Joined: 14 September 2004
Joined: 14 September 2004
CDMA & GSM Cellular Technology
From the beginning, the planners of GSM wanted ISDN compatibility in services offered and control signaling used. The radio link imposed some limitations, however, since the standard ISDN bit rate of 64 Kbps could not be practically achieved.
The digital nature of GSM allows data, both synchronous and asynchronous data, to be transported as a bearer service to or from an ISDN terminal. The data rates supported by GSM are 300 bps, 600 bps, 1200 bps, 2400 bps, and 9600 bps.
The most basic teleservice supported by GSM is telephony. A unique feature of GSM compared to older analog systems is the Short Message Service (SMS). Supplementary services are provided on top of teleservices or bearer services, and include features such as international roaming, caller identification, call forwarding, call waiting, multiparty conversations, and barring of outgoing (international) calls, among others.
Though CDMA's application in cellular telephony is relatively new, it is not a new technology. CDMA has been used in many military applications, such as:
CDMA is a spread spectrum technology, which means that it spreads the information contained in a particular signal of interest over a much greater bandwidth than the original signal. With CDMA, unique digital codes, rather than separate RF frequencies or channels, are used to differentiate subscribers. The codes are shared by both the mobile station (cellular phone) and the base station, and are called pseudo-random code sequences. Since each user is separated by a unique code, all users can share the same frequency band (range of radio spectrum). This gives many unique advantages to the CDMA technique over other RF techniques in cellular communication.
CDMA is a digital multiple access technique and this cellular aspect of the protocol is specified by the Telecommunications Industry Association (TIA) as IS-95. In CDMA, the BSSAP is divided into the DTAP and BSMAP (which corresponds to BSSMAP in GSM).
Joined: 14 September 2004
A mobile phone is a device which behaves as a normal telephone whilst being able to move over a wide area (cf. cordless phone which acts as a telephone only within a limited range). Mobile phones allow connections to be made to the telephone network, normally by directly dialling the other party's number on an inbuilt keypad. Most current mobile phones use a combination of radio wave transmission and conventional telephone circuit switching, though packet switching is already in use for some parts of the mobile phone network, especially for services such as internet access and WAP.
Mobile phone manufacturers include Audiovox, Kyocera (formerly the handset division of Qualcomm), Motorola, Nokia, Panasonic (Matsushita Electric), Samsung, Sanyo, Siemens, Sony Ericsson, Alcatel, LG and Sagem.
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Mobile phones have a long and varied history that stretches back to the early 1970's. Due to their low establishment costs and rapid deployment, mobile phone networks have since spread rapidly throughout the world, outstripping the growth of fixed telephony. Such networks can often be economic, even with a small customer base, as mobile network costs are mostly call volume related, while fixed-line telephony has a much higher subscriber related cost component.
In most of Europe, wealthy parts of Asia, and Australia, mobile phones are now virtually universal, with the majority of the adult, teenage, and even child population owning one. They are less common in the United States — while widely available, market penetration is lower than elsewhere in the developed world (around 66 percent of the U.S. population as of 2003). Reasons advanced for this include incomplete coverage, relatively high minimum monthly service charges (around $30), and the availability of relatively low-cost fixed-line networks (around $30 for unlimited local calling).
Before the phone can be used, a subscription to a mobile phone operator is required. The operator will issue a SIM card which contains the unique subscription and authentication parameters for that customer. Once the SIM card is inserted into the phone, services can be accessed. Mobile phones do not only support voice calls; they can also send and receive data and faxes (if a computer is attached), send short messages (or "text messages"; see SMS), access WAP services, and provide full Internet access using technologies such as GPRS. Mobile phones usually have a clock and a calculator and often one can play some games on them.
Newer models also allow for sending pictures and have a built-in digital camera. This gives rise to some concern about privacy, in view of possible voyeurism, for example in swimming pools. For this reason, Saudi Arabia has banned cameraphones entirely; South Korea has ordered manufacturers to ensure that all new handsets emit a beep whenever a picture is taken.
Newer models have included many features aimed toward personalisation, such as user defined and downloadable ring tones and logos, and interchangeable covers, which have helped in the uptake by the teenage market. Usually one can choose between a ring tone, a vibrating alert, or a combination of both.
A multi-mode (a.k.a dual, tri or quad band) mobile phone is a phone which is designed to work on more than one GSM radio frequency. The multi-mode case occurs mostly in GSM which originated in the 900 MHz band, but expanded to other bands including 1800 and 1900Mhz bands.
Multi mode phones have been valuable to enable roaming but are now becoming most important in allowing the introduction of WCDMA without customers having to give up the wide coverage of GSM. Almost every single true 3G phone sold is actually a WCDMA/GSM dual-mode mobile. This is also true of 2.75G phones such as those based on CDMA-2000 or EDGE.
The special challenge involved in producing a multi-mode mobile is in finding ways to share the components between the different standards. Obviously, the phone keypad and display should be shared, otherwise it would be hard to treat as one phone. Beyond that, though, there are challenges at each level of integration. How difficult these challenges are depends on the differences between systems. The different variants of the GSM system have only different frequencies and so aren't even considered true multi-mode phones but rather are called multi-band phones. When talking about IS-95/GSM multi-mode phones, for example, or AMPS/IS-95 phones, the base band processing is very different from system to system. This leads to real difficulties in component integration and so to larger phones.
