Traffic cases in GSM

Wireless Networks

Unit 7

Unit 7

GSM System Operation and Traffic Cases

Structure: 7.1 Introduction 7.2 GSM System Operations Mobile Telephone Initialization 7.3 Traffic Cases in GSM Network The Mobile Station is in idle mode Roaming in the Network The mobile station is in active mode 7.4 Handover in GSM Network 7.5 Summary 7.6 Answers

7.1 Introduction In unit 6 you have learnt the process of data transmission in GSM network in TDMA format. The discussion of time division multiple access (TDMA) frame hierarchy used in GSM network is discussed. You have also learnt the use of channels in GSM network for transmitting managerial information and data traffic. It is seen how various identifiers in the GSM network facilitate the roaming of subscribers without any difficulty to locate and identify them. In this unit you will learn system operations of GSM network in order to provide services to GSM subscribers. The issues and challenges faced by any mobile network to support mobility are discussed with respect to GSM network. The methods adopted by the GSM network to maintain continuous conversations on movement are also discussed in this unit. The process of handover is also discussed at the end of the unit. Objectives: After studying this unit, you will be able to:  explain operations supported by GSM network  describe the process of handover  describe various traffic cases  explain security enhancement during conversation between two subscribers

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7.2 GSM System Operations To provide services to the GSM subscribers there are various operations carried out by the system components. It has to identify authorize subscribers to use the GSM services, identify them when they request services, initiate a call to another subscribers and many more. The operations of GSM are discussed under few headings like initialization of a call or a mobile device, management of the connection during conversation, and ending the connection. For each case the system has to perform a set of tasks. In this section GSM system operations are discussed. 7.2.1 Mobile Telephone Initialization – is the process of obtaining information that is broad cased from the mobile system that identifies how the mobile device should access the system. There are three main goals of the mobile initialization procedure: 1. Frequency synchronization 2. Timing synchronization 3. Overhead information acquisition Frequency Synchronization: As the terminal is switched on, it scans over the available GSM RF channels and takes several readings of their RF levels to obtain an accurate estimate of the signal strengths. Starting with the channel with the highest level, the terminal searches for the frequency correction burst on the (Broadcast Control Channel) BCCH. If no frequency correction burst is detected, it then moves to the next highest level signal and repeats the process until it is successful. In this event, the terminal will then synchronize its local oscillator with the frequency reference of the base station transceiver. Timing Synchronization: After frequency synchronization has been achieved, the terminal will search for the synchronization burst for the timing information present on the SCH. If it is not successful, it then moves to the next highest level signal and repeats the process starting from the frequency synchronization procedure until it is successful. In this event, it moves to the BCCH (Broadcast Control Channel) to acquire overhead system information. Overhead Information Acquisition: After timing synchronization has been achieved, the terminal will search for overhead information on the BCCH. If the BCCH information does not include the current BCCH number, it will Sikkim Manipal University

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restart the mobile initialization procedure. In a successful event, the terminal will have acquired, from the BCCH and through the system information message present on the BCCH, the following main information:  Country code  Network code  Location area code  Cell identity  Adjacent cell list  BCCH (Broadcast Control Channel) location  Minimum received signal strength  The terminal checks if the acquired identification codes coincide with those in the SIM card. In a successful event, it will maintain the link and monitor the PCH. Otherwise, it will start a location update procedure. Details about PCH are available in unit 6 (section 6.3). Updating Location – is the process of sending a registration message to the system. A location update procedure is carried out in one of the following events:  The terminal is switched on and verifies that the identification codes present on the current BCCH do not coincide with those in the SIM card.  The terminal moves into a location area different from that within which it is currently registered.  There has been no activity for a reestablished amount of time. As part of the process used to speed the paging procedure, location reports are used. These location reports are periodic reports used to update the location of the terminal so that, in the event of a page, the latest reported location is used as an initial guess to locate the terminal. The time span between location reports constitutes a system parameter whose value is indicated on the BCCH, varying in accordance with the network loading. The location update procedure starts with the uplink channel request message on the random access channel (RACH). The network answers with an immediate assignment message on the AGCH (Access Grant Channel) indicating the SDCCH (Standalone Dedicated Control Channel) number to be used throughout the location update procedure. The terminal moves to this SDCCH and sends a location updating request message with its identification (IMSI or, preferably, TMSI). An authentication procedure is Sikkim Manipal University

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then carried out. In case the authentication is unsuccessful, the procedure is aborted. In a successful event, the ciphering procedure is performed. The network then uses the location updating accept message to assign a new TMSI to the terminal. The terminal stores its TMSI and responds with a TMSI allocation complete message. The location update is concluded with a channel release message from the network to the terminal. The terminal then resumes its PCH monitoring procedure. Waiting for Calls (Idle) – is the process of waiting for call alerts or other messages. Cell Reselection – is the process of determining when a different cell site should be monitored. A MS listens for beacons, and if it detect one, it adds it to the active beacon kernel table. If the device determines that it needs to communicate via a new BS, kernel modulation initiates the handoff process. The MS locates the nearest BS via user-level processing. System Access – is the process of coordinating how to get the attention of the system and receive channel assignments. When a new mobile station tries to access the service of a BS, the BS performs the identity verification of the new user as a first step of the process. Then the BS searches the MSC with which the mobile user is registered. It also checks the billing information of the user and kind of access permission granted to the user by its home MSC. The home MSC sends an appropriate authentication response to BS currently serving the user and stored in corresponding VLR of serving MSC (two way pointers between HLR-VLR pairs). The BS at the visited location approves or disapproves user access. Mobile Call Origination – is the process of requesting access to the GSM system after a user has initiated a call. The terminal detects a useroriginated call. It then accesses the Random Access Channel (RACH) to send a channel request message. An immediate assignment with the number is sent by the network on the Access Grant Channel (AGCH). The terminal moves to this channel and the following occurs. The terminal transmits a paging response message indicating the reason for the specific message (call setup). The base station responds with an unnumbered acknowledgment message. An authentication procedure is carried out, as already described. In a successful event, a ciphering procedure is performed, as already described. Sikkim Manipal University

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The terminal then sends a setup message. The base station responds with a call confirmed message followed by an alerting message in which case the terminal applies the ring-b k .A p y’s p , the network sends an assignment command message informing the traffic channel number to be used for the conversation. The subscriber, still on the SDCCH, responds with an assignment acknowledgment message and moves to the traffic channel that has been assigned. The network confirms the acceptance of the call by the other party by means of a connect acknowledgment message on the FACCH of the assigned TCH. And the conversation proceeds on the TCH. Transferring Calls Between Call Sites – is the process of identifying when a connection should be transferred to another cell site and coordinating the handover process. Mobile Assisted Handoff – is the process of transferring information (such as RF signal quality) from the mobile device to the system which can be used to assist in the transfer process. Receiving a Call on a Mobile Telephone – is the process of alerting a mobile device that an incoming call is waiting to be answered and connecting the audio path if the mobile user answers the call. The call is also switched from signaling to voice. Conversation Mode – is the process of maintaining an audio connection with a user during a call. MSC informs the BSS about the allocated voice circuit. Connected Mode – is the process of maintaining the availability of a device that is transferring bursts of data (packet data) on the network. Authentication – is the validation of the identity of the device through the use of security processes and secret information. An authentication procedure may be required at the location update procedure or at the request of a new service. The GSM encryption is shown in the figure 7.2, authentication procedure starts with the network sending an authentication request message to the terminal; the message conveys a 128-bit random number (RAND). The terminal uses the RAND, the secret key, Ki, stored at SIM, and the encryption algorithm, referred to as A3 (specifies the confidentiality algorithm), to compute a 32-bit number, referred to as a Sikkim Manipal University

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signed response (SRES). Another 64-bit key, the ciphering key, Kc, is computed using another encryption algorithm, referred to as A8 (specifies the confidentiality algorithm). The Kc parameter is later used in the ciphering procedure. After these computations, the terminal responds with an authentication response message, which contains the SRES. The network uses the same parameters and the same algorithm to compute another SRES. The terminal SRES and the network SRES are then compared with each other. If a match occurs, the network accepts the user as an authorized subscriber. Otherwise, the authentication is rejected.

