ch13_cyganski

7/27/2019 ch13_cyganski

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Chapter 13

Cyganski Book

Monica Stoica, [email protected]


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The Original (Analog) Telephone System

The original telephone system was entirely analog and

substantial pieces remain analog to this day.

the components out of which a working telephone system

can be constructed:

1.Microphone: a small amount of carbon granules, a

container for the granules, a diaphragm making up one sideof the container, and two metal contacts.

The action of sound waves on the diaphragm alternately

compresses and relaxes pressure on the granules, varyingtheir electrical resistance in synchronism with the sound

waves. Hence, a mechanical quantity (sound or air

pressure variations) is converted into an electrical

quantity (resistance).


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Receiver and Transmission System Receiver: a permanent magnet and a coil of wire

attached to a paper diaphragm. When an electricalcurrent passes through the coil, a magnetic fieldresults, which interacts with the permanent magnetfield to cause the diaphragm to move. If the electriccurrent varies at the same speed as the soundpressure waves for voice, the diaphragm movesat that same speed, and produces new air

pressure variations also at the same speed, andhence with the same sound. This describes theoperation of all loudspeakers.

Transmission System : two lengths of wire and a

flashlight battery.


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The Switching System The wires, the battery, the microphone, and the receiver are

connected into an electrical circuitso that the currentcaused by the battery varies due to the variation inresistance of the microphone in response to sound waves.The receiver (loudspeaker) moves in synchronism with the

electrical current and hence produces new sound waves thatmatch the original sound waves.

Switching System: the system above is a working telephone(actually one direction of a telephone) but it does not permit

the transmitter and receiver to change. The switchingsystem breaks the electrical wires from one end andconnects them to the desired telephone at the other end. Thisconnection happens with many intervening switches asvarious point-point transmission systems are connectedtogether to reach between the two desired telephones.


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Analog and Digital Phone Connections

it is a fact that most telephone calls today are really digitaltelephone calls. How can this be? It is quite simple: the twoends of the call are analog, and the middle section is digital.Conversions from analog to digital, and back to analog, aremade in such a way that it is essentially impossible todetermine that they were made at all.

At present, most telephone calls are analog from thetelephone in the home to the first telephone switchingoffice. In areas of moderate or greater population density,most telephones are within about five miles of the telephone

central office. At the central office, most incomingtelephone lines are connected to equipment that converts theincoming voice to digital (A/D conversion) and converts theoutgoing (to the telephone set) voice to analog (D/A

conversion).


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T1 If the telephone call needs to be routed from one

central office to another (across town or across theworld) the call is combined (using time divisionmultiplexing) with many other calls for efficiency.

The smallest unit of channel combination(in theU.S.) is 24 channels, which corresponds to a datarate of 1.544 Mbits/second. This is the so-called T1rate, which has become well known.

Actually, some bits are ``stolen'' from the voice dataso that synchronization bits may be included in the1.544 Mbits/sec rate. This is also referred to as theDS1 rate in the hierarchy of digital transmission.


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All digital one day At present essentially all of the transmission facilities

among telephone central offices are digital. One of themajor advantages of digital transmission is that afterdigitization one signal is exactly like another: they are all

just bits. Hence T1 or other digital transmission facilities

may be used to carry telephone calls, Internet data, or anyother data that will fit in the bit rate.

The nature of the digital revolution appears to be toconstantly expand the realm of the digital signal, replacing

more and more cases where analog signal processing ortransmission has been done. Within the first years of thenew millennium, most telephones will become purelydigital, with A/D and D/A conversion being done within thetelephone set to accommodate the analog beings (humans)who are using the telephone.


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Radio- telephone ``Radio-telephones" have existed for over 60 years, but until

the invention of the ``cellular radio system," the number of

users in a given area such as a city was severely limited (onthe order of only a few hundred users!). There are severalfactors that created this limit:

The radio spectrum is limited in size (frequency range), and

hence in the number of telephone signals that can be activeat any given time.

The ultimate upper limit of the spectrum, and hence thenumber of telephone channels, is determined by physical

laws. For example, as the frequency becomes very high, thesignals can no longer pass through heavy rain.

Thepracticalupper limit of the spectrum is affected bycurrent technology. Until recently, the electronic equipmentfor very high frequencies was quite expensive.


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Cellular Calls First the user turns on his or her phone. After a few

seconds the phone generally indicates that it is inservice. If no cell site is within range, the phone

indicates ``no service." What happens in that time is

that the phone has automatically communicated with

at least one cell site base station to confirm that

communication is possible, and (very importantly)

to let the telephone system know where the cell

phone is now located.

