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Horizontal Handoff
Vertical Handoff
automatic link transfer

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  1. 1. presentsHandoff / Handover 1
  2. 2. Handoff in Cellular Systems AJAL.A.J Assistant Professor –Dept of ECE, Federal Institute of Science And Technology (FISAT) TM    MAIL: 2
  3. 3. Analysis of handoff The Analytical Approach The Simulation Approach a) Cell model b) Propagation model c) Traffic model d) Mobility model The Emulation Approach 3
  4. 4. OBJECTIVES Handoff Strategies ► When to handoff ► 1G, BS based ► 2G or todays, Mobile-Assisted Prioritizing Handoff ► Guard channels concept ► Queuing handoff requests Practical handoff considerations ► Umbrella cell ► Cell dragging 4
  5. 5. @ Olympics If you’re a fan of the summer Olympics, I am sure you have watched plenty of track events – especially the exciting 400 and 800 meter relays.  Four athletes run in each event and hand a baton off to the next athlete until the race is complete.  Although the handoffs are usually done without mishap. 5
  6. 6. Patient handover process • Handover is ‘the transfer of professional responsibility and accountability for some or all aspects of care for a patient, or group of patients, to another person or professional group on a temporary or permanent basis.’    6
  7. 7. Communication methods  methods  Verbal  Written  Electronic  Combination of them    7
  8. 8. Nursing report and information sharing For a report to be meaningful the information to the receiver has to be given in an effective way. Nursing handover using accurate and documented information promotes effective time management.     8
  9. 9. Design of Wireless Networks• The design is done in two steps – Area coverage planning – Channel (Frequency) allocation •  Outage area • Coverage area 9
  10. 10. • The purpose of wireless networks is to provide wireless access to the fixed network (PSTN) 10
  11. 11. Personal communications network  PSTN: Public Switched HLR: Home Location Register 11Telephone NetworkSTP: Service Transfer Point VLR: Visitor Location Register
  12. 12.    Hexagonal   Network   Coverage   Model for PCN 12
  13. 13. 13
  14. 14. Near-Field Reactive Near Field Region • Near-field region:  – Angular distribution of energy depends on  distance from the antenna;  – Reactive field components dominate (L, C)Radiating Near Field (Fresnel) Region 14
  15. 15. Far Field (Fraunhofer) Region• Far-field region:  – Angular distribution of energy is independent  on distance;  – Radiating field component dominates (R) – The resultant EM field can locally be treated  as uniform (TEM) 15
  16. 16. 16
  17. 17. Basics: Structure Multiple Access Downlink Handoff UplinkMobile Station Distributed  Base Station   Fixed transceivers transceiver Cells  Different  Frequencies or  Codes
  18. 18. Access and backhauling in 5GHz Broadband Hotspot Feeding Leased Lines Video Surveillance VoIP Oil & Gas Telecommunications Secured VPN Nomadic Applications Shopping Malls Video Surveillance &  Public Safety Public safety Leased LinesBreezeNET B Backhauling Municipalities &  Education BreezeACCESS VL Traffic Control Video  Surveillance &  Base station Traffic Control Hotspots Feeding Internet  cafés Broadband Access Confidential Information Leased Lines Replacement Gaming VoIP Businesses Residential
  19. 19. Handoff When a mobile moves into a different cell while a conversation is in progress, MSC automatically transfers the call to a new channel belonging to the new base station. Handoff operation not only involves identifying a new base station, but also requires that the voice and control signals be allocated to channels associated with the new base station. Prioritize handoff requests over call initiation requests when allocating unused channels in a cell site. Confidential Information
  20. 20. m p r-s p t -b l  eg: an imperceptible drop in temperature.  Handoffs must be performed As infrequently as possible, and Be imperceptible to the users.  In order to meet these requirements, we must specify a minimum usable signal level for acceptable voice quality at the base station; then a slightly stronger signal level is used as a threshold at which a handoff is made. Confidential Information
  21. 21. Handoff (Handover) Characteristics Handoffs must be: 1. performed quickly  2. performed infrequently 3. imperceptible to users 4. performed successfully Confidential Information
  22. 22. Handoff Strategies Handoff: a mobile user moves to a different cell while conversation is in progress, MSC transfers the call to a new BS. ► Identifying new BS ► New voice and control channels to be allocated Handoff must be performed ► Successfully ► Infrequently Impossible or difficult to perceive by the mind or  ► Imperceptible senses:  an imperceptible drop in temperature.  To achieve this, designer must specify optimum signal level at which handoff initiates 22 22
  23. 23. ( See diagram on slide 16) Once, a signal level is specified as min usable for acceptable voice quality ► A slightly stronger signal level is used as threshold ► Normally taken between -90dBm and -100 dBm. This margin ∆ = Pr_handoff – Pr_min, can not be too large or too small ( See diagram on slide 16) ► If ∆ is too large, unnecessary handoffs, burden on MSC ► If ∆ is too small, insufficient time to complete a handoff before a call is lost due to weak signal ► ∆ should be chosen carefully to meet conflicting requirements 23 23
  24. 24. • An efficient way of managing the radio spectrum is by reusing the same frequency, within the service area, as often as possible• This frequency reuse is possible thanks to the propagation properties of radio waves 24
  25. 25. Handover decision receive level receive level BTSold BTSold HO_MARGIN MS MS BTSold BTSnew 25(#)
  26. 26. 26(#)
  27. 27. CASE 1: Handoff - SuccessfulValue of delta is large enough. When the PHandoff is reached, theMSC initiates the handoff. 27
  28. 28. CASE:2 Handoff - UnsuccessfulIn this case, the MSC was unable to perform the handoff beforethe signal level dropped below the minimum usable level, and sothe call was lost. 28
  29. 29. Handoff – Unsuccessful (cont’d)Reasons for failed handoff: ∆ too small (i.e. PHANDOFF too low) high mobile speeds excessive delay at MSC High traffic level Un-availability of channels 29
  30. 30. Handoff (cont’d)Ways to improve handoff performance Optimize ∆ Account for varying mobile speeds Minimize delay at MSC Prioritize handoffs Mobile assisted handoffs, mobile locators, minimize number of BS’s to search, ... 30
  31. 31. • But, the margin, given by ∆ = Pr handoff − Pr minimum usable cannot be too large or too small.• If ∆ Is too large, unnecessary handoffs which burden the MSC may occur.• If ∆ is too small, there may be insufficient time to complete a handoff before a call is lost due to weak signal conditions. 31
  32. 32. • Of course, the dropped call can happen when there is an excessive delay by the MSC in assigning a channel (during high traffic conditions) as well.• On the other hand, in deciding when to handoff, it is important to ensure that the mobile is actually moving away from the serving base station, (not that the drop in the measured signal level is due to momentary fading.)PRECAUTION. In order to ensure this, the base station monitors the signal level for a certain period of time before a handoff is initiated. 32
  33. 33. momentary fading• Of course, the dropped call can happen when there is an excessive delay by the MSC in assigning a channel (during high traffic conditions) as well.• On the other hand, in deciding when to handoff, it is important to ensure that the mobile is actually moving away from the serving base station, (not that the drop in the measured signal level is due to momentary fading.)• In order to ensure this, the base station monitors the signal level for a certain period of time before a handoff is initiated. mimicking 33
  34. 34. mimicking 34
  35. 35. Dwell time• The length of monitoring needed depends largely on the speed of mobile units.• Dwell time: the time over which a call may be maintained within a cell without handoff. The statistics of dwell time, vary greatly depending on the type of radio coverage and user profiles within a cell, are important in the practical design of handoff algorithms. 35
  36. 36. System Model for the Cellular Handoff
  37. 37. Definition of Handoff Horizontal Handoff  Occurs when the user switches between different network access points of the same kind.  e.g. Handoff among 802.11 APs. Vertical Handoff  Involves two different network interfaces which usually represent different technologies.  e.g. Handoff from 802.11 to 1xRTT (CDMA 2000). 37
  38. 38. Handoff Illustration 38
  39. 39. 39(#)
  40. 40. 40(#)
  41. 41. • Hard Handoff: Break-before-make Old New Old New Break-before-make•Soft Handoff: Make-before-Break. Need to usetwo radios Old New Make-before-Break•Horizontal Handoff: Same radio access technology (RAT)•Vertical Handoff: Different Technologies•Terminal Controlled•Terminal Initiated, Network Assisted•Network Initiated, Network controlled 41
  42. 42. Handoff If the mobile user moves to the coverage area of another BS, the radio link to the old BS is eventually disconnected, and a radio link to the new BS should be established to continue the conversation. This process is variously referred to as automatic link transfer, handover, or handoff. 42(#)
  43. 43. When to handoff ?? When to handoff, ► Drop in signal level is not due to momentary fading ► Mobile is actually moving away from serving BS ► To ensure this, • BS monitors the signal level for certain period of time • The period depends on the vehicle speed ► If slope of average received signal level is steep, handoff is made quickly 43
  44. 44. @ 1G In 1G, signal level was measured by BS and supervised by MSC ► Each BS constantly monitors the signal strength of all its reverse channels to determine relative location of each mobile user ► In addition, the locator receiver (a spare receiver) is used to scan and measure RSSI of mobile users in neighboring cells and reports to MSC ► Based on these measurements, MSC decides if handoff is necessary 44 44
  45. 45. @ 2G Mobile assisted handoff (MAHO) In 2G, handoff decisions are mobile assisted ► Each mobile measures RSSI of all surrounding BS ► Reports to serving BS ► Handoff is initiated if power of serving BS is lesser than nearby BS by a certain level or for a certain period of time ► Enables calls to be handed over between Base Stations at much faster rate than in 1G ► MSC no longer constantly monitors RSSI. ► More suitable for microcellular where HO is frequent 45 45
  46. 46.  In 1G, ► time to make handoff when signal drops below threshold is 10s. ► This requires that the value of ∆ be on the order of 6 dB to 12 dB. In 2G ► such as GSM, MAHO determines the best handoff candidates and requires only 1 or 2 seconds. ► ∆ is usually between 0 dB and 6 dB. ► Provides MSC substantial time to rescue a call that is in need of handoff 46 46
  47. 47. roaming 47
  48. 48. Handover in network 48(#)
  49. 49. UMTS HandoverIntra-system Handover:Intra-frequency HOInter-frequency HOInter-system HO• Different radio access technologies (e.g. UMTS and GSM/EDGE)• Different radio access modes (e.g. FDD/WCDMA and TDD/TD-CDMA) 49
  50. 50. 1.Hard handoverHard handover `break-before-make is appliedwhen the user’s equipmentcommunicates with onlyjust one Node B.Connection with the oldNode B is broken beforethe new connection isestablished 50
  51. 51. 2.Soft handoverUE is located in the coveragearea of two or more differentNode Bs. The UEsimultaneouslycommunicates with two ormore Node Bs via two ormore radio channels. Areceived signal in Node B isrouted to the RNC (RadioNetwork Controller). TheRNC compares the signal onthe frame by- frame basis.The best frame is selected forthe next processing; theothers frames are discarded. 51
  52. 52. 3G (UMTS) Soft Handoff Procedure 52
  53. 53. Testbed
  54. 54. 54
  55. 55. 3.Softer handoverSofter handover issimilar to softhandover. The maindifference betweenthese two handoversresides in fact that aUE is located in thecoverage area of twosectors of one Node B 55
  56. 56. Handover Phasesa] Handover detectionb] Handover preparationc] Handover execution 56
  57. 57. a] Handover DetectionOne of the responsibilities of thehandover entity is monitoring andtriggering the handover. 57
  58. 58. Handoff Detection Handoff may depend more reliably on WEI of the current channel rather than RSSI. If WEI is good, then handoff is not performed. However, it is necessary to accumulate WEI measurements over a period of time, whereas RSSI is known instantaneously. To make the handoff decision accurately and quickly, it is desirable to use both WEI and RSSI. 58
  59. 59. Handoff Detection RSSI measurements are affected by distance- dependent fading, lognormal fading (i.e., shadow fading), and Rayleigh fading (i.e., multipath fading). – Distance-dependent fading, or path loss, occurs when the received signal becomes weaker due to increasing distance between MS and BS. – Shadow fading occurs when there are physical obstacles (e.g., hills, towers, and buildings) between the BS and the MS, which can decrease the received signal strength. – Multipath fading occurs when two or more transmission paths exist (due to signal being reflected off buildings or mountains) between the MS and BS.59
  60. 60. b] Handover PreparationIn this phase, the handover entityrequests for the other networksresource availability information. 60
  61. 61. c] Handover ExecutionA handover execution message isresponsible for triggering the handoverto another network and is sent by thehandover entity. 61
  62. 62. Handover Procedure MS BTSold BSCold MSC BSCnew BTSnew measurement measurement report result HO decision HO required HO request resource allocation ch. activation HO command HO request ack ch. activation ack HO command HO command HO access Link establishment HO complete HO complete clear command clear command clear complete clear complete 62
  63. 63. Handoff Strategies Used toDetermine Instant of Handoff1. Relative signal strength2. Relative signal strength with threshold3. Relative signal strength with hysteresis4. Relative signal strength with hysteresis and threshold5. Prediction techniques 63(#)
