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GSM ARCHITECTURE

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Includes architecture, and technologies related to GSM like GPRS etc.

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GSM ARCHITECTURE

  1. 1. Section 1 – GSM Architecture OverviewGSM Architecture Overview
  2. 2. Section 1 – GSM Architecture Overview IntroductionIt provides an overview of the GSM network architecture. This includes a brief explanation of the different network subsystems and a description of the functionality of the elements within each of the subsystems. Topics include:• General architecture overview• The Mobile Station (MS) Subsystem and Elements• The Base Station Subsystem (BSS) and Elements• The Network Subsystem (NSS) and Elements• Introduction to network interfaces
  3. 3. Section 1 – GSM Architecture Overview
  4. 4. Section 1 – GSM Architecture OverviewA GSM network is made up of three subsystems:• The Mobile Station (MS)• The Base Station Sub-system (BSS) – comprising a BSC and several BTSs• The Network and Switching Sub-system (NSS) – comprising an MSC and associated registersThe interfaces defined between each of these sub systems include:• A interface between NSS and BSS• Abis interface between BSC and BTS (within the BSS)• Um air interface between the BSS and the MS
  5. 5. Section 1 – GSM Architecture OverviewAbbreviations:MSC – Mobile Switching CenterBSS – Base Station Sub-systemBSC – Base Station ControllerHLR – Home Location RegisterBTS – Base Transceiver StationVLR – Visitor Location RegisterTRX – TransceiverAuC – Authentication CenterMS – Mobile StationEIR – Equipment Identity RegisterOMC – Operations and Maintenance CenterPSTN – Public Switched Telephone Network
  6. 6. Section 1 – GSM Architecture Overview Mobile StationThe Mobile Station (MS) consists of the physical equipment used by a PLMN subscriber to connect to the network. It comprises the Mobile Equipment (ME) and the Subscriber Identity Module (SIM). The ME forms part of the Mobile Termination (MT) which, depending on the application and services, may also include various types of Terminal Equipment (TE) and associated Terminal Adapter (TA).
  7. 7. Section 1 – GSM Architecture Overview
  8. 8. Section 1 – GSM Architecture Overview• The IMSI identifies the subscriber within the GSM network while the MS ISDN is the actual telephone number a caller (possibly in another network) uses to reach that person.• Security is provided by the use of an authentication key and by the transmission of a temporary subscriber identity (TMSI) across the radio interface where possible to avoid using the permanent IMSI identity.• The IMEI may be used to block certain types of equipment from accessing the network if they are unsuitable and also to check for stolen equipment.
  9. 9. Section 1 – GSM Architecture OverviewMS and SIM
  10. 10. Section 1 – GSM Architecture OverviewThe mobile station consists of : • mobile equipment (ME) • subscriber identity module (SIM)The SIM stores permanent and temporary data about the mobile, the subscriber and the network, including : • The International Mobile Subscribers Identity (IMSI) • MS ISDN number of subscriber • Authentication key (Ki) and algorithms for authentication checkThe mobile equipment has a unique International Mobile Equipment Identity (IMEI), which is used by the EIR
  11. 11. Section 1 – GSM Architecture OverviewBase Station Subsystem (BSS)
  12. 12. Section 1 – GSM Architecture OverviewThe BSS comprises:• Base Station Controller (BSC)• One or more Base Transceiver Stations (BTSs)The purpose of the BTS is to:• provide radio access to the mobile stations• manage the radio access aspects of the systemBTS contains:• Radio Transmitter/Receiver (TRX)• Signal processing and control equipment• Antennas and feeder cables
  13. 13. Section 1 – GSM Architecture OverviewThe BSC:• allocates a channel for the duration of a call• maintains the call: monitors quality controls the power transmitted by the BTS or MS generates a handover to another cell when required
  14. 14. Section 1 – GSM Architecture Overview Network Switching System (NSS)The NSS combines the call routing switches (MSCs and GMSC) with database registers required to keep track of subscribers’ movements and use of the system. Call routing between MSCs is taken via existing PSTN or ISDN networks. Signaling between the registers uses Signaling System No. 7 protocol.
  15. 15. Section 1 – GSM Architecture OverviewFunctions of the MSC:• Switching calls, controlling calls and logging calls• Interface with PSTN, ISDN, PSPDN• Mobility management over the radio network and other networks• Radio Resource management - handovers between BSCs• Billing Information
  16. 16. Section 1 – GSM Architecture Overview InterfacesUm VLR Abis A BSC MSC ISDN, TUP
  17. 17. Section 1 – GSM Architecture Overview ExerciseQ1. Name the interfaces used between Mobile and BTS BTS and BSC BSC and MSC
  18. 18. Section 2 – Access NetworkAccess Network
  19. 19. Section 2 – Access Network ObjectiveThe Trainee will be able to understand:• Different BTS configuration commonly used in the network• Advantages of the configuration and optimal use of the trunks• Abis mapping
  20. 20. Section 2 – Access Network IntroductionAccess network is a connection between MS and NSS, comprise of BTSs & BSCs. It is responsible for radio management.BSC looks towards MSC through single A-interface as being the entity responsible for communicating with Mobile Stations in a certain area. The radio equipment of a BSS may support one or more cells.A BSS may consist of one or more base stations, where an A-bis- interface is implemented.
  21. 21. Section 2 – Access Network BSS Configuration• Collocated BTS• Remote BTS• Daisy Chain BTS• Star Configuration• Loop Configuration
  22. 22. Section 2 – Access NetworkCollocated BTS: BTS is situated along with BSC or the MSC and no additional E1 link is required. BTS BSC
  23. 23. Section 2 – Access NetworkRemote BTS : BTS is situated in a stand alone position and additional E1 links are required to connect to BSC. BSC BTS
  24. 24. Section 2 – Access Network Daisy Chain BTS 3 BTS 1 BTS 4 BSCBTS 2 MSC
  25. 25. Section 2 – Access Network Star Configuration BTS 3 BTS 1 BSC BTS 4BTS 2 MSC
  26. 26. Section 2 – Access Network Loop Configuration BTS 3 BTS 1 BTS 4 BSCBTS 2 MSC
  27. 27. Section 2 – Access Network Comparison of Different Configurations• Daisy Chain: Easy to implement, effective utilization of transmission links but if one of the link fails, all the BTSs connected in the chain will went off.• Star Configuration: Easy to implement but poor utilization of links. Each BTS require one E1 to connect to BSC. But if link goes down only individual BTS will be affected.• Loop Configuration: Slightly difficult to implement but effective utilization of E1 links. Even if one link goes off BTS will continue to communicate with the network from the other side.
  28. 28. Section 2 – Access Network BSS Interfaces• Air Interface: Radio Interface between the BTS and Mobile the supports frequency hopping and diversity.• A Interface: Interface carried by a 2-Mb link between NSS and BSS. At this interface level, transcoding takes place.• OMC Interface: X25 Link.
  29. 29. Section 2 – Access Network
  30. 30. Section 2 – Access Network Abis Interface (BTS - BSC)If the BTS and BSC are not combined, A-bis interface will be used. Otherwise, BS interface will be used. Several frame unit channels are multiplexed on the same PCM support and BSC and BTS can be remote from each other. Its main functions are:• Conversion of 260 – bit encoded blocks (corresponding to 160x8 – bit samples for 20ms)• Encoded block synchronization• Vocal activity detection• Alarm dispatch to BSC via PCM• Test loop back operation
  31. 31. Section 2 – Access NetworkTRX 1TRX 2
  32. 32. Section 2 – Access Network ExerciseQ1. In How many ways BTSs can be connected and which configuration gives the optimal solution?Q2. What is a difference between BS interface and Abis interface?Q3. How many time slots are occupied by 1TRX on a PCM frame?
