GSM Technology
Introduction:
What is TDMA?
TDMA (time division multiple access) is a technology used in digital cellular telephone communication to divide each cellular channel into three time slots in order to increase the amount of data that can be carried.
How it Works?
TDMA works by time-division multiplexing: sending multiple signals (each of which has its own time slot) simultaneously on a single carrier in the form of a complex signal, and then recovering the separate signals at the receiving end. For TDMA, the carrier is divided into three time slots, each of which serves one subscriber. The information is broken into tiny data packets, which are transmitted in timed bursts in the 30-megahertz range. At the receiving end, the separate information streams are recovered. See also FDMA (frequency division multiple access) and CDMA (code-division multiple access).
TDMA was developed in response to the basic wireless network problem:
large numbers of users and limited frequency allotments. TDMA increases network efficiency by enabling single connections to carry multiple data channels, offering a three-fold increase in capacity over Advanced Mobile Phone Service (AMPS) networks. Flexible and scalable, TDMA facilitates step-by-step migration to digital operation. TDMA can be implemented seamlessly across both 800- and 1900-MHz networks. Its hierarchical cell structure allows service providers to increase capacity where demand is greatest, in high-use areas.
TDMA is applied in Digital-American Mobile Phone Service, Global System for Mobile communications, and Personal Digital Cellular (PDC). However, each of these systems implements TDMA in a somewhat different and incompatible way. TDMA was first specified as a standard in EIA/TIA Interim Standard 54 (IS-54). IS-136, an evolved version of IS-54, is the United States standard for TDMA for both the cellular (850 MHz) and personal communications services (1.9 GHz) spectrums. TDMA is also used for Digital Enhanced Cordless Telecommunications.
Code Division Multiple Access (CDMA):
The term CDMA refers to any of several protocols used in so-called second-generation (2G) and third-generation (3G) wireless communications. As the term implies, CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. The technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands. CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum technology. Audio input is first digitized into binary elements. The frequency of the transmitted signal is then made to vary according to a defined pattern (code), so it can be intercepted only by a receiver whose frequency response is programmed with the same code, so it follows exactly along with the transmitter frequency. There are trillions of possible frequency-sequencing codes; this enhances privacy and makes cloning difficult.
The CDMA channel is nominally 1.23 MHz wide. CDMA networks use a scheme called soft handoff, which minimizes signal breakup as a handset passes from one cell to another. The combination of digital and spread-spectrum modes supports several times as many signals per unit bandwidth as analog modes. CDMA is compatible with other cellular technologies; this allows for nationwide roaming. The original CDMA standard, also known as CDMA One and still common in cellular telephones in the U.S., offers a transmission speed of only up to 14.4 Kbps in its single channel form and up to 115 Kbps in an eight-channel form. CDMA2000 and wideband CDMA deliver data many times faster.
Global System for Mobile communication (GSM):
What is GSM?
The Global System for Mobile communication, usually called GSM, Telecommunications Standards Institute (ETSI) to describe protocols for second generation (2G) digital cellular networks used by mobile phones. The GSM standard was developed as a replacement for first generation (1G) analog cellular networks, and originally described a digital, circuit switched network optimized for full duplex voice telephony. This was expanded over time to include data communications, first by circuit switched transport, then packet data transport via GPRS (General Packet Radio Services) and EDGE (Enhanced Data rates for GSM Evolution or EGPRS). Further improvements were made when the 3GPP developed third generation (3G) UMTS standards followed by fourth generation (4G)LTE Advanced standards. "GSM" is a trademark owned by the GSM Association.
GSM is a cellular network, which means that mobile phones connect to it by searching for cells in the immediate vicinity.
The ubiquity of the GSM standard makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. GSM differs significantly from its predecessors in that both signalling and speech channels are Digital call quality, which means that it is considered a second generation (2G) mobile phone system. This fact has also meant that data communication was built into the system from the 3rd Generation Partnership Project (3GPP).
