GSM is called as Global System for Mobile Communications, or just Global System for Mobile. The technology was started in 1985 by a French company which is previously recognized as Groupe Special Mobile. The main competitor of GSM is CDMA, which is now in use by Bell Mobility, Telus Mobility and Mobility Canada carriers.
At present, just two are the main carriers of GSM network in Canada. Microcell(Fido,Cityfone) and Rogers Wireless. Fido was the first carrier who starts utilizing the technology, and it was followed by Rogers Wireless mainstream around 2001. There are many companies in the United States of America who have adopted GSM and its spreading rapid among AT&T Wireless, T-Mobile. However, there are only two companies in Canada, and they are GSM-850 and GSM-1900.GSM-850 and GSM-1900 which operate at 1.9Ghz.
The radio spectrum can be shared by different users accessing similar frequency band without causing any kind of obstruction. The techniques used for this are TDMA (Time division multiple access), FDMA (Frequency division multiple access) and CDMA (Code division multiple access).GSM is the combination of both Time and Frequency-Division Multiple Access (TDMA/FDMA). FDMA part involves the partition by frequency of (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart. Each of the carrier frequencies is then alienated in time, using a TDMA plan. The fundamental unit of time in this TDMA system is called a burst period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are grouped into TDMA frame (120/26 ms, or approx. 4.615 ms), which forms essential unit for the definition of logical channels. One physical channel is one burst period per TDMA frame. Thus GSM allows eight concurrent calls on same radio frequency. GSM (Global System for Mobile Communications) is a form of multiplexing, which divides the accessible bandwidth between different channels.GSM operates on four special frequencies worldwide.
GSM calls are based on data or voice. Audio codecs used by voice calls are known as half-rate, full-rate and enhanced full-rate. Data calls can turn cell phone into modem operating at 9600 bps. A comprehensive GSM characteristic is high speed circuit switched data, allowing phone to transmit up to around 40 kbps.
Importance of GSM Technology
GSM service is in more than 200 different countries, so it is quite easy to simply use your GSM phone when you are in one of these countries. The cool thing is that a GSM cell phone will work with any other GSM service anywhere in the world as long as it has the same frequency. If you are on an international business trip to five different countries all you need is one cell phone and a SIM card compatible with the GSM in each country. This is much easier and faster to deal with than trying to lug around five different cell phones or figure out how to use other types of communication.
GSM Modulation Technique
Conventional GSM transmitters utilize quadrature amplitude modulation (QAM) with In-phase (I) and Quadrature (Q) signals that are mixed with a local oscillator operating at the carrier frequency. A typical system, where the baseband I and Q data is converted to analog and then mixed with a local oscillator. This method, although viable, requires mixers, filters and D/A converters to up-convert the baseband I and Q signals to the RF carrier frequency. It is difficult to realize the required analog filters in monolithic form so the system becomes complex and costly. A more elegant solution is direct modulation of a high resolution DS synthesizer as described in. In this architecture, the phase-locked loop (PLL) closed-loop bandwidth is narrow (compared to the reference frequency) to satisfy the PLL noise requirements. This restricts the modulating signal bandwidth since the PLL can only readily track frequencies within its bandwidth so this architecture is only suitable for narrow-band modulation. An alternative for wide-band modulation is to break the loop during transmission. Then the modulation is limited only by the VCO and power amplifier bandwidth. This technique has been used for DECT (Gaussian frequency shift keyed modulation) where the transmission data bursts are relatively short and accurate phase control is not required. The problem with opening the loop is that the VCO is free-running and will drift over time with no phase noise suppression. Another difficulty is avoiding switching transients while breaking the loop. A transient while opening the loop results in a frequency channel offset error during the transmit time. Wide-band modulation of a closed loop is possible if some form of compensation is used to overcome the natural roll off of the PLL loop bandwidth. In principle equalization is possible as long as the true PLL characteristics are known. This tends to be the pitfall since this PLL contains analog filters which cannot be realized to close specifications and therefore the necessary equalization transfer function is not known. The synthesizer proposed is a better architecture to use with equalization since it incorporates mostly digital signal processing which has predictable transfer functions to use in designing the equalizer.
If you need assistance with writing your essay, our professional essay writing service is here to help!Essay Writing Service
GMSK (Gaussian filtered Minimum Shift Keying):
GMSK is a form of modulation used in a variety of digital radio communications systems. It has advantages of being able to carry digital modulation while still using the spectrum efficiently. One of the problems with other forms of phase shift keying is that the sidebands extend outwards from the main carrier and these can cause interference to other radio communications systems using nearby channels.