  33. 33. Section 3 – NSS TopologyNSS Topology
  34. 34. Section 3 – NSS Topology ObjectiveThe Trainee will be able to understand:• Terminology used in Network Sub System• Protocols and Interfaces inside NSS• Call routing and circuit groups• Switching modules• Stand alone and integrated HLR• Echo canceller and TRAU location• Authentication, Ciphering, OMC, Billing center• Transit Switch
  35. 35. Section 3 – NSS Topology IntroductionNetwork Sub System can be considered as a heart of the GSM Network. All the major activities like switching of calls, routing, security functions, call handling, charging, operation & maintenance, handover decisions, takes place within the entities of NSS.Various kinds of interfaces are used to communicate between the different entities. Different methods are used to optimize and provide the quality network with the minimum operating cost.
  36. 36. Section 3 – NSS Topology Network Switching System (NSS)Key elements of the NSS:• Mobile Switching Center (MSC)• Visitor Location Register (VLR)• Home Location Register (HLR)• Authentication Center (AuC)• Equipment Identity Register (EIR)• Gateway MSC (GMSC)These elements are interconnected by means of an SS7 network
  37. 37. Section 3 – NSS Topology NSS IdentifierIMEI – International Mobile Equipment Identifier.The IMEI is an internationally-unique serial number allocated to the MS hardware at the time of manufacture. It is registered by the network operator and (optionally) stored in the AuC for validation purposes. IMEI = TAC + FAC + SNR +sp TAC = Type Approval Code by central GSM body FAC = Final Assembly Code, identifies the manufacturer SNR = Serial Number, unique six digit number sp = spare for future use
  38. 38. Section 3 – NSS TopologyIMSI – International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator, a unique subscriber IMSI identifier is issued and stored in the SIM of the MS as well as in the HLR . An MS can only function fully if it is operated with a valid SIM inserted into an MS with a valid IMEI. IMSI consist of three parts: IMSI = MCC + MNC + MSIN MCC = Mobile Country Code MNC = Mobile Network Code MSIN = Mobile Station Identification Number
  39. 39. Section 3 – NSS TopologyTMSI –Temporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber. It is issued by and stored within a VLR (not in the HLR) when an IMSI attach takes place or a Location Area (LA) update takes place. At the MS it is stored in the MS’s SIM. The issued TMSI only has validity within a specific LA.Since TMSI has local significance, the structure may be chosen by the administration. It should not be more than four octets.
  40. 40. Section 3 – NSS TopologyMSISDN – Mobile Station ISDN NumberThe MSISDN represents the ‘true’ or ‘dialled’ number associated with the subscriber. It is assigned to the subscriber by the network operator at registration and is stored in the SIM.According to the CCITT recommendations, it is composed in the following way: MSISDN = CC + NDC + SN CC = Country Code NDC = National Destination Code SN = Subscriber Number
  41. 41. Section 3 – NSS TopologyMSRN – Mobile Station Roaming NumberThe MSRN is a temporary, location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility. It is stored in the VLR and associated HLR but not in the MS. The MSRN is used by the VLR associated MSC for call routing within the MSC/VLR service area.
  42. 42. Section 3 – NSS TopologyLAI – Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI). The LAI is broadcast regularly by BTSs on the Broadcast Control channel (BCCH), thus uniquely identifying each cell with an associated LA. LAI = MCC + MNC + LAC MCC = Mobile Country Code, same as in IMSI MNC = Mobile Network Code, same as in IMSI LAC = Location Area Code, identifies a location area within a GSM PLMN network. Maximum length of LAC is 16 bits.
  43. 43. Section 3 – NSS Topology Mobile Switching Center (MSC)The Mobile services Switching Center (MSC) performs the telephony switching functions of the system. It also controls calls to and from other telephony and data systems, such as the Public Switched Telephone Network (PSTN) and Public Land Mobile Network (PLMN).Difference between a MSC and an exchange in a fixed network is - MSC has to take into account the impact of the allocation of radio resources and the mobile nature of the subscribers and has to perform in addition, at least the following procedures:
  44. 44. Section 3 – NSS Topology• required for location registration• procedures required for handoverAn MSC can be connected to only one VLR. Therefore, all mobile stations that move around under base stations connected to the MSC are always managed by the same VLR.An MSC would communicate typically with one EIR. While it is possible for an MSC to communicate to multiple EIRs, this is highly unlikely since the EIR provides a centralized and geographic independent function.
  45. 45. Section 3 – NSS TopologyThe MSC consults an HLR to determine how a call should be routed to a given mobile station:• For incoming calls to a mobile station, the MSC would typically consult one HLR.• For mobile-to-mobile calls in larger networks, a MSC could consult HLRs of other systems to help minimize the trunk paths to the other mobile station.A given MSC can be interconnected to other MSCs to support inter-MSC handovers
  46. 46. Section 3 – NSS TopologyThe following are typical MSC functions in a cellular system:• Provide switched connections with PSTN• Provide switched connections between mobile subscribers• Provide coordination over signaling with mobiles• Coordinate the location and handover process• Provide custom services to mobile users• Collect billing data
  47. 47. Section 3 – NSS Topology ProtocolsMSC/BSC MSC/HLR OMC/MSC MSC/Fixed Network MSC/VLR OMC/HLR MSC/EIR OMC/VLR MSC/Voice messaging MSC/GMSC OMC/BSS VLR/VLR VLR/HLR MSC/MSCBSSMAP TCAP+MAP X.225 R2, ISUP other Signaling SCCP SCCP X.224 MTP MTP X.25 MTP SS7 SS7
  48. 48. Section 3 – NSS Topology Switching In MSCSignaling network is separated from the speech network and consists of• signaling Links (SL)• signaling Point (SP)• signaling Transfer Part (STP).
  49. 49. Section 3 – NSS TopologyTelephony system contains:• Group Switch to switch the calls,• ST to perform signaling in accordance with SS7• Trunk interfaces for interfacing the PCM.Group switch provides a semi permanent connection between time slot (PCM) and ST.
  50. 50. Section 3 – NSS Topology Signaling Point (SP)SP provides the functions of signaling and transmit and receive messages to and from different nodes. Each SP in the network will have an identification code termed as signaling Point Code (SPC).
  51. 51. Section 3 – NSS Topology Signaling Transfer Point (STP)Signaling Transfer Part is signaling point that only transfers messages from one signaling point (SP) to another. STP SP SP (SPC) STP (SPC)
  52. 52. Section 3 – NSS Topology Signaling Link (SL)Signaling Link is the 64kbps link interconnecting two signaling Points and provides the functions of message error control and message sequencing. Each signaling Link has an SLC (signaling Link Code), which identifies the signaling Link with in the signaling Link Set.
  53. 53. Section 3 – NSS Topology Service Switching Point (SSP)The MSC contains:• The Service Switching Point• One or more radio control pointSSP handles the usual switching function and can be connected via 2Mbps PCM link with:• Other exchanges of fixed PSTN or mobile PLMN,• Points on the SS7 signaling network,• X.25 networkContinued…..
  54. 54. Section 3 – NSS Topology• The OA&M network,• The Intelligent network,• PSTN via user data channels and signaling channels using ISUP and R2 protocols,• Other elements of the GSM
  55. 55. Section 3 – NSS TopologySwitching Function of SSP:• Main control,• Switching matrix,• PCM multiplex connection,• Service circuits• Operation and maintenance• Establishing and releasing section of the links from and to mobiles,• Finding circuits to the BSS; special circuit groups are created. SSP selects an incoming and outgoing circuit.
  56. 56. Section 3 – NSS Topology Call Routing• If a number received is a national or international number, the address information is passed to the SSP.• If the number received is an HPLMN (Home PLMN), the RCP asks the HLR for a roaming number (MSRN). This MSRN is passed to the SSP for routing.• If the number received is an emergency service number, the originating geographic area is attached to it and the combined information passed to the SSP.Continued…..
  57. 57. Section 3 – NSS TopologyIn the SSP the number received from RCP follow the standard translation process:• Preliminary analysis: Selection of a translator (national, international),• Translation: Determination of a routing depend on the first digits dialled,• Routing: Determination of an outing circuit group.
