Cyganski Book

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Chapter 13

• Cyganski Book
• Monica Stoica, smonica_at_cs.bu.edu

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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 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).  

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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.

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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.  

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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).

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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.

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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.

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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 (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.

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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 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.

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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.

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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 behanded 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.

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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 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.

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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 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.

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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.

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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
  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.

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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 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.

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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 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. 

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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 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.

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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 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. 

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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.

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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.

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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-generation systems 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 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.

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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
  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.

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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
  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! 

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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
  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.

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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 skilled operator (!) 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 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.

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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 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.

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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.

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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).

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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 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.

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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.      

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