Wireless Communications Engineering Lecture 9: GSM Prof. Mingbo Xiao Nov. 25, 2004
Introduction to GSM <ul><li>Short for Global System for Mobile Communications </li></ul><ul><li>Most Popular 2G Cellular S...
Multiple Access Control Power Frequency Time FDMA Frequency Power Time FDMA/TDMA Frequency CDMA Power Time
Frequency Bands / Bandwidth Uplink 890 – 915 MHz 25 MHz Downlink 935 – 960 MHz 25 MHz 100 KHz 200 KHz 100 KHz 1 4 3 124 2 ...
 
 
 
 
 
Basic Components of GSM BTS MS U m BTS BTS BSC Base Station Subsystem (BSS) A-bis A BTS:  Base Transceiver Station BSC:  B...
1. Mobile Station <ul><li>MS  is the physical equipment used by a GSM subscriber (their ‘mobile handset’).  </li></ul><ul>...
2. Subscriber Identity Module <ul><ul><ul><ul><li>( SIM ) is a  smart card  which carries all the subscriber specific info...
3. Mobile Equipment <ul><ul><li>( ME ) provides the  radio and processing needed to access the GSM network , plus a  man m...
4. Base Transceiver Station <ul><ul><li>( BTS )  provides GSM radio coverage within a cell . It comprises radio transmitti...
5. Base Station Controller <ul><ul><li>( BSC ) is a  small switch  with enhanced processing capability. It acts as a  loca...
5. Base Station Controller… 2 <ul><ul><ul><li>radio channel management </li></ul></ul></ul><ul><ul><ul><li>handoff between...
6. Mobile Switching Centre <ul><ul><ul><li>( MSC ) is an  ISDN switch  with (significantly) enhanced processing capability...
7. Visitor Location Register <ul><ul><ul><li>( VLR ) is an  intelligent database  and service control function. It  stores...
8. Interrogating Node <ul><ul><li>( IN ) is the target for calls bound for GSM users. It is responsible for determining th...
9. Home Location Register <ul><ul><ul><li>(HLR) is an  intelligent database  and service control function responsible for ...
10. Authentication Centre <ul><ul><li>( AuC ) is an  intelligent database  concerned with the  regulation of access to the...
GSM Interfaces <ul><ul><li>The component parts of the GSM system interconnect using standard interfaces. These allows an o...
GSM Characteristics
GSM Processes <ul><li>handoff </li></ul><ul><ul><li>brief introduction </li></ul></ul><ul><li>Paging </li></ul><ul><ul><li...
Handoff <ul><ul><ul><li>Handoff  is the means of maintaining a call when a user moves outside the coverage area of the ser...
Paging <ul><ul><ul><li>Paging  is a process of broadcasting a message which  alerts a specific mobile to take some action ...
Location Updating <ul><ul><ul><li>Location updating  is used to reduce the area over which paging must be undertaken in a ...
Location Area <ul><li>GSM Network Areas... </li></ul>Location Area MSC / VLR Area Public Land Mobile Network (PLMN) Cell
GSM Network Areas <ul><ul><li>Public Land Mobile Network </li></ul></ul><ul><ul><ul><li>the area served by one operator, e...
Location Update <ul><ul><ul><li>The MS detects that it has entered a new location area by comparing the last known LA ( st...
 
LA Update Message Sequence <ul><ul><ul><li>MS to NEW MSC (via BSS)  – location updating request </li></ul></ul></ul><ul><u...
 
 
 
 
 
 
 
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Wireless Communications and Networks

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Wireless Communications and Networks

