TELEPHONING SYSTEM
Introduction:
Telephoning systems which include means for generating signals in
response to the occurrence of ringback and busy signals following a dialing
operation and means for controlling the operation of a message broadcast unit
and of a call sequence unit in response to the generated signals. The telephone
is a telecommunications device that is used to transmit and receive electronically
or digitally encoded sound between two or more people conversing. It is one of
the most common household appliances in the developed world today. Most
telephones operate through transmission of electric signals over a complex
telephone network which allows almost any phone user to communicate with
almost any other user.
History:
The telephone was created through the cross-pollination of two fields of
study - electricity and acoustics. Although the invention of the telephone is
usually credited to Alexander Graham Bell, many inventors were working on the
problem throughout the 1860s and 1870s, most notably Elisha Gray, who filed a
patent application for the same device only a few hours after Bell did. Although
the basics of electricity were known by the 1830s, nobody suggested transmitting
speech electrically until 1854. The telephone was finally invented on March 10,
1876. Into it were spoken the famous words, "Mr. Watson, come here, I want to
see you."
Working Principle:
The telephone operates on simple principles. A telephone mouthpiece
contains a thin metallic coating separated from an electrode by a thin barrier
which connects to a wire carrying an electric current. When a person speaks into
the mouthpiece, the acoustic vibrations from her speech push the metallic
coating slightly closer to the electrode, resulting in variations in voltage and
therefore a speedy conversion from acoustic to electric energy. The electric
pulses are conveyed through a wire to the speaker on the other end, where
electric pulses are converted into acoustic energy again.
Converting speech to electrical energy before transmission is far more
efficient than conveying speech through a mechanical channel, for example a
metal pipe, because the walls of mechanical channels absorb so much of the
acoustic energy as it travels. Well-insulated wires, however, are effective at
protecting electrical energy from dispersing before reaching its destination.
Electrical pulses travel at the speed of light, whereas acoustic pulses are limited
by the speed of sound. In his 1627 book New Utopia, Francis Bacon referred to
the possibility of a long-distance network of tubes to transmit human speech.
Many historians consider this the first reference to something like a telephone.
Unfortunately, he lacked the scientific knowledge to understand why this would
be impractical.
Three critical developments were necessary precursors to the invention of
the telephone. First was the understanding that an electrical current can create a
magnetic field, and therefore mechanical or acoustic energy. This is credited to
Danish physicist Christian Oersted in 1820, who demonstrated that a compass
needle can be manipulated by running an electric wire over it. Second was the
understanding that the reverse is possible - electrical induction - that is,
generating a current by placing a moving magnet next to a wire. This insight is
credited to inventor Michael Faraday in the year of 1821. The last necessary
development was that of the battery, a chemical device capable of producing a
continuous source of electricity. The first very crude battery was the "Leyden jar",
a device for storing static electricity invented by two Dutchmen in 1746. Volta and
others developed more sophisticated batteries throughout the 18th and 19th
centuries.
Loop-back test system
The test system of the invention uses suppressed ringing connections
from a test unit to the customer premise equipment via the central office to
establish connections for performing the loop-back test. The suppressed ringing
connections are established over standard voice trunks between a switch in the
central office and the test unit by a central office service unit (COSU) such that
the number of lines that can be tested simultaneously is limited only by the
number of trunks linking the COSU to a switch. Because these trunks are
relatively inexpensive and simple to install and maintain, the test system of the
invention can be readily installed and expanded in the existing network to provide
virtually any level of monitoring capability.
Local loop system
The local loop is the physical link or circuit that connects from the
demarcation point of the customer premises to the edge of the carrier or
telecommunications service provider's network. At the edge of the carrier network
in a traditional PSTN (Public Switched Telephone Network) scenario, the local
loop terminates in a circuit switch housed in an ILEC (Incumbent Local Exchange
Carrier) CO (Central Office). Traditionally, the local loop was wireline in nature
from customer to central office, specifically in the form of an electrical circuit (i.e.,
loop) provisioned as a single twisted pair in support of voice communications.
However, modern implementations may include a digital loop carrier system
segment or fiber optic transmission system known as fiber-in-the-loop. A local
loop may be provisioned to support data communications applications, or
combined voice and data such as digital subscriber line (DSL).
Telephone system
This entry includes a brief general assessment of the system with details on
the domestic and international components. The following terms and
abbreviations are used through out the entry:
Arabsat - Arab Satellite Communications Organization (Riyadh, Saudi
Arabia).
Autodin - Automatic Digital Network (US Department of Defense).
CB - citizen's band mobile radio communications.
Cellular telephone system - the telephones in this system are radio
transceivers, with each instrument having its own private radio frequency and
sufficient radiated power to reach the booster station in its area (cell), from
which the telephone signal is fed to a telephone exchange.
Coaxial cable - a multichannel communication cable consisting of a central
conducting wire, surrounded by and insulated from a cylindrical conducting
shell; a large number of telephone channels can be made available within the
insulated space by the use of a large number of carrier frequencies.
DSN - Defense Switched Network (formerly Automatic Voice Network or
Autovon); basic general-purpose, switched voice network of the Defense
Communications System (US Department of Defense).
Fiber-optic cable - a multichannel communications cable using a thread of
optical glass fibers as a transmission medium in which the signal (voice,
video, etc.) is in the form of a coded pulse of light.
GSM - a global system for mobile (cellular) communications devised by the
Groupe Special Mobile of the pan-European standardization organization,
Conference Europeanne des Posts et Telecommunications (CEPT) in 1982.
HF - high frequency; any radio frequency in the 3,000- to 30,000-kHz range.
Landline - communication wire or cable of any sort that is installed on poles
or buried in the ground.
