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  1. 1. Wireless communication and mobile Programming
  2. 2. Overview of GSM  GSM Stands for global system for mobile Communications  GSM is based on set of standards formulated in early 1980.  GSM was developed by ETSI(European Telecommunications Standards Institute)  GSM offers data services with maximum bit rate 14.4 kb/s  GSM uses combination of FDMA ,TDMA
  3. 3. FDMA/TDMA
  4. 4.  Uplink/Downlink of 25MHz  890 -915 MHz for Up link  935 - 960 MHz for Down link  Combination of frequency division and time division multiplexing  FDMA  124 channels of 200 kHz  TDMA  Burst Air Interface: MS to BTS
  5. 5. Number of channels in GSM  Freq. Carrier: 200 kHz  TDMA: 8 time slots per freq carrier  No. of carriers = 25 MHz / 200 kHz = 125  Max no. of user channels = 125 * 8 = 1000  Considering guard bands = 124 * 8 = 992 channels
  6. 6. Cellular Concept
  7. 7. Cellular Concept  Base stations (BS): implement space division multiplex  Each BS covers a certain transmission area (cell)  Each BS is allocated a portion of the total number of channels available  Cluster: group of nearby BSs that together use all available channels  Mobile stations communicate only via the base station, using FDMA, TDMA, CDMA…
  8. 8. Advantage of FDMA  Simple algorithmically, and from a hardware standpoint  Fairly efficient when the number of stations is small and the traffic is uniformly constant  No need for network timing  Capacity increase can be obtained by reducing the information bit rate and using efficient digital code
  9. 9. Advantage of TDMA  Flexible bit rate  No frequency guard band required  No need for precise narrowband filters  Easy for mobile or base stations to initiate and execute hands off  Extended battery life
  10. 10. GSM System Hieararchy
  11. 11. Architecture of GSM
  12. 12. Mobile Station (MS)  MS consists of following two components  Mobile Equipment (ME)  Mobile Subscriber Identity Module (SIM)  Removable plastic card  Stores Network Specific Data such as list of carrier frequencies and current Location Area ID (LAI).  Stores International Mobile Subscriber Identity (IMSI) + ISDN  Stores Personal Identification Number (PIN) & Authentication Keys.  Also stores short messages, charging information, telephone book etc.
  13. 13. Base Transceiver Station (BTS)  A Base Station Transceiver (BTS) is a radio transceivers station that communicates with the mobile stations. Its backend is connected to the BSC  One per cell  Consists of high speed transmitter and receiver  Function of BTS  Provides two channels Signalling and Data Channel
  14. 14. Base Station Controller (BSC)  Controls multiple BTS  A Base Station Controller (BSC) is a high-capacity switch with radio communication and mobility control capabilities.  The functions of a BSC include radio channel allocation, location update, handover, timing advance, power control and paging.  Time and frequency synchronization signals to BTSs  Time Delay Measurement and notification of an MS to BTS  Power Management of BTS and MS
  15. 15.  The mobile switching center (MSC) performs the telephony switching function.  Switching node of a PLMN (Public Land Mobile Network)  Allocation of radio resource (RR)  Handoff  Mobility of subscribers  Location registration of subscriber  There can be several MSCs in a PLMN Mobile Switching Centre (MSC)
  16. 16.  Connects mobile network to a fixed network  Entry point to a PLMN  Usually one per PLMN  Request routing information from the HLR and routes the connection to the local MSC Gateway MSC (GMSC)
  17. 17. HLR/VLR  HLR - Home Location Register  Contains semi-permanent subscriber information  For all users registered with the network, HLR keeps user profile  MSCs exchange information with HLR  When MS registers with a new GMSC, the HLR sends the user profile to the new MSC  VLR - Visitor Location Register  Contains temporary info about mobile subscribers that are currently located in the MSC service area but whose HLR are elsewhere  Copies relevant information for new users (of this HLR or of foreign HLR) from the HLR  VLR is responsible for a group of location areas, typically associated with an MSC
  18. 18. AuC/EIR  AuC: Authentication Center  The authentication center (AUC) provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call.  The AUC protects network operators from different types of fraud found in today's cellular world.  The GSM has standard encryption and authentication algorithm which are used to dynamically compute challenge keys and encryptions keys for a call.