  1. 1. Wireless Communications Engineering Lecture 9: GSM Prof. Mingbo Xiao Nov. 25, 2004
  2. 2. Introduction to GSM <ul><li>Short for Global System for Mobile Communications </li></ul><ul><li>Most Popular 2G Cellular System: ¾ of the mobile users worldwide are GSM subscribers </li></ul><ul><li>TDMA system with slow frequency hopping (channel=time slot in a band) </li></ul>
  3. 3. Multiple Access Control Power Frequency Time FDMA Frequency Power Time FDMA/TDMA Frequency CDMA Power Time
  4. 4. Frequency Bands / Bandwidth Uplink 890 – 915 MHz 25 MHz Downlink 935 – 960 MHz 25 MHz 100 KHz 200 KHz 100 KHz 1 4 3 124 2 …………… . A 200 kHz carrier spacing has been chosen. Excluding 2x100 kHz edges of the band, this gives 124 possible carriers for the uplink and downlink. The use of carrier 1 and 124 are optional for operators. Multiple Access Technique FDMA/TDMA. The total band is divided into 124x200 kHz bands (FDMA). Each group of 8 users transmit through a 200 kHz band sharing transmission time (TDMA).
  5. 10. Basic Components of GSM BTS MS U m BTS BTS BSC Base Station Subsystem (BSS) A-bis A BTS: Base Transceiver Station BSC: Base Station Controller HLR: Home Location Register VLR: Visited Location Register OMC: Operation & Maintenance Centre EIR: Equipment Identity Register AUC: Authentication Centre Mobile Services Switching Centre (MSC) HLR EIR VLR OMC AUC F H G B C other MSCs E PSTN ISDN CSPDN PSPDN other VLRs other BSSs D
  6. 11. 1. Mobile Station <ul><li>MS is the physical equipment used by a GSM subscriber (their ‘mobile handset’). </li></ul><ul><li>It comprises two parts: </li></ul><ul><ul><li>a Subscriber Identity Module ( SIM ) and </li></ul></ul><ul><ul><li>the Mobile Equipment ( ME ). </li></ul></ul>
  7. 12. 2. Subscriber Identity Module <ul><ul><ul><ul><li>( SIM ) is a smart card which carries all the subscriber specific information used by an MS. Major functions are to identify the current user of an MS and to take part in security and confidentiality procedures. It also stores recent location data and may also store personal information for the user such as abbreviated dialling codes (telephone directory). Specific functions include: </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Permanent storage of a subscriber’s International Mobile Subscriber Identity (IMSI) and Authentication key (Ki) </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Semi permanent storage of system information e.g. current Location Area Identity (LAI), encryption key Kc and lists of preferred / forbidden GSM networks </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Semi permanent storage of user data , ‘telephone directory’, short messages </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Participation in mobility procedures e.g. user authentication, generation of ciphering key, instigation of location updates. </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Protected by PIN </li></ul></ul></ul></ul></ul>
  8. 13. 3. Mobile Equipment <ul><ul><li>( ME ) provides the radio and processing needed to access the GSM network , plus a man machine interface MMI to enable the user to access services. Specific functions </li></ul></ul><ul><ul><ul><li>radio transceiving and signal processing </li></ul></ul></ul><ul><ul><ul><li>radio related operations: power control; timing advance; discontinuous transmission (DTX); slow frequency hopping (SFH). </li></ul></ul></ul><ul><ul><ul><li>Call handling </li></ul></ul></ul><ul><ul><ul><li>man-machine interface, display, keypad, speech transducers . </li></ul></ul></ul><ul><ul><ul><li>interfaces to external equipment e.g. laptops / palmtops </li></ul></ul></ul>
  9. 14. 4. Base Transceiver Station <ul><ul><li>( BTS ) provides GSM radio coverage within a cell . It comprises radio transmitting and receiving equipment (including antennas) and associated signal processing. A ‘sophisticated’ radio modem. Specific functions include: </li></ul></ul><ul><ul><ul><li>antennas (with diversity) </li></ul></ul></ul><ul><ul><ul><li>radio transceiving and signal processing </li></ul></ul></ul><ul><ul><ul><li>sending TDMA timing advance instructions to MS </li></ul></ul></ul><ul><ul><li>implementation of radio related operations: slow frequency hopping (SFH); discontinuous transmission (DTX); ciphering and power control. </li></ul></ul>
  10. 15. 5. Base Station Controller <ul><ul><li>( BSC ) is a small switch with enhanced processing capability. It acts as a local concentrator of traffic and provides local switching to effect handoff between a number of BTSs. It manages the radio channel and undertakes control of a variety of radio related procedures ensuring that reliable radio links are maintained. Its responsibilities include: </li></ul></ul><ul><ul><ul><li>co-ordination and control of a number of BTSs </li></ul></ul></ul><ul><ul><ul><li>traffic concentration </li></ul></ul></ul><ul><ul><ul><li>low level switching operations </li></ul></ul></ul><ul><ul><ul><li>network management interface to all radio elements </li></ul></ul></ul>