Microwave radio relay - transmission of long distance telephone calls and
television programs by highly directional radio microwaves that are received
and sent on from one booster station to another on an optical path.
Orbita - a Russian television service; also the trade name of a packet-
switched digital telephone network.
Radiotelephone communications - the two-way transmission and reception
of sounds by broadcast radio on authorized frequencies using telephone
handsets.
Satellite communication system - a communication system consisting of
two or more earth stations and at least one satellite that provide long distance
transmission of voice, data, and television; the system usually serves as a
trunk connection between telephone exchanges; if the earth stations are in
the same country, it is a domestic system.
SHF - super high frequency; any radio frequency in the 3,000- to 30,000-MHz
range.
Shortwave - radio frequencies (from 1.605 to 30 MHz) that fall above the
commercial broadcast band and are used for communication over long
distances.
Submarine cable - a cable designed for service under water.
Telefax - facsimile service between subscriber stations via the public
switched telephone network or the international Datel network.
Telegraph - a telecommunications system designed for unmodulated electric
impulse transmission.
Telex - a communication service involving teletypewriters connected by wire
through automatic exchange
UHF - ultra high frequency; any radio frequency in the 300- to 3,000-MHz
range.
VHF - very high frequency; any radio frequency in the 30- to 300-MHz range.
CELLULAR TELEPHONE
A mobile phone is also known as cellular telephone. Cellular phone is a
long-range, electronic device used for mobile voice or data communication over a
network of specialized base stations known as cell sites. In addition to the
standard voice function of a mobile phone, telephone, current mobile phones
may support many additional services, and accessories, such as SMS for text
messaging, email, packet switching for access to the Internet, gaming, Bluetooth,
infrared, camera with video recorder and MMS for sending and receiving photos
and video, MP3 player, radio and GPS. Most current mobile phones connect to a
cellular network of base stations,which is in turn interconnected to the public
switched telephone network (PSTN).1
A mobile phone proper typically has a telephone keypad, more advanced
devices have a separate key for each letter. Some mobile phones have a
touchscreen.
Basic Theory and Operation
Cell phone theory is simple. Executing that theory is extremely
complicated. Each cell site has a base station with a computerized 800 or 1900
megahertz transceiver and an antenna. This radio equipment provides coverage
for an area that's usually two to ten miles in radius. Even smaller cell sites cover
tunnels, subways and specific roadways. The area size depends on, among
other things, topography, population, and traffic.
When you turn on your phone the mobile switch determines what cell will
carry the call and assigns a vacant radio channel within that cell to take the
conversation. It selects the cell to serve you by measuring signal strength,
matching your mobile to the cell that has picked up the strongest signal.
Managing handoffs or handovers, that is, moving from cell to cell, is handled in a
similar manner. The base station serving your call sends a hand-off request to
the mobile switch after your signal drops below a handover threshold. The cell
site makes several scans to confirm this and then switches your call to the next
cell. You may drive fifty miles, use 8 different cells and never once realize that
your call has been transferred. At least, that is the goal. Let's look at some details
of this amazing technology, starting with cellular's place in the radio spectrum
and how it began.
There are different techniques used in the cellular phones:-
AMPS
Let's look at how cellular uses data channels and voice channels. Keep in
mind the big picture while we discuss this. A call gets set up on a control channel
and another channel actually carries the conversation. The whole process begins
with registration. It's what happens when you first turn on a phone but before you
punch in a number and hit the send button. It only takes a few hundred
milliseconds. Registration lets the local system know that a phone is active, in a
particular area, and that the mobile can now take incoming calls. What cell folks
call pages. If the mobile is roaming outside its home area its home system gets
notfied. Registration begins when you turn on your phone.
D-AMPS
The most commonly used digital cellular system in America is IS-136,
colloquially known as D-AMPS or digital AMPS.It was formerly known as IS-54,
and is an evolutionary step up from that technology. This system is all digital,
unlike the analog AMPS. IS-136 uses a multiplexing technique called TDMA or
time division multiple access. The TDMA based IS-136 uses puts three calls into
the same 30kz channel space that AMPS uses to carry one call. It does this by
digitally slicing and dicing parts of each conversation into a single data stream,
like filling up one boxcar after another with freight. We'll see how that works in a
bit. TDMA is a transmission technique or access technology, while IS-136 or
GSM are operating systems. In the same way AMPS is also an operating
system, using a different access technology,
Time division multiple access or TDMA handles multiple and simultaneous
calls by dividing them in time, not by frequency. This is purely digital
transmission. Voice traffic is digitized and portions of many calls are put into a
single bit stream, one sample at a time. We'll see with IS-136 that three calls are
placed on a single radio channel, one after another. Note how TDMA is the
access technology and IS-136 is the operating system?
CDMA: Another transmission technique
Code division multiple access is quite a different way to send information,
it's a spread spectrum technique. Instead of concentrating a message in the
smallest spectrum possible, say in a radio frequency 10 kHz wide, CDMA
spreads that signal out, making it wider. A frequency might be 1.25 or even 5
MHz wide, 10 times or more the width a conventional call might use. Now, why
would anyone want to do that?, to go from a seemingly efficient method to a
method that seems deliberately inefficient?
CDMA is a transmission technique, a technology, a way to pass
information between the base station and the mobile. Although called 'multiple
access', it is really another multiplexing method, a way to put many calls at once
on a single channel. As stated before, analog cellular or AMPS uses frequency
division multiplexing, in which callers are separated by frequency, TDMA
separates callers by time, and CDMA separates calls by code. CDMA traffic
includes telephone calls, be they voice or data, as well as signaling and
supervisory information. CDMA is a part of an overall operating system that
provides cellular radio service. The most widespread CDMA based cellular radio
system is called IS-95.