  58. 58. Section 3 – NSS Topology Circuit GroupsCall routes from the MSC through circuit groups. Different circuit groups are created inside it:• Group for the PSTN (according to the exchange)• Group for the BSCs• Group for the Supplementary services• Group for the IWF
  59. 59. Section 3 – NSS Topology CG1 BSC1 CG2 BSC2 CGn BSCnMSC CGa PSTN1 CGx PSTNx CG Supplementary Services CG IWF
  60. 60. Section 3 – NSS TopologyInterfaces
  61. 61. Section 3 – NSS Topology A-Interface (MSC – BSC)The interface between the MSC and its BSS is specified in the 08-series of GSM Technical Specifications. The BSS-MSC interface is used to carry information concerning:• BSS management;• call handling;• mobility management.
  62. 62. Section 3 – NSS Topology B-Interface (MSC – VLR)The VLR is the location and management data base for the mobile subscribers roaming in the area controlled by the associated MSC(s). Whenever the MSC needs data related to a given mobile station currently located in its area, it interrogates the VLR. When a mobile station initiates a location updating procedure with an MSC, the MSC informs its VLR which stores the relevant information. This procedure occurs whenever an MS roams to another location area. Also, when a subscriber activates a specific supplementary service or modifies some data attached to a service, the MSC informs (via the VLR) the HLR which stores these modifications and updates the VLR if required.
  63. 63. Section 3 – NSS Topology C-Interface (HLR - MSC)The Gateway MSC must interrogate the HLR of the required subscriber to obtain routing information for a call or a short message directed to that subscriber.
  64. 64. Section 3 – NSS Topology D-Interface (HLR - VLR)This interface is used to exchange the data related to the location of the mobile station and to the management of the subscriber. The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area. To support this, the location registers have to exchange data. The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station.The HLR sends to the VLR all the data needed to support the service to the mobile subscriber. The HLR then instructs the previous VLR to cancel the location registration of this subscriber. Exchanges of data may occur when the mobile subscriber requires a particular service, when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
  65. 65. Section 3 – NSS Topology E-Interface (MSC - MSC)When a mobile station moves from one MSC area to another during a call, a handover procedure has to be performed in order to continue the communication. For that purpose the MSCs have to exchange data to initiate and then to realize the operation. After the handover operation has been completed, the MSCs will exchange information to transfer A-interface signaling as necessary. When a short message is to be transferred between a Mobile Station and Short Message Service Centre (SC), in either direction, this interface is used to transfer the message between the MSC serving the Mobile Station and the MSC which acts as the interface to the SC.
  66. 66. Section 3 – NSS Topology F-Interface (MSC - EIR)This interface is used between MSC and EIR to exchange data, in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station.
  67. 67. Section 3 – NSS Topology G-Interface (VLR - VLR)When a mobile subscriber moves from a VLR area to another Location Registration procedure will happen. This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR.
  68. 68. Section 3 – NSS Topology H-Interface (HLR - AUC)When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data, the HLR requests the data from the AuC. The protocol used to transfer the data over this interface is not standardized.
  69. 69. Section 3 – NSS Topology Switch ModulesSwitch has three major types of equipment modules:• Switching module (SM)• Communication module (CM)• Administrative module (AM)
  70. 70. Section 3 – NSS TopologySwitching Module (SM):All external lines, trunks, and special services circuits are terminated at the switching module. The analog and digital signals are converted to the digital format used inside the switch. The SM performs almost 95% of the call processing and maintenance functions including:• Line and trunk scanning• Tone generation• Announcements• Call progress supervision• Routine maintenance and self-maintenance.
  71. 71. Section 3 – NSS TopologyThe SM also provides subscriber calling features including:— call waiting— abbreviated dialing— call diversion— conference calls.SM further has two components:9. Control units - Control all activities within the SM, such as call processing and maintenance functions.2. Peripheral units - Perform testing functions and provide customers and other exchanges access to the switch.
  72. 72. Section 3 – NSS TopologyCommunication Module (CM):The CM serves as the hub (focal point) for all inter module communication in a switch. The CM has four main functions:4. Call switching - The CM interconnects the paths between modules to complete telephone calls and to relay data.2. Message switching - The CM provides paths to send information between processors to process calls, maintain records, and perform system tasks.Continued…..
  73. 73. Section 3 – NSS Topology3. Network timing - The CM provides accurate timing and synchronization for the switch.4. Fast pump - The CM provides resources to quickly download (pump) an SM’s software if needed.
  74. 74. Section 3 – NSS TopologyAdministrative Module (AM):The AM controls the CM and communicates with all the SMs (through the CM). The AM monitors itself and the CM for malfunctions. If there are any problems, they are reported to maintenance personnel.The AM performs resource allocation and processing functions that are done more efficiently on a centralized basis such as:• Call routing for inter module and intra module calls• Administrative data processing/billing dataContinued…..
  75. 75. Section 3 – NSS Topology• Traffic measurement reports/system performance reports• Memory management• System maintenance• Maintaining file records of changes to the system Software Release.• Personnel interface/system monitoring• Allocating trunks for call processing.
  76. 76. Section 3 – NSS Topology Switch SM AM CMControl Peripheral MSGS TMS Unit Unit Control I/O Disk Tape MCC Unit Processor Unit Unit
  77. 77. Section 3 – NSS Topology Home Location RegisterHLR is a database that stores subscription and set of functions needed to manage subscriber data in one PLMN area. Any administrative action by the service provider or changes made by subscriber is first carried out on the HLR and then update the VLR. Following are the subscriber data which frequently changes: - Identification number MSISDN & IMSI - Service restriction - Teleservices - Bearer services - Supplementary services
  78. 78. Section 3 – NSS TopologyBeside the permanent data it also include dynamic data of home subscriber including VLR address, call forward number and call barring numbers.Triplets are also stored in the HLR for the authentication purpose.The HLR communicates with other nodes: VLR, AUC, GMSC & SMS – SC via MAP (Mobile Access Protocol). To support this communication HLR needs MTP and SCCP
  79. 79. Section 3 – NSS Topology
  80. 80. Section 3 – NSS Topology MAP (Mobile Application Protocol)The only way via which HLR communicates with other GSM nodes is Mobile Access Protocol. Number of functional blocks exist to support different MAP operations eg HLCAP is used for location cancellation or HLUAP is required for location updating. Other functions defined on the MAP are: - Inter MSC Handover and subsequent handover - Update HLR and VLR - Fault Recovery - Management and handling of supplementary services.Continued…..
  81. 81. Section 3 – NSS Topology - Support of Short Message Services. - Call establishment / delivery - Security related data. - Retrieval of subscriber data during call setup.HLR also needs to communicate with GMSC, VLR, AUC and SMS-SC, for which MTP and SCCP is essential.
  82. 82. Section 3 – NSS Topology SCCP (Signaling Connection Control Point)All MAP messaging use SCCP to analyze the GT (Global Title) of incoming information. If GT belongs to anther node, then SCCP will use the services of MTP (Message Transfer Part) to reroute the message.SCCP must have the GT analysis to terminate and route MAP messages from all nodes it communicates with.To find out the DPC, SCCP looks in a routing case translation table. The information about the DPC is then sent to MTP which sends the message to the appropriate SP.
  83. 83. Section 3 – NSS Topology MTP (Message Transfer Part)MTP must be defined to allow the nodes to communicate with each other.The MTP provides the means for reliable transport and delivery of UP (User Part) information across the No. 7 network eg ISDN User part (ISUP), the Telephone User Part (TUP), Signaling Connection Control Part (SCCP), Interworking function User Part (IWUP) and Data User Part (DUP)Continued…..