An interesting special case of multi-mode phones is the WCDMA/GSM phone. The radio interfaces are very different from each other, but mobile to core network messaging has some quite strong similarities, meaning that software sharing is quite easy. Probably more importantly, the WCDMA air interface has been designed with GSM compatibility in mind. It has a special mode of operation, known as punctured mode, in which, instead of transmitting continuously, the mobile is able to stop sending for a short period and try searching for GSM carriers in the area. This mode allows for safe inter-frequency handovers with channel measurements which can only be approximated using "pilot signals" in other CDMA based systems.
A final interesting case is that of mobiles covering DS-WCDMA and MC-CDMA the 3G variant of CDMA-2000. Initially, the chip rate of these phones was incompatible. As part of the negotiations related to patents, it was agreed to use compatible chip rates. This should mean that, despite the fact that the air and system interfaces are quite different, even on a philosophical level, much of the hardware for each system inside a phone should be common with differences being mostly confined to software.
The use of mobile phones is generally forbidden on aircraft during flight. One reason given for this is that the mobile phone could interfere with the sensitive equipment on the aircraft. This could be restated as "during development these aircraft were not designed to accept signals from mobile phones and there has not been sufficient testing to be sure that they could" as can be seen from plans to improve certification  (http://www.caa.co.uk/docs/389/srg_sys_00002-01-300103.pdf). Some level of electromagnetic interference is theoretically possible from active radio transmitters such as mobile phones on aircraft. Exactly how much and in what way is dependent on the particular phone system in use and the plane component in question. Whether that level of interference should have any influence on electronic systems which should be designed to fly through lightning storms without falling out of the sky is an entirely different question.
One area in which interference would be most likely is in the radio-based audio equipment used for voice communications between the aeroplane and the ground. The mobile phone transmitter is much closer to the receiver on the aircraft than the ground station, but operates at a lower power than the ground station.
Some mobile phone systems such as GSM may cause an irritating buzz (explained in the TDMA article) which would certainly disrupt communications from the pilot to ground. The high speed of air travel may make interference more likely than it would otherwise be. The maximum speed of travel in a mobile phone system is limited by several factors, frequency changes, rate of change of timing offset, etc. The speed of an aeroplane often exceeds these (typically phones are designed for use in a fast car) which means the mobile will fail to register to the network and retry registration repeatedly.
Older analogue systems simply broadcast at a high power of up to several watts. This has the potention to cause more general interference, and since the voice signal is not encoded there may be direct crosstalk into the communication systems of the plane.
Another factor is that from an altitude, distant cells are visible to the mobile with no line-of-sight attenuation from intervening obstacles. This means that the phone could try to establish contact with a far away cell where the signal will not be recognised. This transmission will probably be at maximum power due to the lack of prior response. The U.S. Federal Communications Commission prohibits the use of mobile phones in the air for this reason. This repeated sending of maximum power messages increases the risk of interference with electronic equipment on the aircraft.
All of the above having been said, according to the BBC "most of the evidence is circumstantial and anecdotal. There is no absolute proof mobile phones are hazardous."  (http://news.bbc.co.uk/2/hi/uk_news/399154.stm) Some airlines do allow use of mobiles phones in flight, only restricting their use (and use of all other electronic devices) during take off and landing when communications with the ground are most critical. Meanwhile the passenger aircraft manufacturers, such as Boeing and Airbus, have begun to introduce wireless services on their planes (e.g. WLAN) and radio based satellite phones are a standard installation on aeroplanes. Clearly there is a direct airline industry advantage in having control over communication systems from within an aeroplane, with no clear way for potential competitors to certify their systems as safe for use on board. Some articles have even gone so far as to accuse the airline industry of pushing the ban on mobile phones in order to increase revenue from on board telephones  (http://news.zdnet.co.uk/business/0,39020645,2074198,00.htm). A number of new phones have an "aeroplane mode" feature that presumably stops all incoming and outgoing communications while still allowing the user to play games, type notes etc.
Main article: Mobile phone radiation and health
As with many new technologies, concerns have arisen about the effects on health from using a mobile telephone. There is little scientific evidence for an increase in certain types of rare tumors in long-time, heavy users. More recently a pan-European study provided significant evidence of DNA damage under certain conditions. So far, however, the World Health Organization Task Force on EMF effects on health has no definitive conclusion on the veracity of these allegations. (see also Electromagnetic radiation hazard).
Earlier mobile phones were fairly simple and security wasn't much of a concern, but in 2004, even basic phones can send and receive text messages which makes them vulnerable to attack by worms and viruses. Advanced phones capable of e-mail can be susceptible to viruses that can multiply by sending messages through a phone's address book. Of more important concern, a virus may allow unauthorized users to access a phone to find passwords or corporate data stored on the device. Moreover, they can be used to commandeer the phone to make calls or send messages at the owner's expense. Unlike computers that are restricted to only a few widespread operating systems, cellular phones use a variety of systems that require separate programs to be designed in order to disable each one. While reducing overall compatibilty from an application design standpoint, this has the beneficial effect of making it harder to design a mass attack. However, the rise of cellular phone operating system programming platforms shared by many manufacturers such as Java, Microsoft operating systems, Linux or Symbian OS, may in the future change this status quo.
Bluetooth is a wireless communication feature now found in many higher-end phones, and the virus Cabir hijacked this function, sending Bluetooth phones on a search-and-destroy mission to infect other Bluetooth phones. In early November 2004, several web sites began offering a specific piece of software promising ringtones and screensavers for certain phones. Those who downloaded the software found that it turned each icon on the phone's screen into a skull-and-crossbones and disabled their phones, so they could no longer send or receive text messages or access contact lists or calendars. The virus has since been dubbed "Skulls" by security experts.
There is a great deal of active research and development into mobile phone technology that is currently underway. Some of the improvements that are being worked on are:
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