  19. 19.  is accessed by HLR to authenticate a user for service  Contains authentication and encryption keys for subscribers  EIR: Equipment Identity Register  The equipment identity register (EIR) is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations
  20. 20. Registration Process
  21. 21. Call Establishment
  22. 22. Example: Outgoing call setup  User keys in the number and presses send  Mobile transmits Set Up message on uplink signaling channel (RACH) to the MSC  MSC requests HLR/VLR to get subscriber parameters necessary for handling the call.  VLR/HLR sends Complete Call msg to the MSC  MSC sends an Assignment message to the BSS and asks it to assign TCH for the MS  BSS allocates a radio channel (TCH) and sends an Assignment message to MS over SDCCH  MS tunes to the radio channel (TCH) and sends an Assignment Complete message to the BSS.  BSS deallocates SDCCH. Now voice path is established between MS and MSC  MSC completes the PSTN side of the signaling.
  23. 23. Mobility management  Why mobility and location management is required? Very nature of mobile communications implies that the MS is constantly changing its location, a need for tracking mobile and restructuring existing connection as its moves, mobility management and location management handle the operation require for this purpose.
  24. 24. Location management  Location management involve tracking of the location of the MS as its moves for delivery of voice and data service.  Basic operation:- Location update, paging, location information Dissemination  Location updates are the message sent by the MS regarding its changing points of access to the fix network  Updates may have varying frequency  Types of Location update:-
  25. 25.  Location registration:- Location registration takes place when MS is turned on  Network send TMSI and LAI to MS store in SIM  Generic Location update perform when LAI is different  Chanel request message sent to target MSC/VLR include TMSI,LAI
  26. 26. Generic location update procedure
  27. 27.  Periodic:-periodic location update is carried out when network does not receive any location update request from MS in specified time  Issue:-  Unnecessary cost in terms of both usage of spectrum and network resource if location updates are too frequent  Large area to be paged if location updates are few.  Location update algorithms :- Static and Dynamic  Static:- In static location updates the topology of cellular network decides when the location updates need to be initiated
  28. 28.  Each group of cell assign LAI  BS broad cast LAI periodically  Problem:-ping-pong effect  Solution:-distance based, timer based  Dynamic:- in dynamic location update mobility of users and call pattern is used in initiating the location update  Dynamic location update is state based and user profile based  State based decision are made based on current state information
  29. 29.  state information include time elapsed, distance travel, no of LA crossed  Profile based maintain sequential list of LA
  30. 30.  Paging scheme:-Paging is broadcasting message in cell or group of cell to bring out response from MS  Blanket paging:-paging the MS in all cell of LA simultaneously  Location information Dissemination:- Every MS associated with home network and home database
  31. 31. Handover  Handoff management required to handle on going connection when mobile terminal moves from coverage of one point of access to another  Maintaining the traffic connection with a moving user when crossing cell boundaries  Occurs when the quality or the strength of the radio signal falls below certain parameters (signal quality reason)  Occurs when the traffic capacity of a cell has reached its maximum or is approaching (traffic reason)
  32. 32. Relative Signal Strength
  33. 33. Relative Signal Strength with Threshold
  34. 34.  For a high threshold (e.g., Th1), this scheme performs the same as the relative signal strength scheme  If the threshold is set quite low (e.g., Th3), the mobile may move far into the new cell  Threshold should not be used alone because its effectiveness depends on prior knowledge of the crossover signal strength between the current and the candidate base stations
  35. 35. Relative Signal Strength with Threshold and Hysteresis
  36. 36.  Handover occurs only if the current signal level drops below a threshold, and  the target base station is stronger than the current one by a hysteresis margin H
  37. 37. Hard handover
  38. 38.  “Break before make”  Connection is released before making the new connection  Causes a short cut in the connection  The terminal is linked to no more than one base station at any given time  Primarily used in FDMA and TDMA, where different frequency ranges are used in adjacent cells
  39. 39. Soft handover
  40. 40.  “Make before break”  New connection is established before the old connection is released, avoiding a cut in the connection during handover  After the successful handover, the old connection is released  Used in CDMA, where adjacent cells use the same frequency range
  41. 41. Control over Handover  Network-controlled Handover (NCHO)  Network measures the transmission quality via base stations and decides when handover should be executed  Handover process (including data transmission, channel and network switching) takes 100-200ms  Mobile-assisted Handover (MAHO)  Mobile terminal continuously measures signal strength from serving and neighboring base stations and sends the recorded values to the serving base station  On the basis of these values, the network decides when handover should take place
  42. 42.  Handover time between handover decision and execution is approximately 1 second  Mobile-controlled Handover (MCHO)  Mobile terminal is completely in control of the handover process, i.e., it measures signal strength and decides on handover  Very short reaction time (on the order of 0.1 seconds)