GMS技術的普及介紹與探究
導讀：這篇文章主要選擇一個電信標準（GSM或CDMA）就GSM或CDMA標準進行研究，并簡要介紹其有關特定技術。本文由英國dissertation網、代寫assignment頻道整理提供。
Abstract
   According to our Project, we have to make a report/research on either on GSM or CDMA standard. We have to select one of the telecom standards (GSM or CDMA) and briefly describe about particular technology.

         As we have made our project on GSM standard, after going through some websites and some group discussion. We are finally supposed to work on GSM standard. One of the basic reasons to select this technology is that, it is much familiar or many people are currently using this technology. So, it will make us easy or comfortable. In our research we have concentrate on the given points like:

1.What is the importance of this particular technology?
2.What are the modulation techniques?
3.What are the security issues for this Technology?
4.Where such technology is best suited to implement?
         So in brief we have made this report with a very great effort. Each and every question of a project has been solved very accurately and practically.

Introduction
GSM communication is standardized group, which is globally accepted for digital cellular communication. It was established in 1982 to make a common European mobile telephone standard that would formulate specifications for a pan-European mobile cellular radio system operating at 900 MHz. In the field of communication technologies, various systems were introduced and installed to achieve standardization in mobile industry during the advancement of mobile technology, but all the efforts were unsuccessful. Then, several issues were sustained like incompatibility of systems, development of digital RF. Then, these problems like standardization, incompatibility etc were overcame after introducing the GSM technology. In 1980, it was introduced for the testing purpose in the market. GSM mobile are working on TDMA Systems, in which single RF offers to the users with any interference. Now, GSM is become largest mobile communication technology worldwide (214 countries), and all Mobile phone's manufacturer now developing their products based on GSM technology, and provide their subscribers GSM networks.

Feature of Cellular Telecommunication
Intelecommunication, the termhandofforhandover is common, which mainlyrefers to the process of transferring an ongoing call/data session from one channel connected to another. It can be done by two ways:

1.Hard Handoff
In this process the channel in the source cell is released and after that another channel in the target cell is engaged. During this process, the connection to the source is broken before establishing the connection to the target, such handoffs are also known asBreak-Before-Make. It is proposed to be direct in order to minimize the disruption to the call, which is recognized by network engineers during the call.#p#分頁標題#e#

2.Soft Handoff
This is the second method of Handoff; in this case channel in the source cell is maintained and used parallel with the channel in the target cell. So, the connection to the target is established before the connection to the source is broken; this handoff is known asMake-Before-Break. It is recognized by network engineers as a state of call, rather than a concise event.

Reason for Popularity of GSM Technology
         It has several features due to which, it is now become most leading mobile communication technology in the worldwide.

1.It facilitates with high speed integrated data. Example: - voice data, fax, mail, voice mail and mostly used SMS feature.
2.It ensures the communication between networks are secured and protected from intruders and frauds.
3.It supports multiple frequencies like 900 MHz, 1800 MHz, and 1900 MHz.
         In North America region, GSM technology is working on 1900 MHZ, where as 1800MHz is used in other parts of the world. The frequency bands are differing by different operators.

Architecture and Working of GSM Network
Architecture of GSM Network
         GSM network consists of three major systems, which can be considered as architecture of GSM networks. These are as follows:

Switching System (SS) - It is very operative, where many crucial operations are conducted, it holds five databases (HLR, MSC, VLR, AUC and EIR), which performs different functions. The major task of Switching System is to perform call processing and subscriber related functions.
Base Station System (BSS) - It perform very important role in mobile communication. It is basically outdoor units which consist of iron rods and are usually of high length. They are responsible for connecting subscribers (MS) to mobile networks. All the communication is made in Radio transmission. It is further divided in two systems, which are BSC, and BTS.
Operation and Support System (OSS) - It is use to monitor and control the complex systems. The basic reason for develop this system is to provide cost effective support and solutions to the customers. It helps in managing, centralizing, local and regional operational activities required for GSM networks.
Working of GSM Network
1.Outgoing Call
First of all MS sends dialed number to BSS, then in the second step BSS sends dialed number to MSC, which checks VLR if MS allowed the requested service. If so, then MSC asks BSS to allocate the resources for the call. After that, MSC routes the call to GMSC, which routes the call to the local exchange of called user. Then, Answer back (ring back) tone is routed from called user to MS via GMSC, MSC, and BSS.

