THE MOBILE SYSTEM
The traditional mobile system (even if it some day gets multigigabit endto-end fiber) will still not do able into satisfy a growing group of users: people on the go. Instead we will concentrate on the mobile system, which is used for wide area voice and data communication. In the following sections we will study this topic in some detail. People now expect into make phone calls from airplanes, cars, swimming pools, and while jogging in the park. Wireless telephones come in two basic varieties: cordless phones and mobile phones (sometimes called cell phones).
THE MOBILE TELEPHONE SYSTEM CHAP.
These are never used for networking, so we will not examine them further. Cordless phones are devices consisting of a base station and a handset sold as a set for use within the home. Consequently, there is a tremendous amount of interest in wireless telephony. Within a few years they will also expect toward send e-mail and surf the Web from all these locations and more. Mobile phones have gone into three distin.
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 (analog) 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 government-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 manufacturers 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 Europe is part of the reason, but there is more. A second area where the U.S. and 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 caller
1) First-Generation Mobile Phones: Analog Voice
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 time 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.
Enough about the politics and marketing aspects of mobile phones. Now let us look at the technology, starting with the earliest system. Mobile radiotelephones were used sporadically for maritime and military communication during the early decades of the 20th century. In 1946, the first system for car-based telephones 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.
2) •®Advanced Mobile Phone System
The idea of frequency reuse is illustrated in Fig. 2-41(a). The cells are normally roughly circular, but they are easier to model as hexagons. In Fig. 2-41(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 buffer 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 issue. 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 management :)
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 numbers of mobile users congregate for a few hours. How big the cells should be is a complex matter, which is treated in (Hac, 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
Using better compression algorithms, it is possible to get the speech down to 4 kbps, in which case six users can be stuffed into a frame, as illustrated in Fig. 242(b). From the operator’s perspective, being able to squeeze three to six times as many D-AMPS users into 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 comparable to what can be achieved at 56 kbps, but few PCS operators advertise their hi-fi 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 response, and short messages. But conceptually, it works like AMPS. When a mobile 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.
3)-*Third-Generation Mobile Phones: Digital Voice and Data :
Back in 1992, ITU tried to get a bit more specific about this dream and issued a blueprint for getting there called IMT-2000, where IMT stood for International Mobile Telecommunications. The number 2000 stood for three things:
•(1) the year it was supposed to go into service,
•(2) the frequency it was supposed to operate at (in MHz), and
•(3) the bandwidth the service should have (in kHz). It did not make it on any of the three counts. Nothing was implemented by 2000. ITU recommended that all governments reserve spectrum at 2 GHz so devices could roam seamlessly from country to country. China reserved the required bandwidth but nobody else did. Finally, it was recognized that 2 Mbps is not currently feasible for users who are too mobile (due to the difficulty of performing handoffs quickly enough).
More realistic is 2 Mbps for stationary indoor users (which will compete head-on with ADSL), 384 kbps for people walking,
The basic services that the IMT-2000 network is supposed to provide to its users are:
•1. High-quality voice transmission.
2. Messaging (replacing e-mail, fax, SMS, chat, etc.).
•3. Multimedia (playing music, viewing videos, films, television, etc.).
•4. Internet access (Web surfing, including pages with audio and video).
Much has been written about 3G systems, most of it praising it as the greatest thing since sliced bread. Some references are (Collins and Smith, 2001; De Vriendt et al., 2002; Harte et al., 2002; Lu, 2002; and Sarikaya, 2000). However, some dissenters think that the industry is pointed in the wrong direction (Garber, 2002; and Goodman, 2000). While waiting for the fighting over 3G to stop, some operators are gingerly taking a cautious small step in the direction of 3G by going to what is sometimes called 2.5G, although 2.1G might be more accurate. One such system is EDGE (Enhanced Data rates for GSM Evolution), which is just GSM with more bits per baud. The trouble is, more bits per baud also means more errors per baud, so EDGE has nine different schemes for modulation and error correction, differing on how much of the bandwidth is devoted to fixing the errors introduced by the higher speed. Another 2.5G scheme is GPRS (General Packet Radio Service), which is an overlay packet network on top of D-AMPS or GSM. It allows mobile stations to send and receive IP packets in a cell running a voice system. When GPRS is in operation, some time slots on some frequencies are reserved for packet traffic. The number and location of the time slots can be dynamically managed by the base station, depending on the ratio of voice to data traffic in the cell. The available time slots are divided into several logical channels, used for different purposes. The base station determines which logical channels are mapped onto which time slots. One logical channel is for downloading packets from the base station to some mobile station, with each packet indicating who it is destined for. To send an IP packet, a mobile station requests one or more time slots by sending a request to the base station. If the request arrives without damage, the base station announces the frequency and time slots allocated to the mobile for sending the packet. Once the packet has arrived at the base station, it is transferred to the Internet by a wired connection. Since GPRS is just an overlay over the existing voice system, it is at best a stop-gap measure until 3G arrives. Even though 3G networks are not fully deployed yet, some researchers regard 3G as a done deal and thus not interesting any more. These people are already working on 4G systems (Berezdivin et al., 2002; Guo and Chaskar, 2002
that some people think 3G is not only not a done dealE MOBILE SYSTEM
The traditional mobile system (even if it some day gets multigigabit endto-end fiber) will still not do able into satisfy a growing group of users: people on the go. Instead we will concentrate on the mobile system, which is used for wide area voice and data communication. In the following sections we will study this topic in some detail. People now expect into make phone calls from airplanes, cars, swimming pools, and while jogging in the park. Wireless telephones come in two basic varieties: cordless phones and mobile phones (sometimes called cell phones).
