Every cellular system digital or analog is comprised of four parts.
1 ) Cells and cell sites ( base stations )
2 ) Switching station ( mobile telephone switching office or MTSO )
3 ) System operator and its local office
4 ) Cellular telephones
The
heart of the system is made up of individual radio coverage areas
called " cells. " Each cell is a self-contained calling area. Within
the cell, a cell site is strategically positioned as a base station for
receiving, sending and routing the radio signals of cellular phone
calls. Because the cellular system is a radio system,
no exact boundary
can be drawn on a map. In most cases calls can be place and received
throughout the service area, except for certain enclosed areas such as
underground parking garages. The No Svc ( No Service ) indicator will
illuminate on the cellular phone when in one of those areas or is
outside of the service area.
The cell site's transmitter is low powered
and does not reach much beyond that cell's boundaries. That makes it
possible to reuse channels ( frequencies ) - a given channel can be
used at the same time in different cells, as long as the cells do not
border one another, without causing signal interference. This is
particularly valuable in urban areas where lots of cellular phones are
in use at the same time. All cell sites are connected to the Mobile
Telephone Switching Office ( MTSO ), which provides connection into the
Public Switched Telephone network ( PSTN ) - the local telephone
company. The MTSO also provides other central functions, including call
processing, traffic management, and transferring calls as a phone moves
between cell sites.
Making
a Call
When
a cellular user makes a call from a cellular phone, radio signals are
transmitted to the cell site. The cell site alerts the Mobile Telephone
Switching Office ( MTSO ) switching station. The MTSO, in turn,
provides an open channel ( frequency ) and connects the call to the
Public Switched Telephone Network ( PSTN ).
The PSTN put the call
through to the number to be reached. This process takes the same amount
of time that it takes to make a call from a land line phone.
Receiving
a Call
These
are the steps that occur when you receive a call on a cellular phone. A
call placed to a cellular phone may come from either a land line phone
or another cellular phone. Whichever the source, the MTSO is notified
that a call has been placed to a specific cellular telephone number. At
this point, the MTSO searches for the correct cellular phone by sending
out data over the radio waves. Cellular phones that are in standby mode
( i.e., turned on but not being used in a call ) continuously scan the
radio waves being transmitted by the MSTO. If a phone " hears " its
telephone number, it sends back a signal that informs the closest cell
site of its Electronic Serial Number ( ESN ) and its telephone number (
Mobile Identification Number or MIN ). The cell site passes this
information to the MTSO, where the ESN and MIN are verified and a
channel ( frequency ) is assigned for the call. The cellular phone
receives the message directing it to tune to the correct voice channel.
The cell site makes the voice channel available, and the call is
completed.
Hand-off
Hand-off
is the transfer of a call from one cell site to another as the
cellular phone moves through the service coverage area. The cell site
warns the MSTO that the mobile's signal strength is falling below a
predetermined level. The MTSO then alerts all cell sites bordering on
the first one. They measure the mobile's transmitting signal and report
back to the MTSO. The MTSO, which is programmed to select the site
receiving the strongest signal, then switches the call from the weak
cell to the strongest cell without interrupting the call. The whole
process takes a fraction of a second, and the caller usually is unaware
of it. Such hand-offs may occur several times during a single
conversation as the caller moves through the coverage area.
Roaming
Roaming
is a service offered by most cellular service providers that allows
subscribers to use cellular service while traveling outside their home
service area. When they are outside their home service area and come
within range of another cellular system, the ROAM indicator on the
cellular phone will light to show that they are in range. Typically
there are two cellular system operators serving a specific area. One is
a wireline ( local phone company ) and the other is non-wireline. When
starting cellular telephone service, subscribers are assigned to one or
the other. When they roam ( operate outside their home system ), their
cellular phone will seek service from the same type of cellular system
as the one they subscribe to at home. But if that type is not available
where they are roaming, the phone will try to obtain service from the
non-home-type system. A blinking light indicates a non-home-type
system. There is an extra charge for calls placed while roaming.