  11. 16. 5. Base Station Controller… 2 <ul><ul><ul><li>radio channel management </li></ul></ul></ul><ul><ul><ul><li>handoff between BTSs within its domain </li></ul></ul></ul><ul><ul><ul><li>scheduling of paging request </li></ul></ul></ul><ul><ul><ul><li>passing system information e.g. LAI (Location Area Identity) to BTSs for onward transmission </li></ul></ul></ul><ul><ul><ul><li>queuing </li></ul></ul></ul><ul><ul><ul><li>control of radio related operations: slow frequency hopping (SFH); discontinuous transmission (DTX); ciphering and power control </li></ul></ul></ul><ul><ul><ul><li>service transcoding, changing air interface rates to network rates e.g. converting speech from 13 kbit/s to 64 kbit/s </li></ul></ul></ul>
  12. 17. 6. Mobile Switching Centre <ul><ul><ul><li>( MSC ) is an ISDN switch with (significantly) enhanced processing capability. An MSC will parent a number of BSCs. Responsible for call handling of the mobile subscribers within its domain; this includes generating call charging records (for billing). Specific responsibilities of the MSC include: </li></ul></ul></ul><ul><ul><ul><ul><li>call control </li></ul></ul></ul></ul><ul><ul><ul><ul><li>generation of call records </li></ul></ul></ul></ul><ul><ul><ul><ul><li>supplementary service execution </li></ul></ul></ul></ul><ul><ul><ul><ul><li>inter BSC and inter MSC handoff </li></ul></ul></ul></ul><ul><ul><ul><li>Whilst they are identified as distinct entities within the specifications, the Mobile-services Switching Centre and Visitor Location Register (VLR) are invariably implemented as a single MSC/VLR unit. </li></ul></ul></ul>
  13. 18. 7. Visitor Location Register <ul><ul><ul><li>( VLR ) is an intelligent database and service control function. It stores (on a temporary basis) the information needed to handle calls set up or received by MSs registered with it and controls certain services (primarily those associated with outgoing calls ). This includes their International Mobile Subscriber Identities (IMSI), current Location Area Identities (LAIs) and supplementary service entitlements. Responsibilities of the VLR include: </li></ul></ul></ul><ul><ul><ul><ul><li>executing supplementary service programs (outgoing calls barred) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>initiating authentication and ciphering </li></ul></ul></ul></ul><ul><ul><ul><ul><li>initiating paging </li></ul></ul></ul></ul><ul><ul><ul><ul><li>mapping of various identities (MSISDN, IMSI, TMSI, MSRN) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>passing location information to HLR </li></ul></ul></ul></ul>
  14. 19. 8. Interrogating Node <ul><ul><li>( IN ) is the target for calls bound for GSM users. It is responsible for determining the location of a called subscriber and for routeing calls accordingly. The IN is commonly combined with an MSC forming what is known as a Gateway MSC ( GMSC ). </li></ul></ul>
  15. 20. 9. Home Location Register <ul><ul><ul><li>(HLR) is an intelligent database and service control function responsible for management of each subscriber’s records and control of certain services (primarily those associated with incoming calls ). It carries subscription details for a subscriber and location information enabling the routeing of incoming calls towards the subscriber, i.e. the MSC/VLR currently serving the MS. Responsibilities of the HLR include: </li></ul></ul></ul><ul><ul><ul><ul><li>management of service profiles </li></ul></ul></ul></ul><ul><ul><ul><ul><li>mapping of subscriber identities (MISDN, IMSI) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>supplementary service control and profile updates </li></ul></ul></ul></ul><ul><ul><ul><ul><li>execution of supplementary service logic e.g. incoming calls barred. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>passing subscription records to VLR </li></ul></ul></ul></ul>
  16. 21. 10. Authentication Centre <ul><ul><li>( AuC ) is an intelligent database concerned with the regulation of access to the network ensuring that services can be used only by those who are entitled to do so and that the access is achieved in a secure way . </li></ul></ul><ul><ul><li>The principle is that the AuC and the SIM have a unique key for every subscriber (K i ) which is used as the basis for generating a response (SRES) to a random number (RAND) generated by the AuC. Only the true SIM will be able to generate the correct response and thus gain access to the network. </li></ul></ul><ul><ul><li>The AuC is generally integrated with the HLR. </li></ul></ul><ul><ul><li>[Also EIR the Equipment Identity Register - another database that holds a list of ‘allowed’ equipment identities, i.e. ME numbers: white / grey / black lists] </li></ul></ul>
  17. 22. GSM Interfaces <ul><ul><li>The component parts of the GSM system interconnect using standard interfaces. These allows an operator to purchase different parts of the system competitively, i.e. from different manufacturers. </li></ul></ul><ul><ul><li>The more important interfaces are </li></ul></ul><ul><ul><ul><li>U m – the air interface </li></ul></ul></ul><ul><ul><ul><li>A interface – between the BSC and MSC </li></ul></ul></ul>
  18. 23. GSM Characteristics
  19. 24. GSM Processes <ul><li>handoff </li></ul><ul><ul><li>brief introduction </li></ul></ul><ul><li>Paging </li></ul><ul><ul><li>Location Area </li></ul></ul><ul><ul><li>Location Update </li></ul></ul><ul><ul><li>Location Area Message Sequence as example </li></ul></ul><ul><li>Incoming Call </li></ul><ul><li>Outgoing Call </li></ul>
