 4 illustrates a seven-cell cluster. Fig u r e 4 : A Se v e n - Ce ll Clu st e r Fr e qu e n cy Re u se Because only a small number of radio channel frequencies were available for m obile system s, engineers had to find a way to reuse radio channels in order to carry m ore than one conversation at a tim e. The solution the industry adopted was called frequency planning or frequency reuse. Frequency reuse was im plem ented by restructuring the m obile telephone system architecture into the cellular concept. Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 5/ 23 The concept of frequency reuse is based on assigning to each cell a group of radio channels used within a sm all geographic area. Cells are assigned a group of channels that is com pletely different from neighboring cells. The coverage area of cells are called the footprint. This footprint is lim ited by a boundary so that the sam e group of channels can be used in different cells that are far enough away from each other so that their frequencies do not interfere (see Figure 5). Fig u r e 5 : Fr e q u e n cy Re u se Cells with the sam e num ber have the sam e set of frequencies. Here, because the num ber of available frequencies is 7, the frequency reuse factor is 1/ 7. That is, each cell is using 1/ 7 of available cellular channels. Ce ll Split t in g Unfortunately, econom ic considerations m ade the concept of creating full system s with m any sm all areas im practical. To overcom e this difficulty, system operators developed the idea of cell splitting. As a service area becomes full of users, this approach is used to split a single area into sm aller ones. In this way, urban centers can be split into as m any areas as necessary in order to provide acceptable service levels in heavy-traffic regions, while larger, less expensive cells can be used to cover rem ote rural regions (see Figure 6). Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 6/ 23 Fig u r e 6 : Ce ll Sp lit t in g H a n doff The final obstacle in the developm ent of the cellular network involved the problem created when a m obile subscriber traveled from one cell to another during a call. As adjacent areas do not use the sam e radio channels, a call m ust either be dropped or transferred from one radio channel to another when a user crosses the line between adjacent cells. Because dropping the call is unacceptable, the process of handoff was created. Handoff occurs when the m obile telephone network autom atically transfers a call from radio channel to radio channel as a m obile crosses adjacent cells (see Figure 7). Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 7/ 23 Fig u r e 7 : H a n d of f b e t w e e n Ad j a ce n t Ce lls During a call, two parties are on one voice channel. When the m obile unit m oves out of the coverage area of a given cell site, the reception becom es weak. At this point, the cell site in use requests a handoff. The system switches the call to a stronger-frequency channel in a new site without interrupting the call or alerting the user. The call continues as long as the user is talking, and the user does not notice the handoff at all. 4 . N or t h Am e r ica n An a log Ce llu la r Syst e m s Originally devised in the late 1970 s to early 1980 s, analog system s have been revised som ewhat since that tim e and operate in the 80 0 -MHz range. A group of governm ent, telco, and equipm ent m anufacturers worked together as a com m ittee to develop a set of rules (protocols) that govern how cellular subscriber units (m obiles) com m unicate with the "cellular system ." System developm ent takes into consideration m any different, and often opposing, requirem ents for the system , and often a com prom ise between conflicting requirem ents results. Cellular developm ent involves som e basic topics: 1. frequency and channel assignm ents Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 8/ 23 2. type of radio m odulation 3. m axim um power levels 4. m odulation param eters 5. messaging protocols 6. call-processing sequences Th e Adv a n ce d M obile Ph on e Se r v ice ( AM PS) AMPS was released in 1983 using the 80 0 -MHz to 90 0 -MHz frequency band and the 30 kHz bandwidth for each channel as a fully autom ated m obile telephone service. It was the first standardized cellular service in the world and is currently the m ost widely used standard for cellular com m unications. Designed for use in cities, AMPS later expanded to rural areas. It m axim ized the cellular concept of frequency reuse by reducing radio power output. The AMPS telephones (or handsets) have the fam iliar telephone-style user interface and are com