Wireless refers to a communications system in which waves
carry a signal through air rather than through a wire. Most wireless systems use radio
frequency (RF) or infrared (IR) waves.
The first "wireless" instrument was the wireless telegraph, invented the early
20th century. This later came to be known as radio when amplitude modulation
(AM), made it possible to transmit voice and music via wireless. Today, wireless devices
are being developed to serve various telecommunications markets, such as paging services,
personal communications service (PCS), cellular telephone service (CTS), low earth
orbiting satellite networks (LEOs), 2-way radios and more.
Although some of these technologies such as paging services have been around for over a
decade, others such as LEOs are relatively new and are still in development. Before
cellular telephones were available in 1984, mobile phones were the only way one could
place a call from a car. These were costly with a limited capacity due to only one
transmitter and receiver available for the entire area. There was also considerable
static. A mobile radio service is ½ duplex, which means that although it is a 2-way
communication, only one person can transmit while the other person receives at any one
time. Mobile operators can contact each other, but not anyone from a public network, or a
service not their own. Mobile services were popular with taxicabs, police departments and
fire departments.
In order for wireless systems to work, the waves are transmitted on a radio frequency.
Frequency refers to the number of times a radio wave completes a cycle in a second.
Different companies use different portions of the spectrum to communicate with wireless
radio service. Due to the fact that there are a limited amount of radio frequencies
available, the FCC allocates frequencies. This is to stop different companies transmitting
over the same frequency. For example, 2 radio channels transmitting over the same
frequency would result in listeners hearing an overlap of broadcasts. This is why the FCC
breaks up the spectrum and assigns the frequencies. The higher the frequency is the
smaller the wave. Higher frequencies are more susceptible to weather conditions like rain
since raindrops are larger in comparison to a smaller wave. That very raindrop is smaller
in comparison to a larger wave. Lower frequencies are not as affected by weather
conditions.
Pagers, first introduced by Motorola in 1956, started out as tone-only pagers. As the name
suggests, a tone was sent to the pager. In order to find out the telephone number of the
person trying to reach the receiver, a paging operator had to be called. Since then,
paging services and their sophistication have increased dramatically.
Advancements such as telephone numbers now being captured instead of having to call a 3rd
party to find out the number or alphanumeric paging have played a part in helping to
increase sales. In addition, paging services are relatively cheap as compared to other
services like CTS. Paging usually has monthly fees that rarely rise above $20, including
usage.
Pagers are also easy to use, small, and fit conveniently on a belt or in a purse.
Batteries last for a month, compared to the PCS batteries that last for 3 days while on
stand-by. New pagers even have battery saving features such as having the pager sample the
air at specific intervals so that the battery is not constantly in use.
Traditional paging services continue to grow due to various factors – strategic
marketing, availability of new spectrum for advanced features and product innovations.
Pagers first sold as business and professional tools and are now sold to a widening range
of customers. They come in bright colors and are easily attainable from retail outlets. In
certain parts of the world, pagers are seen as status symbols.
Advancements in paging services include the 2-way pager that can respond to short messages
with pre-programmed canned messages by pressing buttons on the pager. Another advancement,
the VoiceNow pager, offered by a Dallas company allows customers to get 30 second messages
on special pagers which save up to eight 30 second voice messages within the pager itself.
When the cellular telephone service (CTS), was first introduced in the late 1980s, many
experts felt that this might mean the dearth of paging services since cellular is a 2-way
service. People did not have to run around searching for a telephone every time someone
was trying to reach them.
Contrary to this prediction however, paging is still thriving today. The experts did not
realize that the differences between paging services and CTS actually complemented one
another. It is estimated that 25% of people with pagers have cellular telephones. This
enables the customer to return calls using the cellular telephone whenever paged, and to
screen calls, avoiding paying for nuisance calls.
Cellular Telephone Service
(CTS)
Cellular telephone service (CTS) is the analog cellular telephone service available in
urban and rural areas of the US. CTS has a greater capacity than mobile telephone service
in which only 25-35 conversations can take place simultaneously. The greater capacity is
due to the fact that frequency is re-used in hexagonal-shaped cells with each cell able to
support 57 conversations. When more capacity is needed, the cells are split into smaller
sizes and their frequencies are re-used since more cells equals to more capacity.