In view of the efficient use of the spectrum in this way, GMSK modulation has been used in a number of radio communications applications. Possibly the most widely used is the GSM cellular technology which is used worldwide and has well over 3 billion subscribers.
GSM Output Spectrum:
The spectral requirements of a GSM modulated carrier are quite stringent due to the narrow channel spacing. Without careful spectral control, excessive RF power would spread into adjacent channels. The RF spectrum exceeds the GSM spectral requirements and the spurious response is not difficult to meet since this architecture has no mixers and associated analog filters to introduce spurs. The only potential source of spurs is from limit cycles in the DS modulator with a DC input. These are inherently avoided since the modulation data keeps the DS modulator busy enough to randomize the quantization errors.
Open Loop Gain Control:
The open-loop response and gain k of a conventional indirect synthesizer using analog loop filters and a VCO is generally unknown. The reason is that the filters cannot be realized to close specifications and the VCO sensitivity may vary due to process tolerances. Traditionally, the solution to this problem is to provide some means of adjusting the filter response and open-loop gain k by using an active loop filter. In this architecture, the synthesizer loop gain is the only unknown parameter and is caused by deviation of the VCO sensitivity kv due To process variations. A -20% gain error results in significant closing of the eye and a reduced noise margin. Additionally, the zero crossing are spread over a larger time period and this makes the receiver more sensitive to timing errors. Conversely, a +20% gain error has a larger peak distortion with an adequate eye opening but the zero crossings remain spread leading to similar timing sensitivity. Therefore, some form of external adjustment is necessary to compensate for the gain error caused by the unknown VCO sensitivity. Practically, this needs to be done once since the VCO sensitivity wouldn’t drift far from the initial process value although periodic corrections are possible. A suitable compensation method is to measure the actual VCO sensitivity and compensate for it using the existing DSP. This can be done by tuning the synthesizer to the upper and lower GSM transmit frequencies and measuring the tuning voltage in each case. Conversion of the analog tuning voltage to a digital value can be accomplished using the same D/A converter that forms part of the loop filter so minimal extra hardware is required. Once the two tuning voltages have been obtained, a new kv can be computed and the digital loop parameters adjusted accordingly.
The purpose of security for GSM system is to make the system as secure as possible. Various authentication techniques have been incorporated into GSM make it the most secure mobile communication standard currently available. GSM security is based mainly on authentication and encryption techniques.
Some of the security services provided by GSM are:
Anonymity: So that it may not be easy to identify the user of the system. It is provided by using temporary identifiers. When a user first switches on his set, the real identity is used, and after that a temporary identifier is issued. To know the temporary identity the user has to be tracked.
Authentication: So that the operator knows who is using the system for billing purposes. It is used to identify the user (or holder of a Smart Card) to the network operator. It is performed by a “Challenge and Response” mechanism. A random challenge is issued which is encrypted by the mobile using authentication algorithms. A key is assigned to the mobile which sends back a response to the operator who checks that the key is correct.
Signaling Protection & User Data Protection: So that susceptible information on the signaling channel, e.g. telephone numbers, may be protected over the radio path and also that data passed by the user over the radio path is protected.
The response is then passed through an algorithm A8 by both the mobile and the network to derive the key Kc (in figure below) used for encrypting the signaling and messages to provide privacy (A5 series algorithms).
Encryption for GSM
Other GSM security mechanisms
There is always the risk that the SIM card can be compromised. This is considered doubtful, especially as some operators use their own version of A3. Keys Ki (in figure) and the matching IMSI could be compromised by someone who sells information for money.
In GSM the customer subscription and authentication is contained in a smart card known as SIM, Subscriber Identity Module. When a smart card is inserted on any mobile it will take the identity of the subscriber. The mobiles become attractive items to steal, as they can be used with another SIM card.
To prevent this, GSM has specified an International Mobile Equipment Identifier (IMEI).
An Equipment Identity Register (EIR) is placed in each network, with Black, White and Grey Lists for stolen or unapproved mobiles, valid mobiles and mobiles which require tracking respectively. Grey lists represent local tracking of mobiles within a network.
GSM has defined a procedure so that approved; lost or stolen mobile IMEIs can be communicated to all other operators. A Central Equipment Identity Register has been (CEIR) proposed. Type approval authorities issue white list numbers (random ranges of valid IMEIs) to mobile manufacturers, and manufacturers inform the CEIR when the mobiles are released to market. All operators are able to post their black lists to the CEIR, and in return collect a consolidated list of all operators black and white lists.
By this method stolen or invalid mobiles can be quickly barred throughout the world.
