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Read operation. A read operation is used to retrieve the current settings used by the GSM/GPRS modem or mobile phone for certain tasks.
Execution operation. An execution operation is used to perform an action or retrieve information/status about the GSM/GPRS modem or mobile phone.
The command syntax for performing an operation will be described in detail in the following sections.
17.1. Test Command -- Checks Whether a Certain AT Command is Supported
A test operation is used to check whether a certain AT command is supported by the GSM/GPRS modem or mobile phone. All extended AT commands support the test operation. The syntax is:
command=?
where command is an AT command. When an AT command is used in the above syntax to perform a test operation, it is called a test command.
Here is an example. The AT command +CGMI (command name in text: Request Manufacturer Identification) is used to get the manufacturer name of the GSM/GPRS modem or mobile phone. To test whether +CGMI is supported, you can make use of the test command "+CGMI=?". The complete command line that should be entered is:
AT+CGMI=?
If the GSM/GPRS modem or mobile phone supports the AT command +CGMI, the result code "OK" will be returned, like this:

AT+CGMI=?
OK
If the GSM/GPRS modem or mobile phone does not support the AT command +CGMI, the result code "ERROR" will be returned, like this:
AT+CGMI=?
ERROR
In the above example, the AT command +CGMI does not have any parameters. If the AT command to be tested has parameter(s), the parameter value(s) supported by the GSM/GPRS modem or mobile phone may be printed additionally. Below is an example that illustrates the format of the response. +COMMAND1 is a fictitious AT command that has four parameters.
AT+COMMAND1=?
+COMMAND1: (0,1),(0-10),(0,1,5-10),("GSM","UCS2")
OK
The supported values of each of the four parameters are enclosed in parentheses. Commas are used to delimit the parentheses and the values inside parentheses. A hyphen is used to indicate a range of values. The values inside parentheses can be of the string type.
In the above example, the response of the test command "+COMMAND1=?" provides us the following information:
(0,1). The first parameter accepts either 0 or 1.
(0-10). The second parameter accepts any integer between 0 and 10.
(0,1,5-10). The third parameter accepts 0, 1 or any integer between 5 and 10.
("GSM","UCS2"). The fourth parameter accepts either the string "GSM" or "UCS2".
To a few AT commands, the test operation does not return the parameter values supported. Instead, it returns the values that are allowed to appear in the information response of the AT command. An example is the +CBC AT command (command name in text: Battery Charge). The +CBC command is used to retrieve the connection status and charge level of the battery of the mobile device. Two values are returned in the information response of the +CBC AT command. The format is:
+CBC: connection_status,charge_level
For example, if the battery is placed in the mobile device with no charger connected and the charge level is 80%, the result of the execution of the +CBC AT command will be:
AT+CBC
+CBC: 0,80
OK
If you run the test command "+CBC=?", all the supported values that are allowed to appear in the connection status field and charge level field will be provided. With my Nokia 6021, the result is:

AT+CBC=?
+CBC: (0,1),(0-100)
OK
"(0,1)" means the connection status field in the information response of the +CBC AT command can contain either 0 or 1, while "(0-100)" means the charge level field can contain any integer between 0 and 100.

In addition to the basic equipment there is new cell phone technology to provide the many new services that are now available, enabling cell phone users to enjoy many new applications from games, and ringtone downloads to picture and video downloads. With technologies ranging from GSM, GPRS, EDGE to UMTS or W-CDMA and cdmaOne (IS-95) to CDMA2000 1X, EV-DO and EV-DV and mobile TV technologies such as MediaFLO, DMB, and DVB-H, there are plenty of technologies in use.
There is plenty of terminology associated with cellular telecommunications and cell phone technology. Our terminology glossary explains the most commonly used terms associated with cellular telecommunications / cell phone technology:
The technology behind cellular systems and cell phones has developed from the first generation (1G) systems to the second geenration (2G) systems and then to the third generation (3G) systems. At each stage the performance improved and further facilities were available, from SMS messaging to video downloads.