GSM is a digital mobile telephone system that is widely used in Europe and other parts of the world. GSM uses a variation of time division multiple access (Time Division Multiple Access) and is the most widely used of the three digital wireless telephone technologies (TDMA, GSM, and CDMA). GSM digitizes and compresses data, then sends it down a channel with two other streams of user data, each in its own time slot. It operates at either the 900 MHz or 1800 MHz frequency band.
GSM is the de facto wireless telephone standard in Europe. GSM has over 120 million users worldwide and is available in 120 countries, according to the GSM MOU Association. Since many GSM network operators have roaming agreements with foreign operators, users can often continue to use their mobile phones when they travel to other countries.
American Personal Communications (APC), a subsidiary of Sprint, is using GSM as the technology for a broadband personal communications service (personal communications services). The service will ultimately have more than 400 base stations for the palm-sized handsets that are being made by Ericsson, Motorola, and Nokia. The handsets include a phone, a text pager, and an answering machine.
GSM together with other technologies is part of an evolution of wireless mobile telecommunication that includes High-Speed Circuit-Switched Data (High-Speed Circuit-Switched Data), General Packet Radio System (General Packet Radio Services), Enhanced Data GSM Environment (Enhanced Data GSM Environment), and Universal Mobile Telecommunications Service (Universal Mobile Telecommunications System).
The Generations of Mobile Networks
The idea of cell-based mobile radio systems appeared at Bell Laboratories in the United States in the early 1970s. However, mobile cellular systems were not introduced for commercial use until a decade later. During the early 1980’s, analog cellular telephone systems experienced very rapid growth in Europe, particularly in Scandinavia and the United Kingdom. Today, cellular systems still represent one of the fastest growing telecommunications systems. During development, numerous problems arose as each country developed its own system, producing equipment limited to operate only within the boundaries of respective countries, thus limiting the markets in which services could be sold.
First-generation cellular networks, the primary focus of the communications industry in the early 1980’s, were characterized by a few compatible systems that were designed to provide purely local cellular solutions. It became increasingly apparent that there would be an escalating demand for a technology that could facilitate flexible and reliable mobile communications. By the early 1990’s, the lack of capacity of these existing networks emerged as a core challenge to keeping up with market demand. The first mobile wireless phones utilized analog transmission technologies, the dominant analog standard being known as “AMPS”, (Advanced Mobile Phone System). Analog standards operated on bands of spectrum with a lower frequency and greater wavelength than subsequent standards, providing a significant signal range per cell along with a high propensity for interference. Nonetheless, it is worth noting the continuing persistence of analog (AMPS) technologies in North America and Latin America through the 1990’s.
Initial deployments of second-generation wireless networks occurred in Europe in the 1980’s. These networks were based on digital, rather than analog technologies, and were circuit-switched. Circuit-switched cellular data is still the most widely used mobile wireless data service. Digital technology offered an appealing combination of performance and spectral efficiency (in terms of management of scarce frequency bands), as well as the development of features like speech security and data communications over high quality transmissions. It is also compatible with Integrated Services Digital Network (ISDN) technology, which was being developed for land-based telecommunication systems throughout the world, and which would be necessary for GSM to be successful. Moreover in the digital world, it would be possible to employ very large-scale integrated silicon technology to make handsets more affordable.
To a certain extent, the late 1980’s and early 1990’s were characterized by the perception that a complete migration to digital cellular would take many years, and that digital systems would suffer from a number of technical difficulties (i.e., handset technology). However, second-generation equipment has since proven to offer many advantages over analog systems, including efficient use of radio-magnetic spectrum, enhanced security, extended battery life, and data transmission capabilities. There are four main standards for 2G networks: Time Division Multiple Access (TDMA), Global System for Mobile Communications (GSM) and Code Division Multiple Access (CDMA); there is also Personal Digital Cellular (PDC), which is used exclusively in Japan. (See Figure 1.1) In the meantime, a variety of 2.5G standards (to be discussed in Section 2.7) have been developed. ‘Going digital’ has led to the emergence of several major 2G mobile wireless systems.
