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Wireless computing

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  • 1. 1CSE-302: Wireless Computing
  • 2. 2Personal Communication SystemPersonal Communication System(PCS)(PCS) Architecture Wireless communication Bandwidth limitations Frequency reuse
  • 3. 3What is a mobile connectivity?A mode in which a client or a server can establishcommunication with each other whenever needed.Intermittent connectivity is a special case of mobileconnectivity.
  • 4. 4What is intermittentconnectivity?A node in which only the client can establishcommunication whenever needed with the server but theserver cannot do so.
  • 5. 5Personal Communication SystemPersonal Communication System(PCS)(PCS)A system where wired and wireless networks are integratedfor establishing communication.PSTN: Public Switched Network.MSC: Mobile Switching Center. Also called MTSO(Mobile Telephone Switching Office).BS: Base Station.MS: Mobile Station. Also called MU (Mobile Unit)or Mobile Host (MH).HLR: Home Location Register.VLR: Visitor Location Register.EIR: Equipment Identify Register.AC: Access Chanel.PSTNBSVLRHLREIRACMSC (MTSO)MSC (MTSO)MSMS Wireless component
  • 6. 6Personal Communication SystemPersonal Communication System(PCS)(PCS)Wireless ComponentsBSMSC (MTSO)MSMS Wireless componentBase Station (BS): A switch, which serves ascommunication link between MU and the entirenetworkMobile Units (MU): Also called Mobile Systems (MS) orMobile Hosts (MH). A mobile component, whichcommunicates with BS through a limited number ofwireless channels.
  • 7. 7Personal Communication SystemPersonal Communication System(PCS)(PCS)Wireless channels are limitedItem Europe (MHz) US (MHz) Japan (MHz)MobilePhonesNMT: 453-457, 463-467GSM: 890-915, 935-960,1710-1785, 1805-1880AMPS, TDMA, CDMA824-849, 869-894GSM, TDMA, CDMA1850-1910, 1930-1990PDC: 810-826940-956,1429-1465,1477-1513.CordlessPhonesCT1+: 885-887, 930-932CT2: 864-868DECT: 1880-1900PACS1850-1910,1930-1990;PACS-UB: 1910-1930PHS1895-1918;JCT: 254-380NMT: Nordic Mobile TelephonePDC: Pacific Digital CellularPACS: Personal Access Communications SystemPHS: Personal Handyphone SystemPACS-UB: PACS Unlicensed BandJCT: Japanese Cordless Telephone(Taken from Mobile Communications by Jochen Schiller)
  • 8. 8Personal Communication SystemPersonal Communication System(PCS)(PCS)Limited channels must be utilized efficiently. It is doneso byFrequency reuseThe same radio frequency is used forcommunication by more than one cellsessions.Mobile cellsTo achieve frequency reuse, the entirewireless coverage area is divided into cells.
  • 9. 9Personal Communication SystemPersonal Communication System(PCS)(PCS)Mobile cellsMetropolitan area Metropolitan areaCoverage area in one cell Coverage area in three cellsBSBSBSBase StationLarge cells.Low densitySmall cells.High densitySmaller cells.Higher density
  • 10. 10Personal Communication SystemPersonal Communication System(PCS)(PCS)Mobile cellsThe entire coverage area is a group of a number ofcells. The size of cell depends upon the power ofthe base stations.PSTNMSC
  • 11. 11Personal Communication SystemPersonal Communication System(PCS)(PCS)Frequency reuse617543261754326175432D AAAAAAANRD3=D = distance between cells using the same frequencyR = cell radiusN = reuse pattern (the cluster size, which is 7).Thus, for a 7-cell group with cell radius R = 3 miles, the frequency reusedistance D is 13.74 miles.