  84. 84. Section 3 – NSS TopologyMTP has the ability to react to system and network failure that affect the user information.MTP further has three functional levels:4. MTP Level 1 – Signaling data link5. MTP Level 2 – Signaling link6. MTP Level 3 – Signaling network
  85. 85. Section 3 – NSS TopologyHLR connects with MSC via C interface, VLR via D interface
  86. 86. Section 3 – NSS TopologyHLR can be configured in two ways:2. Integrated with MSC
  87. 87. Section 3 – NSS Topology• Hs• Stand Alone HLR (External Database)
  88. 88. Section 3 – NSS Topology Integrated Vs Stand Alone HLRThe Integrated HLR is accessed by other MSC’s/ VLR’s via MAP, and the switch can use MAP to query other off switch HLRs. The main advantages with an integrated HLR solution at this early stage are:• Efficient use of HW and lower HW investments• Fewer physical connections required due to fewer physical nodes• Less capacity required in No. 7 network as major part of HLR signaling is internal within MSC/VLR/HLR
  89. 89. Section 3 – NSS Topology• A single fault will affect a smaller number of subscribers than if standalone HLR is usedMajor drawbacks are:• Less processing capacity available for MSC/VLR. Additional Switching capacity will be required earlier• Migration to standalone HLR (which is to be preferred in a mature larger network) will induce major changes in the network• Administration of subscriptions• Operation and maintenance for HLR geographically distributed
  90. 90. Section 3 – NSS TopologyIn Stand Alone HLR, call processing activities are not performed by the switch. Only HLR queries are handled via the GSM standard MAP messages coming over signaling links from other Mobile Switching Centers (MSCs) in the wireless network.
  91. 91. Section 3 – NSS TopologyBenefits:• All HLR data is centralized, thus simplifying its ongoing maintenance and operation• High HLR Capacity• High processing capacity• On going enhancementThere are some drawbacks with standalone HLR• A fault in a HLR will affect many subscribers• A fault in a HLR will increase the signaling substantially inthe whole signaling network
  92. 92. Section 3 – NSS Topology HLR is responsible for:• Connection of mobile subscribers and definition of corresponding subscriber data.• Subscription to basic services.• Registration/deletion of supplementary services.• Activation/deactivation of supplementary services.• Interrogation of supplementary services status.Continued…..
  93. 93. Section 3 – NSS Topology• Functions for analysis of mobile subscriber numbers (MSISDN, IMSI, additional MSISDN) and other types of addresses.• Statistical functions for collecting data regarding the performance of the system.• Functions for communication with GMSC and VLR using the No. 7 signaling system and MAP• Handling of authentication and ciphering data for mobile subscribers including communication with an authentication center.Continue…..
  94. 94. Section 3 – NSS Topology• Get Password/Register Password• Alert Service Center• Provide Roaming Number• Send Routing Information for SMS• Send Routing Information for GMSC• Set Message Waiting Data
  95. 95. Section 3 – NSS Topology Visitor Location RegisterIt is a subscriber database containing the information about all the MS currently located in the MSC service area. VLR can be considered as a distributed HLR in the case of a roaming subscriber. If MS moves into a new service area (MSC), VLR requests the HLR to provide the relevant data and store it, for making the calls for that MS.VLR is always integrated with MSC to avoid the signaling load on the system.It can also be viewed as a subset of a HLR.
  96. 96. Section 3 – NSS TopologyVLR connects with MSC via B interface, HLR via D interface and with another VLR via G interface. G
  97. 97. Section 3 – NSS TopologyVLR is responsible for• Setting up and controlling calls along with supplementaryservices.• Continuity of speech (Handover)• Location updating and registration• Updating the mobile subscriber data.• Maintain the security of the subscriber by allocating TMSIContinued…..
  98. 98. Section 3 – NSS Topology• Receiving and delivering short messages• Handling signaling to and from - BSC and MSs using BSSMAP - other networks eg PSTN, ISDN using TUP• IMEI check• Retrieve data from HLR like authentication data, IMSI,ciphering keyContinued…..
  99. 99. Section 3 – NSS Topology• Retrieve information for incoming calls.• Retrieve information for outgoing calls.•Attach/Detach IMSI• Search for mobile subscriber, paging and complete the call.
  100. 100. Section 3 – NSS Topology Security FeatureBoth the users and the network operator must be protected against undesirable intrusion of third party. As a consequence, a security feature is implemented in the telecommunication services. The following parts of the system have been reinforced and provide the various security features:2. Access to the network authentication3. Radio part ciphering4. Mobile equipment equipment identification5. IMSI temporary identity
  101. 101. Section 3 – NSS Topology Authentication Center (AUC)AUC is always integrated with HLR for the purpose of the authentication. At subscription time, the Subscriber Authentication Key (Ki) is allocated to the subscriber, together with the IMSI. The Ki is stored in the AUC and used to provide the triplets, same Ki is also stored in the SIM.AUC stores the following information for each subscriber4. The IMSI number,5. The individual authentication key Ki,6. A version of A3 and A8 algorithm.Continued…..
  102. 102. Section 3 – NSS TopologyAuthentication is required at each registration, at each call setup attempt (mobile originated or terminated), at the time of location updating, before supplementary service activation, de- activation , registration.HLR uses the IMSI to communicate with AUC, triplets are requested in sets of five.Continued…..
  103. 103. Section 3 – NSS TopologyIn AUC following steps are used to produce one triplet:4. A non- predictable random number, RAND, is produced5. RAND & Ki are used to calculate the Signed Response (SRES) and the Ciphering Key (Kc)6. RAND, SRES and Kc are delivered together to HLR as one triplet.HLR delivers these triplets to MSC/VLR on request in such a way that VLR always has at least one triplet.
  104. 104. Section 3 – NSS TopologyAuthentication Procedure:The MSC/VLR transmits the RAND (128 bits) to the mobile. The MS computes the SRES (32 bits) using RAND, subscriber authentication key Ki (128 bits) and algorithm A3. MS sends back this SERS to AUC and is tested for validity.
  105. 105. Section 3 – NSS Topology SIM Card RAND A4IMSIKi SERS A4 =?A3 IMSI RAND KiA8 Ki Kc Triplets A3 A8 Triplets Kc A2 Generation Ciphering Ciphering RAND Function Function SERS A5 A5 Kc MS BTS MSC/VLR HLR AUC OMC
  106. 106. Section 3 – NSS Topology CipheringThe user data and signaling data passes over the radio interface are ciphered to prevent intrusion. The ciphered key (Kc) previously computed by the AUC is sent from the VLR to the BSS after the mobile has been authenticated. The Kc is also computed in the MS and in the way both ends of the radio link (MS and BSS) possess the same key.
  107. 107. Section 3 – NSS TopologyProcedure:For the authentication procedure, when SRES is being calculated, the Ciphering Key (Kc), is calculating too, using the algorithm A8.The Kc is used by the MS and the BTS in order to cipher and decipher the bit stream that is sent on the radio path.
  108. 108. Section 3 – NSS Topology SIM AUC Choice of random no RAND (128 bits) Ki RAND Ki A3 A3 A3 A3 SERS SERS =? A8 OK A8 A8 A8 Ciphering Command Kc (64 Kc Speech, data,sigSpeech, data,sig bits) Ciphered Data A5 A5 Ciphering/Deciphering
  109. 109. Section 3 – NSS Topology Subscriber ConfidentialityThe subscriber identity (IMSI), since is considered sensitive information, is not normally transmitted on the radio channel. A local, temporary identity is used for all interchanges. The identity (TMSI) is assigned after each change of authenticated location. For other cases:• Call set-up• Use of supplementary services• Use of SMSContinued…..
  110. 110. Section 3 – NSS TopologyA TMSI is allocated when the one supplied by the MS is considered out of date or when the MS does not provide the TMSI.Transmission of the TMSI over the traffic channel is ciphered.
  111. 111. Section 3 – NSS Topology Equipment Identification Register (EIR)Purpose of this feature is to make sure that no stolen or unauthorized mobile equipment is used in the network.EIR is a database that stores a unique International Mobile Equipment Identity (IMEI) number for each item of mobile equipment.