If more than one cell site is within range, the one

with the strongest signal is selected, and the control

system directs the other cell site(s) to ignore the call.


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Cell calls

When the user enters a number on the cell phoneand presses ``send," a channel is dedicated to thatuser, and then a number is processed at the cell siteand sent into the regular telephone network (calledthe Public Switched Telephone Network, or PSTN).

Assuming the called number is a wired telephone,the call is completed in the normal manner. If it is

another cell phone, the cellular system control centeris queried to determine whether the called cell phoneis in service, and if so, what cell site (nearby oraround the world) it is currently accessing.


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What happens if either (or both) cell phones in a

conversation move from one cell site to another?

The call must be``handed off" from one site to another

without losing the connection.This (usually) works

correctly, and the user may notice (with traditional analog

cell phones) that the connection first becomes noisy, and

then becomes clear again when the transfer is made. This hand-off is possible because each cell site

continuously monitors all the cell phone signals it hears,

even if that site is not handling the call. As the signal

becomes weaker at the active site, the central control unitsearches for another site that is receiving that phone's signal

with more power.

Upon locating such a site, the controller makes the hand-off.

h b i


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What about ``roaming?" This mode reflects a combination of non ideal technical

design of the cell system, and the competitive nature of

telecommunications! When your cell phone indicates that it is in ``roam" mode, it

means that you are not within range of the cell system towhich you subscribe (to which you pay your monthly bill)

but you are within range of another system.Your callswill go through with no problem, but you may be chargedextra for using that different system.

In the past, a greater problem was in receiving calls in this

mode. When you were off the home system, there was oftenno way to know where the cell phone was located, andhence no way to connect an incoming call. This problem hasessentially been eliminated at present, with better real-timecommunication of cell phone status and location amongsystems.

Th Al h b t S f C ti C ll l S t


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The Alphabet Soup of Competing Cellular Systems:

AMPS, GSM, TDMA, CDMA, and PCS

The cell phone concept originated in 1947, but

commercial service in the United States did notbegin until 1979. Shortly thereafter, the originalsystem design was improved, and this design (still inwide use today) was referred to as AMPS(Advanced Mobile Phone System).

Today, the ``A" is often defined as referring to``Analog" because this is in fact an analog system,

and the newer systems are all digital. AMPS was the only system in the United States

until about 1997, and as of 2000 is still in commonuse in the U.S.


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AMPS

The voice transmission part of the AMPS system iscompletely conventional, essentially the same ascould be used in any ``walkie talkie."

AMPS uses the 800 MHz frequency band.It was thecellular system design and the overall controlfunctions that were technically novel.

The analog format made cell phone conversationalmost completely non private. Anyone with asimple scanning receiver could listen to theconversations.


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GSM The next major cellular system to be developed, and the one

with the greatest worldwide use, is referred to as GSM.

Originally this stood for the name (in French) of thecommittee that approved the system design, Groupe Special

Mobile . This was originally a European standard, but hasspread worldwide (including the U.S.) and the initials have

been redefined to represent Global System for MobileCommunications .

This is a digital system, meaning that the voices aredigitized and processed to minimize the bit rate before

transmission. The digital signals are transmitted over similar RF channels

as in the analog case, in the 900 and 1800 MHz bands.

The frequencies for AMPS and GSM are different so that

both systems may operate simultaneously in a given area.


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AMPS and GSM differences the means by which individual calls are kept

separate during radio transmission are different.

In AMPS, each user is simply assigned anindividual frequency (actually two frequencies, onefor each direction of voice transmission) for the

duration of the call. This is exactly the same as themanner in which individual radio stations areseparated in the frequency spectrum and on the radiodial. This is calledfrequency division multipleaccess or FDMA. It is conceptually simple, but hassome technical drawbacks.

An alternative used by GSM is called time division

multiple access or TDMA.


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TDMA

In this scheme the radio spectrum is not divided into

channels for each user.

Rather, each user occupies the entire radio spectrum, but

only for a brief time. After one user transmits a burst of

information, that user is quiet for a time and another usertransmits a burst. This continues for all the users until it is

time for the first user to transmit again.

Because speech is continuous, this system obviously

requires a means to store the information for each userduring the periods in which that user cannot transmit. With

digitized voice data, this storage is quite easy.

TDMA lends itself naturally to digital signal processing.


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CDMA The final basic type of common cellular system is called

CDMA, referring to its channel separation scheme, which

is Code Division Multiple Access rather than FDMA orTDMA.

In CDMA each user occupies the entire radio channel as inTDMA, but the user also transmits all the time, as in

FDMA. In other words, the users are not separated ineither time or frequency.

What does keep the users separate? Each user is assigned aunique digital code (the ``Code" in CDMA) which is used to

encode the data from the voice digitized beforetransmission.At the receiver the same code is used to decodethe incoming signal, and the result contains two terms: theoriginal voice coder data bits, and a (hopefully) small

amount of interference from the other users with different

CDMA ti d


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CDMA continued In FDMA the number of available radio channels

determines the number of simultaneous users. Similarly, in

TDMA the number of available time slots determines thenumber of simultaneous users. In CDMA the maximumnumber of users is determined by the amount ofinterference that can be tolerated (the total interferenceis the sum of the interference contributions from all theother users).

In practice this number of users is somewhat greater thanwould be the case with FDMA or TDMA on a given pieceof radio spectrum. This is the fundamental advantage ofCDMA; the principal disadvantage is greater systemcomplexity. There is also considerable controversy over justhow great the extra capacity is. Claims of a channel gain onthe order of a factor of 10 have been made, but in actual use

the increase appears to be somewhat less than a factor of 2.


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Sprint PCS Personal Communications Services, and it was intended to

encompass an overall vision for telephony as distinctionsamong wired service, cellular service, and pagingdisappeared.

For example, a person might have a small handset which he

or she always carried, and a telephone number associatedwith the person rather than with a conventional telephone.

The telephone system always keeps track of the person'slocation for call delivery. In the home or office, the handset

operates as a cordless phone working inside buildings, andnot taking up expensive cellular bandwidth. Outside itoperates as cell phone. At all times it also incorporates

paging functions. It may also work in planes and trains in amicrocellularmode.


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Generations of cell phones The various digital cellular systems described above

represent thesecond generation of cellular service. Analog

systems were the first generation, and these first generationsystems are still in widespread use, and will remain so forsome years. The digital services are superior to analog inessentially all ways, and so over time analog will disappear.

This change of generations is facilitated by the availabilityof dual-mode cell phones, which can operate on twosystems, such as analog AMPS and digital-GSM. From theservice provider's point of view, greater capacity represents

the major benefit of the second generation digital systems. From the users' point of view, along with better audio

quality, the second generation systems add some featuressuch as Caller ID and integrated paging.


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Future?

There is still substantial room for improvement,

however, and that is where ``third generation

cellular" comes in. Desired features include higher(much higher) data rates for video, Internet access,

Web browsing, complete worldwide operability,

and usability inside aircraft and buildings.

Widespread introduction of third-generationsystems is expected to begin by 2005.


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Facts about cell phones

Why are base station cellular antennas so ugly?The antenna on a hand held cell phone is a simplerod, about 6 inches long. Base station antennas

could be as simple and unobtrusive as this, buttechnically they work better if they are arranged ingroups of three so that each antenna transmits to onethird (120 degrees) of the complete cell. Eachantenna must be physically separate from the otherantennas. Hence we see rather complicatedarrangements of equipment on top of most cellular

towers.

Wh t hi h t hil


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Why are some antennas on high towers, while

some are fairly low to the ground?

This relates to the desired size of the cell. As you

drive along the Interstate highways in the midwestof the United States, you will see occasional hightowers with cellular antennas on top. These serve

large (long and narrow) cells along the highway,that may be 20 miles or more in size.

Conversely, in cities the cells must be small tohandle the large number of users, and it is desired

to keep the radio energy from propagating outsidethose cells. Because the energy travels only instraight lines, keeping the antennas low

accomplishes this goal.


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Cordless Phones?

What is the difference between a cell phone and a

cordess phone?

A cordess phone is more properly called a``cordess handset" because it must be connected to

regular telephone service. The cordless handset

must stay within range of its base station, which in

turn is connected to the wired network.

The cordless phone has no capability to travel

from one base station to another.


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FCC Why is it illegal to use a cell phone in an airplane?

There are two answers to this question: First, during critical phases of flight, the use of any

devices that can emit radio energy is not permittedbecause of possible (highly unlikely) disturbances to

the aircraft control and navigation systems. Specifically for cell phones, the problem is that from

a high altitude the signal would be received by many

cell sites, potentially causing confusion, andcertainly tying up channels on unneeded sites.

