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Wireless Services (pg. 2)
Personal Communications
Service (PCS)
PCS is a wireless telephone service that is similar to CTS, offering customers the same
features as digital cellular. However, it transmits within the range of 1.8 to 1.9 GHz.
This means that PCS towers need to be spaced more closely together than digital and analog
cellular towers since higher frequency airwaves are smaller and cannot travel as far as
lower frequency airwaves. Fortunately, the towers are inexpensive to construct, being
shorter, and use up less power.
PCS services are deployed over
incompatible multiplexing schemes with services developed by various manufacturers using
different, incompatible methods of digitizing calls. All digital wireless services use
multiplexing techniques to carry more information on the same amount of spectrum. So, even
though all digital add more capacity than analog cellular, the ways that they do so are
incompatible with each other.
There are basically three ways to multiplex calls - Code division Multiple Access (CDMA),
Time Division Multiple Access (TDMA) and Group Speciale Mobile (GSM), now known as Global
System for Mobile Communications.
CDMA is a "spread spectrum" technology introduced by a US company called
Qualcomm in 1989. However, actual implementation of CDMA was only in 1996. Each
conversation transmitted is sent over multiple frequencies. This is made possible using
unique 40-bit codes assigned to each data or voice transmission, allowing multiple users
to share spectrum. CDMA handsets not only have more capacity, they also use less power.
Lower emission translates to less threats of causing cancer, not to mention a longer
battery life.
The standard that had been agreed upon by the Telecommunications Industry Association
(TIA) when CDMA was announced is time division multiple access (TDMA). Using this method
of multiplexing, the transmission channel is broken down into various time slots. Half of
these time slots carry data from devices, and the other half is not used. The unused half
ensures no interference on the channels carrying traffic.
The incompatible methods of adding capacity onto the PCS network are the reason why not
all callers are able to use their PCS telephones when they travel.
Wireless for Local Telephone
Service
Long distance carriers hoping to provide local calling services are considering wireless
options as a way to link homes and businesses to their own central office switches.
Wireless spares service providers the hassle of laying cable, which saves money and labor
costs. One option that may be considered by companies looking to provide wireless where
copper and fiber facilities are at capacity is fixed wireless.
Fixed wireless provides customers with a fixed antenna on their premises, and AT&T is
currently exploring this option to save money on reaching local customers. The cost of
providing a wireless link is roughly 50% that of providing wire-based lines.
AT&T is testing a service consisting of a 13-inch box, containing 2 telephone lines
and a data channel for Internet connection capable of transmitting at 128,000 bps, the
speed of 2 channels of a BRI ISDN line. This box is attached to the outside of a home to
act as an antenna, while neighborhood antennas capable of serving 2,000 homes beam radio
signals to these premises.
Although this is system is not ready for deployment yet, it shows how wireless service can
be used for local calling, and how AT&T can link callers to its existing switches
providing local and long distance calls.
Specialized Mobile Radio
Initially, the Specialized Mobile Radio (SMR) spectrum, 800-900 MHz range, was given out
for voice applications and were used by contractors who wanted to communicate with people
on the other end of a mobile unit privately. These specialized networks aren’t
interconnected with anyone else’s network, or to the public switched telephone
network, so the SMR spectrum was adapted for new analog data only.
Eventually certain companies used them
for digital services selling voice, data and short messaging services. SMR speed ranges
from 4,800 bps to 19,200 bps, making them suitable for small bursts of text like e-mail,
bar codes and license look-ups.
Traditional data communication networks transmit data between company sites at stationary
locations, but certain organizations needed data communications for mobile employees, like
delivery, transportation and field services.
In the late 1970s, various organizations started using radio frequency networks to
transmit data from mobile workers. An example is Federal Express, which used its private
network to track the location of their packages, giving it an edge over other competitors.
Each package to be delivered is assigned a bar code, which is scanned into the computer
system at each strategic point of the delivery system. A driver scans the package with a
hand-held device that transmits the bar-code information to the main Federal Express
office. This tracks the location of the package practically every step of the way.
Portable computers and hand-held devices or scanners for data entry was a technology that
helped stimulate the use of radio networks. Improvements in flash technology, lighter
batteries, and micro-circuitry made scanners or computers more efficient and portable.
Rather than developing private radio networks, specialized radio networks can be leased
from companies.
Low Earth Orbiting Satellite Networks
(LEOs)
Telephone service is not readily available in a large number of countries.
As such, a system that manages to reach all areas of the world without requiring laying of
miles and miles of cable is a major improvement. Large sums of money are being spent on
the design of satellite systems that can deliver telecommunication services like
telephone, Internet access or facsimile transmission, to hard-to-reach places with rugged
terrain. Although still few of the systems are in place, the systems are now being built,
and licenses are being obtained for spectrum.
One of the technologies in place for delivering these services to all locations worldwide
is low earth orbiting satellite networks (LEOs). These satellites orbit around the earth
from 435 to 1,000 miles above the earth’s surface. This closer distance, as compared
to traditional geosynchronous satellites orbiting at 22,300 miles above the earth’s
surface, help solve the problems of delayed transmission.
This can be compared to holding a flashlight in the air pointing towards the ground. The
higher up the flashlight, the larger the area covered but the weaker the light. When the
flashlight is lowered, the area covered does decrease, but the light gets more intense.
The cost of LEOs systems are slightly higher due to the fact that a greater number of
satellite systems are needed.
Take the wireless quiz!
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