Implementation of GSM
Implementation in industries:
GSM-R or GSM-Railway:
It is an international wireless communications standard developed for railway communication and applications. It is a technology used for communication between train and Railway Regulation Control centers. The system is based on GSM qualifications which ensure performance at speeds up to 500 km/h (310 mph), without any data loss. It is a cost efficient digital replacement for current in-track cable and analogue railway radio networks.
Other uses of GSM – R:
· PtP Call: Point-to-Point Call, the same type of call like in regular GSM calls.
· VGCS: Voice Group Call System, similar to walkie-talkie communication.
Nokia Siemens Networks has proposed India’s first GSM-R communications solution into operation under the Mobile Train Radio Communication (MTRC) project for North Central Railways.
Local use of GSM:
SMS: Short Message Service (SMS) is part of the GSM specification and allows short text messages to be sent or received via mobile phones.
Voicemail: This service functions just like a conventional answer machine. If the line is engaged or the user doesn’t answer an incoming call after some specified number of rings, the operator of the mobile diverts the caller to a voicemail system. The caller is welcomed by a pre-recorded greeting and given the opportunity to leave a message.
MMS stands for Multimedia Messaging Service is a messaging service that allows subscribers to exchange multimedia messages. MMS supports the transmission of additional media types: text, picture, audio, and video. The sender can easily create a Multimedia Message, using the phone, or can use images and sounds stored previously in the phone. To send or receive a MMS, the user must have a compatible phone that is running over a GPRS or 3GSM network.
Problems Faced in implementation of GSM:
Much Authentication techniques have been applied in security of GSM but they do have certain flaws.
The user identity confidentiality is violated by transmitting the user identities (IMSI) in unprotected form through the intermediate transport networks between the GSM registers.
In the GSM authentication phase, two related parameters, RAND and SRES, are transmitted on the air interface in the clear. So any listener on the air interface can perform a known plaintext attack on the RAND – SRES pair to obtain the authentication key. Private Key based security protocols are used which has some flaws like; they cannot guarantee confidentiality of the calls within the network.
Advantages and Disadvantages of GSM
Advantages of GSM:
GSM is used all over the world with more than 450 million users.
International roaming facility permits the use of one phone throughout the world unlike CDMA which will work in Asia, but not European nations.
GSM is established, started in the mid-80s due to which a more stable network with healthy features are available.
The accessibility of Subscriber Identity Modules (SIM cards), which are smart cards; provide secure data encryption to give GSM m-commerce advantages.
GSM service is in more than 200 different countries, so it is quite easy to simply use your GSM phone when you are in one of these countries.
Disadvantages of GSM:
In case of GSM, SIM cards, individual authentication keys of the users are stored in the authentication centers. Any person with the rights and qualifications to access to authentication center can manipulate these to impersonate that mobile user.
Security algorithms used in GSM e.g. A3, A5, and A8 are all undisclosed algorithms. But researchers have proved that these algorithms cannot guarantee 100% security.
Privacy of a call and mystery of a subscriber are only assured on the air interface among the mobile station and the base station subsystem leading to likelihood of eavesdropping of voice data.
Anonymous. (anonymous). Benefits of GSM. Available: http://ezinearticles.com/?Benefits-of-GSM-Phone-Service&id=950625. Last accessed 14-03-2010.
Anonymous. (anonymous). GSM Technology. Available: http://www.canadiancontent.net/mobile/gsm_technology.php. Last accessed 14-03-2010.
Anonymous. (anonymous). General Information. Available: http://www.naavi.org/cl_editorial_04/edit_dec_04_04_01.htm. Last accessed 16-02-2010 .
Charles Brookson. (1994). GSM (and PCN ) Security and Encryption. Available: http://www.brookson.com/gsm/contents.htm. Last accessed 03 march 2010.
http://www.mcs.csueastbay.edu/~lertaul/ICW3016.pdf. Last accessed 22 march 2010.
Ed Sutherland. (2005). GSM vs CDMA . Available: http://www.m-indya.com/gsm/CDMA-vs-GSM.htm. Last accessed 12 February 2010.
EE Times-India. (2008). Nokia Siemens launches GSM-R in India. Available: http://www.eetindia.co.in/ART_8800525363_1800005_NT_1aa8c7bf.HTM. Last accessed 14 February 2010.
GSM world. (2005). Messaging . Available: http://www.gsmworld.com/technology/services/messaging.htm. Last accessed 2 February 2010.
Cite This Work
To export a reference to this article please select a referencing stye below:
Related ServicesView all
DMCA / Removal Request
If you are the original writer of this essay and no longer wish to have your work published on UKEssays.com then please: