GSM Network Aspects

SIM Information System (PTA) (Please click to view your information)

1. Handover
2. Location updating and call
3. Authentication and security

Ensuring the transmission of voice or data of a given quality over the radio link is only half the problem in a cellular mobile network. The fact that the geographical area covered by the network is divided into cells necessitates the implementation of a handover mechanism. Also, the fact that the mobile can roam nationally and internationally in GSM requires that registration, authentication, call routing and location updating functions exist in the GSM network.
The signalling protocol in GSM is structured in three layers , shown in Figure 3. Layer 1 is the physical layer, which uses the channel structures discussed above. Layer 2 is the data link layer. Across the Um interface, the data link layer uses a slight modification of the LAPD protocol used in ISDN, called LAPDm. Across the A interface, the lower parts of Signalling System Number 7 are used. Layer 3 is subdivided into 3 sublayers.

Radio Resources Management
controls the setup, maintenance, and termination of radio channels
Mobility Management
manages the location updating, handovers, and registration procedures, discussed below
Connection Management
handles general call control, similar to CCITT Recommendation Q.931, and provides supplementary services.

Signalling between the different entities in the network, such as between the HLR and VLR, is accomplished throught the Mobile Application Part (MAP). Application parts are the top layer of Signalling System Number 7. The specification of the MAP is complex. It is one of the longest documents in the GSM recommendations, said to be over 600 pages in length . Described below are the main functions of the Mobility Management sublayer.

1. Handover

Handover, or handoff as it is called in North America, is the switching of an on­going call to a different channel or cell. There are four different types of handover in the GSM system, which involve transferring a call between

• channels (time slots) in the same cell,
• cells (Base Transceiver Stations) under the control of the same Base Station Controller (BSC),
• cells under the control of different BSCs, but belonging to the same Mobile services Switching Center (MSC)
• cells under the control of different MSCs.
  

The first two types of handover, called internal handovers, involve only one Base Station Controller (BSC). To save signalling bandwidth, they are managed by the BSC without involving the Mobile service Switching Center (MSC), except to notify it at the completion of the handover. The last two types of handover, called external handovers, are handled by the MSCs involved. Note that call control, such as provision of supplementary services and requests for further handoffs, is handled by the original MSC.

Handovers can be initiated by either the mobile or the MSC (as a means of traffic load balancing). During its idle time slots, the mobile scans the Broadcast Control Channel of up to 16 neighboring cells, and forms a list of the six best candidates for possible handover, based on the received signal strength. This information is passed to the BSC and MSC, and is used by the handover algorithm.

The algorithm for when a handover decision should be taken is not specified in the GSM recommendations. There are two basic algorithms used, both closely tied in with power control. This is because the BSC usually does not know whether the poor signal quality is due to multipath fading or to the mobile having moved to another cell. This is especially true in small urban cells.

The algorithm for when a handover decision should be taken is not specified in the GSM recommendations. There are two basic algorithms used, both closely tied in with power control. This is because the BSC usually does not know whether the poor signal quality is due to multipath fading or to the mobile having moved to another cell. This is especially true in small urban cells.

The 'power budget' method uses handover to try to maintain or improve a certain level of signal quality at the same or lower power level. It thus gives precedence to handover over power control. It avoids the 'smeared' cell boundary problem and reduces co­channel interference, but it is quite complicated.

2. Location updating and call routing

The MSC provides the interface between the GSM mobile network and the public fixed network. From the fixed network's point of view, the MSC is just another switching node. However, switching is a little more complicated in a mobile network since the MSC has to know where the mobile is currently roaming - and in GSM it could even be roaming in another country. The way GSM accomplishes location updating and call routing to the mobile is by using two location registers: the Home Location Register (HLR) and the Visitor Location Register (VLR).

Location updating is initiated by the mobile when, by monitoring the Broadcast Control Channel, it notices that the location­area broadcast is not the same as the one previously stored in the mobile's memory. An update request and the IMSI or previous TMSI is sent to the new VLR via the new MSC. A Mobile Station Roaming Number (MSRN) is allocated and sent to the mobile's HLR (which always keeps the most current location) by the new VLR. The MSRN is a regular telephone number that routes the call to the new VLR and is subsequently translated to the TMSI of the mobile. The HLR sends back the necessary call­control parameters, and also sends a cancel message to the old VLR, so that the previous MSRN can be reallocated. Finally, a new TMSI is allocated and sent to the mobile, to identify it in future paging or call initiation requests.

