Cell Phone is became a part of our life. Each call or SMS we make from our mobile phone reach to the receiver through the network. But how does this mobile network work over a large area? How does a call carried from sender to receiver? We will discuss the full mobile network system here.
In case of a telephone network every telephone is connected with its exchange stations by copper wire or by optical fibers, but in case of mobile network some stations are connected by optical fibers where the final end connection that is the link between our mobile phone to the network is completely wireless medium. There are mobile towers in every area. Each mobile tower covers some area and responsible for every phone call made in that area. Each local tower has some GSM antennas set at the upper end and has BTS(Base transceiver station) present at the base of the tower.
GSM antennas are directly connected with the BTS by cable. Every local towers are connected with BSC(Base Station Controller). For better understanding I may say that in a district there is a BSC and every Local tower of different localities are connected to that BSC by radio through Microwaves or by optical fibers. Basically BSC controls the BTS present at the local towers. Then many numbers of BSC are connected to a MSC(Mobile switching centre). In a State there may be 2-3 MSC and all the BSC present in that state are connected to the MSC by Optical fibers.
MSC is the central part of the network or the exchange. Every call or SMS a sender make travels to the MSC via Local Tower and then BSC, then MSC validate and pass it to the appropriate receiver. It has many Systems integrated in it to validate user, Switch call to proper user, Security modules etc.
Several units of MSC
HLR(Home Location Register) : It is a Data Base present in MSC which stores the permanent data about a SIM card. Like the SIM number, To whom a number is assigned, SIM card validity etc.
VLR(Visitors Location Register) : It store any change in location of every users. Basically it tracks the location of every user and store the data that currently we are now under which local tower? So that MSC can switch and redirect a call to our phone through that local tower.
EIR(Equipment Identity Register) : It contains Information which numbers are blocked now, which are active and which are partially blocked to use some services.
AUC(Authentication Centre) : It contains the key number of every SIM. This Unit authenticates and passes a call after checking the SIM identity/key.
IN(Intelligent Network) : This Unit of MSC controls the billing section. It holds the information about current Balance, subscribed value added services of every SIM and deduct balance from a SIM for any call or SMS or any Value added Service.
SMSC(Short Message Service center) : SMS sent or received to a number is controlled by this unit. Every SIM has a Short message service number. By which SMSC identify and exchange messages to our number. It stores the message came from sender and then forward to the recipient number and then return back a delivery report to the Sender.
These are the several modules present in a MSC and then MSC is connected with PSTN and GMSC.
PSTN (public switched telephone Network) : This is the connection to the public network. If we try to call to a Telephone then the call will forward to the PSTN and will be redirected to the receiver. Or if we will dial a mobile number from a Telephone then that call will come to MSC through PSTN and the from the MSC to that mobile phone.
GMSC (Gateway MSC) : Every MSC is connected to GMSC. It manages international calls. For example if we want to make a call to America from India, then our call will pass through GMSC of India and then enter through GMSC of America.
When we make a call the transmitter of our mobile phone start transmitting our voice which is caught by the GSM Antenna of the local mobile tower. Then that signal travels to the BTS of that Local tower. As BTS is connected to BSC the signal travels to BSC and then to the MSC. Real operations take place at MSC end. HLR Unit and VLR unit find out the current location of the dialed Number or the receiver. If the dialed number is a Telephone Number then it will be transferred to PSTN Unit, otherwise the call will send to the BSC and then to the Sender’s mobile via Local Tower from MSC.
In later posts we will discuss about Microwaves and its link, GSM antenna and about types of BTS in details.