History of GSM:
Early European analog cellular networks consisted of a mix of technologies and protocols that varied from country to country, meaning that phones did not necessarily work on different networks. In addition, manufacturers had to produce different equipment to meet various standards across the markets.
In 1982, work began to develop a European standard for digital cellular voice telephony when the European Conference of Postal and Telecommunications Administrations (CEPT) created the Groupe Spécial Mobile committee and provided a permanent group of technical support personnel, based in Paris. Five years later in 1987, 15 representatives from 13 European countries signed a memorandum of understanding in Copenhagen to develop and deploy a common cellular telephone system across Europe, and European Union rules were passed to make GSM a mandatory standard. The decision to develop a continental standard eventually resulted in a unified, open, standard-based network which was larger than that in the United States. In 1989, the Groupe Spécial Mobile committee was transferred from CEPT to the European Telecommunications Standards Institute(ETSI).
In parallel, France and Germany signed a joint development agreement in 1984 and were joined by Italy and the UK in 1986. In 1986 the European Commission proposed reserving the 900 MHz spectrum band for GSM.
Phase I of the GSM specifications were published in 1990. The world's first GSM call was made by the Finnish prime minister Harri Holkeri to Kaarina Suonio (mayor in city ofTampere) on 1 July 1991 on a network built by Telenokia and Siemens and operated by Radiolinja. The following year in 1992, the first short messaging service (SMS or "text message") message was sent and Vodafone UK and Telecom Finland signed the first international roaming agreement.
Work begun in 1991 to expand the GSM standard to the 1800 MHz frequency band and the first 1800 MHz network became operational in the UK by 1993. Also that year, Telecom Australia became the first network operator to deploy a GSM network outside Europe and the first practical hand-held GSM mobile phone became available.
In 1995, fax, data and SMS messaging services were launched commercially, the first 1900 MHz GSM network became operational in the United States and GSM subscribers worldwide exceeded 10 million. Also this year, the GSM Association was formed. Pre-paid GSM SIM cards were launched in 1996 and worldwide GSM subscribers passed 100 million in 1998.
In 2000, the first commercial GPRS services were launched and the first GPRS compatible handsets became available for sale. In 2001 the first UMTS (W-CDMA) network was launched and worldwide GSM subscribers exceeded 500 million. In 2002 the first multimedia messaging services (MMS) were introduced and the first GSM network in the 800 MHz frequency band became operational. EDGE services first became operational in a network in 2003 and the number of worldwide GSM subscribers exceeded 1 billion in 2004.
By 2005, GSM networks accounted for more than 75% of the worldwide cellular network market, serving 1.5 billion subscribers. In 2005, the first HSDPA capable network also became operational. The first HSUPA network was launched in 2007 and worldwide GSM subscribers exceeded two billion in 2008.
The GSM Association estimates that technologies defined in the GSM standard serve 80% of the global mobile market, encompassing more than 5 billion people across more than 212 countries and territories, making GSM the most ubiquitous of the many standards for cellular networks.
Macau phased out their GSM network in January 2013 (except for roaming services), making it the first region to decommission a GSM network.
Architecture of the GSM network:
A GSM network is composed of several functional entities, whose functions and interfaces are specified. Figure 1 shows the layout of a generic GSM network. The GSM network can be divided into three broad parts. The Mobile Station is carried by the subscriber. The Base Station Subsystem controls the radio link with the Mobile Station. The Network Subsystem, the main part of which is the Mobile services Switching Center (MSC), performs the switching of calls between the mobile users, and between mobile and fixed network users. The MSC also handles the mobility management operations. The Mobile Station and the Base Station Subsystem communicate across the Um interface, also known as the air interface or radio link. The Base Station Subsystem communicates with the Mobile services Switching Center across the A interface.
