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Title: Cyganski Book


1
Chapter 13
  • Cyganski Book
  • Monica Stoica, smonica_at_cs.bu.edu

2
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).  

3
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.

4
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.  

5
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).

6
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.

7
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.

8
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.

9
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10
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.

11
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.

12
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.

13
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.

14
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.

15
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.

16
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.

17
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.

18
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. 

19
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.

20
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. 

21
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.

22
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.

23
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.

24
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.

25
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.

26
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. 

27
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! 

28
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.

29
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.  

30
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.

31
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.

32
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.

33
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).

34
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35
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.

36
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.      

37
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