Cellular network basics
a summary or tutorial about cellular network basics, detailing the main
elements within it - BTS, BSC, HLR, VLR, etc.
The network forms the heart of any cellular telephone system.
The cellular network fulfils many requirements. Not only does the cellular
network enable calls to be routed to and from the mobile phones as well as
enabling calls to be maintained as the cell phone moves from one cell to
another, but it also enables other essential operations such as access to the
network, billing, security and much more. To fulfil all these requirements the
cellular network comprises many elements, each having its own function to
complete.
The most obvious part of the cellular network is the base
station. The antennas and the associated equipment often located in a container
below are seen dotted around the country, and especially at the side of highways
and motorways. However there is more to the network behind this, as the system
needs to have elements of central control and it also needs to link in with the
PSTN landline system to enable calls to be made to and from the wire based
phones, or between networks.
Different cellular standards often take slightly different
approaches for the cellular network required. Despite the differences between
the different cellular systems, the basic concepts are very similar.
Additionally cellular systems such as GSM have a well defined structure, and
this means that manufacturers products can be standardised.
Basic cellular network structure
An overall cellular network contains a number of different
elements from the base transceiver station (BTS) itself with its antenna back
through a base station controller (BSC), and a mobile switching centre (MSC) to
the location registers (HLR and VLR) and the link to the public switched
telephone network (PSTN).
Of the units within the cellular network, the BTS provides
the direct communication with the mobile phones. There may be a small number of
base stations then linked to a base station controller. This unit acts as a
small centre to route calls to the required base station, and it also makes some
decisions about which of the base station is best suited to a particular call.
The links between the BTS and the BSC may use either land lines of even
microwave links. Often the BTS antenna towers also support a small microwave
dish antenna used for the link to the BSC. The BSC is often co-located with a
BTS.
The BSC interfaces with the mobile switching centre. This
makes more widespread choices about the routing of calls and interfaces to the
land line based PSTN as well as the HLR and VLR.
Base transceiver station, BTS
The base transceiver station or system, BTS consists of a
number of different elements. The first is the electronics section normally
located in a container at the base of the antenna tower. This contains the
electronics for communicating with the mobile handsets and includes radio
frequency amplifiers, radio transceivers, radio frequency combiners, control,
communication links to the BSC, and power supplies with back up.
The second part of the BTS is the antenna and the feeder to
connect the antenna to the base transceiver station itself. These antennas are
visible on top of masts and tall buildings enabling them to cover the required
area. Finally there is the interface between the base station and its controller
further up the network. This consists of control logic and software as well as
the cable link to the controller.
BTSs are set up in a variety of places. In towns and cities
the characteristic antennas are often seen on the top of buildings, whereas in
the country separate masts are used. It is important that the location, height,
and orientation are all correct to ensure the required coverage is achieved. If
the antenna is too low or in a poor location, there will be insufficient
coverage and there will be a coverage "hole". Conversely if the antenna is too
high and directed incorrectly, then the signal will be heard well beyond the
boundaries of the cell. This may result in interference with another cell using
the same frequencies.
The antennas systems used with base stations often have two
sets of receive antennas. These provide what is often termed diversity
reception, enabling the best signal to be chosen to minimise the effects of
multipath propagation. The receiver antennas are connected to low loss cable
that routes the signals down to a multicoupler in the base station container.
Here a multicoupler splits the signals out to feed the various receivers
required for all the RF channels. Similarly the transmitted signal from the
combiner is routed up to the transmitting antenna using low loss cable to ensure
the optimum transmitted signal.
Mobile switching centre (MSC)
The MSC is the control centre for the cellular system,
coordinating the actions of the BSCs, providing overall control, and acting as
the switch and connection into the public telephone network. As such it has a
variety of communication links into it which will include fibre optic links as
well as some microwave links and some copper wire cables. These enable it to
communicate with the BSCs, routing calls to them and controlling them as
required. It also contains the Home and Visitor Location Registers, the
databases detailing the last known locations of the mobiles. It also contains
the facilities for the Authentication Centre, allowing mobiles onto the network.
In addition to this it will also contain the facilities to generate the billing
information for the individual accounts.
In view of the importance of the MSC, it contains many backup
and duplicate circuits to ensure that it does not fail. Obviously backup power
systems are an essential element of this to guard against the possibility of a
major power failure, because if the MSC became inoperative then the whole
network would collapse.
Summary
While the cellular network is not seen by the outside world
and its operation is a mystery to many, the cellular network is at the very
centre of the overall cellular system and the success of the whole end to end
system is dependent largely on its performance.
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