Chapter 13

Cyganski Book

Monica Stoica, smonica@cs.bu.edu

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 side of the container, and two metal contacts.

• The action of sound waves on the diaphragm alternately compresses and relaxes pressure on the granules, varying their electrical resistance in synchronism with the sound waves. Hence, a mechanical quantity (sound or air pressure variations) is converted into an electrical quantity (resistance).  

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

attached to a paper diaphragm. When an electrical current passes through the coil, a magnetic field results, which interacts with the permanent magnet field to cause the diaphragm to move. If the electric current varies at the same speed as the sound pressure waves for voice, the diaphragm moves at that same speed, and produces new air pressure variations also at the same speed, and hence with the same sound. This describes the operation of all loudspeakers.  

• Transmission System : two lengths of wire and a flashlight battery.

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

connected into an electrical circuit so that the current caused by the battery varies due to the variation in resistance of the microphone in response to sound waves. The receiver (loudspeaker) moves in synchronism with the electrical current and hence produces new sound waves that match 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 switching system breaks the electrical wires from one end and connects them to the desired telephone at the other end. This connection happens with many intervening switches as various point-point transmission systems are connected together to reach between the two desired telephones.  

Analog and Digital Phone Connections

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

• At present, most telephone calls are analog from the telephone in the home to the first telephone switching office. 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 incoming telephone lines are connected to equipment that converts the incoming voice to digital (A/D conversion) and converts the outgoing (to the telephone set) voice to analog (D/A conversion).

T1• If the telephone call needs to be routed from one

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

• The smallest unit of channel combination(in the U.S.) is 24 channels, which corresponds to a data rate of 1.544 Mbits/second. This is the so-called T1 rate, which has become well known.

• Actually, some bits are ``stolen'' from the voice data so that synchronization bits may be included in the 1.544 Mbits/sec rate. This is also referred to as the DS1 rate in the hierarchy of digital transmission.

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

among telephone central offices are digital. One of the major advantages of digital transmission is that after digitization 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 any other data that will fit in the bit rate.

• The nature of the digital revolution appears to be to constantly expand the realm of the digital signal, replacing more and more cases where analog signal processing or transmission has been done. Within the first years of the new millennium, most telephones will become purely digital, with A/D and D/A conversion being done within the telephone set to accommodate the analog beings (humans) who are using the telephone.

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 (on the order of only a few hundred users!). There are several factors that created this limit:

• The radio spectrum is limited in size (frequency range), and hence in the number of telephone signals that can be active at any given time.

• The ultimate upper limit of the spectrum, and hence the number of telephone channels, is determined by physical laws. For example, as the frequency becomes very high, the signals can no longer pass through heavy rain.

• The practical upper limit of the spectrum is affected by current technology. Until recently, the electronic equipment for very high frequencies was quite expensive.

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

seconds the phone generally indicates that it is in service. 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.

Cell calls• When the user enters a number on the cell phone

and presses ``send," a channel is dedicated to that user, and then a number is processed at the cell site and sent into the regular telephone network (called the 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 center is queried to determine whether the called cell phone is in service, and if so, what cell site (nearby or around the world) it is currently accessing.

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 unit searches for another site that is receiving that phone's signal with more power.

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

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 to which you subscribe (to which you pay your monthly bill) but you are within range of another system.Your calls will go through with no problem, but you may be charged extra 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 often no way to know where the cell phone was located, and hence no way to connect an incoming call. This problem has essentially been eliminated at present, with better real-time communication of cell phone status and location among systems.

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 not begin until 1979. Shortly thereafter, the original system design was improved, and this design (still in wide 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 common use in the U.S.

AMPS

• The voice transmission part of the AMPS system is completely conventional, essentially the same as could be used in any ``walkie talkie."

• AMPS uses the 800 MHz frequency band.It was the cellular system design and the overall control functions that were technically novel.

• The analog format made cell phone conversation almost completely non private. Anyone with a simple scanning receiver could listen to the conversations.

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 the

committee that approved the system design, Groupe Special Mobile . This was originally a European standard, but has spread worldwide (including the U.S.) and the initials have been redefined to represent Global System for Mobile Communications .

• This is a digital system, meaning that the voices are digitized 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.

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 an

individual frequency (actually two frequencies, one for each direction of voice transmission) for the duration of the call. This is exactly the same as the manner in which individual radio stations are separated in the frequency spectrum and on the radio dial. This is called frequency division multiple access or FDMA. It is conceptually simple, but has some technical drawbacks.

• An alternative used by GSM is called time division multiple access or TDMA.

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 user transmits 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 user during 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. 

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 or TDMA.

