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The traditional telephone system (even when broadband ISDN is fully
operating) will still not be able to satisfy people on the go. Consequently,
there is increasing competition from systems that use wireless technologies
for communication. They are already creating a huge market. Many companies
in the computer, telephone, satellite, and other industries want a piece
of action. The result is a chaotic market, with numerous overlapping and
incompatible products and services, all rapidly changing.
2.7.1. Paging Systems
The first paging systems used loudspeakers within a single building. Nowadays, people who want to be paged wear small beepers, usually with tiny screens for displaying short incoming messages.
A person wanting to page a beeper wearer can call the beeper company and enter a security code, the beeper number, and the number the beeper wearer is to call (or another short message). The request is then broadcasted from a hilltop antenna (for local paging) or from a satellite (for long distance paging). When a beeper detects its unique number in the incoming radio stream, it beeps and displays the number to be called.
Most current paging systems are one-way systems, from a single computer out to a large number of receivers. There is no problem about who will speak next, and no contention among many competing users.
Paging systems require little bandwidth since each message requires only a single burst of about 30 bytes. At this rate, a 1 Mbps satellite channel can handle over 240000 pages per minute. The older paging systems run in the 150 - 174 MHz band, the modern ones in the 930 - 932 MHz band. (Fig. 2-53).
Fig. 2-53. (a) Paging systems are one way.
(b) Mobile telephones are two way.
A cordless telephone consists of two parts: a base station and a telephone. The base station has a standard phone jack and is connected by a wire to the telephone system. The telephone communicates with the base station by low-power radio. The range is typically 100 - 300 m.
Some of cheaper models of cordless telephones used a fixed frequency, selected at the factory. If, by accident, someone in the neighborhood of a user had the telephone with the same frequency, he could listen user's calls. More expensive models avoided this problem by allowing the user to select the transmission frequency.
The generations of cordless telephones:
Mobile radiotelephones were used sporadically for maritime and military communication during the early decades of the 20th century.
Push-to-talk systems (installed in several big cities in the late 1950s) had a single channel used for both sending and receiving. They used a large transmitter on top of a tall building. To talk, the user had to push a button that enabled the transmitter and disabled the receiver. The users could hear each other.
IMTS (Improved Mobile Telephone System - in the 1960s) also used a high-powered
(200 watt) transmitter, on top of a hill, but it used different frequencies
for sending and for receiving, so no push-to-talk button was necessary. 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. The adjacent systems had to be several hundred km apart. So the
system was impractical due to limited capacity.
2.7.4. Advanced Mobile Phone System
AMPS (Advanced Mobile Phone System) was invented by Bell Labs, first installed in US in 1982. It is also used in England, where it is called TACS, and in Japan, where it is called MCS-L1.
In AMPS, a geographic region is divided up into cells, typically 10 to 20 km across, each using some set of frequencies. The key idea that gives AMPS far more capacity than all previous systems, is using relatively small cells, and reusing transmission frequencies in nearby (but not adjacent) cells. The idea of frequency reuse is illustrated in Fig. 2-54(a). The cells are normally roughly circular, but they are easier to model as hexagon. In Fig. 2-54(a), the cells are all the same size. They are grouped in units of seven cells. Each letter indicates a group of frequencies.
Fig. 2-54. (a) Frequencies are not reused in adjacent cells.
(b) To add more users, smaler cells can be used.
In an area where the number of users has grown to the point where the system is overloaded, the power is reduced and the overloaded cells are split into smaller ones to permit more frequency reuse (Fig. 2-54(b)).
At the center of each cell, there 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. The base stations are connected to MTSO (Mobile Telephone Switching Office). In larger areas, several MTSOs may be needed, all of which are connected to a second-level MTSO, and so on. The MTSO system is 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 leaves
a cell, its ownership is transferred to the cell getting the strongest signal
from it. If a call is in progress, it will be asked to switch to a new
channel. This process is called handoff and takes about 300 msec. The channel
assignment is done by MTSO.
2.7.5. Channels
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 simplex channels is 30 kHz wide. AMPS uses FDM to separate the channels.
In the 800 MHz band, radio waves travel in straight lines. They are absorbed by trees and plants and bounce off the ground and buildings. This may lead to an echo effects or signal distortion.
The 832 channels are divided into four categories:
Since the same frequencies cannot be reused in nearby cells, the actual number
of voice channels per cell is much smaller than 832, typically about 45.
2.7.6. Call Management
Each mobile phone in AMPS has a 32 bit serial number and 10 digit telephone number in its PROM. When a phone is switched on, it scans a preprogrammed list of 21 control channels to find the most powerful signal. From the control channel, it learns the number of paging and access channels.
The phone then broadcasts its serial number and telephone number several times. When the base station hears the announcement, it tells the MTSO, which records the existence of its new customer and also inform the customer's home MTSO of his current location. During the normal operation, the mobile telephone reregisters about once every 15 minutes.
To make a call, the user enters the number to be called, and hits the send button. The phone sends the number and its own identity on the access channel. When the base station gets the request, it informs the MTSO. The MTSO looks for idle channel for the call. If one is found, the channel number is sent back on the control channel. The mobile phone then automatically switches to the selected voice channels and waits until the called party picks up the phone.
As for incoming calls, all idle phones continuously listen to the paging
channel to detect messages directed at them. When a call is placed to a
mobile phone, a packet is sent to the callee's home MTSO to find out
where it is. A packet is then sent to the base station in its current
cell, which then sends a broadcast on the paging channel of the
form: "Unit 14, are you here?" The called phone then responds with "Yes"
on the control channel. The base then says something like: "Unit 14, call
for you on channel 3." The called phone switches to channel 3 and starts
making ringing sounds.
2.7.7. Security Issues
Analog cellular phones are totally insecure. Anyone with an all-band radio receiver (scanner) can tune in and hear everything going in a cell.
Another major problem is theft of air time, again based on the possibility of monitoring the transmitted information.
Some of these problems could be solved by encryption, but then the police
could not easily perform "wiretaps" on wireless criminals.
2.7.8. Digital Cellular Telephones
First generation cellular systems were analog. The second generation is digital. In Europe, an agreement on a common digital system, call GSM (Global System for Mobile communication) was achieved.
GSM operates in a new frequency band (1.8 GHz) and uses both FDM and TDM. The available spectrum is broken up into 50 200 kHz bands. With each band TDM is used to multiplex multiple users.
Some GSM telephones use smart cards (credit card sized devices containing a
CPU). The serial number and telephone number are contained there, not
in telephone, making for better security. Encryption is also used.
2.7.9. Personal Communication Services
Everyone would like to have a small cordless phone that works around the house and also anywhere in the world. Such a system is currently under vigorous development. In the US it is called PCS (Personal Communication Services), everywhere else it is called PCN (Personal Communication Network).
PCS will use cellar technology, but with microcells, perhaps 50 - 100 m wide. This allows very low power (1/4 watt), which makes it possible to build very small, light phones. On the other hand, it requires many more cells. The small base stations in these cells are sometimes called telepoints.