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GSM Carrier Frequencies
    GSM Carrier Frequencies
    GSM carrier frequencies

    GSM networks operate in a number of different carrier frequency ranges (separated into GSM frequency ranges for 2G and UMTS frequency bands for 3G). Most 2G GSM networks operate in the 900 MHz or 1800 MHz bands. Some countries in the Americas (including Canada and the United States) use the 850 MHz and 1900 MHz bands because the 900 and 1800 MHz frequency bands were already allocated. Most 3G GSM EDGE networks in Europe operate in the 2100 MHz frequency band.

    The rarer 400 and 450 MHz frequency bands are assigned in some countries where these frequencies were previously used for first-generation systems.

    GSM-900 uses 890C915 MHz to send information from the mobile station to the base station (uplink) and 935C960 MHz for the other direction (downlink), providing 124 RF channels (channel numbers 1 to 124) spaced at 200 kHz. Duplex spacing of 45 MHz is used.

    In some countries the GSM-900 band has been extended to cover a larger frequency range. This 'extended GSM', E-GSM, uses 880C915 MHz (uplink) and 925C960 MHz (downlink), adding 50 channels (channel numbers 975 to 1023 and 0) to the original GSM-900 band. Time division multiplexing is used to allow eight full-rate or sixteen half-rate speech channels per radio frequency channel. There are eight radio timeslots (giving eight burst periods) grouped into what is called a TDMA frame. Half rate channels use alternate frames in the same timeslot. The channel data rate for all 8 channels is 270.833 kbit/s, and the frame duration is 4.615 ms.

    The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.

    [edit] Voice codecs

    GSM has used a variety of voice codecs to squeeze 3.1 kHz audio into between 6.5 and 13 kbit/s. Originally, two codecs, named after the types of data channel they were allocated, were used, called Half Rate (6.5 kbit/s) and Full Rate (13 kbit/s). These used a system based upon linear predictive coding (LPC). In addition to being efficient with bitrates, these codecs also made it easier to identify more important parts of the audio, allowing the air interface layer to prioritize and better protect these parts of the signal.

    GSM was further enhanced in 1997[13] with the Enhanced Full Rate (EFR) codec, a 12.2 kbit/s codec that uses a full rate channel. Finally, with the development of UMTS, EFR was refactored into a variable-rate codec called AMR-Narrowband, which is high quality and robust against interference when used on full rate channels, and less robust but still relatively high quality when used in good radio conditions on half-rate channels.

    [edit] Network structure

    The structure of a GSM network

    The network behind the GSM seen by the customer is large and complicated in order to provide all of the services which are required. It is divided into a number of sections and these are each covered in separate articles.

    [edit] Subscriber Identity Module (SIM)

    One of the key features of GSM is the Subscriber Identity Module, commonly known as a SIM card. The SIM is a detachable smart card containing the user's subscription information and phone book. This allows the user to retain his or her information after switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM, or only a SIM issued by them; this practice is known as SIM locking and is illegal in some countries.

    [edit] Phone locking

    Sometimes mobile phone operators restrict handsets that they sell for use with their own network. This is called locking and is implemented by a software feature of the phone because the purchase price of the mobile phone to the consumer is typicallysubsidised with revenue from subscriptions, and operators must recoup this investment before a subscriber terminates service. A subscriber may usually contact the provider to remove the lock for a fee, utilize private services to remove the lock, or make use of free or fee-based software and websites to unlock the handset themselves.

    In some territories (e.g., BangladeshHong KongPakistanIndia) all phones are sold unlocked. In others (e.g., BelgiumFinland) it is unlawful for operators to offer any form of subsidy on a phone's price.

    [edit] GSM service security

    GSM was designed with a moderate level of service security. The system was designed to authenticate the subscriber using a pre-shared key and challenge-response. Communications between the subscriber and the base station can be encrypted. The development of UMTS introduces an optional Universal Subscriber Identity Module (USIM), that uses a longer authentication key to give greater security, as well as mutually authenticating the network and the user - whereas GSM only authenticates the user to the network (and not vice versa). The security model therefore offers confidentiality and authentication, but limited authorization capabilities, and no non-repudiation.

    GSM uses several cryptographic algorithms for security. The A5/1 and A5/2 stream ciphers are used for ensuring over-the-air voice privacy. A5/1 was developed first and is a stronger algorithm used within Europe and the United States; A5/2 is weaker and used in other countries. Serious weaknesses have been found in both algorithms: it is possible to break A5/2 in real-time with a ciphertext-only attack, and in February 2008, Pico Computing, Inc revealed its ability and plans to commercialize FPGAs that allow A5/1 to be broken with a rainbow table attack.[14] The system supports multiple algorithms so operators may replace that cipher with a stronger one.

    On 28 December 2009 German computer engineer Karsten Nohl announced that he had cracked the A5/1 cipher.[15] According to Nohl, he developed a number of rainbow tables (static values which reduce the time needed to carry out an attack) and have found new sources for known plaintext attacks. He also said that it is possible to build "a full GSM interceptor ... from open source components" but that they had not done so because of legal concerns.[16]

    In 2010, reported that "A group of cryptographers has developed a new attack that has broken Kasumi, the encryption algorithm used to secure traffic on 3G GSM wireless networks. The technique enables them to recover a full key by using a tactic known as a related-key attack, but experts say it is not the end of the world for Kasumi."[17] Kasumi is the name for theA5/3 algorithm, used to secure most 3G GSM EDGE traffic.

    Although security issues remain for GSM newer standards and algorithms may address this. New attacks are growing in the wild which take advantage of poor security implementations, architecture and development for smart phone applications. Some wiretapping and eavesdropping techniques hijack[18] the audio input and output providing an opportunity for a 3rd party to listen in to the conversation. Although this threat is mitigated by the fact the attack has to come in the form of a Trojan, malware or a virus and might be detected by security software.

    [edit] Standards information

    The GSM systems and services are described in a set of standards governed by ETSI, where a full list is maintained.[19]

    [edit] Example specifications

    • GSM 07.07 "AT command set for GSM Mobile Equipment (ME)" describes the Main AT commands to communicate via a serial interface with the GSM subsystem of the phone.[20] For more, see Hayes command set.
    • 3GPP TS 27.007 - AT command set for User Equipment (UE).[21]
    • GSM 07.05 has additional AT commands for SMS and CBS.[22][23]

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