No one can ever forget Alexander Graham Bell, who is credited as the inventor of “Telephone”. With innovation and advancement in technology, the simple two way communicating device has taken a new form popularly known as “mobile phone”. We have reached an era where it is very common to hear words like “I can’t live without mobile phone”. Cell phone has become a part of our daily life. Let it be business deals, any personal work or meetings everything today can be done with the help of mobile phone.
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A Mobile Phone, Cell Phone or Hand Phone is an electronic device which lets the user to be mobile and make or receive telephone calls across a wide geographical area, served by many public cells. The calls could be made to and received from either a fixed line or another mobile. Mobile phone uses a wireless network for communication i.e. a network which is which is not connected by cables of any kind.
Base Station (BS).Each base station is in turn is connected to a central hub and is controlled by this switching office, also known as Mobile Switching Center (MSC). MSC is nothing else but a computerized center that is responsible for connecting calls, recording call information and billing. The communication between all the base stations and the telephone central office is coordinated by MSC. PSTN connects all conventional telephone switching centers with MSCs throughout the world.
Also, the cell size is not fixed and can be increased or decreased depending on the population of the area. Generally, the radius is kept 1 to 12 mi. In comparison to low density areas, high density areas require more geographically smaller cells to meet the traffic demands. Once the cell is determined, the cell size is optimized so that the adjacent cells signals are not interfered. For this purpose, the transmission power of each cell is kept low to prevent its signal from interfering with those of other cells.
Block Diagram of cellular Network
Difference between Fixed Line and Mobile Phone
In case of public switched telephone network (PSTN), the landline trunked lines (trunks) are used for transfer of information. These trunks comprise of fiber optic cables, copper cables, microwave links and satellite links. The network configuration in the PSTN is virtually static as the changes in the network connections would be required only when the subscriber would change the residence. Wireless networks, on the other hand are highly dynamic, wherein the network configuration is to be rearranged every time the subscriber moves into the coverage region of new base station. Unlike fixed networks which are difficult to change, wireless networks must reconfigure themselves for users within small intervals of time (on the fraction of seconds) to provide roaming and imperceptible handoffs between calls as a mobiles moves about. The available channel bandwidth for fixed networks can be increased by installing high capacity cables whereas wireless networks are constrained by RF cellular bandwidth provided for each user.
Frequency Reuse Principle
Increasing the capacity and coverage area is the key feature of any cellular network and this can be accomplished by re-using the frequency. It is true that to avoid interference, neighboring cells should not use same frequency but as the frequencies available is limited, they can be reused. A frequency reuse pattern is nothing else but a collection of N cells arranged together where N is the reuse factor, in which each cell uses a unique set of frequencies. The frequencies can be reused whenever the pattern is repeated.
F1,F2, F3,and F4 in the above diagram define the pattern with the reuse factor of 4.The cells that use the same frequencies(ones with the same frequency number) are known as reusing cells.
Transmitting and Receiving Calls
Whenever user makes a call from cell phone, the mobile station searches for a band with a strong signal to setup a channel and send the data to the closest base station using that channel. The base station then connects to Mobile Switching Center (MSC) and passes the data to it. MSC further passes the data to the telephone central office. A connection is established and result is sent back to MSC, if the called party is available. At this point MSC assigns an unused voice channel to the call and mobile station automatically adjusts its tuning to the new channel so that communication can take place.
Whenever a call is made to a mobile phone , the central office of the telephone from where the call has been initiated sends the number to the MSC.MSC in turn sends the query signals to each cell in process to find out the location of mobile station. This process is called paging. Once the mobile station is found MSC sends the ringing signal to it and after the station answers, a voice channel is assigned to the call for the communication to begin.
The process of transferring the connected or on going call from one channel to another in order to prevent the termination of call is known as Handoff or Handover. Whenever the mobile station moves from one cell to another and signal becomes weak, handover is required. MSC monitors the level of signal every few seconds and if it finds the strength of signal diminishing then it searches for a new cell wherein better communication can take place. There are two types of handoff:
Hard Handoff: In this case, a mobile station communicates only with one base station. When the mobile station moves from one cell to another, the connection with the target cell is established only when the connection from the source is broken. Thus, such handovers are also known as break-before-make.
Soft Handoff: In this case, a mobile station can communicate with two base stations so there is no need of breaking from the source cell to make a connection to a target cell. Instead, a mobile station may continue with a new connection while retaining the older one for a while. Thus, such handovers are known as make-before-break.
Frequency Division Multiple Access (FDMA): FDMA is a technique in which the spectrum is divided into frequencies and is assigned to users. The channel is assigned to only one subscriber or user at a time i.e. A channel will remain blocked until the call which was initially made has completed. A “full duplex” FDMA requires two channels, one for sending or transmitting the data and another for receiving. FDMA technology was used in first generation systems, which were analog systems.
Time Division Multiple Access (TDMA): TDMA splits each frequency into time slots thus improving spectrum capacity. In TDMA technology, each user is allowed to access the entire radio frequency channel for the short period of a call. The same frequency channel can be shared by other users as well at different time slots. The base station continually switches from user to user on the channel. The second generation mobile cellular network is dominated by TDMA.