- Major Mobile Phone Systems (a tabular overview)
- The Route from Analogue to 3G
Cellular telecommunications basics.
There are a number of basic concepts behind cellular telecommunications systems. These include the idea of cells themselves as well as how the networks are set up, what is in a mobile phone and how some of the transmission technologies such as CDMA, TDMA and the like operate.
- The basic concept of a cellular telecommunications system
- The multiple access schemes used by mobile phones
- The duplex schemes used by mobile phone networks - FDD and TDD
- The operation and electronics within a mobile phone
- The basics of a cellular network
- The way in which a mobile phone registers onto a network
- Handover or handoff, the way in which cellular calls are transferred from one cell to another
- Code Division Multiple Access (CDMA) What it is and how it works
- Orthogonal Frequency Division Multiplex (OFDM) What it is and how it works
- MIMO - Multiple Input Multiple Output What it is and how it works
Cellular testing
With vast numbers of phones and cellular systems deployed and new ones beong launched all the time, it is essential that new equipment is thoroughly tested ebfore it is launched ontoth e market. Additionally ones systems are deployed it is essential to esnure that they are operating to their best.
- Cellular conformance testing overview
- Conformance testing for GSM and UMTS
- cdma interoperability test
Global System for Mobile Communications (GSM).
GSM is the system that was developed in Europe and is now the most popular system in use around the globe.
- The development of the Global System for Mobile communications - GSM
- GSM technical overview (5 pages)
- General Packet Radio Service, GPRS, tutorial (5 pages)
- Enhanced Data for GSM Evolution (EDGE)
- UMA - unlicensed mobile access and GAN generic access network
- UMA / GAN conformance testing
CDMA System including cdmaOne (IS-95) and CDMA2000 (IS-2000)
The CDMA family of technologies including cdmaOne (IS-95) to CDMA 2000 were the first technologies to use CDMA technology.
- What is CDMA - cdmaOne / CDMA2000? A Guide to the CDMA system and its evolution.
- IS-95 (cdmaOne)
- CDMA2000 1xEV-DO
- CDMA Band Classes and Frequency Allocations
- UMB Ultra-Mobile Broadband the 3.99G evolution for cdam2000
Universal Mobile Telecommunications System (UMTS) / Wideband CDMA (W-CDMA)
- UMTS / W-CDMA Tutorial (5 pages)
- High Speed Downlink Packet Access (HSDPA)
- High Speed Uplink Packet Access (HSUPA) The companion to HSDPA.
- UMTS TDD The scheme being used for mobile broadband.
- 3G LTE - Long Term Evolution an overview of the developments being undertaken for the next generation cellualr systems The scheme being used for mobile broadband.
Miscellaneous sytems and features
- Pacific or Personal Digital Cellular (PDC)
- i-mode for e-mails and internet surfing
- TD-SCDMA the 3G system being developed in China
Location services:
- Assisted GPS
Mobile video:
- Mobile video broadcasting video to phones using T-DMB and DVB-H
- DVB-H Digital Video Broadcasting - Handheld
- DMB Digital Multimedia Broadcasting
Private Mobile Radio PMR
Private or Professional Mobile Radio (PMR), or Public Access Mobile Radio (PAMR) is used in many countries around the world, and new technology is being developed to ensure it keeps up with today's needs. It is widely used by the emergency services, and this is particularly true of the new Tetra system that is being widely deployed.
An overview of private mobile radio (PMR) - the various standards that are available to use.
An overview of the basic "local" PMR system
Trunking using the MPT1327 system
An Overview of TETRA private mobile radio (PMR)

To overcome this, a variety of voice coding systems or vocoders are used. These systems involve analysing the incoming data that represents the speech and then performing a variety of actions upon it to reduce the data rate. At the receiving end the reverse process is undertaken to re-constitute the speech data so that it can be understood. In GSM a variety of vocoders are used, including LPC-RPE, EFR, etc as described in the following paragraphs.
The vocoder that was originally used in the GSM system was the LPC-RPE (Linear Prediction Coding with Regular Pulse Excitation) vocoder. This vocoder took each 20 mS block of speech and then represented it using just 260 bits. This actually equates to a data rate of 13 kbps.
In GSM it is recognised that some bits are more important than others. If some bits are missed or corrupted, it is more important to the voice quality than others. Accordingly the different bits are classified:
Class Ia 50 bits - most important and sensitive to bit errors
Class Ib 132 bits - moderately sensitive to bit errors
Class II 78 bits - least sensitive to bit errors
The 50 Class 1a bits are given a 3 bit Cyclic Redundancy Code (CRC) so that errors can be detected. This makes a total length of 53 bits. If there are any errors, the frame is not used, and it is discarded. In its place a version of the previously correctly received frame is used. These 53 bits, together with the 132 Class Ib bits with a 4 bit tail sequence, are entered into a 1/2 rate convolutional encoder. The total length is 189 bits. The encoder encodes each of the bits that enter as two bits, the output also being dependent upon a combination of the previous 4 input bits. As a result the output from the convolutional encoder consists of 378 bits. The remaining 78 Class II bits are considered the least sensitive to errors and they are not protected and simply added to the data. In this way every 20 ms speech sample generates a total of 456 bits. Accordingly the overall bit rate is 22.8 kbps. Once in this format the data is interleaved to add further protection against interference and noise.
The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time-slots, i.e. a total of four bursts as each burst takes two sets of data.
Later another vocoder called the Enhanced Full Rate (EFR) vocoder was added in response to the poor quality perceived by the users. This new vocoder gave much better sound quality and was adopted by GSM. Using the ACELP (Algebraic Code Excitation Linear Prediction) compression technology it gave a significant improvement in quality over the original LPC-RPE encoder. It became possible as the processing power that was available increased in mobile phones as a result of higher levels of processing power combined with their lower current consumption.
There is also a half rate vocoder. Although this gives much inferior voice quality, it does allow for an increase in network capacity. It is used in some instances when network loading is very high to accommodate all the calls.

These are then used by different logical channels to transfer information. These channels may either be used for user data (payload) or signalling to enable the system to operate correctly.
Common and dedicated channels
The channels may also be divided into common and dedicated channels. The forward common channels are used for paging to inform a mobile of an incoming call, responding to channel requests, and broadcasting bulletin board information. The return common channel is a random access channel used by the mobile to request channel resources before timing information is conveyed by the BSS.
The dedicated channels are of two main types: those used for signalling, and those used for traffic. The signalling channels are used for maintenance of the call and for enabling call set up, providing facilities such as handover when the call is in progress, and finally terminating the call. The traffic channels handle the actual payload.
The following logical channels are defined in GSM:
TCHf - Full rate traffic channel.
TCH h - Half rate traffic channel.
BCCH - Broadcast Network information, e.g. for describing the current control channel structure. The BCCH is a point-to-multipoint channel (BSS-to-MS).
SCH - Synchronisation of the MSs.
FCHMS - frequency correction.
AGCH - Acknowledge channel requests from MS and allocate a SDCCH.
PCHMS - terminating call announcement.
RACHMS - access requests, response to call announcement, location update, etc.
FACCHt - For time critical signalling over the TCH (e.g. for handover signalling). Traffic burst is stolen for a full signalling burst.
SACCHt - TCH in-band signalling, e.g. for link monitoring.
SDCCH - For signalling exchanges, e.g. during call setup, registration / location updates.
FACCHs - FACCH for the SDCCH. The SDCCH burst is stolen for a full signalling burst. Function not clear in the present version of GSM (could be used for e.g. handover of an eight-rate channel, i.e. using a "SDCCH-like" channel for other purposes than signalling).
SACCHs - SDCCH in-band signalling, e.g. for link monitoring.

The GSM system uses digital TDMA technology combined with a channel bandwidth of 200 kHz. Accordingly the system is able to offer a higher level of spectrum efficiency that that which was achieved with the previous generation of analogue systems. As there are many carrier frequencies that are available, one or more can be allocated to each base station. The system also operates using Frequency Division Duplex and as a result, paired bands are needed for the up and downlink transmissions. The frequency separation is dependent upon the band in use.
The carrier is modulated using Gaussian Minimum Shift Keying (GMSK). GMSK was used for the GSM system because it is able to provide features required for GSM. It is resilient to noise when compared to some other forms of modulation, it occupies a relatively narrow bandwidth, and it has a constant power level.
The data transported by the carrier serves up to eight different users under the basic system. Even though the full data rate on the carrier is approximately 270 kbps, some of this supports the management overhead, and therefore the data rate allotted to each time slot is only 24.8 kbps. In addition to this error correction is required to overcome the problems of interference, fading and the like. This means that the available data rate for transporting the digitally encoded speech is 13 kbps for the basic vocoders.
Power levels
A variety of power levels are allowed by the GSM standard, the lowest being only 800 mW (29 dBm). As mobiles may only transmit for one eighth of the time, i.e. for their allocated slot which is one of eight, the average power is an eighth of the maximum.
Additionally, to reduce the levels of transmitted power and hence the levels of interference, mobiles are able to step the power down in increments of 2 dB from the maximum to a minimum 13 dBm (20 milliwatts). The mobile station measures the signal strength or signal quality (based on the Bit Error Rate), and passes the information to the BTS and hence to the BSC, which ultimately decides if and when the power level should be changed.
A further power saving and interference reducing facility is the discontinuous transmission (DTx) capability that is incorporated within the specification. It is particularly useful because there are long pauses in speech, for example when the person using the mobile is listening, and during these periods there is no need to transmit a signal. In fact it is found that a person speaks for less than 40% of the time during normal telephone conversations. The most important element of DTx is the Voice Activity Detector. It must correctly distinguish between voice and noise inputs, a task that is not trivial. If a voice signal is misinterpreted as noise, the transmitter is turned off an effect known as clipping results and this is particularly annoying to the person listening to the speech. However if noise is misinterpreted as a voice signal too often, the efficiency of DTX is dramatically decreased.
It is also necessary for the system to add background or comfort noise when the transmitter is turned off because complete silence can be very disconcerting for the listener. Accordingly this is added as appropriate. The noise is controlled by the SID (silence indication descriptor).
Multiple access and channel structure
GSM uses a combination of both TDMA and FDMA techniques. The FDMA element involves the division by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart as already described.
The carriers are then divided in time, using a TDMA scheme. The fundamental unit of time is called a burst period and it lasts for approximately 0.577 mS (15/26 mS). Eight of these burst periods are grouped into what is known as a TDMA frame. This lasts for approximately 4.615 ms (i.e.120/26 ms) and it forms the basic unit for the definition of logical channels. One physical channel is one burst period allocated in each TDMA frame.
There are different types of frame that are transmitted to carry different data, and also the frames are organised into what are termed multiframes and superframes to provide overall synchronisation.

Mobile station
Mobile stations (MS), mobile equipment (ME) or as they are most widely known, cell or mobile phones are the section of a GSM cellular network that the user sees and operates. In recent years their size has fallen dramatically while the level of functionality has greatly increased. A further advantage is that the time between charges has significantly increased.
There are a number of elements to the cell phone, although the two main elements are the main hardware and the SIM.
The hardware itself contains the main elements of the mobile phone including the display, case, battery, and the electronics used to generate the signal, and process the data receiver and to be transmitted. It also contains a number known as the International Mobile Equipment Identity (IMEI). This is installed in the phone at manufacture and "cannot" be changed. It is accessed by the network during registration to check whether the equipment has been reported as stolen.
The SIM or Subscriber Identity Module contains the information that provides the identity of the user to the network. It contains are variety of information including a number known as the International Mobile Subscriber Identity (IMSI).
Base station subsystem
The Base Station Subsystem (BSS) section of the GSM network is fundamentally associated with communicating with the mobiles on the network. It consists of two elements, namely the Base Transceiver Station (BTS) and the Base Station Controller (BSC).
The BTS used in a GSM network comprises the radio transmitter receivers, and their associated antennas that transmit and receive to directly communicate with the mobiles. The BTS is the defining element for each cell. The BTS communicates with the mobiles and the interface between the two is known as the Um interface with its associated protocols.
The BSC forms the next stage back into the GSM network. It controls a group of BTSs, and is often co-located with one of the BTSs in its group. It manages the radio resources and controls items such as handover within the group of BTSs, allocates channels and the like. It communicates with the BTSs over what is termed the Abis interface.
Network subsystem
The network subsystem contains a variety of different elements, and is often termed the core network. It provides the main control and interfacing for the whole mobile network. It includes elements including the MSC, HLR, VLR, Auc and more as described below:
The main element within the core network is the Mobile switching Services Centre (MSC). The MSC acts like a normal switching node within a PSTN or ISDN, but also provides additional functionality to enable the requirements of a mobile user to be supported. These include registration, authentication, call location, inter-MSC handovers and call routing to a mobile subscriber. It also provides an interface to the PSTN so that calls can be routed from the mobile network to a phone connected to a landline. Interfaces to other MSCs are provided to enable calls to be made to mobiles on different networks.
To enable the MSC to perform its functions it requires data from a number of databases. One is known as the Home Location Register (HLR). It contains all the administrative information about each subscriber along with their last known location.
When a user switches on their phone, the phone registers with the network and from this it is possible to determine which BTS it communicates with so that incoming calls can be routed appropriately. Even when the phone is not active (but switched on) it re-registers periodically to ensure that the network (HLR) is aware of its latest position.
There is one HLR per network, although it may be distributed across various sub-centres to for operational reasons.
Another of the databases is known as the Visitor Location Register (VLR). This contains selected information from the HLR that enables the selected services for the individual subscriber to be provided.
The VLR can be implemented as a separate entity, but it is commonly realised as an integral part of the MSC, rather than a separate entity. In this way access is made faster and more convenient.
The third register is the Equipment Identity Register (EIR). The EIR is the entity that decides whether a given mobile equipment may be allowed onto the network. Each mobile equipment has a number known as the International Mobile Equipment Identity. This number, as mentioned above, is installed in the equipment and is checked by the network during registration. Dependent upon the information held in the EIR, the mobile may be allocated one of three states - allowed onto the network, barred access, or monitored in case its problems.
The final register is the Authentication Centre (AuC). The AuC is a protected database that contains the secret key also contained in the user's SIM card. It is used for authentication and for ciphering on the radio channel.
Another element in the network is the Gateway Mobile Switching Centre (GMSC). The GMSC is the point to which a ME terminating call is initially routed, without any knowledge of the MS's location. The GMSC is thus in charge of obtaining the MSRN (Mobile Station Roaming Number) from the HLR based on the MSISDN (Mobile Station ISDN number, the "directory number" of a MS) and routing the call to the correct visited MSC. The "MSC" part of the term GMSC is misleading, since the gateway operation does not require any linking to an MSC.
The SMS-G or SMS gateway is the term that is used to collectively describe the two Short Message Services Gateways defined in the GSM standards. The two gateways handle messages directed in different directions. The SMS-GMSC (Short Message Service Gateway Mobile Switching Centre) is for short messages being sent to an ME. The SMS-IWMSC (Short Message Service Inter-Working Mobile Switching Centre) is used for short messages originated with a mobile on that network. The SMS-GMSC role is similar to that of the GMSC, whereas the SMS-IWMSC provides a fixed access point to the Short Message Service Centre.

System idea
The GSM system was designed as a second generation (2G) cellular communication system. One of the basic aims was to provide a system that would enable greater capacity to be achieved than the previous first generation analogue systems. GSM achieved this by using a digital TDMA (time division multiple access approach). By adopting this technique more users could be accommodated within the available bandwidth. In addition to this, ciphering of the digitally encoded speech was adopted to retain privacy. Using the earlier analogue systems it was possible for anyone with a scanner receiver to listen to calls and a number of famous personalities had been "eavesdropped" with embarrassing consequences.