Of course, similar problems can occur from tallbuildings or mountains, but the FCC has not found it

practical to regulate these uses!


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Safe to use? There is a potential concern whenever radio frequency

energy is absorbed by humans. The concern increases as

frequency increases. As we reach X-ray (so-called ionizing)frequencies the danger is quite serious.

However, cell phone frequencies are well below theionizing range, and the limits are stated in terms of how

much heating of tissue the energy creates. Hand-held cell phones (and all other cell phone equipment

such as base stations and car-mounted phones) meet thislimit.

distance from the antenna is the most significant factor, withany risk falling off rapidly with distance. Hence, any

possible concern relates to the users of hand-held cellphones (because the antenna is within inches of the brain),

notto cellular base stations in the neighborhood.


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No Satellite Phones What about those situations where there is no wired

telephone, and not even a cellular system is within reach?

A good example is on a large ship or small boat out at sea.

Not too many years ago, the only alternative would be use

of some sort of two-way radio system.

In fact, since the time of the Titanic, and continuing until

1999, all commercial sea-going vessels were required to

have a licensed radio operator on board, who could

communicate in Morse code as well as voice. A skilledoperator (!) was required because the type of radio that was

used was very different in its operation from a telephone

system.


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Radio transmission The system was called ``HF'' because it used so-called

``high frequencies.''

These frequencies lie between the AM and FM radiobands(between about 1 MHz and 30 MHz in fact), and arenot high at all by today's standards.

In the days before satellites, however, these frequencies hadone important, and unique property: under the correctconditions they can travel all the way around the world,and hence can support communications between any

two points on earth. At any given time a few frequencies would perform much

better than any others for communications between thedesired points. The selection of the proper frequency was

one of the reasons for requiring a trained operator.


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Radio HF This long-distance communication was possible because

with the proper frequency, the radio energy would not just

travel in straight line (out into space) but would reflect offof the ionosphere,which envelopes the earth above theatmosphere.

This reflection would enable some of the energy to travel

beyond the horizon of the transmitter, and the energy mightreflect off the surface of the earth and head back for theionosphere.

Several of these reflections may occur if conditions are just

right, resulting in around-the-world propagation. It isinteresting to note that this ionospheric reflection isanalogous to the total internal reflection that occurs insideoptical fibers, with dimensions many orders of magnitude

different.

Satellite Phones


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Satellite Phones The ionospheric conditions described above are constantly

changing, requiring constant returning of the radiotransmitter and receiver, and may permit only poor-qualitycommunications.

All of this changed with the advent of the communicationssatellite. Now a ship at sea is as easily reachable as any

point on land.

For some years a corporation called INMARSAT (forInternational Marine Satellite) has provided satellite radiocommunications for ships at sea, and somewhat as a sideline

has made their facilities available to other users,typicallythose in very remote areas. This system has twodisadvantages: the ground terminals are rather bulky (bytoday's standards) and service is expensive. The smallestavailable terminal is the size of a briefcase, and it requiresthat an antenna be set up and aimed at the satellite.

M t l


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Motorola Taking this system to the next step is theIridium system,

which was conceived by Motorola Corporation.

This system was planned to consist of a constellation of 66

satellites in low earth orbit, 780 km high.

The low orbit was selected (rather than high-altitude

geosynchronous orbit) to reduce the power required to reachfrom handset to satellite.

This reduces the battery power required, as well as the

antenna size, and makes a hand-held satellite telephone

possible (though the handset is substantially larger than

today's terrestrial cell phones).


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Iridium The Iridium system would operate in a manner quite similar

to that of terrestrial cellular systems, the difference being

that the cell sites are overhead, and are moving!

The voice signals are digitized in a manner similar to that

used in the GSM cellular system. Frequencies of about 1.6

GHz are used between the cellular telephone and thesatellites, and frequencies of 20 to 30 GHz are used between

satellites, and between satellites and ground stations.

These latter frequencies are very high, and suffer rain

attenuation, but this can be compensated by extra power

because these frequencies are not used to the hand sets.


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Iridium

The Iridium system began operation in 1999, andrepresented the first generally available global telephone

system.

It had two significant drawbacks: the relatively high cost of

service and the fact that it was intended for analog voice,

not data transmission.

After operating commercially for about a year and attracting

few customers, the Iridium system went into bankruptcy andceased operations early in 2000. With the service's high cost

and inability to handle data transmission, it could not attract

a viable customer base in competition with the rapidly-

spreading cellular systems across the globe.


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