2.Incoming Call
When GSM user call to another user, then call is forwarding to the GMSC and signal is setup to the HLR, after that it will made request from the VLR. All the responses are forwarded from MSC to GMSC, and then call is forwarded to the current MSC and gets the all status of MS. Then remaining process are proceed one by one like paging of MS, MS answer, Security Check and in last Setup the connection between both the subscriber.#p#分頁標題#e#

Importance of GSM Technology
         In cellular service there are two main competing network technologies: GSM and CDMA (Code Division Multiple Access). But, in our point of view GSM technology is far better than CDMA technology either technology point view or people choice's point view. Due, to user's satisfaction with this technology, currently it has approximately 450 million users around the world, which means more than 3 times of the CDMA user. Below, we have mentioned some points which make GSM important to us:

1.Outstanding Voice and Call Quality
         GSM uses Enhanced Full Rate voice codec standard to improve the voice clarity and call quality. Its quality of speech is less by modern standards, but was a best negotiation between complication and quality. EFR is the enhancement version of Full Rate, which was working at the bit rate of 13kb/s. FR codec is still usually used in networks around the world. But, that also outstandingly improve the voice quality.

2.Service Migration
         GSM phones have a SIM (Subscriber Identity Module) card that carries all the information of a particular user. That particular user has rights to lock their information by the help of secret codes that are entered into the phone. It is the biggest benefit of GSM technology because phone works everywhere our need is SIM, for that particular country. If the user travel regularly to one different country, in this case user replace their old SIM card with a new one, and ready to start calling. These case users need not to worry about changing their cell phone.

3.International Roaming
Most of country around the world only supports GSM, although of 4 different frequencies. The phone, which we buy must support the frequency (1 of the 4) used in the country where user wish to visit. GSM technology is the more internationally accepted and has more international roam-ability than CDMA. As we can see that, 200 countries use GSM technology, which means 70% of all wireless handsets worldwide are GSM and 1 billion customers are using GSM around the worldwide.

4.Talk Time
As we know that, the GSM technology has lower talk-range (35 KMs) from a tower in comparison with CDMA (110 KMs). But, on the other hand, GSM has higher power output (2W) as per CDMA phone's 200 mW. That means CDMA implies lesser radiation hazard. So that the reason the talk time is generally higher in a GSM phone due to its pulse nature of transmission.

5.Market Size
The market size of GSM technology is increasing every second in both, either we see the number of subscribers or amount of total area covered. In India, GSM technology spread out and establishes a large market either in a city or village.

6.Limited active calls per cell
GSM technology has a hard limited number of calls per cell. Once the limit is exceeded we cannot use the cell. On the other hand, CDMA has no hard limit of number of calls per cell. So, the quality goes down the more calls are active and the provider may restrict the total number of active users.#p#分頁標題#e#

7.Improved Security and Privacies
In the GSM special attention is paid to security aspects, such as security against forgery and theft, security of speech and data transmission, and security of the subscriber's identity. Use of a radio transmission makes the PLMNs particularly sensitive to the misuse of resources by unauthorized persons and the eavesdropping of information exchanged on the radio path. GSM networks adopted the Subscriber Identity Module (SIM) card architecture for mobile handsets, which can provide encryption and digital signatures for transactions.

The Modulation Technique in GSM
         GSM technology uses a digital modulation format called0.3 Gaussian minimum shift keying, or0.3GMSK, which is a continuous-phase frequency-shift keying modulation scheme. Here, 0.3 represents the bandwidth of the Gaussian filter in relation to the bit rate. In this Modulation technique, we use two different frequencies to represent ones and zeroes are calledFrequency Shift Keying and the data rate is exactly four times the RF frequency shift. By which we minimize the modulation spectrum and improve channel efficiency. The spectrum is further reduced by using Gaussian pre-modulation filter, which slows down the rapid frequency transitions; otherwise it spread energy into adjacent channels.

         Afrequency modulationscheme in which digital information is transmitted through discrete frequency changes of acarrier wave. The simplest FSK isbinaryFSK(BFSK). BFSK literally implies using a pair of discrete frequencies to transmit binary (0s and 1s) information. With this scheme, the "1" is called the mark frequency and the "0" is called the space frequency.

How we can generate GMSK Modulation（英國dissertation網http://www.ttc-switch.com/）
GMSK Modulation can be done by two different ways:

1.To Generating the GMSK modulation, we use Gaussian filter and Voltage Controlled Oscillator to filter the modulating signals and apply it to frequency modulator, where modulation index must exactly equal to 0.5. This analog method is very easy and basic, but it is not practically suitable because component tolerances drift cannot be set exactly.
Generating GMSK using a Gaussian filter and VCO
2.In the second method, we use I/Q (In-phase/Quadrature) modulator, which is widely in use for GMSK modulation. It is an efficient way to transfer information, and it also works well with digital formats. It can create AM, FM and PM. The term quadrature means that the phase of a signal is in 90 degrees to another one. It uses one signal that is said to be in-phase and another that is in 90 degrees to this. In view of the in-phase and quadrature elements this type of modulator is often said to be an I-Q modulator, which we use, to maintain the modulation index at exactly 0.5 without any settings or adjustments. That why, this method is easier to use, and capable of providing the required level of performance. If we reverse the all process, then that could use for demodulation.#p#分頁標題#e#
Why GMSK Modulation require
We have several reasons to use GMSK modulation for a radio communications system:

