Model Of Mobile Communication Network (Theory)

The basic elements in a communication network

       As you all know a communication network consists of the users and the network elements to provide the service to the users. These network elements consists of the BTS, BSC, MSC, VLR, HLR, GMSC, PSTN etc. Hey dont worry seeing all these! I will explain it one by one to you all.

Mobile Station (MS)
       This refers to the physical phone itself. The phone must be able to operate on a GSM network. Older phones operated on a single band only. Newer phones are dual-band, triple-band, and even quad-band capable. Each phone is uniquely identified by the International Mobile Equipment Identity (IMEI) number. This number is burned into the phone by the manufacturer. The IMEI can usually be found by removing the battery of the phone and reading the panel in the battery well.

Base Transceiver Station (BTS)

       The BTS is the Mobile Station's access point to the network. It is responsible for carrying out radio communications between the network and the MS. It handles speech encoding, encryption, multiplexing (TDMA), and modulation/demodulation of the radio signals. It is also capable of frequency hopping. A BTS will have between 1 and 16 Transceivers (TRX), depending on the geography and user demand of an area. Each TRX represents one ARFCN. One BTS usually covers a single 120 degree sector of an area. Usually a tower with 3 BTSs will accommodate all 360 degrees around the tower. However, depending on geography and user demand of an area, a cell may be divided up into one or two sectors, or a cell may be serviced by several BTSs with redundant sector coverage. A BTS is assigned a Cell Identity. The cell identity is 16-bit number (double octet) that identifies that cell in a particular Location Area. The cell identity is part of the Cell Global Identification (CGI).The CGI is a number that uniquely identifies a specific cell within its location area, network, and country. The interface between the MS and the BTS is known as the Um Interface or the Air Interface.

Base Station Controller (BSC)

        The BSC controls multiple BTS. It handles allocation of radio channels, frequency administration, power and signal measurements from the MS, and handovers from one BTS to another (if both BTSs are controlled by the same BSC). A BSC also functions as a "funneler". It reduces the number of connections to
the Mobile Switching Center (MSC) and allows for higher capacity connections to the MSC.A BSC may be collocated with a BTS or it may be geographically separate. It may even be collocated with the Mobile Switching Center (MSC). The interface between the BTS and the BSC is known as the Abis Interface. The Base Transceiver Station (BTS) and the Base Station Controller (BSC) together make up the Base Station System (BSS). 

Mobile Switching Center (MSC)
        The MSC is the heart of the GSM network. It handles call routing, call setup, and basic switching functions. An MSC handles multiple BSCs and also interfaces with other MSC's and registers. It also handles inter-BSC handoffs as well as coordinates with other MSC's for inter-MSC handoffs. The interface between the BSC and the MSC is known as the A Interface. 

Gateway Mobile Switching Center (GMSC)
         There is another important type of MSC, called a Gateway Mobile Switching Center (GMSC). The GMSC functions as a gateway between two networks. If a mobile subscriber wants to place a call to a regular land line, then the call would have to go through a GMSC in order to switch to the Public Switched Telephone Network (PSTN).The interface between two Mobile Switching Centers (MSC) is called the E Interface. 

Home Location Register (HLR)
        The HLR is a large database that permanently stores data about subscribers. The HLR maintains subscriber-specific information such as the MSISDN, IMSI, current location of the MS, roaming restrictions, and subscriber supplemental features. There is logically only one HLR in any given network, but generally speaking each network has multiple physical HLRs spread out across its network.

Visitor Location Register (VLR)

          The VLR is a database that contains a subset of the information located on the HLR. It contains similar information as the HLR, but only for subscribers currently in its Location Area. There is a VLR for every Location Area. The VLR reduces the overall number of queries to the HLR and thus reduces network traffic. VLRs are often identified by the Location Area Code (LAC) for the area they service. A LAC is a fixed-length code (two octets) that identifies a location area within the network. Each Location Area is serviced by a VLR, so we can think of a Location Area Code (LAC) being assigned to a VLR. The VLR also has one other very important function: the assignment of a Temporary Mobile Subscriber Identity (TMSI). TMSIs are assigned by the VLR to a MS as it comes into its Location Area. TMSIs are only allocated when in cipher mode. The interface between the MSC and the VLR is known as the B Interface and the interface between the VLR and the HLR is known as the D Interface. The interface between two VLRs is called the G Interface.

        Now we know about various components in the communication network.  So lets move on to the next question "How a call gets connected?". Various steps behind this process is as follows:

• When a subscriber initiates a call by dialling a number in his mobile it directly send a request to the BTS which he comes under.

• BTS there by sends the request to the BSC to which it is connected.

• From the BSC the request is made to the MSC to which it is connected.

• Then MSC sends a request to the HLR to check the information about the caller like account balance (if prepaid), live user, area of the caller etc.

• After checking all the details the HLR sends an acknowledgement message to the MSC that the caller is O.K. to make a call or not.

• If that message is O.K. the operation continues or else the MSC sends a NOT O.K. message to the BSC which has made the request and BSC to the BTS and BTS to the Mobile. At the next instance the call get disconnected.

• If the message received by the MSC is O.K. then it again sends a request to the HLR to find the called party (the subscriber to whom call is made).

• The called party and the caller may come under the same MSC or different MSC. If both come under the same MSC then the HLR finds the subscriber easily and informs the MSC that to which area the called party belongs. The MSC therefore establishes an air link between the both parties. Thus the call gets connected.

• If both parties comes under different MSC then the HLR again search for the MSC to which the called party belongs and inform the same to the MSC  of the caller. Thus the MSC of the caller establishes an air link between the  both parties. Thus the call gets connected.

• After the call gets disconnected the Billing is made the Billing Server which is present in the HLR.

     So now all of you know the theory. In the following posts I will tell you how we developed a system to depict all these and the interesting electronics behind it!