  • 12. 12Personal Communication SystemPersonal Communication System(PCS)(PCS)Problems with cellular structure How to locate of a mobile unit in the entirecoverage area?Solution: Location management How to maintain continuous communicationbetween two parties in the presence of mobility?Solution: Handoff How to maintain continuous communicationbetween two parties in the presence of mobility?Solution: Roaming
  • 13. 13Personal Communication SystemPersonal Communication System(PCS)(PCS)HandoffA process, which allows users to remain in touch, evenwhile breaking the connection with one BS andestablishing connection with another BS.Old BS New BSMSCOld BS New BSMSCMSCOld BS New BS New BSOld BSMSC
  • 14. 14Personal Communication SystemPersonal Communication System(PCS)(PCS)HandoffTo keep the conversation going, the Handoffprocedure should be completed while the MS (thebus) is in the overlap region.GOld BS New BSCell overlap region
  • 15. 15Personal Communication SystemPersonal Communication System(PCS)(PCS)Handoff issues Handoff detection Channel assignment Radio link transfer
  • 16. 16Personal Communication SystemPersonal Communication System(PCS)(PCS) Mobile-Controlled handoff (MCHO) Network-Controlled handoff (NCHO) Mobile-Assisted handoff (MAHO)Handoff detection strategies
  • 17. 17Personal Communication SystemPersonal Communication System(PCS)(PCS)Mobile-Controlled Handoff (MCHO)In this strategy, the MS continuously monitors theradio signal strength and quality of the surroundingBSs. When predefined criteria are met, then the MSchecks for the best candidate BS for an availabletraffic channel and requests the handoff to occur.MACHO is used in DECT and PACS.
  • 18. 18Personal Communication SystemPersonal Communication System(PCS)(PCS)Network-Controlled Handoff (NCHO)In this strategy, the surrounding BSs, the MSC orboth monitor the radio signal. When the signal’sstrength and quality deteriorate below a predefinedthreshold, the network arranges for a handoff toanother channel. NCHO is used in CT-2 Plus andAMPS.
  • 19. 19Personal Communication SystemPersonal Communication System(PCS)(PCS)Mobile-Assisted Handoff (MAHO)It is a variant of NCHO strategy. In this strategy,the network directs the MS to measure the signalfrom the surrounding BSs and to report thosemeasurements back to the network. The networkthen uses these measurements to determinewhere a handoff is required and to which channel.MACHO is used in GSM and IS-95 CDMA.
  • 20. 20Personal Communication SystemPersonal Communication System(PCS)(PCS)Handoff types with reference to the network Intra-system handoff or Inter-BS handoffThe new and the old BSs are connected tothe same MSC.Old BS New BSMSC
  • 21. 21Personal Communication SystemPersonal Communication System(PCS)(PCS)Handoff types with reference to the network Intersystem handoff or Inter-MSC handoffThe new and the old BSs are connected todifferent MSCs.Old BS New BSMSCMSC
  • 22. 22Personal Communication SystemPersonal Communication System(PCS)(PCS)Handoff types with reference to link transfer Hard handoffThe MS connects with only one BS at a time,and there is usually some interruption in theconversation during the link transition. Soft handoffThe two BSs are briefly simultaneouslyconnected to the MU while crossing the cellboundary. As soon as the mobiles link withthe new BS is acceptable, the initial BSdisengages from the MU.
  • 23. 23Personal Communication SystemPersonal Communication System(PCS)(PCS)Handoff types with reference to link transferHard handoff1. MU temporarily suspends the voice conversationby sending a link suspend message to the old BS.2. MU sends a handoff request message through anidle time slot of the new BS to the network.3. The new BS sends a handoff ack message andmarks the slot busy.4. The MU returns the old assigned channel bysending a link resume message to the old BS.
  • 24. 24Personal Communication SystemPersonal Communication System(PCS)(PCS)Handoff types with reference to link transferHard handoff5. MU continues voice communication while thenetwork prepares for the handoff.6. Upon receipt of a handoff request message, thenew BS sends a handoff ack message andreconfigures itself to effect the handoff.7. The MSC inserts a bridge into the conversationpath and bridges the new BS.8. Finally, the network informs the MU to executethe handoff via both the new and old BSs bysending the handoff execute message.
  • 25. 25Personal Communication SystemPersonal Communication System(PCS)(PCS)Handoff types with reference to link transferHard handoff9. MU releases the old channel by sending anaccess release message to the old BS.10. Once the MU has made the transfer to the newBS, it sends the network a handoff completemessage through the new channel, and resumesthe voice communication. The network removesthe bridge from the path and frees up theresources associated with the old channel.
  • 26. 26Personal Communication SystemPersonal Communication System(PCS)(PCS)Handoff types with reference to link transferSoft handoff1. MU sends a pilot strength measurement messageto the old BS, indicating the new BS to be added.2. The old BS sends a handoff request message tothe MSC. If the MSC accepts the handoff request,it sends a handoff request message to the newBS.3. The BS sends a null traffic message to the MU toprepare the establishment of the communicationlink.
  • 27. 27Personal Communication SystemPersonal Communication System(PCS)(PCS)Handoff types with reference to link transferSoft handoff4. The new BS sends a join request message to theMSC. The MSC bridges the connection for thetwo BSs, so that the handoff can be processedwithout breaking the connection.5. The new BS sends a handoff ack message to theold BS via the MSC. The old BS instructs the MUto add a link to the new BS by exchanging thehandoff command and handoff completemessages.
  • 28. 28Personal Communication SystemPersonal Communication System(PCS)(PCS)Handoff types with reference to link transferSoft handoff6. The old BS and the MSC conclude this procedureby exchanging the required handoff information.The quality of the new link is guaranteed by theexchange of the pilot measurement request andthe pilot strength measurement message pairbetween the MU and the new BS.
  • 29. 29Personal Communication SystemPersonal Communication System(PCS)(PCS)RoamingRoaming is a facility, which allows a subscriber toenjoy uninterrupted communication from anywhere inthe entire coverage space.A mobile network coverage space may be managedby a number of different service providers. Theymust cooperate with each other to provide roamingfacility.Roaming can be provided only if some administrativeand technical constraints are met.
  • 30. 30Personal Communication SystemPersonal Communication System(PCS)(PCS)RoamingAdministrative constraints Billing. Subscription agreement. Call transfer charges. User profile and database sharing. Any other policy constraints.
  • 31. 31Personal Communication SystemPersonal Communication System(PCS)(PCS)RoamingTechnical constraints Bandwidth mismatch. For example, European900MHz band may not be available in otherparts of the world. This may preclude somemobile equipment for roaming. Service providers must be able tocommunicate with each other. Needs somestandard. Mobile station constraints.
  • 32. 32Personal Communication SystemPersonal Communication System(PCS)(PCS)RoamingTechnical constraints Integration of a new service provider into thenetwork. A roaming subscriber must be ableto detect this new provider. Service providers must be able tocommunicate with each other. Needs somestandard. Quick MU response to a service provider’savailability. Limited battery life.
  • 33. 33Personal Communication SystemPersonal Communication System(PCS)(PCS)Location ManagementTwo-Tier SchemeHLR: Home Location RegisterA HLR stores user profile and thegeographical location.VLR: Visitor Location RegisterA VLR stores user profile and the currentlocation who is a visitor to a different cell thatits home cell.
  • 34. 34Personal Communication SystemPersonal Communication System(PCS)(PCS)Location ManagementTwo-Tier Scheme steps. MU1 wants to talk to MU2.MU1MU2Cell 1 Cell 2
  • 35. 35Personal Communication SystemPersonal Communication System(PCS)(PCS)Location ManagementTwo-Tier Scheme steps. MU1 wants to talk to MU2.1. VLR of cell 2 is searched for MU2’s profile.2. If it is not found, then HLR is searched.3. Once the location of MU2 is found, then theinformation is sent to the base station of cell 1.4. Cell 1 establishes the communication.
  • 36. 36Personal Communication SystemPersonal Communication System(PCS)(PCS)Location ManagementTwo-Tier Scheme steps location update1. MU2 moves from cell 1 to cell 2.2. MU2’s location is changed so new locationmust be recorded.3. HLR is updated with the new location address.4. MU2’s entry is deleted from the VLR of cell 1and new entry is made in cell 2’s VLR.
  • 37. 37Personal Communication SystemPersonal Communication System(PCS)(PCS)Location ManagementTwo-Tier Scheme steps location searchSource-mssDestlsSourcelsId LSDest Dest-ls- -Id HLSDest Dest-HLS- -DestHLSId MSSDest Dest-mss- -DestSrc12349568710
  • 38. 38Personal Communication SystemPersonal Communication System(PCS)(PCS)Location ManagementTwo-Tier Scheme steps location updateNew-lsOld-lsHLSMU12310956478Id HLSMU HLS- -Id MSSMU New-mss- -Id LSMU New-ls- -New-mss

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