1.The improved spectral efficiency as compare to other phase shift keyed modes.
2.We can amplify GMSK modulation by a non-linear amplifier without any distortion in a signal. It is possible because there are no elements of the signal that are carried as amplitude variations. As we know, non-linear amplifiers are more efficient in terms of the DC power input from the power rails that they convert into a RF signal. It means, it consume less power for a given output, which results in lower levels of battery consumption, which is important factor for cell phones.
3.Information cannot be carried as amplitude varies, which means it can resist amplitude variations. Therefore, it is more resilient to noise than other forms of modulation, because most noise is mainly amplitude based.
Basic Requirement for Implementing GSM
1.Mobile Station (User Equipment)
2.Base Transceiver Station (Antennas, Radio Transceivers)
3.Distribution Switch Unit
4.Combining and Distribution Unit
5.Energy Control Unit
6.Power Supply Unit
Software Requirements
1.Voice Codec (Data and Speech Compression)
2.Cryptography (Encryption)
Cost
Minimum cost, to implement the GSM technology, would be $500-1000 (approx).

Skills
1.Network Engineer
2.Or GSM Technology experts by any company.
Security in GSM Network
         The GSM technology has special attention for security aspects, for example security against forgery and theft, and security of the subscriber's identity, voice and data transmission. It uses three different security algorithms:

1.A3 Algorithms: - Its validation process used for authentication of the mobile user. It performs mathematical process that combines to generate a secret number (Ki) stored in to the SIM card and a second one is transferred between BS and MS.
RAND (128 bits)
Ki (128 bits)
SRES (32 bits)

2.A5 Algorithms: - Its secret message used to encryption over-the-air. It uses the ciphering key that is calculated by the authentication centre and by the SIM. It is initialized with the session key (Kc) and the number of the frame being de/encrypted, which is used throughout the call, but the 22-bit frame number changes during the call. Thus, it generates a unique key stream for every frame.
3.A8 Algorithms: - It is the key generation algorithm, which generates the session key (Kc) from the random challenge (RAND) received from the MSC and from Ki. Kc is used until the MSC decides to authenticate the MS again.
RAND (128 bits)
Ki (128 bits)
KC (64 bits)

         A3 and A8 are generally implemented together into the SIM cards and in network Authentication Centers. On the other hand, A5 algorithm is use to mixed up the user's voice and data traffic between the handset and the base station to provide privacy. It is implemented in both, the handset and the Base Station Subsystem (BSS).#p#分頁標題#e#

         GSM can provide the protection for the user by the PIN (Personal Identification Number), which is stored into the SIM card and the additional security features. Here we have considered some functions of the GSM Security for the users:

Authentication
For authentication AuC provides authentication triplets to the VLR via the HLR, which includes a signed response, random number, and ciphering key. Each triplet is used only once and AuC delivers new triplets on demand. It is mainly compare the subscriber authentication key Ki in the authentication centre with SIM without ever sending the Ki on the radio path. Then, network sends a random number to the mobile at the beginning of each call, at the same time SIM uses A3 algorithm. The AuC has also computed this response, called signed response (SRES), and delivered it in the authentication triplet to the VLR. The VLR performs a comparison and if they match, the MS grant to use the network.

IMEI Check
The IMEI stands for International Mobile Equipment Identity, which is manufacturer-specific code stored in the mobile, used to ensure that the mobile equipment does not belong to the black list where the EIR stores the serial numbers of stolen mobiles. If an IMEI is found on the black list, a connection cannot be established.

Encryption of Speech and Data
GSM uses A5 algorithms to encrypt the speech and data before forwarding the radio or air interface. For encryption an exclusive-or operation is performed with data and the ciphering sequence. The ciphering key depends on the subscriber-specific key together with the random number that is given to the mobile station at the beginning of each call.

Mobile Subscriber Identity
It is normally addressed over the air interface by using a Temporary Mobile Subscriber Identity, which is allocated for each mobile located inside Local Area. The global identity of the mobile, International MSI, which is stored into SIM, is very hardly ever to sent over the air interface to prevent snooping devices from using it as trigger information. A new TMSI is allocated for the next call when communication is in ciphered mode. IMSI is global subscriber identification but it is not the same as the telephone number. A subscriber may have several telephone numbers.