THE MOBILE TELEPHONE SYSTEM CHAP.
These are never used for networking, so we will not examine them further. Cordless phones are devices consisting of a base station and a handset sold as a set for use within the home. Consequently, there is a tremendous amount of interest in wireless telephony. Within a few years they will also expect toward send e-mail and surf the Web from all these locations and more. Mobile phones have gone into three distin.
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 (analog) 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 government-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 manufacturers 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 Europe is part of the reason, but there is more. A second area where the U.S. and 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 caller
4) First-Generation Mobile Phones: Analog Voice
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 time 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.
Enough about the politics and marketing aspects of mobile phones. Now let us look at the technology, starting with the earliest system. Mobile radiotelephones were used sporadically for maritime and military communication during the early decades of the 20th century. In 1946, the first system for car-based telephones 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.
Advanced Mobile Phone System
The idea of frequency reuse is illustrated in Fig. 2-41(a). The cells are normally roughly circular, but they are easier to model as hexagons. In Fig. 2-41(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 buffer 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 issue. 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 management :)
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 numbers of mobile users congregate for a few hours. How big the cells should be is a complex matter, which is treated in (Hac, 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
Using better compression algorithms, it is possible to get the speech down to 4 kbps, in which case six users can be stuffed into a frame, as illustrated in Fig. 242(b). From the operator’s perspective, being able to squeeze three to six times as many D-AMPS users into 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 comparable to what can be achieved at 56 kbps, but few PCS operators advertise their hi-fi 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 response, and short messages. But conceptually, it works like AMPS. When a mobile 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.
2) Third-Generation Mobile Phones: Digital Voice and Data :
Back in 1992, ITU tried to get a bit more specific about this dream and issued a blueprint for getting there called IMT-2000, where IMT stood for International Mobile Telecommunications. The number 2000 stood for three things:
•(1) the year it was supposed to go into service, •(2) the frequency it was supposed to operate at (in MHz), and
•(3) the bandwidth the service should have (in kHz). It did not make it on any of the three counts. Nothing was implemented by 2000. ITU recommended that all governments reserve spectrum at 2 GHz so devices could roam seamlessly from country to country. China reserved the required bandwidth but nobody else did. Finally, it was recognized that 2 Mbps is not currently feasible for users who are too mobile (due to the difficulty of performing handoffs quickly enough).
More realistic is 2 Mbps for stationary indoor users (which will compete head-on with ADSL), 384 kbps for people walking,
The basic services that the IMT-2000 network is supposed to provide to its users are:
•1. High-quality voice transmission.
2. Messaging (replacing e-mail, fax, SMS, chat, etc.).
•3. Multimedia (playing music, viewing videos, films, television, etc.).
•4. Internet access (Web surfing, including pages with audio and video).
Much has been written about 3G systems, most of it praising it as the greatest thing since sliced bread. Some references are (Collins and Smith, 2001; De Vriendt et al., 2002; Harte et al., 2002; Lu, 2002; and Sarikaya, 2000). However, some dissenters think that the industry is pointed in the wrong direction (Garber, 2002; and Goodman, 2000). While waiting for the fighting over 3G to stop, some operators are gingerly taking a cautious small step in the direction of 3G by going to what is sometimes called 2.5G, although 2.1G might be more accurate. One such system is EDGE (Enhanced Data rates for GSM Evolution), which is just GSM with more bits per baud. The trouble is, more bits per baud also means more errors per baud, so EDGE has nine different schemes for modulation and error correction, differing on how much of the bandwidth is devoted to fixing the errors introduced by the higher speed. Another 2.5G scheme is GPRS (General Packet Radio Service), which is an overlay packet network on top of D-AMPS or GSM. It allows mobile stations to send and receive IP packets in a cell running a voice system. When GPRS is in operation, some time slots on some frequencies are reserved for packet traffic. The number and location of the time slots can be dynamically managed by the base station, depending on the ratio of voice to data traffic in the cell. The available time slots are divided into several logical channels, used for different purposes. The base station determines which logical channels are mapped onto which time slots. One logical channel is for downloading packets from the base station to some mobile station, with each packet indicating who it is destined for. To send an IP packet, a mobile station requests one or more time slots by sending a request to the base station. If the request arrives without damage, the base station announces the frequency and time slots allocated to the mobile for sending the packet. Once the packet has arrived at the base station, it is transferred to the Internet by a wired connection. Since GPRS is just an overlay over the existing voice system, it is at best a stop-gap measure until 3G arrives. Even though 3G networks are not fully deployed yet, some researchers regard 3G as a done deal and thus not interesting any more. These people are already working on 4G systems (Berezdivin et al., 2002; Guo and Chaskar, 2002
that some people think 3G is not only not a done deal.
