FUNDAMENTAL CONCEPTS
The Beginning
Wireless communication began only a little later than the wired variety Morse’s telegraph (1837) and
Early wireless system used crude, though often quite powerful, spark gap transmitter, and were suitable only for radio telegraphy. The invention of the triod vacuum tube by De Forest in 1906 allowed for the modulation of the continues wave signal and made voice transmission practical. There is some dispute about exactly who did what first, but it appears likely that Regional Fessenden make the first public broadcast of voice and music in late 1906. Commercial radio broadcast in both US and
Early transmitter was too cumbering some to build on vehicles. two way police radio, with the equipment occupying most of the car trunk, began in the mid-1930’s. Amplitude modulation was used until late 1930’s, when frequency modulation began to displace it.
Soon after the end of World War II AT&T introduce its Improved Mobile Telephone Service (IMTS) in 1946, featuring automatic connection of subscriber to the PSTN. The next year in 1947, the American government set up the Citizen’s band radio service. Initially it used the frequency near 460Mhz. the service was too short and it did not provide connection to the PSTN .then after another humble appearing appliances has came into the picture, the cordless phone. It was having connectivity to PSTN and was convenient to use with a range of few meters inside the house. It uses the frequency range of 46 and 49 MHz.
Elements of wireless communication system
The following figure shows the block diagram of the wireless communication system.
You are all aware of three types of communication system that are
1) simplex
2) half duplex
3) full duplex
The radio frequency spectrum
| Frequency destination | Frequency range | Wavelength range | Wavelength destination |
| Extremely high frequency | 30-300 GHz | 1mm-1cm | Millimeter wave |
| Super high frequency | 3-30 GHz | 1-10 cm | Microwaves |
| Ultra high frequency | 300Mhz-3Ghz | 10cm -1m | |
| Very high frequency | 30-300 MHz | 1-10 m | |
| High frequency | 3-30 MHz | 10-100 m | Short wave |
| Medium frequency | 300 Khz-3 MHz | 100m -1km | Medium wave |
DIGITAL COMMUNICATION
Several good reasons for digital communication are as follows
1) Digital signal can be manipulated more easily than any analog signal. They are easier to multiplex for instance.
2) Digital signal can be encrypted to ensure privacy.
3) Noise efficiency can be greatly reduced to great extent by applying several error control signals or characters.
4) Data compression can be used with a digital signal to reduce bandwidth requirement.
Distinguish between bit rate and baud rate. The bit rate is simply the number of bits transmitted per second, while the baud rate is the number of symbols per seconds. Therefore, if we let the baud rate be S, then:
C=S log2 M
Where
C=capacity in bits per second
S=baud rate in symbols per second
M=number pf possible states per symbols
BASIC TELEPHONY
The telephone system is often used for the same reason
The PSTN is undoubtedly the largest and probably the most important communication system in the world. The reasons for these are contained in those first two words. It is public in the sense that any one can have access to it. Because it is switched it is possible in theory at least to communicate with any one else.
The addition of wireless communication to the PSTN has made it even more useful. Radio systems that were separate entities that are increasingly becoming extensions of the telephone system. The difference between citizens’ band radio and cellular radio. The former is a separate system, useful only when those who want to call compatible radios and are within radio range. The latter can communicate its user to any telephone in the world, regardless of the distance.
Network Topologies
Switched networks can be divided into two circuit switch and packet switch network. In a circuit switch network there is dedicated physical path from transmitter to receiver for duration of communication. The PSTN is a circuit switched network. Packet switched networks routes short burst of data, called packets, from point to point as needed. A virtual network may exist, but it is merely a record of the address on the network between which communication takes place. Successive packets may take different paths through the network. The internet is a packet switched network.
Wireless networks can be either
1) circuit switched
2) packet switched
PSTN structure
Each subscriber is normally connected via a separate twisted pair line called a local loop to a central office, also called an end office, where circuit switching is done.
Local access and transport area
The local loop and its signals
Normally each individual subscriber telephone is connected to the central office by a single pair of twisted pair of wires. The wires are twisted to help cancel their magnetic fields and reduce interference called crosstalk, between circuits in the same cable. It is common practice to run a four wire cable to each residence, nut only two of these wires used for a single line. The others allow for the installation of a second line without running more cables.
Recently, there has been a trend towards running multiplexed digital signal to junction boxes in neighborhoods, switching at that point to analog signals on individual pairs, in an effort to reduce the total amount of copper cable.