Figure 7.1: Encryption for GSM

7.3 Traffic Cases in GSM Network The traffic cases specify the action taken either by the GSM network elements or by the mobile station to use the services provided by the network. Based on the position and status of the mobile user or the movement area of the mobile user there are various traffic cases available to address in GSM network. A description of each of these cases is given below. 7.3.1 The Mobile Station is in idle mode In this traffic case the mobile device is not in use. It may be either in the switch off mode or in a standby mode. In standby model the device consumes no or little power and performs no operation. If the mobile device is switch off mode then it has to perform some operation automatically when Sikkim Manipal University

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it is switched on. A mobile device on getting switched on performs the following operations: When a mobile station is about to go to the idle state then it executes an IMSI attach procedure which involves the following steps. 1. The mobile station sends an IMSI attach message to the network indicating that it has changed state to idle. 2. The VLR determines whether there is a record for the subscriber already present. If not, the VLR contacts the subs b ’s LR f py f subscription information. 3. The VLR updates the mobile stations status to idle. 4. Acknowledgement is sent to mobile station. When the mobile station is in switched off mode, it can change its location into a different location area. But when it powered on in the new area it has to update its location information with the VLR of the visiting network. The mobile station executes the IMSI attach procedure in order to update the location information with the VLR During IMSI attach, the VLR may determine that the current Location Area Identifier (LAI) of the station is ff f m LAI s M ’s subscription information. If so, the VLR updates the LAI of the MS. 7.3.2 Roaming in the Network Mobile stations move continuously in the cellular network. The HLR keeps track of all the mobile stations under its coverage and the VLR store the information of all visitor nodes in a location area. The ability to move around, changing cells and connections over the air interface for quality reasons is called roaming. The movement of the mobile station may lead to a location change either within the same LA or among different LAs. In both the cases different procedures are executed. Below each of such cases are discussed. Changing Cells within an LA: If a mobile station change its location from one cell to another within the same LA, the network need not update. Because all cells under the same LA share the same HLR/VLR. The mobile station can detect the LA of a particular cell by listening to the BCCH in the new cell. The BCCH broadcasts ’s LAI. T s compares the last LAI received with the new LAI. If they are the same, it means that the mobile

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station has changed his location within the same LA and does not need to inform the network. Changing Location to different LA under same MSC/VLR: If an MS detects a change in LAI on the BCCH, it has to update the information stored in different network elements. On detecting a change in the LA, the mobile station sends location update information to the concerned VLR. On receipt of the location update request from the mobile station, the MSC/VLR s s b s f xs f m b s ’s f m . If the entry if found in the data base then it updates the existing information with new information received through the location update message, otherwise a new entry is made in the data base. The following steps are performed by the MSC/VLR to update the location information of the visitor mobile station. 1. On entering into a new cell, the mobile station listens to BCCH in the new cell and determines the LAI. The received LAI information is compared to the old one. If they differ, the mobile station initiates a location update procedure. 2. The mobile station uses the SDCCH to establish a connection with the network. Once the connection is established, it performs authentication procedure. 3. On successful completion of the authentication, the mobile station sends a Location Updating Request to the concerned MSC/VLR. 4. The MSC/VLR on receipt of the location update request send an acknowledgement to mobile station and release the signaling channel. Changing Location to different LA under different MSC/VLR: When a mobile station moves to a new LA under different MSC/VLR the location updating is performed. When the Location Update Request is received by the new VLR, it executes the procedure as given below. 1. The VLR verify the authenticity of the location update request as a first step of the location update process. If authentication is successful, the VLR checks its database to determine whether or not it has a record for this mobile station. 2. If the VLR do not find any record for the new mobile station, it sends a qu s subs b ’s LR f py f mobile stations subscription information. Sikkim Manipal University

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3. The HLR passes the information to the VLR and updates its location information for the subscriber. The HLR sends a request to the old VLR to delete the information about the mobile station. 4. The VLR stores its subscription information for the MS including the latest location and status. The VLR sends acknowledgement to the mobile station. Periodic Location Update or Periodic Registration: Apart from the location update cases discussed above, the mobile station also has to update its location information with the concerned location registers. If a mobile station does not perform such periodic location update with the system, the network marks the mobile station as detached. This may occur if a station is out of the coverage for long time. Sometimes the network elements also initiates periodic registration through the BCCH and also specifies how often periodic registration must be performed. Periodic registration has an acknowledgment message. The mobile station tries to register until it receives this message. Detaching the mobile station from the Network: IMSI detach enables the mobile station to indicate to the network that it is switched off. At power off, the mobile station sends an IMSI detach message to the network. On reception of the detach message, the VLR marks the corresponding IMSI as detached. The HLR is not informed. No acknowledgement is sent to the mobile station. Implicit Detach: Sometimes the mobile station cannot respond to the explicit IMSI detach process due to poor quality of the radio link. In that case the system might not be able to be detached from the network. The VLR then performs an implicit detach procedure to mark the mobile station as detached from the network. MS Purging: The VLR stores information regarding the visitor mobile stations. It is very obvious as due to movement of the mobile station it changes its VLR from one to another. When a VLR notices that a particular mobile station is not under its coverage area then it decides to purge the information from its data base regarding the station. Before purging the information regarding any mobile station the VLR informs the HLR of the station that the VLR is about to remove a subscriber record from its own Sikkim Manipal University

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database. The HLR then sets the mobile station purged flag and treats the subscriber as unreachable. 7.3.3 The mobile station is in active mode If a mobile station is not in idle mode then it may have any ongoing conversation. In this case the mobile station is said to be in active state. When it is in active state, it may be engaged in a call, or may be in a state of call setup process. Based on these scenarios the following traffic cases are described for active mode of a mobile station. Call from a mobile station: The first traffic case in the active mode of the mobile station is considered where the mobile station is making a phone call to a PSTN subscriber. In order to initiate a call the mobile station performs the following procedure stepwise: 1. It (the mobile station) uses the RACH to request a signaling channel for the base station. 2. The BSC on request and availability allocates a signaling channel to the mobile station using AGCH. 3. The station then sends a call setup request using SDCCH to the MSC or VLR. 4. The MSC/VLR instructs the BSC to allocate an idle TCH. The mobile station is informed to tune to the TCH allocated by the BSC. 5. The MSC/VLR forwards the dialed number to the concerned exchange in the PSTN, which establishes a connection to the subscriber. 6. If the PSTN subscriber answers, the connection is established. Call to a mobile station: The second traffic case in active mode of the mobile station is calling a mobile station by a PSTN subscriber. The major difference between the first and the second case is that, in a call to a mobile station the exact location of the mobile subscriber is not known at the time of requesting the call setup. So, the mobile station needs to be searched using paging before a connection can be established. Below is the procedure that is been followed by different components of the network to complete a call from the PSTN subscriber to a mobile station. 1. The PSTN subscriber dials the mobile telephone number using Mobile Station ISDN number (MSISDN). A connection request is made to the mobile stations home GMSC (Gateway Mobile Switching Centre). Sikkim Manipal University

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2. The home GMSC finds the respective HLR of the called mobile station and sends a query to the HLR for information about the serving MSC/VLR of the called mobile station. 3. The HLR translates MSISDN into IMSI, and determines which MSC/VLR is currently serving the MS. 4. The HLR requests a Mobile Subscriber Roaming Number (MSRN) from the serving MSC/VLR of the called mobile station. 5. The MSC/VLR returns an MSRN via HLR to the GMSC. 6. The GMSC routes the call to the concerned MSC/VLR. 7. The MSC/VLR searches its record to find the current LA of the mobile station and a paging message is sent to the BSC of the controlling the LA. 8. The B

’s uses PCH to page the mobile station.

9. When the mobile station detects the paging message, it sends a request on RACH for a SDCCH. 10. The BSC provides a SDCCH via AGCH. 11. The MSC/VLR instructs the BSC to allocate an idle TCH to the mobile station and the mobile station is informed to tune to the TCH. If the mobile subscriber answers, the connection is established. Self Assessment Questions 1. When a mobile station is about to go to the idle state then it executes an IMSI attach procedure. (True/False) 2. The ____________ store the information of all visitor nodes in a location area. 3. The mobile station listens to ______________ in the new cell and determines the LAI. 4. The mobile station uses ____________ to establish a connection with the network. 5. Network elements may initiates periodic registration through the BCCH. (True/False) 6. The mobile station uses ____________ to request a signaling channel for the base station.

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7. The mobile station sends a call setup request using __________ to the MSC or VLR. 8. The PSTN subscriber dials the mobile telephone number using MSISDN. (True/False)

7.4 Handover in GSM Network During the movement of the mobile station it changes its cell and Location Area (LA). For each of the cell change the station may need to update its location information in the network. In the discussion made above we have seen few scenarios of changing cell and LA by the mobile station and the actions taken by the network or by the station as a reaction to cell changing process. However, we have not discussed anything about the cell change process during a conversation between any two subscribers on the move. In this section the cell changing process of a mobile station during ongoing conversation is discussed briefly. The process of changing cells during an ongoing conversation is called handover or handoff in GSM terminology. The mobile station always measures the strength of the received signal while it is on move. Because based on the signal strength the station can discover a cell change and appropriate action against handover. This type of handover where the mobile station takes the initiative to perform a handover is called Mobile Assisted Handover (MAHO). A mobile station continuously measures the signal strength and quality of its current cell and neighboring cell on the BCCH carriers. The measurements are carried out on the downlink while the station is in active mode. The measured results are sent to the radio base station (RBS) on SACCH at regular intervals. The concerned RBS measures signal strength and quality on the uplink channel. In the measurements are sent to the BSC in the form of measurement reports. Based on these received report the BSC takes the decision to perform handover or not to perform handover. If handover is required then the BSC also decides the cell to which the mobile station to be handed over. This process of deciding the handover and the required cell is called locating. As soon as a neighboring cell is considered to be better than the serving cell, a handover is attempted. Handover can also happen when the Timing Advance (TA) used by mobile station exceeds a threshold value set by the operator. Sometimes handover is initiated for balancing load between two neighboring cells. If a mobile station resides in a congested Sikkim Manipal University

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cell and initiates a call setup, it can be transferred to a nearby cell with less traffic. Provided an acceptable connection quality is likely to be obtained in the new cell. Such forced handover is also noticed in during the maintenance process of the network. In such situation all channels are released from traffic if required. There are several types of handover found in GSM network. These handovers are classified as handover takes place between two cells controlled by the same BSC or handover between two cells con by ff B ’s, bu the under same MSC/VLR or Handover between cells controlled by different MSC/VLRs. There exist a special handover called intra-Cell Handover exists in GSM network. It occurs when the BSC detects that the quality of the channel currently connecting the mobile station is too low, but there is no indication of any existing nearby cell with sufficiently strong signal. In that case the BSC selects another channel in the same cell with better signal quality, and the mobile station is ordered to retune to the new channel. A brief description of various handover processes are discussed below. In each case it is assumed that the decision to perform a handover has already been made and a target cell has been identified. Handover between two cells of the same BSC: To perform a handover between two cells of the same BSC, the MSC/VLR is not involved in the process. However, they are informed that the handover has taken place. If v v v s ff LA’s, up e is performed after completion of the ongoing call. To complete the handover process the following steps are carried out. 1. The BSC instructs the new RBS to activate a TCH. 2. The BSC sends a message to the mobile station to the old RBS using FACCH. The message contains information about the frequency and time slot to be used by the mobile station. 3. On receipt of the message the mobile station tunes to the new frequency, and transmits handover access bursts in the allotted time slot. 4. When the new RBS detects the handover bursts, it sends information about TA via FACCH. 5. The mobile station sends a Handover Complete message to the BSC through the new RBS. 6. The BSC informs the old RBS to release the old TCH. Sikkim Manipal University

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Handover between two diffe B ’s bu u s m M /VLR: I this case the following procedure is executed, and shown in the figure 7.3. 1. The current BSC of the mobile station sends a Handover required message to the MSC containing the identity of the target cell to which the handover will take place. 2. The MSC detects the BSC of the target cell and sends a Handover Request to it. 3. The new BSC instructs the target RBS to activate a TCH. 4. The new BSC sends a message to the mobile station via the MSC and the old RBS. 5. The mobile station then tunes to the new frequency and transmits handover access bursts in the allotted time slot. 6. When the new RBS sends information about TA the mobile station sends a Handover Complete message to MSC via the new BSC. 8. MSC sends the old BSC an instruction to release the old TCH. 9. The old BSC tells the old RBS to release the TCH.

Figure 7.2 : Inter-BSC handover Sikkim Manipal University

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Handover between s by ff M ’s: The procedure to perform handover is given below, and shown in the figure 7.4. 1. The current BSC of the mobile station sends a Handover required message to the serving MSC along with the identity of the target cell to which the mobile station to be handed over. 2. MSC identifies the MSC of target cell communicated through the Handover required sends a handover initiation request. 3. The new MSC allocates a handover number to reroute the call and send a Handover Request new BSC of the new MSC. 4. The new BSC instructs the target RBS to activate a TCH. 5. The old MSC on receipt of the information passes it on to old MSC together with the handover number. 6. A link is established to new MSC. 7. The old MSC sends a handover command to the mobile station via the old BSC. 8. The mobile station tunes to the new frequency and transmits handover access bursts in the allotted time slot.

Figure 7.3: Inter-MSC handover

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Self Assessment Questions 9. The BSC provides a SDCCH via AGCH. (True/False) 10.

The process of changing cells during an ongoing conversation is called handover or handoff in GSM terminology. (True/False)

11. This type of handover where the mobile station takes the initiative to perform a handover is called _________ ____________. 12.

A mobile station continuously measures the signal strength and quality of its current cell and neighbouring cell on the BCCH carriers. (True/False)

13. When the new RBS detects the handover bursts, it sends information about TA via FACCH. (True/False) 14. When a mobile station is about to go to the idle state then it executes an IMSI attach procedure. (True/False) 15. The _____________ store the information of all visitor nodes in a location area.

7.5 Terminal Questions 1. Explain system operations of GSM in details? 2. Briefly explain various traffic cases in GSM network? 3. What do you understand by the term handover in GSM network? Briefly explain the handovers in GSM network?

7.6 Summary We have discussed the GSM system operations and different traffic cases in this unit. To support users in GSM network the system components has to perform various tasks. The device and call initialization, connection management, and ending a call are main operations performed by a GSM network. Due to the movement of user in the network, the subscriber changes their locations vary frequently. This change of location during an ongoing call is called handover. Maintaining a uninterrupted call during handover is a challenging task in mobile network. To accomplish this task the network maintains the location information of the entire subscriber in the network. Various traffic cases leads to update different levels of registers maintained by the GSM system. Describe various traffic cases. Authenticating a subscriber before providing a GSM service to it is another essential task carried out by the network. Along with the identity verification, Sikkim Manipal University

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the system also checks the authorized services for the subscriber before serving any mobile user.

7.7 Answers Self Assessment Questions 1. True 2. The VLR 3. BCCH. 4. the SDCCH 5. True 6. the RACH 7. SDCCH 8. True. 9. True 10. True 11. Mobile Assisted Handover (MAHO). 12. True 13. True 14. True 15. VLR Terminal Questions 1. Answer to provide services to the GSM subscribers there are various operations carried out by the system components. It has to identify authorize subscribers to use the GSM services, identify them when they request services, initiate a call to another subscribers and many more. Refer to section 7.2. 2. Answer. The traffic cases describe the action taken either by the GSM network elements or by the mobile station to use the services of the network. Based on the movement pattern and status of the mobile user or the movement area of the mobile user there are various traffic cases available in GSM network Refer to section 7.2. 3. Answer. The process of changing cells during an ongoing conversation is called handover or handoff in GSM terminology. The mobile station always measures the strength of the received signal while it is on move. Sikkim Manipal University

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Because based on the signal strength the station can discover a cell change and appropriate action against handover. For rest of the answer refer to section 7.4. References  Second Edition, Tata McGraw Hills, 1995.  Michel Daoud Yacoub, Wireless Technology, CRC Press, 2002.  Dharma Prakash Agarwal, Introduction to Wireless Communication, Second Edition, Thomson Learning 2007. E-References  http://www.scribd.com/doc/6606382/Traffic-Cases  http://www.ericsson.com/hr/services/competence/course_gsm/LZU_108 _852_en.pdf

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