patible with any AMPS base station. This m akes m obility between service providers (roam ing) sim pler for subscribers. Lim itations associated with AMPS include: 1. low calling capacity 2. lim ited spectrum 3. no room for spectrum growth 4. poor data communications 5. m inim al privacy 6. inadequate fraud protection AMPS is used throughout the world and is particularly popular in the United States, South Am erica, China, and Australia. AMPS uses frequency m odulation (FM) for radio transm ission. In the United States, transm issions from m obile to cell site use separate frequencies from the base station to the m obile subscriber. N a r r ow ba n d An a log M obile Ph on e Se r v ice ( N AM PS) Since analog cellular was developed, system s have been im plem ented extensively throughout the world as first-generation cellular technology. In the second generation of analog cellular system s, NAMPS was designed to solve the problem Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 9/ 23 of low calling capacity. NAMPS is now operational in 35 U.S. and overseas m arkets and NAMPS was introduced as an interim solution to capacity problem s. NAMPS is a U.S. cellular radio system that com bines existing voice processing with digital signaling, tripling the capacity of today's AMPS system s. The NAMPS concept uses frequency division to get three channels in the AMPS 30 -kHz single channel bandwidth. NAMPS provides three users in an AMPS channel by dividing the 30 -kHz AMPS bandwidth into three 10 -kHz channels. This increases the possibility of interference because channel bandwidth is reduced. 5 . Ce llula r Syst e m Com pone nt s The cellular system offers m obile and portable telephone stations the sam e service provided fixed stations over conventional wired loops. It has the capacity to serve tens of thousands of subscribers in a m ajor m etropolitan area. The cellular com m unications system consists of the following four m ajor com ponents that work together to provide m obile service to subscribers (see Figure 8): 1. public switched telephone network (PSTN) 2. m obile telephone switching office (MTSO) 3. cell site with antenna system 4. m obile subscriber unit (MSU) Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 10/ 23 Fig u r e 8 : Ce llu la r Sy st e m Com p on e n t s PSTN The PSTN is m ade up of local networks, the exchange area networks, and the long-haul network that interconnect telephones and other com m unication devices on a worldwide basis. M ob ile Te le p h on e Sw it ch in g Of f ice ( M TSO) The MTSO is the central office for mobile switching. It houses the mobile switching center (MSC), field monitoring and relay stations for switching calls from cell sites to wireline central offices (PSTN). In analog cellular networks, the MSC controls the system operation. The MSC controls calls, tracks billing inform ation, and locates cellular subscribers. Th e Ce ll Sit e Th e term cell site is used to refer to the physical location of radio equipment that provides coverage within a cell. A list of hardware located at a cell site includes power sources, interface equipm ent, radio frequency transm itters and receivers, and antenna system s. Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 11/ 23 M obile Su bscr ibe r Un it s ( M SUs) The m obile subscriber unit consists of a control unit and a transceiver that transm its and receives radio transm issions to and from a cell site. Three types of MSUs are available: 1. the m obile telephone (typical transm it power is 4.0 watts) 2. the portable (typical transm it power is 0 .6 watts) 3. the transportable (typical transm it power is 1.6 watts) The mobile telephone is installed in the trunk of a car, and the handset is installed in a convenient location to the driver. Portable and transportable telephones are hand-held and can be used anywhere. The use of portable and transportable telephones is lim ited to the charge life of the internal battery. 6 . D igit a l Syst e m s As dem and for m obile telephone service has increased, service providers found that basic engineering assum ptions borrowed from wireline (landline) networks did not hold true in m obile system s. While the average landline phone call lasts at least ten m inutes, m obile calls usually run ninety seconds. Engineers who expected to assign fifty or m ore m obile phones to the sam e radio channel found that by doing so they increased the probability that a user would not get dial tone—this is known as call-blocking probability. As a consequence, the early system s quickly becam e saturated, and the quality of service decreased rapidly. The critical problem was capacity. The general characteristics of TDMA, GSM, PCS190 0 , and CDMA prom ise to significantly increase the efficiency of cellular telephone system s to allow a greater num ber of sim ultaneous conversations. Figure 9 shows the com ponents of a typical digital cellular system . Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 12/ 23 Fig u r e 9 : D ig it a l Ce llu la r Sy st e m The advantages of digital cellular technologies over analog cellular networks include increased capacity and security. Technology options such as TDMA and CDMA offer m ore channels in the sam e analog cellular bandwidth and encrypted voice and data. Because of the enorm ous am ount of m oney that service providers have invested in AMPS hardware and software, providers look for a m igration from AMPS to DAMPS by overlaying their existing networks with TDMA architectures. Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 13/ 23 Ta b le : AM PS/ D AM PS Com p a r ison Analog Digital Standard EIA–553 (AMPS) IS–54 (TDMA + AMPS Spectrum 824 MHz to 891 MHz 824 MHz to 891 MHz Channel Bandwidth 30 kHz 30 kHz Channels 21 CC / 395 VC 21 CC / 395 VC Conversations per Channel 1 3 or 6 Subscriber Capacity 40 to 50 Conversations per cell 125 to 300 Conversations per cell TX / RCV Type Continuous Time shared bursts Carrier Type Constant phase Variable frequency Constant frequency Variable phase Mobile/Base Relationship Mobile slaved to base Authority shared cooperatively Privacy Poor Better—easily scrambled Noise Immunity Poor High Fraud Detection ESN plus optional password (PIN) ESN plus optional password (PIN) Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 14/ 23 Tim e D iv ision M u lt iple Acce ss ( TD M A) North American digital cellular (NADC) is called DAMPS and TDMA. Because AMPS preceded digital cellular system s, DAMPS uses the sam e setup protocols as analog AMPS. TDMA has the following characteristics: 1. IS– 54 standard specifies traffic on digital voice channels 2. Initial im plem entation triples the calling capacity of AMPS system s 3. Capacity im provem ents of 6 to 15 tim es that of AMPS are possible 4. Uses many blocks of spectrum in 80 0 MHz and 190 0 MHz 5. All transm issions are digital 6. TDMA/ FDMA application 7. 3 callers per radio carrier (6 callers on half rate later), providing three tim es the AMPS capacity TDMA is one of several technologies used in wireless com m unications. TDMA provides each call with time slots so that several calls can occupy one bandwidth. Each caller is assigned a specific tim e slot. In som e cellular system s, digital packets of inform ation are sent during each tim e slot and reassem bled by the receiving equipm ent into the original voice com ponents. TDMA uses the sam e frequency band and channel allocations as AMPS. Like NAMPS, TDMA provides three to six tim e channels in the sam e bandwidth as a single AMPS channel. Unlike NAMPS, digital system s have the m eans to com press the spectrum used to transm it voice inform ation by com pressing idle tim e and redundancy of norm al speech. TDMA is the digital standard and has 30 -kHz bandwidth. Using digital voice encoders, TDMA is able to use up to six channels in the sam e bandwidth where AMPS uses one channel. Ex t e n de d Tim e D ivision M u lt iple Acce ss ( E– TD M A) The extended TDMA (E– TDMA) standard claim s a capacity of fifteen tim es that of analog cellular system s. This capacity is achieved by com pressing quiet tim e during conversations. E– TDMA divides the finite num ber of cellular frequencies into m ore tim e slots than TDMA. This allows the system to support m ore sim ultaneous cellular calls. Fix e d W ir e le ss Acce ss ( FW A) Fixed wireless access (FWA) is a radio-based local exchange service in which telephone service is provided by com m on carriers (see Figure 10 ). It is prim arily a Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 15/ 23 rural application—that is, it reduces the cost of conventional wireline. FWA extends telephone service to rural areas by replacing a wireline local loop with radio com m unications. Other labels for wireless access include fixed loop, fixed radio access, wireless telephony, radio loop, fixed wireless, radio access, and Ionica. FWA system s em ploy TDMA or CDMA access technologies. Fig u r e 1 0 : Fix e d W ir e le ss Acce ss Pe r son a l Com m u n ica t ion s Se r v ice s ( PCS) The future of telecom m unications includes personal com m unications services. PCS at 190 0 MHz (PCS190 0 ) is the North Am erican im plem entation of DCS180 0 (Global System for Mobile com m unications, or GSM). Trial networks were operational in the United States by 1993, and in 1994 the Federal Com m unications Com m ission (FCC) began spectrum auctions. As of 1995, the FCC auctioned com m ercial licenses. In the PCS frequency spectrum the operator's authorized frequency block contains a definite num ber of channels. The frequency plan assigns specific channels to specific cells, following a reuse pattern which restarts with each nth cell. The uplink and downlink bands are paired m irror im ages. As with AMPS, a channel num ber im plies one uplink and one downlink frequency: e.g., Channel 512 = 1850 .2 MHz uplink paired with 1930 .2 MHz downlink. Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 16/ 23 Code D iv ision M u lt iple Acce ss ( CD M A) Code division m ultiple access (CDMA) is a digital air interface standard, claim ing eight to fifteen tim es the capacity of analog. It em ploys a com m ercial adaptation of m ilitary spread-spectrum single- sideband technology. Based on spread spectrum theory, it is essentially the sam e as wireline service—the prim ary difference is that access to the local exchange carrier (LEC) is provided via wireless phone. Because users are isolated by code, they can share the sam e carrier frequency, elim inating the frequency reuse problem encountered in AMPS and DAMPS. Every CDMA cell site can use the sam e 1.25 MHz band, so with respect to clusters, n = 1. This greatly sim plifies frequency planning in a fully CDMA environm ent. CDMA is an interference lim ited system . Unlike AMPS/ TDMA, CDMA has a soft capacity lim it; however, each user is a noise source on the shared channel and the noise contributed by users accum ulates. This creates a practical lim it to how m any users a system will sustain. Mobiles that transm it excessive power increase interference to other mobiles. For CDMA, precise power control of mobiles is critical in m axim izing the system 's capacity and increasing battery life of the m obiles. The goal is to keep each m obile at the absolute m inim um power level that is necessary to ensure acceptable service quality. Ideally, the power received at the base station from each m obile should be the sam e (m inim um signal to interference). Se lf- Te st 1. Interference effects in cellular system s are a result of _ _ _ _ _ _ _ _ _ _ _ . a. the distance between areas b. the power of the transm itters c. the ratio of the distance between areas to the transm itter power of the areas d. the height of the antennas 2. Larger cells are m ore useful in _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . a. densely populated urban areas b. rural areas c. lightly populated urban areas d. m ountainous areas Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 17/ 23 3. The m ost widely used standard for cellular com m unications is _ _ _ _ _ _ _ _ _ _ . a. the advanced m obile phone service (AMPS) b. the m obile subscriber unit (MSU) c. the mobile telephone switching office d. code division m ultiple access (CDMA) 4. How many conversations per channel can TDMA digital cellular carry at once? a. 1 b. 2 c. 3 d. 10 5. Which of the following is not a limitation of AMPS? a. low calling capacity b. poor privacy protection c. Lim ited spectrum d. wide coverage area 6. Digital cellular technologies offer increased capacity and security . a. true b. false 7. TDMA, a digital air interface standard, has twice the capacity of analog. a. true b. false 8. Cells are always hexagonal in shape. a. true b. false Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 18/ 23 9. Frequency reuse was m axim ized by increasing the size of cells. a. true b. false 10 . Fixed wireless access is primarily a rural application. a. true b. false Cor r e ct An sw e r s 1. Interference effect s in cellular systems are a result of _ _ _ _ _ _ _ _ _ . a. the distance between areas b. the power of the transm itters c . th e ratio o f th e d is tan ce be tw e e n are as to th e tran s m itte r p o w e r o f th e a re a s d. the height of the antennas See Topic 2 2. Larger cells are m ore useful in _ _ _ _ _ _ _ _ _ _ _ _ _ . a. densely populated urban areas b. ru ra l a re a s c. lightly populated urban areas d. m ountainous areas See Topic 3 3. The m ost widely used standard for cellular com m unications is _ _ _ _ _ _ _ . a . th e ad van ce d m o bile p h o n e s e rvice ( AMPS) b. the m obile subscriber unit (MSU) c. the mobile telephone switching office d. code division m ultiple access (CDMA) Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 19/ 23 See Topic 4 4. How m any conversations per channel can TDMA digital cellular car ry at once? a. 1 b. 2 c. 3 d. 10 See Topic 6 5. Which of the following is