The central office supply is called the battery. The voltages does, in fact, derive from a storage battery that is constantly under charge. This allows the telephone system to function during electrical power outages, whether they occur at central office or at subscriber house and has resulted in a well observed reputation for reliability.
Telephone dialing
Dialing can be accomplish din one of the two ways. The ld-fashioned rotary dial functions by breaking the local loop circuit at 10Hz rate, with the number of interruptions equal to the number dialed. That is dialing the number 5 causes five interrupts in the loop current. This technique is called pulse dialing and can be emulated by some electronics telephone. The second and much more efficient way is for the phone to transmit a combination of two tones for each number. This is officially known as dual-tone multi frequency (DTMF) dialing and is commonly referred to as touch tone or just tone dialing. The DTMF frequencies are as in table below
| Frequencies | 1209 | 1336 | 1477 | 1633 |
| 697 | 1 | 2 | 3 | A |
| 770 | 4 | 5 | 6 | B |
| 852 | 7 | 8 | 9 | C |
| 941 | * | 0 | # | D |
The local loop voltages and current
| On hook voltages | 48 v dc |
| Off hook voltages | 5-10 v dc depending on loop resistance |
| Off hook current | 23-80 mA dc depending on loop resistance |
| Dial tone | 350 and 440 MHz |
| Ringing voltages | 100 v ac,20 Hz, superimposed on 48 V dc |
| Ringing voltages | 80 v ac, superimposed on 48 v dc |
| Ring back voltage | 440 and 480 Hz, 2 s on 4 s off |
| Busy signal | 480 Hz and 620 Hz, 0.5 s on 0.5 s off |
Digital local loop
The capacity of a twisted pair is much greater than is required for the analog signal, with a bandwidth under 4 KHz that it usually carries. Most of the cost of the local loop is in the labor to install it, rather than the cost of the wire itself, so when greater bandwidth is required it makes good sense redesign the system to increase the capacity of the existing wiring rather than replace the wire.
ISDN
As the telephone network gradually goes digital, it seems logical to send data directly over telephone lines without modems. If the local loop could be made digital, with the codec installed in the telephone instrument, there is no reason why the 64 kb/s data rate required for PCM voice could not also be used for data, at the user’s discretion. There are two types of channels in this line. One of these channels is the D (data) channel and is used for common signaling, that is, for setting up and monitoring calls. The other 23 channels are called B (bearer) channels and can be used for voice or data or combined, to handle high speed data or digitized video signals, for example. Basic interface users have two 64 kb/s B channels for voice or data, one 16 kb/s D channels, and 48 kb/s for network overhead. Several reasons can be advanced for this. First, converting local loops to digital technology is expensive, and it is questionable whether the results justify the cost for most small users. Residential telephone users would not notice the difference.
Asymmetrical digital subscriber line
The idea behind the asymmetrical digital subscriber line is to use the frequencies above the voice range for high speed data while leaving the use of the local loop for the analog telephone intact. This allows the subscriber to use conventional analog telephones without special adapters, while simultaneously sending and receiving high speed data. The word asymmetrical in the name refers to the fact that the system is designed for the faster communication from the network to the subscriber than from the subscriber to the network. Typical uses for the ADSL include internet access and interactive television and for both of these the subscriber needs to receive data at a faster rate than it needs to be transmitted.
Telephone lines
Sending data require bandwidth. Amount of bandwidth needed is directly related to data rate that desired. An analog voice signal contains its data in a relatively narrow bandwidth, in proportion to amount of data carries. This because an analog signal is an efficient way to encode.
A telephone system as it historical develops is designers for voice signal. Many decisions and design were implemented on that basis. Some of those decisions, retrospect, where not the best for sending digital data. A surprising number however, were forced by unavoidable constraint of the ways answers and signal behaves long distance and the need to make a reliable, rugged phone network. This choice was the correct one and still is correct in many ways. The important point is to understand the true nature of the telephone line and to learn to work with to support digital signal properly.
There are two inherent characteristic of its signal line that are very important carrying signal with minimal distortion or corruption. These are amplitude response versus frequency (called frequency response) and the time delay versus frequency. Various amount of imperfection in this two aspect can cause widely diffe5ent amount distortions and problems in signal if they are received being transmitted down a wire.