Figure . General architecture of a GSM network
Mobile Station
The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card called the Subscriber Identity Module (SIM). The SIM provides personal mobility, so that the user can have access to subscribed services irrespective of a specific terminal. By inserting the SIM card into another GSM terminal, the user is able to receive calls at that terminal, make calls from that terminal, and receive other subscribed services.
The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret key for authentication, and other information. The IMEI and the IMSI are independent, thereby allowing personal mobility. The SIM card may be protected against unauthorized use by a password or personal identity number.
Base Station Subsystem
The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the Base Station Controller (BSC). These communicate across the standardized Abis interface, allowing (as in the rest of the system) operation between components made by different suppliers.
The Base Transceiver Station houses the radio transceivers that define a cell and handles the radio-link protocols with the Mobile Station. In a large urban area, there will potentially be a large number of BTSs deployed, thus the requirements for a BTS are ruggedness, reliability, portability, and minimum cost.
The Base Station Controller manages the radio resources for one or more BTSs. It handles radio-channel setup, frequency hopping, and handovers. The BSC is the connection between the mobile station and the Mobile service Switching Center (MSC).
Network Subsystem
The central component of the Network Subsystem is the Mobile services Switching Center (MSC). It acts like a normal switching node of the PSTN or ISDN, and additionally provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. The MSC provides the connection to the fixed networks (such as the PSTN or ISDN). Signaling between functional entities in the Network Subsystem uses Signaling System Number 7 (SS7), used for trunk signaling in ISDN and widely used in current public networks.
The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC, provide the call-routing and roaming capabilities of GSM. The HLR contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the mobile. The location of the mobile is typically in the form of the signaling address of the VLR associated with the mobile station. There is logically one HLR per GSM network, although it may be implemented as a distributed database.
The Visitor Location Register (VLR) contains selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. The geographical area controlled by the MSC corresponds to that controlled by the VLR. Note that the MSC contains no information about particular mobile stations --- this information is stored in the location registers.
The other two registers are used for authentication and security purposes. The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where each mobile station is identified by its International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it has been reported stolen or is not type approved. The Authentication Center (AuC) is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and encryption over the radio channel.
GSM frequencies using around the world:
In North America, GSM operates on the primary mobile communication bands 850 MHz and 1,900 MHz. In Canada, GSM-1900 is the primary band used in urban areas with 850 as a backup, and GSM-850 being the primary rural band. In the United States, regulatory requirements determine which area can use which band.
GSM-1900 and GSM-850 are also used in most of South and Central America, and both Ecuador and Panama use GSM-850 exclusively (Note: Since November 2008, a Panamanian operator has begun to offer GSM-1900 service). Venezuela and Brazil use GSM-850 and GSM-900/1800 mixing the European and American bands. Some countries in the Americas use GSM-900 or GSM-1800, some others use three: GSM-850/900/1900, GSM-850/1800/1900, GSM-900/1800/1900 or GSM-850/900/1800. Soon some countries will use GSM-850/900/1800/1900 MHz like the Dominican Republic, Trinidad & Tobago and Venezuela.
In Brazil, the 1,900 MHz band is paired with 2,100 MHz to form the IMT-compliant 2,100 MHz band for 3G services. The result is a mixture of usage in the Americas that requires travelers to confirm that the phones they have are compatible with the band of the networks at their destinations.Frequency compatibility problems can be avoided through the use of multi-band (tri-band or, especially, quad-band) phones.
Africa, Europe, Middle East and Asia
In Africa, Europe, Middle East and Asia, most of the providers use 900 MHz and 1800 MHz bands. GSM-900 is most widely used. Fewer operators use DCS-1800 and GSM-1800. A dual-band 900/1800 phone is required to be compatible with almost all operators. At least the GSM-900 band must be supported in order to be compatible with many operators. However, Thailand has also approved for some time now the use of the GSM-1900 band in an attempt to alleviate network congestion.
GSM SECURITY:
The security features in the GSM network can be divided into three sub parts: subscriber identity authentication, user and signaling data confidentiality, and subscriber identity confidentiality. The security mechanisms include secret keys, algorithms and computed numbers.
Some definitions:
• Authentication – any technique that enables the receiver to automatically identify and reject messages that have been altered deliberately or by channel errors
• Confidentiality – only the sender and intended receiver should be able to understand the contents of the transmitted message.
• Cipher text – plaintext is encrypted to cipher text with the help of a key and an encryption algorithm
• Key – a string of numbers or characters as input to the encryption algorithm
The base mechanism shows where the different keys and algorithms are stored. The secret key Ki is used to authenticate the identity of a subscriber. The key Ki is given to the subscriber when he opens a new network account. Only the network operator knows the key. The Ki is stored in the subscribers SIM card and the authentication center (AuC) of the subscribers home network. The Ki is never transmitted over the network.
Figure: Base of the security mechanism.
A3 is the algorithm used to authenticate the subscriber. Data transmitted between the MS (Mobile Station) and the BTS (Base Transceiver Station) is encrypted by the A5 algorithm. The A8 algorithm generates the needed ciphering key Kc used by A5.
Subscriber Identity Authentication. The procedure consists of three phases, (1) the network must identify the subscriber, (2) needed security parameters from the home network are asked for and (3) the actual authentication is taking place.
Figure. Subscriber identification process.
In order to identify the subscriber the MS sends the IMSI (International Mobile Subscriber Identity) to the visited network. With the IMSI the subscriber is identified to the system. The IMSI is up to 15 digits and comprises the following parts:
• A 3-digit Mobile Country Code (MCC). This identifies the country where the GSM system operates. Finland has number 244.
• A 2-digit Mobile Network Code (MNC). This uniquely identifies each cellular provider. Sonera has number 91.
• The Mobile Subscriber Identification Code (MSIC).This uniquely identifies each customer of the provider. The length is 10 digits.
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So called security triplets are calculated in the AuC. The triplets consist of a random number (RAND), a signed response (SRES) and a ciphering key (Kc). The SRES is used to authenticate the subscriber and Kc is used as input by the ciphering algorithm A5.
As the visited network has received the security triplets the actual authentication can take place (see Figure 5). If the number sent by the MS to the BTS is the same as the one calculated by the AuC, the subscriber is authenticated.
Figure. Authentication the subscriber
User and Signaling Data Confidentiality: The Ciphering key (Kc) is used for the final encryption of the radio link. One copy of the needed Kc is stored in the VLR and another copy is calculated in the MS by the A8 algorithm. The same Ki and RAND numbers are used as in the authentication process. The A5 algorithm creates 114-bit sequence. This sequence is then XORed with every 114 user data bits and the resulting bit streams are sent over the two 57 bit parts of every GSM slot. All traffic between the MS and the BTS is then secured.
Subscriber Identity Confidentiality: The IMSI is the primary key for subscriber identification. However a temporary identity, TMSI (Temporary Mobile Subscriber Identity) can be given to a subscriber for identification. After initial registration done with the IMSI, the serving network stores the IMSI in the VLR and generates a TMSI for the subscriber. The TMSI is then transmitted back to the MS and it will be used for identification as long as the subscriber is registered in that specific network.
Solutions to Current Security Issues
A corrected version of the COMP 128 has been developed; however, the cost to replace all SIM chips and include the new algorithm is too costly to cellular phone companies. The new release of 3GSM will include a stronger version of the COMP 128 algorithm and a new A5 algorithm implementation. The A5/3 is expected to solve current confidentiality and integrity problems [4]. Fixed network transmission could be fixed by simply applying some type of encryption to any data transferred on the fixed network.
Channel structure:
Depending on the kind of information transmitted (user data and control signaling), we refer to different logical channels which are mapped under physical channels (slots). Digital speech is sent on a logical channel named TCH, which during the transmission can be a allocated to a certain physical channel. In a GSM system no RF channel and no slot is dedicated to a priori to the exclusive use of anything (any RF channel can be used for number of different uses).
Logical channels are divided into two categories:
i) Traffic Channels (TCHs)
ii)Control Channels .
Traffic Channels (TCHs)
A traffic channel (TCH) is used to carry speech and data traffic. Traffic channels are defined using a 26-frame multiform, or group of 26 TDMA frames. The length of a 26-frame multiform is 120 ms, which is how the length of a burst period is defined (120 ms divided by 26 frames divided by 8 burst periods per frame). Out of the 26 frames, 24 are used for traffic, 1 is used for the Slow Associated Control Channel (SACCH) and 1 is currently unused. TCHs for the uplink and downlink are separated in time by 3 burst periods, so that the mobile station does not have to transmit and receive simultaneously, thus simplifying the electronics
TCHs carry either encoded speech or user data in both up and down directions in a point to point communication.
There are two types of TCHs that are differentiated by their traffic rates.
They are:
i. Full Rate TCH
ii. Half Rate TCH
Full Rate TCH(Also represented as Bm)
It carries information at a gross rate of 22.82 Kbps.
Half Rate TCH
It carries information with half of full rate channels.
Control Channel
Basic structure of Control channel
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Actually in the above diagram S will be at slot 1 of next frame, F is frequency correction channel which occurs every 10th burst. The next frame to S contains service operator’s information.
Logical Control Channel (LCC) s are of three types
They are of the following types:
•Broadcast Control Channel(BCCH)
•Common Control Channel(CCCH)
• Dedicated Control Channel(DCCH)
Broadcast Control Channel (BCCH)
The BCCH is a point-to-multipoint unidirectional control channel from the fixed subsystem to MS that is intended to broadcast a variety of information to MSs, including information necessary for the MS to register in the system. BCCH has 51 bursts. BCCH is dedicated to slot1 and repeats after every 51 bursts.
Broadcast Control Channel (BCCH) continually broadcasts, on the downlink, information including base station identity, frequency allocations, and frequency-hopping sequences. This provides general information per BTS basis (cell specific information) including information necessary for the MS to register at the system. After initially accessing the mobile, the BS calculates the requires MS power level and sets a set of power commands on these channels. Other information sent over these channels includes country code network code, local code, PLMN code, RF channels used within the cell where the mobile is located, and surrounding cells, hopping sequence number, mobile RF channel number for allocation, cell selection parameters, and RACH description. One of the important messages on a BCCH channel is CCCH_CONF, which indicates the organization of the CCCHs. This channel is used to down link point-to-multipoint communication and is unidirectional; there is no corresponding uplink. The signal strength is continuously measured by all mobiles which may seek a hand over from its present cell and thus it is always transmitted on designated RF channel using time slot 0(zero). This channel is never kept idle-either the relevant messages are sent or a dummy burst is sent.
The BCCH includes :
-- Frequency correction channel (FCCH) which is used to allow an MS to accurately tune to a BS. The FCCH carries information for the frequency correction of MS downlink. It is required for the correct operation of radio system. This is also a point-to multipoint communication. This allows an MS to accurately tune to a BS.
-- Synchronization channel (SCH), which is used to provide TDMA frame oriented synchronization data to a MS. When a mobile recovers both FCCH and SCH signals, the synchronization is said to be complete. SCH repeats for every 51 frames. SCH carries information for the frame synchronization (TDMA frame number of the MS and the identification of BTS ) .This is also required for the correct operation of the mobile.
The Synchronization Channel contains 2 encoded parameters:
•BTS identification code (BSIC)
•Reduced TDMA frame number (RFN).
Common Control Channel (CCCH)
A CCCH is a point-to-multipoint (bi-directional control channel) channel that is primarily intended to carry signaling information necessary for access management functions (e.g., allocation of dedicated control channels).
The CCCH includes:
-- paging channel (PCH), which is used to search (page) the MS in the downlink direction
-- random access channel (RACH) which is used by MS to request of an SDCCH either as a page response from MS or call origination/ registration from the MS. This is uplink channel and operates in point-point mode(MS to BTS).This uses slotted ALOHA protocol. This causes a possibility of contention. If the mobiles request through this channel is not answered within a specified time the MS assumes that a collision has occurred and repeats the request. Mobile must allow a random delay before re-initiating the request to avoid repeated collision.
-- access grant channel(AGCH) which is a downlink channel used to assign a MS to a specific SDCCH or a TCH. AGCH operates in point-to-point mode. A combined paging and access grant channel is designated as PAGCH.
Dedicated Control Channel (DCCH)
A DCCH is a point to point, directional control channel.
Two types of DCCHs used are:
Standalone DCCH (SDCCH) is used for system signaling during idle periods and call setup before allocating a TCH, for example MS registration, authentication and location updates through this channel. When a TCH is assigned to MS this channel is released. Its data rate is one-eighth of the full rate speech channel which is achieved by transmitting data over the channel once every eighth frame. The channel is used for uplink and downlink and is meant for point-to-point usage.
Associated Control Channel (ACCH) is a DCCH whose allocation is linked to the allocation of a CCH. A FACCH or burst stealing is a DCCH obtained by pre-emptive dynamic multiplexing on a TCH.
SACCH is data channel carrying information such as measurement reports from the mobile of received signal strength for a serving cell as well as the adjacent cells. This is necessary channel for the assisted over hand over function.
SACCH is also used for power regulation of MS and time alignment and is meant for uplink and down link. It is used for point-to-point communication. SACCH can be linked to TCH or an SDCCH. A FACCH is also associated to TCH .FACCH works in a stealing mode. This means that if suddenly during a speech transmission it is necessary to exchange signaling information with the system at a rate much higher than the SACCH can handle, then 20 ms speech (data) bursts are stolen for signaling purposes. This is the case at the case at the hand over. The interruption of the speech will not be heard by the user since it lasts only for 20 ms and cannot sense by human ears.
Data Transmission:
The GSM standard also provides separate facilities for transmitting digital data. This allows a mobile phone to act like any other computer on the Internet, sending and receiving data via the Internet Protocol. The mobile may also be connected to a desktop computer, laptop, or PDA, for use as a network interface (just like a modem or Ethernet card, but using one of the GSM data protocols described below instead of a PSTN-compatible audio channel or an Ethernet link to transmit data). Some GSM phones can also be controlled by a standardised Hayes AT command set through a serial cable or a wireless link (using IRDA or Bluetooth). The AT commands can control anything from ring tones to data compression algorithms. In addition to general Internet access, other special services may be provided by the mobile phone operator, such as SMS.
Circuit-switched data protocols
A circuit-switched data connection reserves a certain amount of bandwidth between two points for the life of a connection, just as a traditional phone call allocates an audio channel of a certain quality between two phones for the duration of the call. Two circuit-switched data protocols are defined in the GSM standard: Circuit Switched Data (CSD) and High-Speed Circuit-Switched Data (HSCSD). These types of connections are typically charged on a per-second basis, regardless of the amount of data sent over the link. This is because a certain amount of bandwidth is dedicated to the connection regardless of whether or not it is needed. Circuit-switched connections do have the advantage of providing a constant, guaranteed quality of service, which is useful for real-time applications like video conferencing.