With the above location­updating procedure, call routing to a roaming mobile is easily performed. The most general case is shown in Figure 4, where a call from a fixed network (Public Switched Telecommunications Network or Integrated Services Digital Network) is placed to a mobile subscriber. Using the Mobile Subscriber's telephone number (MSISDN, the ISDN numbering plan specified in the ITU­T E.164 recommendation), the call is routed through the fixed land network to a gateway MSC for the GSM network (an MSC that interfaces with the fixed land network, thus requiring an echo canceller). The gateway MSC uses the MSISDN to query the Home Location Register, which returns the current roaming number (MSRN). The MSRN is used by the gateway MSC to route the call to the current MSC (which is usually coupled with the VLR). The VLR then converts the roaming number to the mobile's TMSI, and a paging call is broadcast by the cells under the control of the current BSC to inform the mobile.

2. Authentication and security

Since the radio medium can be accessed by anyone, authentication of users to prove that they are who they claim to be, is a very important element of a mobile network. Authentication involves two functional entities, the SIM card in the mobile, and the Authentication Center (AC). Each subscriber is given a secret key, one copy of which is stored in the SIM card and the other in the Authentication Center. During authentication, the AC generates a random number that it sends to the mobile. Both the mobile and the AC then use the random number, in conjuction with the subscriber's secret key and a ciphering algorithm called A3, to generate a number that is sent back to the AC. If the number sent by the mobile is the same as the one calculated by the AC, the subscriber is authenticated.

The above calculated number is also used, together with a TDMA frame number and another ciphering algorithm called A5, to encipher the data sent over the radio link, preventing others from listening in. Enciphering is an option for the very paranoid, since the signal is already coded, interleaved, and transmitted in a TDMA manner, thus providing protection from all but the most persistent and dedicated eavesdroppers.

Another level of security is performed on the mobile equipment, as opposed to the mobile subscriber. As mentioned earlier, each GSM terminal is identified by a unique International Mobile Equipment Identity (IMEI) number. A list of IMEIs in the network is stored in the Equipment Identity Register (EIR).

EIR is one of the following:

white­listed
The terminal is allowed to connect to the network
grey­listed
Under observation from the network, possible problems
black­listed
The terminal has either been reported as stolen, or it is not type approved (the correct type of terminal for a GSM network). The terminal is not allowed to connect to the network.

Global System for Mobile Communication (GSM) Part-IIb

Speech coding

GSM is a digital system, so speech signals, inherently analog, have to be digitized. The method employed by ISDN, and by current telephone systems for multiplexing voice lines over high speed trunks and optical fiber lines, is Pulse Coded Modulation (PCM). The output stream from PCM is 64 kbps, too high a rate to be feasible over a radio link. The 64 kbps signal contains much redundancy, although it is simple to implement. The GSM group studied several voice coding algorithms on the basis of subjective speech quality and complexity (which is related to cost, processing delay, and power consumption once implemented) before arriving at the choice of a Regular Pulse Excited - Linear Predictive Coder (RPE­LPC) with a Long Term Predictor loop. Basically, information from previous samples, which does not change very quickly, is used to predict the current sample. The coefficients of the linear combination of the previous samples, plus an encoded form of the residual, the difference between the predicted and actual sample, represent the signal. Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps.

Due to natural or man­made electromagnetic interference, the encoded speech or data transmitted over the radio interface must be protected as much as is practical. The GSM system uses convolutional encoding and block interleaving to achieve this protection. The exact algorithms used differ for speech and for different data rates. The method used for speech blocks will be described below.

Recall that the speech codec produces a 260 bit block for every 20 ms speech sample. From subjective testing, it was found that some bits of this block were more important for perceived speech quality than others. The bits are thus divided into three classes:

Class Ia 50 bits - most sensitive to bit errors

Class Ib 132 bits - moderately sensitive to bit errors

Class II 78 bits - least sensitive to bit errors

Class Ia bits have a 3 bit Cyclic Redundancy Code added for error detection. If an error is detected, the frame is judged too damaged to be comprehensible and it is discarded. It is replaced by a slightly attenuated version of the previous correctly received frame. These 53 bits, together with the 132 Class Ib bits and a 4 bit tail sequence (a total of 189 bits), are input into a 1/2 rate convolutional encoder of constraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits. The convolutional encoder thus outputs 378 bits, to which are added the 78 remaining Class II bits, which are unprotected. Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps.