Frequency shows how each frequency component of signals far as it passes through wire and system. Any wire or cable is electrically is equivalent in many ways to a filter circuit. A filter has property of being able to pass frequency from it’s input to out put side4, but some frequency passed with more or less attention then others. This is the purpose of the filter. The frequency response is actually used to characterize to virtual which must pass signals. Recall from the study of the Fourier analysis concept that the shape of a signal is determined by frequency by it has and their amount. A digital signal with sharp corner needs significant high frequency to provide those corners. If the high frequency components are attended there is the rounding of the digital signal. Similarly, the low and medium frequency contribute to the correct shape of the signal. If this frequency component are attended different distortion results. The out put wave form does not look like original wave form.
Attributing delay characteristics any electrical has the effect of changing resultant amplitude versus frequency, but also introduce some time delay in the various frequency component of the signal. Time delay phase shift is not same for each frequency, however. Depending upon the exact nature of filter or wire, there is some combination of frequency response and phase shift. The effect of this phase shift is similar frequency response characteristic. As furrier analysis the time and frequency domain are linked together any change in one also change in other.
The distortion caused in variation in frequency response in time delay and make the signal as receive differ from that transmitted signal. The amount of difference depends on exact amount of frequency and time that occurs. For voice signal relatively large amount of distortion is acceptable. Since the human ear can understand voice even with the distortion that looks severe to the eyes. For digital signal, this distortion may cause receiver to misinterpret that so send and produce error.
Standard telephone lines
The regular telephone loop from the local office to phone is guaranteed by Phone Company to some specific characteristic this type of line is lowest performance line called the voice grade conditioning. Figure shows frequency and time delay characteristic of this type line.
The least attention and time delay occur 1.5 to 1.8 kHz. At one kHz the signal component are about to 2 to 3 db and the frequency 3 kHz is attended about 16 db. The time delay at this point is one ms and more than two ms respectively.
Similarly large characteristic are offered by Telephone Company on lines that go between phone company offices. This inters office are called trunks. Any phone interconnection that goes 1 three digit prefix (called the exchange) to another require such a trunk, since each local office serve only one prefix. The exchange within smaller region all has the same area code different areas code is used to connect different region. A system user must be consulted about the signal quality from one end to other, not only local exchange. To meet the inter office trunk performance is also carefully defined.
Because any phone line can connect one user to another user through phone system, the user has a line assigned randomly, through the phone office this is called dialed up or switch network. Any phone can be used to dial up the other end, and the phone company can switch in any suitable line to complete the connection. Lower performance application can use the switch system with acceptable error. Certainly the public dial up; lines are very convenient and easy access.
For application which need greater performance than this dial up line can offers special condition lines? This line both from phone to office and between phone office- provide better frequency and time delay characteristic. This kind of condition is leased by the user, who pays more for it than for a standard line. The term “dedicated” and “leased” are used when Phone Company has send aside a condition line for communication link. Different grades of leased line with each grade of increasing performance better condition usually available. This higher grade can support higher data rate as high as RS232 allows or even 50 to 100 kbits /sec at a much higher price.
Any receiver circuitry that use a telephone network must also be design to cope with signal that arrive with much lower amplitude the they had at transmitter regard less of frequency. A long length of phone links can cause drop in signal amplitude and wire resistance. Some of this law can be corresponded for by amplifier along the way, from one office to another. The final loop to phone is very difficult to manage. The phone may be few yards from exchange or several miles. If the phone company boosted the signal at the office, a separate amplifier end adjustment readjusted whenever phone number is reassigned o changed and this would create a maintenance problem. The simply boosted signal from all phones would not work. Since the phones closed would too much signal and be overloaded. Also simple amplification would boost any noise that would boost any noise that had been picked up along way and matter works.
A result is that actual signal level the receiver must accommodate very considerably, and receiver must error free performance in all this cases. How much attention occurs? Figure shows the amount. The figure indicate the three distant range the percentage of calls with received attention less then the number of x-axis. For example e132-360mi link. There is attention fo5 20 db but not for the 60 percent of the calls. The other 40 percent on average suffer an attention of more than 20 db. Similar number can be obtaining from the graph for the other cases.
Any designer for system for communication over telephone line has a performance goal that defined what percentage of interconnection should be successful. A typical goal makes sure that circuitry work for 90 percent of long distance connection. Using the chart, the designer can see circuitry must accommodate attention value for top 90 percent of user, and not for lowest 10 percent. This 10 percent receive signal with attention with 25 degree, so designer design for only for attention 0 to -25 db and ignore that below -25 degree.
Telephone echoes
This is another characteristic of telephone circuit that can cause problem with reliable, error free communication. This is called echo. Echo occurs as result of unavoidable irregularity in the imperfection in the signal path which causes impedance bump. A signal part of signal coming down the wire reflect back from the bone is a wave running into through as a small back wave if through has sudden narrowing neck. The transmitted wave which the intended receiving end a small part as energy reflect back to transmitter. This is called talker echo. It normally does not create problem for data because the not transmitted circuit is set up to look for signal transverse direction. It is important to note that talker echo may be very enjoying to person using, system to talk, if the echo time is greater than few, millisecond.
When the talker echo reflect back and reach another irregular line it then produce second small echo back to intended receiver. This second echo is listener echo. It look as original signal but lower in amplitude and delay in time. The intended listener may confuse with the original signal.
The most common location of this echo causing irregularity in the part is at each end of the link. The hybrid circuitry to convert 2 to 4 wires conversion can not always perfectly match to the line. The echo distant therefore usually entity signals length minus local loop length.
For short distance communication echo time is usually small enough that does not cause problem in addition the path is essentially, same is timer short distant connection is made so that echo can be turned out by some special circuitry. For long distance this echo time can be large enough to cause signal confusion and error. Consider a signal from one coast to US to the other. The distance is 3000 miles the signal propagates at speed 70% of speed of light.
3000mi * 2 trips
_______________ =0.023s
186,000mi/s * 70 percent
The long distance path may not be same each time a connection is completed, however. The network to Los Angles may be cabled to shortest path (a middle of US). It can be enabled from
Experience has shown that whenever echo time is greater than 45 ms echo can be a problem. Several solutions are used in communication. One solution is to solve a scheme to suppress the echo by employing circuitry which cut the signal transmission path from listener to talker is echo suppression is effective but make effective but make full duplex communication difficult, since the data signal is allowed to flow in only one direction at time. The user id restricted to half duplex which is completely adequate for much application. The real draw back that the circuitry tools which the echo path out can not be switch instantaneously on and off. It takes about 100 ms to echo suppression a instruction to turn on or off and implement it. This 100 ms period time wasted in communication system. If the half duplex operation involves frequent change, a large percentage of time available is lost.
Another solution is to use echo cancellation, made possible by sophisticated IC’s. in echo cancellation a portion of the original signal taken delay and subtracted echo signal itself. It is done perfectly, the echo is done perfectly. The echo circuitry must contain must adjust both time delay and amplitude of this opposed signal so that is exactly equal and opposite to undecided echo. It is possible to do a very good job of this with resent circuitry.
TRANSMISSISON LINES AND WAVE GUIDES
Electromagnetic waves
Radio waves are one form of electromagnetic radiation. Other forms include infrared, visible light, ultraviolet, x-radiation, and gamma rays. Electromagnetic radiations as the name implies involves the creation of electric and magnetic fields in free space or in some physical medium. When the waves propagate in a transmission line consisting of a pair of conductors, the fields can be represented as voltage and currents.
Dielectric tubes and cables
When high power is used it is important to keep the interior of the line dry, so these lines are sometimes pressurized with nitrogen to keep out moisture. Coaxial cables are referred to as unbalanced lines because of their lack of symmetry with respect to ground.
Waveguides
Tuning screw
The type of lines called waveguides; consist of a hollow air filled tube made of conducting material. Rectangular waveguides of brass or aluminum sometimes silver plated on the inside are most common but elliptical and circular cross sectioned are also used. Shorted stubs of adjustable length can be used, but a simpler method is to add capacitance of inductance by inserting a tunnning screw into the guide, screw is inserted farther into the guide; the effect is first capacitance and then series resonant, and finally inductive.
Flexible waveguide
RADIO PROPOGATION
1) free space propagation
2) terrestrial propagation
3) satellite propagation
Free Space propagation
The simplest source of electromagnetic waves would be a point in space. Waves would radiate equally from this source in all directions. A surface on which all the waves have the same phase, would be surface of sphere.
Terrestrial propagation
At low to medium frequency radio wave can follow the curvature of earth, a phenomenon known as ground wave propagation, and in the high frequency range the waves may be returned to earth from an ionized region in the atmosphere called the ionosphere.
The line-of-sight propagation
In spite of title of this section, the maximum distance is actually greater than the eye can see because refraction in the atmosphere tends to bend radio waves slightly towards the earth. The dielectric constant of air usually decreases with increasing height, because of the reduction in pressure, temperature, and humidity with increasing distance from the earth. The effect varies with whether conditions, but is usually results in radio communication being possible over a distance approximately one-third greater than the visual line of sight.
Multipath prorogation
When the surface is highly reflective, the reduction in signal strength can be 20 db or more. This effect is called fading. Generally this type of propagation is usually used whenever there ever availability of reflecting surface like water is there. The multipath propagation will have more than one line of sight to send signals.
Repeater and cellular system
Communication directly between mobile or portable users therefore has limited range and is subject top a great deal of multipath and shadowing even at close range. A good example of such a system is citizens’ band radio. The base system in this system is called a repeater, and must transmit and receive simultaneously on at least two frequencies. Normally, the same antennae is used for both transmitting and receiving, and a high Q-filter is called a duplexer, using resonant cavities is used to separate the transmit and receive frequencies. Since, each transmitter operates at low power, it is possible to reuse frequencies over a relatively short distance. Typically a repeating pattern of either 12 or 7 cell is used, and the available bandwidth is divided among this cells. The frequencies can be reused in the next pattern.
Since, cell sites are expensive; however, the seven cell pattern is more economical to build. As the number of user’s increases, cell sites can be made smaller by installing more cell sites, and frequencies can be reused over close intervals. This technique is called cell splitting and gives cellular system great flexibility. To adopt to change in demand over both space and time.
Here is how cell splitting works the number of cell required for a given area is given by
N = A / a
Where
N=number of cell
A= total area to be covered
a=area of one cell
(In practice cell will not be perfectly hexagonal)
Many callas from to moving vehicles must be transferred ands handed off from one cell site to another as the vehicle proceeds. This require a change in frequency, since, frequency are not reused in adjacent site. The system has to instruct a mobile unit to change frequency.
Satellite propagation
Many communication satellites used the geosynchronous orbit that is they occupy a circular orbit above the earth at a distance of 3578km above the earth surface. At this height the satellite orbit period is equal to time taken by earth to rotate once. Approximately 24 hours. The direction of satellite motion same as that of the earth rotation the satellite appears to remain almost stationary above one spot on the earth surface. The satellite can then be said to be geo stationary.
Satellites that are not geostationary are called orbit satellite. Though of course all satellite are in orbit. They are further categorized by height of the orbit about the earth. Low earth orbit below above 1500 km while medium earth orbit systems are from 10000 to 15000 km in altitude.
ANTENNAES
Types of different antennas
1) simple antennas
2) folded dipole
3) monopole antennae
4) five eight wavelength antennas
5) Discone antennae
6) Helical antennae
7) slot antennae
8) horn antennae
9) patch antennae
Reflectors
A reflector may consist of one or more planes or parabolic in shape. In order to reduce wind and snow loads reflectors are often constructed with mesh or closely spaced rods as long as spacing is small compared with wavelength. The effect on antennae pattern compared with a solid reflector is negligible.
CELLULAR RADIO
Citizens’ band radio
Improved mobile telephone service
IMTS is trunked system; that is radio channels are assigned by the system to mobile users as needed, rather than having one channel or pair of channel, permanently associated with each user, narrowband FM technology is used. Two frequency ranges at about 150 and 450 MHz were used for IMTS, with an earlier system called MTS operating at around 40 MHz.
IMTS is capable of full duplex operation using two channels per telephone calls. Direct dialing is also possible, so using a mobile phone is almost as simple as using an ordinary telephone at home.
Advanced Mobile Phone System (AMPS)
Instead of one repeater there are many located in a gird pattern. Each repeater is responsible for coverage in a small cell. As shown the cell are hexagons. But of course in a real situation the antenna patterns will not achieve this precision the cell are more likely to be approximately circular, with some overlap. All the cell sites in a region are connected by copper cable fiber optics or microwave link to central office called a mobile switching center or mob9ile telephone switching office and mobile switching centers (MSC), and themselves interconnected so that the system can keep track of its mobile phones. The cellular system is connected at a point of presence to wireless to the network so the cellular customer can speak to wire line customers.
Note that there is no provision for direct for mobile to mobile radio communication. Even if two cell phones are in the same room, a call from one to the other must go through a cell site and an MSC provided both portable phones are connected to the same network there would be no need to go through the PSTN.
Channel allocation:
In the early day of cell phones it often took some time a minute or more to find mobile by it improved communication within the system has reduced the time a to a few seconds in most cases. The phone is instructed to ring and once it is answered the system assigns it via voice channel. After that the system follows the phone as it moves from one cell to the next, as explained earlier.
AMPS control system
The mobile identification number is stored in the number assignment module in the phone. The
The other identification number is as electronic serial number (ENS) which is a unique 32 bit number assigned to the phone at the factory. It is not supposed to changeable without rendering the phone inoperable but in practice it is often stored in ands EPROM that can b ere programmed or replaced by persons with the right equipment and not knowledge.
Originating a call
When the user of a mobile phone keys in a phone numbs and present send. The mobile init transmit unit an origination message on the reverse control channel.. this message in clues the mobile units MIN and ESN and the nimbler it is calling./ the cell site passes the information onto the mobile smirching center for processing.
Once authorization is complete the cell site sends a message to the mobile on the forward control channel, telling it which vice chinned to use for the call. It also sends the digital color code which identifies the cell site and a control mobile attenuation code (CMAC).
Cell site equipment
The combination of the mobile cellular phone and cellular radio equipment is known as the air interface. There is much more to cellular telephone than radio, however. The administration included keeping track of phone location billing setting up and handing off call and so on. The substantial computing g resources required to do all this are at least in part responsible for the delay in the introduction of cellular telephone for many years after the idea was first proposed.
Fax and data communication using cellular phones
Many modem cards for notebook and laptop computers will work with cell phones. Cellular modems are advertised as having speed are usually 9600 b/s or less. Performance is improved by operation from a stationary vehicle as this eliminates handoffs and reduces fading
An error correcting protocol called MNP 10 is usually with cellular connection. It must be used at both ends of the connection. MNP10 in corporate some special cellular enhancements.
Facsimile transmission is also possible with cell phone. A fax modem and notebook computer cangue used, or a conventional fax machine can be used with especial adapter. The adapter allows the cell phone to simulate a conventional two
PERSONAL COMMUNICATION SYSYTEMS
Broad band communication system
The term broadband here is relative. It is refers to bandwidth sufficient for voice communication and distinguishes this service form such narrowband services as paging, which will be discussed alerter in this book. Sometimes the term broadband communication is used to refer to video and sigh speed data is not sense in which it is used here.
Broadband PCS band plan
| Allocation | Forward channel or downlink | Reverse channel or uplink |
| A | 1850-1865 | 1930-1945 |
| B | 1870-1885 | 1950-1965 |
| C | 1895-1910 | 1975-1990 |
| D | 1865-1870 | 1945-1950 |
| E | 1885-1890 | 1965-1970 |
| F | 1890-1895 | 1970-1975 |
Many of the major wireless manufactures included Ericsson, Nokia and Motorola have now combined to create a set of de facto standards for creating this content know a the wireless application protocol. The idea is to included a small program called a micro browser in the wireless device with most of the required computing done on network server.
SETELLITE BASED WIRELESS SYSTEM:
Orbital calculation
Satellites are held in orbit by their momentum,. Gravity continually bends a satellites path toward the earth,. But the satellite momentum is sufficient to prevent is trajectory form reaching the earth. This phenomenon is commonly called centrifugal force, through technically there is no such thing.
Any satellite orbit in the earth must satisfy this equation.
V=SQRT (4 * 1011 / d * 6400 )
Where
V= velocity in meter/sec
D= distance above earth surface
Hence farther the satellite is from the earth the slower it travels. The propagation delay is about 0.25 s for a round trip to a geostationary satellite. This delay while not fatal is annoying in real time conversation. This cause time delay in data transmission whenever protocol require acknowledgement the receiving station transmission can continue.
PAGING AND WIRELESS DATA NETWORKING
One way paging
The fact that all pages are sent from alltranmitters means that there is no need for the system to know the location of nay pager, in fact since the pager are receive only devices with no transmission capability there is o wary for the system to tell where and individual pager is or when whether it is turned on.
Voice paging
Voice paging has been available for many years byte was not widely used until recently. It allows on a pager to function in a manner similar to a telephone answering machines.
WIRELESS LOCAL
Wireless local lop using radio transmission in the VHF and UHF bands have been employed in isolated inn many area for some time because they can transmit signal for several kilometers without the need for testing call until recently however this was definitely a niche marker.
TO DOWN LOAD REPORT AND PPT