not a limitation of AMPS? a. low calling capacity b. poor privacy protection c. lim ited spectrum d . w id e co ve rage are a See Topic 4 6. Digital cellular technologies offer increased capacity and security. a . tru e b. false See Topic 6 7. TDMA, a digital air interface standard, has twice the capacity of analog. a. true b. fals e See Topic 6 8. Cells are always hexagonal in shape. a. true b. fals e Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 20/ 23 See Topic 3 9. Frequency reuse was m axim ized by increasing the size of cells. a. true b. fals e See Topic 3 10 . Fixed wireless access is prim arily a rural application. a . tru e b. false See Topic 6 Acr onym Guide AMPS advanced m obile phone service; another acronym for analog cellular radio B TS base transceiver station; used to transm it radio frequency over the air interface CDMA code division m ultiple access; a form of digital cellular phone service that is a spread spectrum technology that assigns a code to all speech bits, sends scrambled transmission of the encoded speech over the air, and reassembles the speech to its original format DAMPS digital advanced m obile phone service; a term for digital cellular radio in North Am erica DCS digital cellular system ESN electronic serial number; an identity signal that is sent from the mobile to the MSC during a brief registration transm ission ETDMA extended TDMA; developed to provide fifteen tim es the capacity over analog systems by compressing quiet time during conversations Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 21/ 23 FCC Federal Com m unications Com m ission; the governm ent agency responsible for regulating telecom m unications in the United Sates FCCH frequency control channel FD MA frequency division m ultiple access; used to separate m ultiple transm issions over a finite frequency allocation; refers to the m ethod of allocating a discrete am ount of frequency bandwidth to each user to perm it m any sim ultaneous conversations FM frequency m odulation; a m odulation technique in which the carrier frequency is shifted by an am ount proportional to the value of the m odulating signal FRA fixed radio access GS M global system for m obile com m unications; standard digital cellular phone service in Europe and J apan; to ensure interpretability between countries, standards address m uch of the network wireless infrastructure, including radio interfaces, switching, signaling, and intelligent networks Hz hertz; a m easurem ent of electrom agnetic energy, equivalent to one wave or cycle per second kH z kilohertz; thousands of hertz. MH z m egahertz; millions of hertz. MS o r MSU m obile station unit; handset carried by the subscriber MSC m obile services switching center; a switch that provides services and coordination between m obile users in a network and external networks MTSO m obile telephone switching office; the central office for the m obile switch, which houses the field m onitoring and relay stations for switching calls from cell sites to wireline central offices (PSTN) Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 22/ 23 MTX m obile telephone exchange NADC North American digital cellular (also called United States digital cellular, or USDC); a tim e division m ultiple access (TDMA) system that provides three to six tim es the capacity of AMPS N AMPS narrowband advanced m obile phone service; NAMPS was introduced as an interim solution to capacity problem s; NAMPS provides three tim es the AMPS capacity to extend the usefulness of analog system s PCS personal com m unications service; a lower-powered, higher-frequency com petitive technology that incorporates wireline and wireless networks and provides personalized features P S TN public switched telephone network; a PSTN is m ade of local networks, the exchange area networks, and the long-haul network that interconnect telephones and other communication devices on a worldwide basis RF radio frequency; electrom agnetic waves operating between 10 kHz and 3 MHz propagated without guide (wire or cable) in free space S IM subscriber identity m odule; a smartcard, which is inserted into a mobile phone to get it going SN SE supernode size enhanced TDMA tim e division m ultiple access; used to separate m ultiple conversation transm issions over a finite frequency allocation of through-the-air bandwidth; used to allocate a discrete am ount of frequency bandwidth to each user; to perm it m any simultaneous conversations, each caller is assigned a specific timeslot for transm ission Web ProForum Tut orials ht t p: / / www.iec.org Copyright © The I nt ernat ional Engineering Consort ium 23/ 23