Title: Cyganski Book
1Chapter 13
- Cyganski Book
- Monica Stoica, smonica_at_cs.bu.edu
2The 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).
3Receiver 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.
4The 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.
5Analog 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).
6T1
- 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.
7All 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.
8Radio- 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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10Cellular 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.
11Cell 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.
12What 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.
13What 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.
14The 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.
15AMPS
- 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.
16GSM
- 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.
17AMPS 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.
18TDMA
- 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.
19CDMA
- 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.
20CDMA 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.
21Sprint 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.
22Generations 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.
23Future?
- 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.
24Facts 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.
25Why 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.
26Cordless 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.
27FCC
- 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!
28Safe 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.
29No 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.
30Radio 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.
31Radio 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.
32Satellite 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.
33Motorola
- 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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35Iridium
- 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.
36Iridium
- 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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