To further protect against the burst errors common to the radio interface, each sample is diagonally interleaved. The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time­slot bursts. Since each time­slot burst can carry two 57 bit blocks, each burst carries traffic from two different speech samples.

Recall that each time­slot burst is transmitted at a gross bit rate of 270.833 kbps. This digital signal is modulated onto the analog carrier frequency, which has a bandwidth of 200 kHz, using Gaussian­filtered Minimum Shift Keying (GMSK). GMSK was selected over other modulation schemes as a compromise between spectral efficiency, complexity of the transmitter, and limited spurious emissions. The complexity of the transmitter is related to power consumption, which should be minimized for the mobile station. The spurious radio emissions, outside of the allotted bandwidth, must be strictly controlled so as to limit adjacent channel interference, and allow for the co­existence of GSM and the older analog systems (at least for the time being).

Multipath equalization

At the 900 MHz range, radio waves bounce off everything - buildings, hills, cars, airplanes, etc. Thus many reflected signals, each with a different phase, can reach an antenna. Equalization is used to extract the desired signal from the unwanted reflections. Equalization works by finding out how a known transmitted signal is modified by multipath fading, and constructing an inverse filter to extract the rest of the desired signal. This known signal is the 26­bit training sequence transmitted in the middle of every time slot burst. The actual implementation of the equalizer is not specified in the GSM specifications.

Frequency hopping

The mobile station already has to be frequency agile, meaning it can move between a transmit, receive, and monitor time slot within one TDMA frame, which may be on different frequencies. GSM makes use of this inherent frequency agility to implement slow frequency hopping, where the mobile and BTS transmit each TDMA frame on a different carrier frequency. The frequency hopping algorithm is broadcast on the Broadcast Control Channel. Since multipath fading is (mildly) dependent on carrier frequency, slow frequency hopping helps alleviate the problem. In addition, co­channel interference is in effect randomized

Discontinuous transmission

Minimizing co­channel interference is a goal of any cellular system, since it allows better service for a given cell size, or the use of smaller cells, thus increasing the overall capacity of the system. Discontinuous transmission (DTX) is a method that takes advantage of the fact that a person speaks less that 40 percent of the time in normal conversation , by turning the transmitter off during silence periods. An added benefit of DTX is that power is conserved at the mobile unit.

The most important component of DTX is, of course, Voice Activity Detection. It must distinguish between voice and noise inputs, a task that is not as trivial as it appears, considering background noise. If a voice signal is misinterpreted as noise, the transmitter is turned off and a very annoying effect called clipping is heard at the receiving end. If, on the other hand, noise is misinterpreted as a voice signal too often, the efficiency of DTX is dramatically decreased. Another factor to consider is that when the transmitter is turned off, there is a very silent silence heard at the receiving end, due to the digital nature of GSM. To assure the receiver that the connection is not dead, comfort noise is created at the receiving end by trying to match the characteristics of the transmitting end's background noise.

Discontinuous reception

Another method used to conserve power at the mobile station is discontinuous reception. The paging channel, used by the base station to signal an incoming call, is structured so that the mobile station knows when it needs to check for a paging signal. In the time between paging signals, the mobile can go into sleep mode, when almost no power is used.

Power control

There are five classes of mobile stations defined, according to their peak transmitter power, rated at 20, 8, 5, 2, and 0.8 watts. To minimize co­channel interference and to conserve power, both the mobiles and the Base Transceiver Stations operate at the lowest power level that will maintain an acceptable signal quality. Power levels can be stepped up or down in steps of 2 dB from the peak power for the class down to a minimum of 13 dBm (20 milliwatts).

The mobile station measures the signal strength or signal quality (based on the Bit Error Ratio), and passes the information to the Base Station Controller, which ultimately decides if and when the power level should be changed. Power control should be handled carefully, since there is the possibility of instability. This arises from having mobiles in co­channel cells alternatingly increase their power in response to increased co­channel interference caused by the other mobile increasing its power. This in unlikely to occur in practice but it is (or was as of 1991) under study.