Implementation of GSM Technology
Global System for Mobile-Railway (GSM-R)
         It is an internationalwireless communicationstandard forrailway communicationand applications. A sub-system ofEuropean Rail Traffic Management System(ERTMS), which is used for communication betweentrainand railway regulation control centers. It is based onGSMand EIRENE - MORANEspecifications which ensure performance at speeds up to 500 km/h, without any communication loss. It is ensuring for secure voice and data communication between railway operational staff (like dispatchers, drivers, train engineers, shunting team members, and station controllers). This will support applications such as cargo tracking,video surveillancein trains and at stations, and passenger information services. It is implemented using dedicatedbase stationtowers close to the railway. Every base station is separated by the distance of 7-15 km, to ensure for greater redundancy, availability and reliability. The train maintains acircuit switcheddigitalmodemconnection to the train control centre at all times, which operates with higher priority than normal users. For any circumstances, if the modem connection between train and train control centre is lost, then the train will automatically stop.#p#分頁標題#e#

Frequency band（英國dissertation網http://www.ttc-switch.com/）
It uses a specific frequency band:

876MHz 880MHz: transmission
921MHz 925MHz: reception
GSM-R Features
VGCS (Voice Group Call Service)
VBS (Voice Broadcast Service)
REC (Railway Emergency Call)
3G (3rd Generation)
         It is a family of standards formobile telecommunicationsfulfilling specifications by theInternational Telecommunication Union. Services include wide-area wireless voicetelephone,video calls, and wireless data, all in a mobile environment. It allows simultaneous use of speech and data services and higher data rates (at least 200 Kb/s) up to 14.0Mb/s. It offers faster connectivity, music entertainment with the best quality and faster internet access. User can experience easy video calling, video call facility, clearer communication experiences and faster speeds when both parties are 3G user.

3G Features
Mobile TV
Video on demand
Video conferencing
Tele-medicine
Location-based services
Education
         Many schools & colleges could improve the academic performance of their students with a district wide broadband wireless internet access network. The technology for this is available and affordable. It can connect schools, faculties and staff anywhere in the district. This implementation can focuses on the three A's (Access, Affordability, and Applications) when considering the installation of 3G technology. Implementing a district wireless internet network by utilizing the GSM technology, this will affect by boosting student reading writing and numeracy skills by the Video Conferencing.

Advantages and Disadvantages of GSM Technology
Advantages of GSM Technology
•Dominant global market share ofGSMgives benefits:
Large number of infrastructure and handset vendor's.
Innovations ahead of CDMA e.g. single chip handsets.
•Widest choice of handsets with widespread sales and service distribution network.
•Handset availability in all price segments and ongoing introduction of wide variety to meet customer requirements.
•Easy to subscribe (SIM based)
•Widespread prepaid solutions
•Seamless interoperability between networks and handsets.
•Global footprint for international roaming including all services.
Disadvantages of GSM Technology
Higher call cost.
Higher roaming cost.
IMEI numbers which is used to lock the phone permanently but by the use of some that can be tamper.
If SIM is lost then all data is lost unless that is stored in memory card.
Problem during implementation of GSM Technology
1.GSM-R may not deliver in high-density areas. The problem of high-density could be two-fold. Since GSM-R is also a data caterer for the European Train Control System there may be problems when transmitting data to many trains' in complex stations. There may also be voice communication overload. One of the main problems for the GSM is spectral limitation.#p#分頁標題#e#
2.3G services require more than 15-20 MHz bandwidth for transmission of data. If we compare it with 2G communication's bandwidth, which is 30-200 KHz. As we can see that 3G required bandwidth is 500 time as compare to 2G communication, which make radio spectrum such a precious and scarce resource in the information age - everybody from television broadcasters to the military wants spectrum, and it is in short supply.
3.3G services require an expensive fee required to earn a 3G service license. This means it is hard for providers who can afford to provide the 3G services. Also there are a large number of differences in the various licensing terms.
4.The necessary infrastructure for 3G is very difficult to build. This is because there is a large amount of debt that is sustained by manytelecommunication companiesas of now.
5.There is no member state support for the financially troubled 3G operators. Also the 3G phones are really expensive. There are not many 2G mobile users who want to convert to the new 3G wireless services. There also is a lack of coverage in the 3G communication service, because it is still comparatively new.
6.3Gtechnologyis that the 3G mobile services are very highly priced. Lack of 3G voice and data services available in a in a mobile phone device.
7.The 3Gtechnologyis really a new thing and it has brought a revolutionary innovation in the mobile technologybut it still is in an immature stage. It will take some time before everything can be settled. 3G service is a wonderful mobiletechnology, it will take some time to set in but it will definitely come up as the most wonderful mobiletechnology.
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