Services provided
Speech or voice calls are obviously the primary function for the GSM system. To achieve this the speech is digitally encoded and later decoded using a vocoder. A variety of vocoders are available for use, being aimed at different scenarios.
In addition to the voice services, GSM supports a variety of other data services. Although their performance is nowhere near the level of those provided by 3G, they are nevertheless still important and useful. A variety of data services are supported with user data rates up to 9.6 kbps. Services including Group 3 facsimile, videotext and teletex can be supported.
One service that has grown enormously is the short message service. Developed as part of the GSM specification, it has also been incorporated into other cellular systems. It can be thought of as being similar to the paging service but is far more comprehensive allowing bi-directional messaging, store and forward delivery, and it also allows alphanumeric messages of a reasonable length. This service has become particularly popular, initially with the young as it provided a simple, low fixed cost.
Basic concept
The GSM system had a number of design aims when the development started. It should offer good subjective speech quality, have a low phone or terminal cost, terminals should be able to be handheld, the system should support international roaming, it should offer good spectral efficiency, and the system should offer ISDN compatibility.
The system that developed provided for all of these. The overall system definition for GSM describes not only the air interface but also the network. By adopting this approach it is possible to define the operation of the whole network to enable international roaming as well as enabling network elements from different manufacturers to operate alongside each other, although this last feature is not completely true, especially with older items.
GSM uses 200 kHz RF channels. These are time division multiplexed to enable up to eight users to access each carrier. In this way it is a TDMA / FDMA system.
The base transceiver stations (BTS) are organised into small groups, controlled by a base station controller (BSC) which is typically co-located with one of the BTSs. The BSC with its associated BTSs is termed the base station subsystem (BSS).
Further into the core network is the main switching area. This is known as the mobile switching centre (MSC). Associated with it is the location registers, namely the home location register (HLR) and the visitor location register (VLR) which track the location of mobiles and enable calls to be routed to them. Additionally there is the Authentication Centre (AuC), and the Equipment Identify Register (EIR) that are used in authenticating the mobile before it is allowed onto the network and for billing. The operation of these are explained in the following pages.
Last but not least is the mobile itself. Often termed the ME or mobile equipment, this is the item that the end user sees. One important feature that was first implemented on GSM was the use of a Subscriber Identity Module. This card carried with it the users identity and other information to allow the user to upgrade a phone very easily, while retaining the same identity on the network. It was also used to store other information such as "phone book" and other items. This item alone has allowed people to change phones very easily, and this has fuelled the phone manufacturing industry and enabled new phones with additional features to be launched. This has allowed mobile operators to increase their average revenue per user (ARPU) by ensuring that users are able to access any new features that may be launched on the network requiring more sophisticated phones.
Specification Summary of GSM Cell Phone System
Multiple Access Technology
FDMA / TDMA
Duplex Technique FDD
Uplink frequency band 933 - 960 MHz(basic 900 MHz band only)
Downlink frequency band 890 - 915 MHz(basic 900 MHz band only)
Channel spacing 200 kHz
Modulation GMSK
Speech coding Various - Original was RPE-LTP/13
Speech channels per RF channel 8
Channel data rate 270.833 kbps
Frame duration 4.615 mS

Multimedia Messaging Service
Mobile Intranet/Extranet Access
Mobile Internet Access
Location-based Services
Rich Voice
According to a whitepaper form Nokia, 3G applications can be divided into:
Wireless Advertising
Mobile Information
Business Solutions
Mobile Transactions
Mobile Entertainment
Person-to-Person Communications
Bearer Entrance and Periodics
No matter how you classify those, the bottom line is that computer and Internet application will merge with communication and location based services. Network operators, vendors and third party providers are already creating the applications and demos. Here are some sample what might soon come to a phone near you:
3G companies have also formed initiatives, co-operations, organisations and special interest groups to create a framework and specification to get different pieces of the puzzle together. Here are a list of web sites of organisations, which are involved to 3G application working: