Transcript Document

Chapter 13
Cyganski Book
Monica Stoica, [email protected]
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.
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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.
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
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 rapidlyspreading cellular systems across the globe.