The concept for CTS was first discovered by AT&T’s Bell laboratories in 1947, but
it was first trialed in late 1983. FCC set aside 825–890 MHz for cellular service,
with two providers in each area to encourage competition. By 1987, 200 cellular systems
were in place in 127 cities. Most were initially purchased for salespeople and business
executives.
Analog CTS was slow to become popular in the US. However, as the number of subscribers
increased, the average monthly bills decreased. This is not due to decreasing costs, but
due to the fact that people used their telephones less often due to the increasing cost.
Initially purchased by businessmen and professionals, who needed to make frequent, daily
calls, cellular services are now purchased by many for safety reasons so that they can be
contacted in case of emergencies.
The public switched telephone network was created for voices, as is the analog CTS. In
fact, according to Motorola, less than ½ of the owners of credit-card-sized modems use
them with analog cellular services that are slow, and unreliable. They rarely transmit at
their top speeds of 14.4 Kbps when used over cellular service, but at a speed of 9600 bps,
which is very much lower than speeds achieved over wire lines.
When used for facsimile transmission there are a large number of errors in the form of
skipped or black lines. This is because the ½ duplex nature of the facsimile transmission
allows data to be transmitted in only one direction. This means that no error detection
messages can be sent from the receiving fax machine to the sending unit, which thus
continues sending data, oblivious of the errors that have occurred.
Even voice users complain about poor transmission and noticeable interference despite that
fact that cellular service has been available since 1984. Thus, analog CTS has several
limitations crippling its expansion. Cellular Digital Packet Data (CDPD) was first
developed by IBM as a way to transmit wireless data over excess capacity in cellular
provider’s networks and was first offered in 1995. Although expectations were high
prior to the introduction of CDPD, results were disappointing with CDPD capturing only 1%
of the mobile data market in 1996. This is probably due to the fact that people are slow
to accept this service due to high initial costs, lack of universal availability and
distrust of a new service. They also know that digital PCS networks could probably offer
greater speeds for data communication, especially since they were designed taking data
transmission into consideration.
During the 1990s, analog cellular service gained in popularity, leaving carriers faced
with the need to add capacity. Although breaking cells up into smaller sizes helped
increase capacity initially, smaller cells lead to more dropped calls and areas
overlapping into adjacent cells mean that customers are unable to place calls.
Customer dissatisfaction is a problem for cellular providers since it leads to
discontinuation of services and possibly even changing carriers. The need for more
capacity is a major driving force in existing carrier’s moves to upgrade their analog
cellular service to digital cellular telephony (D-AMPs).
Other motivations include enhanced features such as Caller ID, call waiting, alphanumeric
paging, longer battery life and other call-enhancing services which in turn attract more
customer. From the sale of these enhanced features carriers hope to gain greater revenue.
Digital cellular not only improves capacity but also privacy. Analog signals are easily
tapped into with scanners that are easily available. The digital bits in a digital
transmission are scrambled when multiplexed by time division multiple access (TDMA) and
code division multiple access (CDMA) schemes. This makes it more difficult to listen in on
conversations. However, privacy is not guaranteed, since there are more expensive scanners
available on the market that can be used to listen in on these digital cellular calls.
Wireless
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Furthermore, digital cellular is
dual-mode, switching between analog mode and digital cellular according to what is
available in an area.
A number of cellular suppliers offer digital cellular service over the same frequencies as
their analog service, setting aside channels for digital service which use either time
division multiplexing or code division multiplexing. With TDMA, existing towers and mobile
central offices can be used with new digital transmitters and receivers to carry the
digital signals, so carriers designate a portion of their total channels to transport
calls in digital format. CDMA, however, requires extensive upgrades to the cellular
foundation.
Also, with the introduction of SS7, alphanumeric paging and caller ID are made possible.
Using separate out-of-band signaling channels to carry information needed, SS7 has the
capability to access the database in the cellular network for information or features like
repeat dialing, call-waiting and call return.