  20. 25. Handoff <ul><ul><ul><li>Handoff is the means of maintaining a call when a user moves outside the coverage area of the serving cell. </li></ul></ul></ul><ul><ul><ul><li>The call must be switched to an alternative cell to provide service, automatically and without loss of service. </li></ul></ul></ul><ul><ul><ul><li>handoff is a complex process requiring synchronisation of events between the mobile station and the network. </li></ul></ul></ul><ul><ul><ul><li>In particular, there is the need to route the call to the new cell before handoff can be effected whilst maintaining the old connection until the new connection is known to have succeeded. </li></ul></ul></ul><ul><ul><ul><li>handoff is a time critical process requiring action to be taken before the existing radio link degrades to such an extent that the call is lost. </li></ul></ul></ul>
  21. 26. Paging <ul><ul><ul><li>Paging is a process of broadcasting a message which alerts a specific mobile to take some action , for example if there is an incoming call to be received. </li></ul></ul></ul><ul><ul><ul><li>If the system does not know the precise cell in which a mobile is located it must perform paging in a number of cells. </li></ul></ul></ul><ul><ul><ul><li>An extreme approach would be to undertake paging throughout the entire coverage area of a cellular system whenever a mobile is to be alerted; however, in anything but the smallest system this would be wasteful of valuable signalling capacity, particularly over the air interface. </li></ul></ul></ul><ul><ul><ul><li>The problem is addressed by the use of location areas and location updating . </li></ul></ul></ul>
  22. 27. Location Updating <ul><ul><ul><li>Location updating is used to reduce the area over which paging must be undertaken in a cellular system. </li></ul></ul></ul><ul><ul><ul><li>Coverage area is divided up into many location areas. </li></ul></ul></ul><ul><ul><ul><li>All cells broadcast the identity of the Location Area (LAI). </li></ul></ul></ul><ul><ul><ul><li>Each time a mobile station observes that it has moved into a new location area it informs the network by performing a location update; this enables the network to perform paging over a smaller area than would otherwise be necessary. </li></ul></ul></ul><ul><ul><ul><li>In the extreme case each cell could be a location area, the system would know very precisely where a mobile was but at the expense of a very high level of location update signalling. As a compromise location areas are generally defined as a group of cells . </li></ul></ul></ul>
  23. 28. Location Area <ul><li>GSM Network Areas... </li></ul>Location Area MSC / VLR Area Public Land Mobile Network (PLMN) Cell
  24. 29. GSM Network Areas <ul><ul><li>Public Land Mobile Network </li></ul></ul><ul><ul><ul><li>the area served by one operator, e.g. Vodafone / Orange etc. </li></ul></ul></ul><ul><ul><li>MSC / VLR Area </li></ul></ul><ul><ul><ul><li>a group of Location Areas served by one MSC / VLR, i.e. all the cells connected to that MSC / VLR </li></ul></ul></ul><ul><ul><li>Location Area </li></ul></ul><ul><ul><ul><li>a Group of Cells within one MSC/VLR Area identified by its Location Area Identity (LAI) </li></ul></ul></ul><ul><ul><li>Cell </li></ul></ul><ul><ul><ul><li>identified by its Cell Global Identity number (CGI) corresponds to the radio coverage are of one base transceiver station </li></ul></ul></ul>
  25. 30. Location Update <ul><ul><ul><li>The MS detects that it has entered a new location area by comparing the last known LA ( stored on the SIM ) with the information broadcast by the local cell. </li></ul></ul></ul><ul><ul><ul><li>The MS gains access to a radio channel and requests a location update. </li></ul></ul></ul><ul><ul><ul><ul><li>If the serving MSC/VLR is unchanged the network can immediately authenticate the MS and note the change of LA. </li></ul></ul></ul></ul><ul><ul><ul><li>If the MS has moved MSC/VLR, the MSC/VLR addresses a message to the HLR. </li></ul></ul></ul><ul><ul><ul><li>The HLR notes the new location (VLR) and downloads security parameters to allow the network to authenticate the mobile, it also passes on subscription details of the user to the new VLR and informs the old VLR to delete its records. </li></ul></ul></ul>
  26. 32. LA Update Message Sequence <ul><ul><ul><li>MS to NEW MSC (via BSS) – location updating request </li></ul></ul></ul><ul><ul><ul><li>New MSC to NEW VLR – update location area </li></ul></ul></ul><ul><ul><ul><li>New VLR to HLR – update location </li></ul></ul></ul><ul><ul><ul><li>HLR to OLD VLR – cancel location </li></ul></ul></ul><ul><ul><ul><li>OLD VLR to HLR – cancel location ACK </li></ul></ul></ul><ul><ul><ul><li>HLR to NEW VLR – insert subscriber data </li></ul></ul></ul><ul><ul><ul><li>NEW VLR to HLR – insert subscriber data ACK </li></ul></ul></ul><ul><ul><ul><li>HLR to NEW VLR – update location ACK </li></ul></ul></ul><ul><ul><ul><li>NEW VLR to NEW MSC – update location area ACK </li></ul></ul></ul><ul><ul><ul><li>NEW VLR to MS – location updating accept </li></ul></ul></ul><ul><ul><ul><li>ACK = acknowledge </li></ul></ul></ul>
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