General Packet Radio Service (GPRS)
The General Packet Radio Service (GPRS) is a packet-switched data transmission protocol, which was incorporated into the GSM standard in 1997. It is backwards-compatible with systems that use pre-1997 versions of the standard. GPRS does this by sending packets to the local mobile phone mast (BTS) on channels not being used by circuit-switched voice calls or data connections. Multiple GPRS users can share a single unused channel because each of them uses it only for occasional short bursts. The advantage of packet-switched connections is that bandwidth is only used when there is actually data to transmit. This type of connection is thus generally billed by the kilobyte instead of by the second, and is usually a cheaper alternative for applications that only need to send and receive data sporadically, like instant messaging.
Short Message Service (SMS)
Short Message Service (more commonly known as text messaging) has become the most used data application on mobile phones, with 74% of all mobile phone users worldwide already as active users of SMS, or 2.4 billion people by the end of 2007. SMS text messages may be sent by mobile phone users to other mobile users or external services that accept SMS. The messages are usually sent from mobile devices via theShort Message Service Centre using the MAP protocol. The SMSC is a central routing hubs for Short Messages. Many mobile service operators use their SMSCs as gateways to external systems, including the Internet, incoming SMS news feeds, and other mobile operators (often using the de facto SMPP standard for SMS exchange).
STRUCTURE OF TDMA SLOT WITH A FRAME:
There are five different kinds of bursts in the GSM system.
They are
• Normal Burst
• Synchronization Burst
• Frequency Correction Burst
• Access Burst
• Dummy Burst
Normal Burst
This burst is used to carry information on the TCH and on control channels. The lowest bit number is transmitted first. The encrypted bits are 57 bits of data or (speech + 1 bit stealing flag) indicating whether the burst was stolen for FACCH signaling or not. The reason why the training sequence is placed in the middle is that the channel is constantly changing. By having it there, the chances are better that the channel is not too different when it affects the training sequence compared to when the information bits were affected. If the training sequence is put at the beginning of the burst, the channel model that is created might not be valid for the bits at the end of a burst there are 8 training sequences shown at the diagram. The 26 bits equalization patterns are determined at the time of the call setup.
Tail Bits (TB) always equal (0,0,0), which has bit location from 0 to 2 and 145 to 147 . The Guard Period are the empty spaced bits and are used to synchronize the burst with exact accuracy and makes sure that different time slots does not overlap during transmission.
Synchronization Burst
This burst is used for the time synchronization of the mobile. It contains 64 bit synchronization sequence. The encrypted 78 bits carry information of the TDMA frame number along with the BSIC. It is broadcast together with the correction burst. The TDMA frame is broadcast over SCH, in order to protect the user information against eavesdropping, which is accomplished is ciphering the information before transmitting. The algorithm that calculates the ciphering key uses a TDMA frame number as one of the parameters and therefore, every frame must have a frame number. By knowing the TDMA frame number, the mobile will know what kind of logical channel is being transmitted on the control channel TS0. BSIC is also used by the mobile to check the identity of the BTS when making signal strength measurements (to prevent measurements on co-channel cells).
Frequency Correction Burst
This burst is used for frequency synchronization of the mobile. It is equivalent to an un-modulated channel with a specific frequency offset. The repetition of these bursts is called FCCH.
Access Burst
This burst is used for random access and longer GP to protect for burst transmission from a mobile that does not know the timing advance when it must access the system. This allows for a distance of 35 km from base to mobile. In case the mobile is far away from the BTS, the initial burst will arrive late since there is no timing advance on the first burst. The delay must be shorter to prevent it from overlapping a burst in the adjacent time-slot following this.
Dummy Burst
It is sent from BTS on some occasions as discussed previously which carries no information and has the format same as the normal burst.
Conclusion
GSM has many benefits over current cellular systems. The main problem now involves the COMP 128 algorithm problem. This problem will be solved as newer technology gets phased in. The lack of extra encryption on the telecommunications network doesn’t pose as a major problem because any data transfer on there will have the same security as the current public switched telephone networks. Despite the current problems more and more cellular companies will switch to GSM based standards. An estimated one billion subscribers are expected by the end of 2003. As GSM slowly moves towards 3GSM, more problems and security issues will be resolved.
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