• In CDMA each user occupies the entire radio channel as in TDMA, but the user also transmits all the time, as in FDMA. In other words, the users are not separated in either time or frequency.

• What does keep the users separate? Each user is assigned a unique digital code (the ``Code" in CDMA) which is used to encode the data from the voice digitized before transmission.At the receiver the same code is used to decode the incoming signal, and the result contains two terms: the original voice coder data bits, and a (hopefully) small amount of interference from the other users with different codes.

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 the number of simultaneous users. In CDMA the maximum number of users is determined by the amount of interference that can be tolerated (the total interference is the sum of the interference contributions from all the other users).

• In practice this number of users is somewhat greater than would be the case with FDMA or TDMA on a given piece of radio spectrum. This is the fundamental advantage of CDMA; the principal disadvantage is greater system complexity. There is also considerable controversy over just how great the extra capacity is. Claims of a channel gain on the 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. 

Sprint PCS• Personal Communications Services, and it was intended to

encompass an overall vision for telephony as distinctions among wired service, cellular service, and paging disappeared.

• For example, a person might have a small handset which he or she always carried, and a telephone number associated with the person rather than with a conventional telephone.

• The telephone system always keeps track of the person's location for call delivery. In the home or office, the handset operates as a cordless phone working inside buildings, and not taking up expensive cellular bandwidth. Outside it operates as cell phone. At all times it also incorporates paging functions. It may also work in planes and trains in a microcellular mode.

Generations of cell phones• The various digital cellular systems described above

represent the second generation of cellular service. Analog systems were the first generation, and these first generation systems are still in widespread use, and will remain so for some years. The digital services are superior to analog in essentially all ways, and so over time analog will disappear.

• This change of generations is facilitated by the availability of dual-mode cell phones, which can operate on two systems, such as analog AMPS and digital-GSM. From the service 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 features such as Caller ID and integrated paging.

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-generation systems is expected to begin by 2005.

Facts about cell phones

• Why are base station cellular antennas so ugly? The antenna on a hand held cell phone is a simple rod, about 6 inches long. Base station antennas could be as simple and unobtrusive as this, but technically they work better if they are arranged in groups of three so that each antenna transmits to one third (120 degrees) of the complete cell. Each antenna must be physically separate from the other antennas. Hence we see rather complicated arrangements of equipment on top of most cellular towers.

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 midwest of the United States, you will see occasional high towers 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 to handle the large number of users, and it is desired to keep the radio energy from propagating outside those cells. Because the energy travels only in straight lines, keeping the antennas low accomplishes this goal.

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. 

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 permitted because 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, and certainly tying up channels on unneeded sites.

• Of course, similar problems can occur from tall buildings or mountains, but the FCC has not found it practical to regulate these uses! 

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 the ionizing 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 this limit.

• distance from the antenna is the most significant factor, with any risk falling off rapidly with distance. Hence, any possible concern relates to the users of hand-held cell phones (because the antenna is within inches of the brain), not to cellular base stations in the neighborhood.

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 skilled operator (!) was required because the type of radio that was used was very different in its operation from a telephone system.  

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

``high frequencies.'' • These frequencies lie between the AM and FM radio

bands(between about 1 MHz and 30 MHz in fact), and are not high at all by today's standards.

• In the days before satellites, however, these frequencies had one important, and unique property: under the correct conditions 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 the desired points. The selection of the proper frequency was one of the reasons for requiring a trained operator.

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 off of the ionosphere,which envelopes the earth above the atmosphere.

• This reflection would enable some of the energy to travel beyond the horizon of the transmitter, and the energy might reflect off the surface of the earth and head back for the ionosphere.

• Several of these reflections may occur if conditions are just right, resulting in around-the-world propagation. It is interesting to note that this ionospheric reflection is analogous to the total internal reflection that occurs inside optical fibers, with dimensions many orders of magnitude different.

Satellite Phones• The ionospheric conditions described above are constantly

changing, requiring constant returning of the radio transmitter and receiver, and may permit only poor-quality communications.

• All of this changed with the advent of the communications satellite. Now a ship at sea is as easily reachable as any point on land.

• For some years a corporation called INMARSAT (for International Marine Satellite) has provided satellite radio communications for ships at sea, and somewhat as a sideline has made their facilities available to other users,typically those in very remote areas. This system has two disadvantages: the ground terminals are rather bulky (by today's standards) and service is expensive. The smallest available terminal is the size of a briefcase, and it requires that an antenna be set up and aimed at the satellite.

Motorola• Taking this system to the next step is the Iridium 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 reach from 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).

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 the satellites, 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.

Iridium

• The Iridium system began operation in 1999, and represented 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 and ceased 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.