  112. 112. Section 3 – NSS TopologyProcedure:• The MSC/VLR requests the IMEI from the MS and sends it to a EIR.• On request of IMEI, the EIR makes use of three possible defined lists: - A white list: containing all number of all equipment identities that have been allocated in the different participating countries. - A black list: containing all equipment identities that are considered to be barred. - A grey list: containing (operator’s decision) faulty or non- approved mobile equipment.• Result is sent to MSC/VLR and influences the decision about access to the system.
  113. 113. Section 3 – NSS Topology EIR MSC/VLR MS Storage of all number Storage of theseries mobile equipment equipmentthat have been allocated identity IMEI in the different GSM -countries Call SetupStorage of all grey/black– listed mobile equipment IMEI Request Sends IMEI Check IMEI Continues/Stops call setup Access/ barring info procedure
  114. 114. Section 3 – NSS Topology Echo CancellerIn order to eliminate echo effects (noticeable by the mobile subscribers while in conversation with PSTN subscribers) caused by the time delay due to coding and decoding of signal processing, group of echo cancellers are installed even for local calls.This is rarely a problem when communicating between two MSs. However, when connecting to a PSTN telephone, the signal must pass through a 4-wire to 2-wire hybrid transformer.Continued…..
  115. 115. Section 3 – NSS TopologyThe function of this transformer is - some of the energy at the 4- wire receive side from the mobile is coupled back to the 4-wire transmit side and thus speech is retransmitted back to the mobile.As a result, all calls on to the PSTN must pass through an echo canceller to remove what would otherwise be a noticeable and annoying echo.Continued…..
  116. 116. Section 3 – NSS TopologyThe process of canceling echo involves two steps:• First, as the call is set up, the echo canceller employs a digital adaptive filter to set up a model or characterization of the voice signal and echo passing through the echo canceller. As a voice path passes back through the cancellation system, the echo canceller compares the signal and the model to dynamically cancel existing echo. It removes more than 80 to 90 percent of the echo across the network.• The second process utilizes a non-linear processor (NLP) to eliminate the remaining residual echo by attenuating the signal below the noise floor.
  117. 117. Section 3 – NSS Topology Transcoder and Rate Adaptor Unit (TRAU)The primary function of the TRAU is to convert 16kps (inc signaling) GSM speech channels to 64kbps PCM channels in the uplink direction and the reverse in the downlink direction. The reason this process is necessary is because MSCs only switch at the 64kbps channel level.
  118. 118. Section 3 – NSS Topology TRAU LocationsTRAU can be physically located in the BTS, BSC or MSC and hence leads to a variety of installation configurations.
  119. 119. Section 3 – NSS Topology Advantages of Different ConfigurationsCase 1, TRAU at BTS: If the TRAU is installed at the BTS, each 16kbps GSM channel would need to be mapped to its own 64kbps PCM channel. This results in 75% of the transmission bandwidth being wasted across both the Abis (BTS-BSC) and A (BSC-MSC) interface.Case 2, TRAU at BSC: If the TRAU is installed at the BSC, 16kbps GSM channel mapped to 64kbps at the A (BSC-MSC) interface, which increases the requirement of the Transmission trunks.
  120. 120. Section 3 – NSS TopologyCase 3, TRAU at MSC: If the TRAU is placed at the MSC, as is generally the case in current networks, a multiplexer can be placed at the BTS which enables 4 x 16kbps GSM channels to be multiplexed onto one 64kbps PCM channel, using 4 x 16kbps ISDN D-channels. In this configuration, only at arrival at the MSC is the 16-64kbps channel conversion necessary, thereby maximizing the efficient usage of the transmission medium by increasing the GSM channel throughput per PCM 2048 bearer from 30 to 120 channels.
  121. 121. Section 3 – NSS Topology Operation And Maintenance Center (OMC)The OMC centralizes all operations and maintenance activities for the MSCs and BSSs using remote software control. It provides remote testing, operations, and maintenance capabilities for the entire system from one central location. Each BSS, MSC, HLR, VLR, EIR, and AUC can be monitored and controlled from the OMC.
  122. 122. Section 3 – NSS Topology OMC Functional Architecture Event/ Alarm Management Security Management MMI Operating Database Fault System ConfigurationManagement Management Communications Handler Performance Management
  123. 123. Section 3 – NSS TopologyThe OMC supports the following network management functions:• Event Management - General functions of the OMC include operator input and output messages, application input commands, and application output reports.• Fault Management - The OMC provides fault management such as diagnostics and alarms for the MSC and BSS. It provides the means to isolate and minimize the effects of faults in the network thereby enabling the network to operate in efficient manner.Continued…..
  124. 124. Section 3 – NSS Topology• Security Management – It provides an extensive range of features to ensure that access to the OMC functions is restricted to relevant personnel. The security features are as follows: Password Authentication of OMC operator Logging of OMC access attempt Configurable user access restrictions Automatic logoff
  125. 125. Section 3 – NSS Topology• Configuration Management - Configuration management for the BSS consists of generic download, non-volatile memory download, database administration, and translations download. For the MSC, software release updates, database administration (route analysis, IMSI analysis table), and subscriber administration (connect/disconnect) are supported.• Performance Management - Performance management supports data collection (such as traffic data, handovers, statistics, plant measurements, and volume data) and basic reporting.
  126. 126. Section 3 – NSS Topology Billing CenterCharging analysis is the process of analysing the Charging Case and then ultimately generating the TT (Toll Ticketing) record so that an itemised bill can be produced and then sent on to the customer.The tariff structure consists of two parts:• The network access component• The network utilization component
  127. 127. Section 3 – NSS TopologyThe network utilization component is registered on a per call basis. Charging starts at the moment the subscriber answers, or on connection to an answering machine internally in the network.The main elements are:• Use of GSM PLMNs• Use of national / international PSTNs• Use of connection between different networks• Use of the signaling system no.7
  128. 128. Section 3 – NSS TopologyDepending on the type of call, one or more call tickets can be generated:• Outgoing call to fixed network: a call tickets is generated by the originating MSC.• Incoming call from the fixed network: two call tickets are created: one in the GMSC and another in the destination MSC. If a call forwarding supplementary service is in operation, other call tickets are generated in the MSC and the GMSC.Continued…..
  129. 129. Section 3 – NSS Topology• Outgoing call from a mobile subscriber to another mobile subscriber belonging to same PLMN: three call tickets are created: one in the originating MSC, one in the GMSC (which is in this case is the originating MSC) and another in the destination MSC.Call tickets mainly register the following information:4. IMSI5. Identity (MSISDN) and type (MSC or GMSC)6. Mobile subscriber location identity
  130. 130. Section 3 – NSS Topology1. Other party’s identity2. Call type (incoming, outgoing, forwarded etc)3. Call status4. Teleservices and bearer service5. Date and time6. Call duration
  131. 131. Section 3 – NSS Topology Call Detail Record (CDRs)• Each call within the PLMN creates one or more call records• These records is generated by the MSC/GMSC originating the call• The records are known as a ‘Call Detail Records’ (CDRs)• CDRs contain the following information: - Subscriber Identity - Number called - Call Length - Route of call• Often referred to as ‘Toll Tickets’
  132. 132. Section 3 – NSS Topology Call Charge Procedure• Network supplies originating MS with CAI details• MS calculates AOC record using CAI details• This record acts as a ‘toll ticket’ which tracks the call on its route through various networks• Each call component can generate a separate CDR• The record passes along the backbone to the home network• Billing computer generates bills based on cumulative CDRs• HPLMN collects the charges• HPLM reimburses VPLMN using TAPs in accordance with roaming agreement
  133. 133. Section 3 – NSS Topology
  134. 134. Section 3 – NSS TopologyThe Transferred Account Procedure (TAP) is the mechanism by which operators exchange roaming billing information. This is how roaming partners are able to bill each other for the use of networks and services through a standard process.