Code Division Multiple Access (CDMA): CDMA simply means communicating with different codes. It allows all the users to occupy all channels at the same time thus increasing the spectrum capacity. Each voice or data call is assigned a unique code, so that calls could be differentiated from each other while being carried over the same spectrum. In case of CDMA terminals can communicate with various base stations using the feature known as “soft hand-off”.
Mobile Generations: Evolution from 1G to 3G
With the rapid advances in technology and the greater selection of new wireless services and applications the mobile has grown through various generations fulfilling the increasing demands of its users. First Generation (1G) mobile phones were analog which had only voice facility. Additional facilities like messaging and data services, fax were introduced in digital phones known as Second Generation (2G) mobile phones. The third generation mobile phones (3G) which are being talked about a lot nowadays, includes high speed of internet browsing, picture and video messaging facilities. In short, multimedia facilities when added to the second generation mobiles (2G) gave birth to the third Generation mobile phones.
First Generation Mobile System: The cellular networks were introduced in 1980’s and since then it has not stopped growing. The first generation mobiles were analog systems with the capability of transmitting at the speed of 9.6 kbps max. At that time there was no worldwide coordination for the development of technical standards worldwide. AMPS (Advanced Mobile Phone System) was invented in Bell Labs and was installed in United States in 1982.When used in England and Japan it was known as TACS and MCS-L1.In 1G mobile systems roaming was not possible and efficient use of frequency spectrum was not there.
Second Generation Mobile System: ETSI was created in Europe in mid 1980s to standardize the mobile communication sector. This standardization lead to the beginning of new network which was based on digital technology and popularly known as Global System for Mobile Communication (GSM).GSM was actually introduced to add more services to the actual network as well as meet the requirements of data traffic. GSM consists of the following three main components:
1. Base Station Subsystem 2.Network Subsystem 3.Network Management System
Base Station Subsystem: It consists of Base Transreceiver Station (BTS), Base Station Controller (BSC) and Transcoder sub-multiplexer (TCSM).BTS is the network component that manages the interface between mobile station and the network. Mobile terminals are connected to the BTS through the air interface. BTS makes use of omnidirectional or directional antennas for transmission and reception. BSC provides major functions like handover and managing radio resources.
Network Subsystem: Network subsystem is basically an interface between the public network and GSM network. All the communication between mobile users and other users (ISDN, fixed, mobile) is managed by NSS. The databases of subscriber and is also kept with NSS to manage users mobility. Various components of NSS are described below:
1) Mobile Switching Center (MSC): It is the most important component that performs switching functions necessary for interconnections between mobile users and other users (fixed or mobile).
2) GMSC: To connect the cellular network with PSTN, a gateway known as GMSC is used.
3) Home Location Register (HLR): All the information about the subscriber such as the coverage area, services provided to the user, current location and mobile equipment status etc is maintained in this register. The database remains same until the termination of subscription.
4) Visitor Location Register (VLR): The information of subscriber is uploaded in this register whenever he enters the coverage region so that necessary services could be provided to him. VLR of the new region is updated with the database whenever the subscriber moves to the new region. VLR is dynamic in nature as it keeps the data of the subscriber temporarily and interacts with HLR for recording the data.
5) Authentication Center (AUC): It takes care of the security by providing standards for encryption and authentication of users. Encryption key is kept in mobile equipment as well as AUC to protect the network from unauthorized access.
6) Equipment Identity Register (EIR): All mobiles are identified using IMEI (International Mobile Equipment Identity) number which is written on the battery of the phone. EIR keeps the list of all valid IMEI number and whenever a call is initiated, the network checks the IMEI number and call is connected if the number is valid. No calls are allowed from unauthorized terminals.
7) GSM Interworking Unit (GIWU): GIWU is an element or a unit that is a combination of hardware and software and synchronizes the information. It lets the user to choose between message and speech mode by acting as an interface to different networks.
Network Management System: The third element of GSM basically monitors the different elements and functions of the network. It performs the following main tasks:
- Network Monitoring
- Network Development
- Network Measurement
- Fault Management
NMS continuously monitors the performance of the network to ensure it runs smoothly. The performance can be measured by collecting the data from the individual elements for analysis and storing it in the database. The network operator can then compare the data collected in the database with the one which was actually expected. If any fault occurs,
the fault alarm is generated .These faults are then required to be corrected either by NMS or manually.
Interfaces in GSM
Different interfaces are used in GSM. These are Air , Abis and A interface.Air interface is an interface between Mobile station and Base Transreceiver Station (BTS),Abis connects Base Transreceiver Station (BTS) to Base Station Controller (BSC), and A interface is the one which is present between TCSM and MSC.
Value added services such as voice mail and short message services (sms) were added to GSM along with intelligent services like Pre-paid and fraud management. Later on, GPRS (General Packet Radio Services) was introduced so that packet switched data service could be provided. SGSN (Serving GPRS) and GGSN (Gateway GPRS) were new elements which were introduced in the existing system so that packets could be sent to air interface. IP routers, firewall servers and Domain name servers were also used along with these elements. Finally, to increase the data rate better coding methods were used.
Third Generation Mobile System
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