Global System for Mobile Communication (GSM) Part-IIa

During the early 1980s, analog cellular telephone systems were experiencing rapid growth in Europe, particularly in Scandinavia and the United Kingdom, but also in France and Germany. Each country developed its own system, which was incompatible with everyone else's in equipment and operation. This was an undesirable situation, because not only was the mobile equipment limited to operation within national boundaries, which in a unified Europe were increasingly unimportant, but there was a very limited market for each type of equipment, so economies of scale, and the subsequent savings, could not be realized.

The Europeans realized this early on, and in 1982 the Conference of European Posts and Telegraphs (CEPT) formed a study group called the Groupe Spécial Mobile (GSM) to study and develop a pan­European public land mobile system. The proposed system had to meet certain criteria:

• good subjective speech quality


• low terminal and service cost


• support for international roaming


• ability to support handhald terminals


• support for range of new services and facilities


• spectral efficiency, and ISDN compatibility

In 1989, GSM responsibility was transferred to the European Telecommunication Standards Institute (ETSI), and phase I of the GSM specifications were published in 1990. Commercial service was started in mid­1991, and by 1993 there were 36 GSM networks in 22 countries, with 25 additional countries having already selected or considering GSM . This is not only a European standard - South Africa, Australia, and many Middle and Far East countries have chosen GSM. By the beginning of 1994, there were 1.3 million subscribers worldwide . The acronym GSM now (aptly) stands for Global System for Mobile telecommunications.

The developers of GSM chose an unproven (at the time) digital system, as opposed to the then­standard analog cellular systems like AMPS in the United States and TACS in the United Kingdom. They had faith that advancements in compression algorithms and digital signal processors would allow the fulfillment of the original criteria and the continual improvement of the system in terms of quality and cost. The 8000 pages of the GSM recommendations try to allow flexibility and competitive innovation among suppliers, but provide enough guidelines to guarantee the proper interworking between the components of the system. This is done in part by providing descriptions of the interfaces and functions of each of the functional entities defined in the system.

GSM -> Services Provided by GSM

From the beginning, the planners of GSM wanted ISDN compatibility in services offered and control signalling used. The radio link imposed some limitations, however, since the standard ISDN bit rate of 64 kbps could not be practically achieved.

Using the ITU­T definitions, telecommunication services can be divided into bearer services, teleservices, and supplementary services. The digital nature of GSM allows data, both synchronous and asynchronous, to be transported as a bearer service to or from an ISDN terminal. Data can use either the transparent service, which has a fixed delay but no guarantee of data integrity, or a non­transparent service, which guarantees data integrity through an Automatic Repeat Request (ARQ) mechanism, but with a variable delay. 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. There is an emergency service, where the nearest emergency­service provider is notified by dialling three digits (similar to 911).

Group 3 fax, an analog method described in ITU­T recommendation T.30, is also supported by use of an appropriate fax adaptor. A unique feature of GSM compared to older analog systems is the Short Message Service (SMS). SMS is a bidirectional service for sending short alphanumeric (up to 160 bytes) messages in a store­and­forward fashion. For point­to­point SMS, a message can be sent to another subscriber to the service, and an acknowledgement of receipt is provided to the sender. SMS can also be used in a cell­broadcast mode, for sending messages such as traffic updates or news updates. Messages can be stored in the SIM card for later retrieval .

Supplementary services are provided on top of teleservices or bearer services, and include features such as caller identification, call forwarding, call waiting, multi­party conversations, and barring of outgoing (international) calls, among others.

Mobile Station

The mobile station (MS) consists of the physical equipment, such as the radio transceiver, display and digital signal processors, and a smart card called the Subscriber Identity Module (SIM). The SIM provides personal mobility, so that the user can have access to all subscribed services irrespective of both the location of the terminal and the use of a specific terminal. By inserting the SIM card into another GSM cellular phone, the user is able to receive calls at that phone, make calls from that phone, or 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), identifying the subscriber, a secret key for authentication, and other user information. The IMEI and the IMSI are independent, thereby providing 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 specified A­bis interface, allowing (as in the rest of the system) operation between components made by different suppliers.

The Base Transceiver Station houses the radio tranceivers that define a cell and handles the radio­link protocols with the Mobile Station. In a large urban area, there will potentially be a large number of BTSs deployed. 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 radio­channel setup, frequency hopping, and handovers, as described below. The BSC is the connection between the mobile and the Mobile service Switching Center (MSC). The BSC also translates the 13 kbps voice channel used over the radio link to the standard 64 kbps channel used by the Public Switched Telephone Network or ISDN.

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 in addition provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. These services are provided in conjuction with several functional entities, which together form the Network Subsystem. The MSC provides the connection to the public fixed network (PSTN or ISDN), and signalling between functional entities uses the ITU­T Signalling System Number 7 (SS7), used 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 call­routing and (possibly international) 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 current location of the mobile is in the form of a Mobile Station Roaming Number (MSRN) which is a regular ISDN number used to route a call to the MSC where the mobile is currently located. There is logically one HLR per GSM network, although it may be implemented as a distributed database.

The Visitor Location Register 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. Although each functional entity can be implemented as an independent unit, most manufacturers of switching equipment implement one VLR together with one MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR, simplifying the signalling required. 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 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 ciphering of the radio channel.

GSM -> Radio Links

• structure


• Speech coding


• Channel coding and modulation
  
• Multipath equalization
  
• Frequency hopping
  
• Discontinuous transmission


• Discontinuous reception


• Power control
  

The International Telecommunication Union (ITU), which manages the international allocation of radio spectrum (among other functions) allocated the bands 890-915 MHz for the uplink (mobile station to base station) and 935-960 MHz for the downlink (base station to mobile station) for mobile networks in Europe. Since this range was already being used in the early 1980s by the analog systems of the day, the CEPT had the foresight to reserve the top 10 MHz of each band for the GSM network that was still being developed. Eventually, GSM will be allocated the entire 2x25 MHz bandwidth. Since radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible. The method chosen by GSM is a combination of Time­ and Frequency­Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the total 25 MHz bandwidth into 124 carrier frequencies of 200 kHz bandwidth. One or more carrier frequencies are then assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme, into eight time slots. One time slot is used for transmission by the mobile and one for reception. They are separated in time so that the mobile unit does not receive and transmit at the same time, a fact that simplifies the electronics.

In the rest of this section, the procedure involved in digitally transmitting a voice signal in a GSM network is examined, along with some of the features, such as discontinuous transmission and reception, used to improve voice quality, reduce the mobile unit's power consumption, and increase the overall capacity of the network.

Channel structure

The structure of the most common time­slot burst is shown in Figure 2. A total of 156.25 bits is transmitted in 0.577 milliseconds, giving a gross bit rate of 270.833 kbps. There are three other types of burst structure for frame and carrier synchronization and frequency correction. The 26­bit training sequence is used for equalization, as described below. The 8.25 bit guard time allows for some propagation time delay in the arrival of bursts.

Each group of eight time slots is called a TDMA frame, which is transmitted every 4.615 ms. TDMA frames are further grouped into multiframes to carry control signals. There are two types of multiframe, containing 26 or 51 TDMA frames. The 26­frame multiframe contains 24 Traffic Channels (TCH) and two Slow Associated Control Channels (SACCH) which supervise each call in progress. The SACCH in frame 12 contains eight channels, one for each of the eight connections carried by the TCHs. The SACCH in frame 25 is not currently used, but will carry eight additional SACCH channels when half­rate traffic is implemented. A Fast Associated Control Channel (FACCH) works by stealing slots from a traffic channel to transmit power control and handover­signalling messages. The channel stealing is done by setting one of the control bits in the time slot burst.

In addition to the Associated Control Channels, there are several other control channels which (except for the Stand­alone Dedicated Control Channel) are implemented in time slot 0 of specified TDMA frames in a 51­frame multiframe, implemented on a non­hopping carrier frequency in each cell. The control channels include:

• Broadcast Control Channel (BCCH): Continually broadcasts, on the downlink, information including base station identity, frequency allocations, and frequency­hopping sequences.


• Stand­alone Dedicated Control Channel (SDCCH): Used for registration, authentication, call setup, and location updating. Implemented on a time slot, together with its SACCH, selected by the system operator.


• Common Control Channel (CCCH): Comprised of three control channels used during call origination and call paging.


• Random Access Channel (RACH): A slotted Aloha channel to request access to the network


• Paging Channel (PCH): Used to alert the mobile station of incoming call.
  
• Access Grant Channel (AGCH): Used to allocate an SDCCH to a mobile for signalling, following a request on the RACH.