  135. 135. Section 3 – NSS Topology Gateway MSC (GMSC)Gateway MSC (GMSC) connects the PLMN with other networks and the entry point for the mobile subscriber calls having the interrogation facility. It has the function to obtain the information from the HLR about the subscriber’s current location and reroute the calls accordingly.In case of the network having only on MSC, the same MSC work as the GMSC, while in the case having more than one MSC, one dedicated MSC works as GMSC.
  136. 136. Section 3 – NSS Topology
  137. 137. Section 3 – NSS Topology Roaming NumberA MSRN is used during the call setup phase for mobile terminating calls. Each mobile terminating call enters the GMSC in the PLMN. The call is then re- routed by the GMSC, to the MSC where the called mobile subscriber is located. For this purpose, a unique number (MSRN) is allocated by the MSC and provided to the GMSC.
  138. 138. Section 3 – NSS TopologyCall Setup
  139. 139. Section 3 – NSS Topology1. GMSC receives a signaling message "Initial Address Message" for the incoming call (MSISDN).2. GMSC sends a signaling message to the HLR where the subscriber data is stored (MSISDN).3. The VLR address that corresponds to the subscriber location and the IMSI are retrieved. HLR sends a signaling message using the VLR address as the destination (IMSI).4. VLR having received the message, requests MSC to seize an idle MSRN and to associate it with the IMSI received. VLR sends back the result to the HLR (MSRN).
  140. 140. Section 3 – NSS Topology1. HLR sends back the result to the GMSC (MSRN).2. GMSC uses MSRN to re-route the call to the MSC. MSC performs digit analysis on the received MSRN and find the association with IMSI. The MSRN is released and the IMSI is used for the final establishment of the call.
  141. 141. Section 3 – NSS Topology Transit SwitchWhen planning the trunk network architecture, it is important to take into consideration the future expansion of the network.Some factors that influence the trunk network configuration are:• Number of MSCs• Transmission costs• Traffic distribution• Traffic volume• PSTN tariffs
  142. 142. Section 3 – NSS TopologyIn case of a medium networks (having 5 - 10 MSCs), some of the MSCs are used as transits for the others and the number of direct links between the MSCs are restricted.In case of large networks (having more than 10 MSCs), separate transit exchanges are used. These are connected to all MSCs and are working with load sharing.Transit functionality is used for passing on calls to another node. This provides a hierarchical structured network.
  143. 143. Section 3 – NSS TopologyHigh Usage trunk
  144. 144. Section 3 – NSS TopologyTraffic between MSCs and from MSCs to other networks is routed over two MSCs in a similar way as is used for the small network. The TGMSCs are used as interconnecting exchanges, since they have trunks to all MSCs in the operators PLMN.MSCs located in the same city area or in close cities are likely to be interconnected by high usage routes, while traffic between distant MSCs is likely more economically routed over the TMSCs.
  145. 145. Section 3 – NSS Topology ADVANTAGES OF USING TRANSIT EXCHANGESThe use of transit exchanges implies a more stable network structure and some of the most important benefits are:• increased flexibility• enhanced reliability• easily expandable network• platform for functional development• lower handling costs• improved signaling network
  146. 146. Section 3 – NSS Topology Value Added ServicesValue Added Services includes the following:• Point-to-Point Short Message Services• Cell Broadcast Short Message Service• Voice/Fax Mail• Pre-Paid SIMThe products associated with each of these services as well as the required interfaces into the core network elements are defined as:
  147. 147. Section 3 – NSS Topology
  148. 148. Section 3 – NSS Topology Short Message Services (SMS)The Point-to-Point and Cell Broadcast Short Message Services are implemented using the Short Message Service Center (SMSC) and Cell Broadcast Center (CBC).SMSC is built around proven Open Systems Platforms from the UNIX based computer platform to the MSC/HLR/VLR interfaces utilizing SS7.
  149. 149. Section 3 – NSS TopologyFollowing are the services and functions for which SMSC is capable of:• Alerting services to indicate call or message waiting• Paging interfaces providing full industry standard TAP interworking• Information services - subscription to financial, weather, traffic, etc. services• DTMF message entry via interactive voice prompts• E-mail• Network administration including bill reminders, statements on demand, network• service information and handset reprogramming.
  150. 150. Section 3 – NSS TopologyThe CBC product is based on the same Open Systems Platforms with an X.25 interface to BSC components. It offers a wide range of applications, which include advertising, general and specialist information distribution services along with other non-mobile terminal applications. The services and functionality that the CBC can provide includes:• Customer care information• Weather and traffic reports• Free advertising• Variable re-transmission rates• Distributed network interface units to handle varying network loads• Local and remote message submission facility.
  151. 151. Section 3 – NSS TopologySMS Network Components
  152. 152. Section 3 – NSS TopologyCallers which cannot reach the MS are given the option (by the VMS) to leave either a short message or a voice mail message. Message waiting notification will be delivered to the MS when the MS is reachable. The VMS (voice mail system) communicates with the SMS SC via TCP/IP or X.25.The VMS has a trunk and signaling interface to the PSTN (e.g., R2, ISUP signaling). The VMS has a trunk and signaling interface to the MSC for mobile subscriber to access his/her voice mail.
  153. 153. Section 3 – NSS Topology SMS Applications• SMS up to 160 alphanumeric characters.• Alert services (MT-SMS) — Voice Message Alert — FAX/Telex Message Alert — E-mail System Alert — Paging Bureau Emulation Services.• Information Services — Financial Services (stock market queries and alerts) — Weather or traffic information (e.g., from TV/radio station data feeds)
  154. 154. Section 3 – NSS Topology• Network Administration — Bill reminders (MT-SMS), bill payment — Statements on demand (MO and MT-SMS) — Handset re-programming and much more.
  155. 155. Section 3 – NSS Topology VMSIt supports a wide range of innovative applications including:• Call answering• Voice and fax bulletin boards• Information on demand• One number services• Voice and fax messaging• Interactive voice response• Prepaid calling cards• Voice activated dialing
  156. 156. Section 3 – NSS Topology Pre Paid SIMThe functionality of the Pre-Paid SIM feature includes:• Provision of pre-defined limits based on air time or talk time• Service provisioning including various provisioning options (point of sale, service providers, etc.) and definitions of pre-paid categories (throw away, top up, etc.)• Service execution for air and talk time credit usage• GSM MAP services, teleservice, bearer services and supplementary services will all be available to the Pre-Paid SIM subscriber, with possible limitations, as required by the network operator.
  157. 157. Section 3 – NSS Topology Supplementary ServicesWide range available in GSM standard and Operators can also define their ownIn GSM it is possible for the subscribers to check and modifythe parameters and status of their Supplementary Services
  158. 158. Section 3 – NSS TopologySome of the Supplementary Services are:• Calling Line Identification/Restriction• Connected Line Identification/Restriction• Call Forwarding• Call Waiting• Call Hold• Conference Calling• Conference Calling• Advice of charge• Call barring
  159. 159. Section 3 – NSS Topology ExerciseQ1. Write a full form of following : IMEI, TMSI, MSRN, LAI, ST, STP, SSPQ2. How many circuit groups are required for 3 BSCs and 10 PSTN?Q3. List down the three functions of each HLR & VLR.Q4. Fill in the following: E interface is used between ------ H interface is used between-------
  160. 160. Section 3 – NSS Topology Algorithm A8 is used for ---------- Algorithm A3 is used for ---------- Transit exchanges are used to reduce the ---------Q5. List down the different locations of TRAU and explain the best position.Q6. What information is contained in the CDRs?Q7. 2 advantages of transit switch.Q8. Name some of supplementary services.
  161. 161. Section 4 – GSM SignalingGSM Signaling
  162. 162. Section 4 – GSM Signaling ObjectiveThe Trainee will be able to understand:• signaling between MSC/VLR and BSS• Concept of DTAP• Concept of BSSMAP• signaling between BSC and BTS• Functions of LAPDm• Functions of LAPD• Frame structure of LAPDm And LAPD
  163. 163. Section 4 – GSM Signaling IntroductionThere are two different types of communication channels:• Traffic channel at 64 Kbps, carrying speech or data for radio channels.• signaling channels at 64Kbps, carrying signaling information.In PCM one time slot is reserved for signaling and remaining are used for transmitting speech or data. As the entire siganlling is done on 64Kbps , there should be special function converting the information to 64Kbps format and back at the receiving end.
  164. 164. Section 4 – GSM Signaling Protocols in GSM Networks VLR AUC MAP MAPISDN ISUP VLR HLR EIRGMSC ISUP MAP MAP MAP MAP MSCMSC TUPPSTN Switching System BSSAP BSC Base LAPD Station System MS LAPDm BTS
  165. 165. Section 4 – GSM Signaling GSM Signaling Matrix DatabaseDTAP BSSMAP MAP BSS DTAP RR MAP RIL3 RSM RSM TCAP BSSAP BSSAPRIL3 ISUP SCCP SCCP MTP2 &3 MTP2 & 3LAPDm LAPDm LAPD LAPD MTP1 MTP1 MS BTS BSC MSC
  166. 166. Section 4 – GSM Signaling• MSC uses ISUP/TUP protocols for PSTN signaling.• MAP siganlling for database applications like HLR, VLR, EIR, AUC, SMS-SC, GMSC.• GSM specific protocol as BSSAP, which comprises of DTAP and BSSMAP.• The BSC on layer 2 uses LAPD protocol, which is an ISDN.• BTS has LAPDm as layer 2 protocol.• Mobile has DTAP for MSC and RR for Radio Resource signaling.
  167. 167. Section 4 – GSM Signaling MAP (Mobile Application Part)MAP is a protocol specially designed for GSM requirement. It is installed in MSC, VLR, HLR, EIR and communicates in case of:• Location registration• Location cancellation• Handling/management/ retrieval of subscriber data.• Handover• Transfer of security/ authentication data.
  168. 168. Section 4 – GSM Signaling BSS Application Part (BSSAP)BSSAP is used for signaling between MSC/VLR and BSS. Three groups of signals belong to BSSAP3. DTAP4. BSSMAP5. Initial MS messages
  169. 169. Section 4 – GSM Signaling Transparent to BSSM DTAP M Initial MS MessageS BSSMAP LAPDm SC BSC/BTS
  170. 170. Section 4 – GSM Signaling Direct Transfer Application Part (DTAP)DTAP is a messages between the MSC and MS, passes through the BSS transparently. These are call control and mobility management messages directed towards a specific mobile.3 main type of DTAP messages are:• Messages for mobility management like location update, authentication, identity request• Messages for circuit mode connections call control• Messages for supplementary services
  171. 171. Section 4 – GSM Signaling BSSMAPBSS management messages (BSSMAP) between MSC and BSS (BSC/ BTS), which are necessary for resource management, handover control, paging order etc. The BSSMAP messages can either be connection less or connection oriented.
  172. 172. Section 4 – GSM Signaling Initial MS MessagesThese messages are passed unchanged through BSS, but BSS analyses part of the messages and is not transparent like DTAP messages.Between BSS and MSC, the initial MS message is transferred in the layer 3 information in the BSSMAP.The Initial MS messages are:• CM Request• Location update request• Paging response
  173. 173. Section 4 – GSM Signaling LAPDmLink Access Procedures on the Dm channel (LAPDm) is the layer 2 protocol used to convey signaling information between layer 3 entities across the radio interface. Dm channel refers to the control channels, independent of the type including broadcast, common or dedicated control channels.LAPDm is based on the ISDN protocol LAPD, used on the Abis interface. Due to the radio environment, the LAPD protocol can not be used in its original form. Therefore, LAPDm segments the message into a number of shorter messages.
  174. 174. Section 4 – GSM SignalingData exchanged between the data link layer and the physical layer is 23 octets for BCCH, CCCH, SDCCH and FACCH. For SACCH only, 21octets are sent from layer 2 to layer 1.LAPDm functions include:• LAPDm provides one or more data link connections on a Dm channel. Data Link Connection Identifier (DLCI) is used for discriminating between data link connections.• It allows layer 3 message units be delivered transparently between layer 3 entities.• It provides sequence control to maintain the sequential order of frames across the data link connections.
  175. 175. Section 4 – GSM Signaling LAPDm Frame Structure info length command addressN(R) P/F N(S) 0 0 0 1 SAPI CR 1
  176. 176. Section 4 – GSM SignalingSequence Number: N(S) send sequence number of the transmitted frame. N(R) is receive sequence number.P/F : All frames contain the Poll/Final bit. In command frames, the P/F bit is referred to as the P bit. In response frames, the P/F bit is referred to as the F bit.Service Access Point Identifier: Service Access Points (SAPs) of a layer are defined as gates through which services are offered to an adjacent higher layer.SAP is identified with the Service Access Point Identifier (SAPI). SAPI = 0 for normal signaling of DTAP & RR SAPI = 3 for short message services
  177. 177. Section 4 – GSM SignalingLAPDm has no error detection and correction. It is used in two modes:• Acknowledge &• Unacknowledgedand having a different structure for both.
  178. 178. Section 4 – GSM Signaling LAPDAll signaling messages on the Abis interface use the Link Access Procedures on the D-channel. (LAPD protocol). LAPD provides two kinds of signaling:• unacknowledged information• acknowledged informationLAPD link handling is a basic function to provide data links on the 64 kbps physical connections between BSC and BTS.
  179. 179. Section 4 – GSM SignalingLinks are provided for operation and maintenance (O&M) of the links, for O&M of the BTS equipment and for transmission of layer 3 Abis messages.Each physical connection can support a number of data links(logical connections). On each physical connection each datalink is identified by a unique TEI/SAPI
  180. 180. Section 4 – GSM SignalingLAPD has three sub signaling channels3. RSL (Radio signaling Link), deals with traffic management, TRX signaling.4. OML (Operation & Maintenance Link), serves for maintenance related info and transmission of traffic statistics.5. L2M (Layer 2 Management), used for management of the different signaling on the same time slot.
  181. 181. Section 4 – GSM Signaling LAPD Frame StructureFlag FCS info length command address Flag N(R ) P/F N(S) 0 TEI 1 SAPI CR 0
  182. 182. Section 4 – GSM SignalingLAPD Frame structure is made up of:Flag: Indicates the beginning and end of each frame unit. Flag has a pattern of 01111110.FCS: Frame Check Sequence, provides the error checking for the frame. If error is found frame will be retransmitted.Command: It has two types of structure, in acknowledge mode it has N(S) and N(R ). N(S) is a sequence number of frame sent and N(R ) is the sequence number of the frame expected to receive next.
  183. 183. Section 4 – GSM SignalingC/R: This bit indicates whether it is command or response.P/F: In command frames, the P/F bit is referred to as the P bit and the other end transmits the response by setting this bit to F.TEI: Terminal Endpoint Identifier, is a unique identification of each physical entity on either side like each TRX within a BTS have a unique TEI.
  184. 184. Section 4 – GSM SignalingSAPI: Service Access Point Identifier, used to identify the type of link. SAPI = 0 for RSL SAPI = 62 for OML SAPI = 63 for L2MLEach LAPD link is identify by SAPI/TEI pair.
  185. 185. Section 4 – GSM Signaling ExerciseQ1. Name the protocol which is transparent to BSS and what information is used to transfer on this protocol?Q2. Name the protocols used between Mobile and BTS BTS and BSC BSC to MSC MSC to PSTN
  186. 186. Section 5 – Call HandlingCall Handling
  187. 187. Section 5 – Call Handling ObjectiveThe Trainee will be able to understand:• Basic call concepts• Location Area concepts• Call setup in different scenarios• SMS routing• Intra and Inter MSC handovers
  188. 188. Section 5 – Call Handling IntroductionCall setup is required to establish communication between a Mobile Station and Network Subsystem (NSS). The NSS is responsible for establishing a connection with the corresponded. Different types of calls require different teleservices.For the optimum utilization of the network, different location areas will be defined to reduce the paging load on the system.
  189. 189. Section 5 – Call Handling Basic Types of CallsThere are three basic types of call:3. Mobility Management calls: Such as Location update. These are used to collect information about the MS and only signaling channels are used.4. Service calls: Such as SMS. These calls passes very small information, therefore signaling channels are used.5. User traffic calls: Such as speech or data. Large amount of data is exchanged hence traffic channels are used.
  190. 190. Section 5 – Call HandlingBasic Call Setup
  191. 191. Section 5 – Call HandlingSubscriber on switch A places a call to a Subscriber on switch B:3. Switch A analyzes the dialed digits and determines that it needs to send the call to switch B.4. Switch A selects an idle trunk between itself and switch B and formulate IAM6. STP W receives a message, inspects its routing label, and determines that it is to be routed to switch B.7. Switch B receives the message. On analyzing the message, it determines that it serves the called number and that the called number is idle.8. Switch B formulates an address complete message (ACM), which indicates that the IAM has reached its proper destination.
  192. 192. Section 5 – Call Handling2. Switch B picks one of its links and transmits the ACM over the link for routing to switch A.3. STP X receives the message, inspects its routing label, and determines that it is to be routed to switch A.4. On receiving the ACM, switch A connects the calling subscriber5. When and/or if the called subscriber picks up the phone, switch B formulates an answer message (ANM),6. Switch B selects the same link it used to transmit the ACM7. STP X recognizes that the ANM is addressed to switch A and forwards it over link
  193. 193. Section 5 – Call Handling2. Switch A ensures that the calling subscriber is connected and conversation can take place.3. If the calling subscriber hangs up first switch A will generate a release message (REL) addressed to switch B.4. STP W receives the REL5. Switch B receives the REL, disconnects the trunk from the subscriber line, returns the trunk to idle status.6. STP X receives the RLC, determines that it is addressed to switch A.7. On receiving the RLC, switch A idles the identified trunk.
  194. 194. Section 5 – Call Handling Location RegistrationWhen the mobile is turned on first time in the network, it has no indications in its data about an old Location Area Identity. MS immediately inform the network and request for the Location Update to the MSC/ VLR. After registration MSC/ VLR will consider the MS as active and marked the MS as “attached”.
  195. 195. Section 5 – Call Handling Location UpdateWhen the MS moves from one LA to another, it has to register. This registration is performed when the MS detects another LAI than the one stored. This is called location updating. This function provides mobile subscribers with uninterrupted service throughout the GSM coverage area so that they can:• Be called on a permanent directory number irrespective of their location at the time of call.• Access the network whatever their position
  196. 196. Section 5 – Call HandlingThere are four different types of location updating:• Normal• IMSI detach• IMSI attach• Periodic registration
  197. 197. Section 5 – Call Handling Normal Update• The Base Transceiver Station (BTS) of every cell continually transmits the Location Area Identity (LAI) on BCCH.• If MS detects LAI is different from the one stored in the SIM- card, it is forced to do a location update.• If the mobile subscriber is unknown in the MSC/VLR (new subscriber) then the new MSC/VLR must be updated, from the HLR, with subscriber information.• It also consider the case of the location update timer runs out.
  198. 198. Section 5 – Call Handling
  199. 199. Section 5 – Call Handling2. The MS requests a location update to be carried out in the new MSC/VLR. The IMSI is used to identify the MS.3. In the new MSC/VLR, an analysis of the IMSI number is carried out. The result of this analysis is a modification of the IMSI to a Mobile Global Title (MGT)4. The new MSC/VLR requests the subscriber information for the MS from the HLR.5. The HLR stores the address of the new MSC/VLR and sends the subscriber data to the new MSC/VLR.
  200. 200. Section 5 – Call Handling5. The HLR also orders the old serving MSC/VLR to cancel all information about the subscriber since the MS is now served by another MSC/VLR.2. When the new MSC/VLR receives the information from the HLR, it will send a location updating confirmation message to the MS.
  201. 201. Section 5 – Call Handling IMSI DetachThe MS must inform the network when it is entering aninactive state (detach).3. At power off or when the SIM card is taken out, the MS asks for a signaling channel4. The MS uses this signaling channel to send the IMSI detach message to the MSC/VLR.5. In the VLR, an IMSI detach flag is set for the subscriber which is used to reject incoming calls to the MS.The detach will not be acknowledged.
  202. 202. Section 5 – Call HandlingOnly the VLR is updated with the “detached” information.
  203. 203. Section 5 – Call Handling IMSI AttachThe attach procedure is performed only when the MS is turned on and is in the same LA as it was when it sent the detach message. If the MS changes location area while being switched off, it is forced to do a normal location update. The procedure is as follows3. The MS requesting a signaling channel.4. The MSC/VLR receives the IMSI attach message from the MS.5. The MSC/VLR sets the IMSI attach in the VLR, that is, the mobile is ready for normal call handling.6. The VLR returns an acknowledgment to the MS.
  204. 204. Section 5 – Call Handling
  205. 205. Section 5 – Call Handling Periodic Location UpdateTo avoid unnecessary paging of the MS in case the MSC never got the IMSI detach message, there is another type of location updating called periodic registration.The procedure is controlled by timers both in the MS and in the MSC.If the MS does not register within the determined interval plus a guard time, then the scanning function in the MSC detects this and the MS will be marked detached.
  206. 206. Section 5 – Call Handling PagingA call to MS is routed to the MSC/ VLR and send a paging message to the MS. This message is broadcast all over the Location Area (LA), which means that all BTSs with in the LA will send a paging message to the mobile. The MS, moving in the LA and listening to the CCCH information, will hear the paging message and answer it immediately.
  207. 207. Section 5 – Call Handling Paging CapacityPaging capacity is the number of mobiles that can be paged per secondThis depends on: • CCCH configuration • AGCH blocks reservation • Type of paging message used• Paging message takes 4 bursts (1 CCCH block)• This can page up to 4 mobiles depending on the message type used
  208. 208. Section 5 – Call Handling Paging Message TypesType 1: can address up to two mobiles using either IMSI or TMSIType 2: can address up to 3 mobiles, one by IMSI and other 2 by TMSI.Type 3: can address up to 4 mobiles using the TMSI only.If the network does not use TMSI then only type 1 is used in the network.
  209. 209. Section 5 – Call Handling Calculation Of Paging CapacityX = number of mobiles paged per paging message (1 to 4)Y = number of possible paging messages per multiframeDuration of channel multiframe = 0.235 seconds (235 ms)• X depends on paging message type• Y depends on CCCH configuration in the multiframe (e.g.3 or 9) and the number of AGCH blocks reserved
  210. 210. Section 5 – Call Handling PCH DimensioningPaging channel requirement in blocks per multiframe is given by:Calls = Number of calls predicted for the location area duringbusy hourMT = Fraction of calls which are mobile terminatedPF = Paging Factor = number of pages required per callM = safety marginPMF = Paging Message Factor = number of pages permessageNumber of control channel multiframes per second = 4.25
  211. 211. Section 5 – Call Handling ExampleA particular location area contains 50 000 subscribers. It is predicted that 30% of these will receive a call during the busy hour. On average 2 pages are needed per call and only type 3 paging messages (TMSI) are used. This gives the following data: Calls = 50 000 MT = 0.3 PF = 2 PMF = 4
  212. 212. Section 5 – Call HandlingA typical safety margin for peak variations in number of calls is 1.2 • 1 PCH block per multiframe will be adequate
  213. 213. Section 5 – Call Handling Paging ControlThe MSC has to initiate the paging procedure, as it holds the information on the last MS location update.MSC sends a paging message to BSC and sets a timer for response from the MS, which is send as a part of service request message. The paging message from the MSC contains a cell list identifier, identifying the cells in which paging message is to be transmitted.

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