  64. 64. • Handoff must ensure that the drop in the measured signal is not due to momentary fading and that the mobile is actually moving away from the serving base station.• Running average measurement of signal strength should be optimized so that unnecessary handoffs are avoided. – Depends on the speed at which the vehicle is moving. – Steep short term average -> the hand off should be made quickly – The speed can be estimated from the statistics of the received short-term fading signal at the base station• Dwell time: the time over which a call may be maintained within a cell without handoff.• Dwell time depends on – propagation – interference – distance – speed 64
  65. 65. Handoff measurement – In first generation analog cellular systems, signal strength measurements are made by the base station and supervised by the MSC. – In second generation systems (TDMA), handoff decisions are mobile assisted, called mobile assisted handoff (MAHO)• Intersystem handoff: If a mobile moves from one cellular system to a different cellular system controlled by a different MSC.• Handoff requests is much important than handling a new call. 65
  66. 66. Handoff parameters• Word error indicator ( WEI )• Received signal strength indication (RSSI)• quality Indicator (QI) 66
  67. 67. Word error indicator ( WEI )• This is a measure to check the output of the demodulator in the receiver.• When the carrier input to the demodulator is sufficient above the threshold , the output will be with less WEI 67
  68. 68. Received signal strength indication (RSSI) • The Received signal strength must be around - 100 dBm 68
  69. 69. Quality Indicator (QI)• This is a measure of the signal quality with respect to interference.• Usually it is prescribed at 5 dB to 25 dB 69
  70. 70. 4 types of handover 1 2 3 4 MS MS MS MS BTS BTS BTS BTS BSC BSC BSC MSC MSC GMSC 70(#)
  71. 71. Handoff Three strategies have been proposed to detect the need for handoff: o mobile‑ controlled handoff (MCHO) o network‑ controlled handoff (NCHO) o mobile‑ assisted handoff (MAHO) 71(#)
  72. 72. Mobile‑Controlled Handoff (MCHO)  The MS continuously monitors the signals of the surrounding BSs and initiates the handoff process when some handoff criteria are met.  MCHO is used in DECT and PACS. 72(#)
  73. 73. Network‑Controlled Handoff (NCHO)  The surrounding BSs measure the signal from the MS, and the network initiates the handoff process when some handoff criteria are met.  NCHO is used in CT‑ 2 Plus and AMPS. 73(#)
  74. 74. Mobile‑Assisted Handoff (MAHO)  The network asks the MS to measure the signal from the surrounding BSs. The network makes the handoff decision based on reports from the MS.  MAHO is used in GSM and IS‑ 95 CDMA. 74(#)
  75. 75. Two types of handoff  inter‑ cell handoff or inter‑ BS handoff  inter-system handoff or inter‑ MSC handoff 75(#)
  76. 76. Two types of handoff The BSs involved in the handoff may be connected to the same MSC (inter‑ cell handoff or inter‑ BS handoff) The BSs involved in the handoff may be connected to two different MSCs (inter-system handoff or inter‑ MSC handoff ). 76(#)
  77. 77. Inter‑ BS Handoff These handoff schemes can significantly reduce the probability of forced termination as well as the probability of call incompletion (new call blocking plus handoff call forced termination). 77(#)
  78. 78. Intersystem Handoff In intersystem handoff, the new and old BSs are connected to two different MSCs. We trace the intersystem handoff procedure of IS‑41, where network‑controlled handoff (NCHO) is assumed. In this figure, a communicating mobile user moves out of the BS served by MSC A and enters the area covered by MSC B. 78(#)
  79. 79. intersystem handoff intersystem handoff ► If a mobile moves from one cellular system to a different system controlled by a different MSC ► Issues to be addressed • A local call becomes a long-distance call (roaming) • Compatibility between two MSC must be determined • Different systems have different policies and methods for managing handoff requests 79 79
  80. 80. Intersystem Handoff Intersystem handoff requires the following steps:Step 1. MSC A requests MSC B to perform handoff measurements on the call in progress. MSC B then selects a candidate BS2, BS2, and interrogates it for signal quality parameters on the call in progress. MSC B returns the signal quality parameter values, along with other relevant information, to MSC A. 80(#)
  81. 81. Intersystem HandoffStep 2. MSC A checks if the MS has made too many handoffs recently (this is to avoid, for example, numerous handoffs between BS1 and BS2 a where the MS is moving within the overlapped area) or if intersystem trunks are not available. If so, MSC A exits the procedure. Otherwise, MSC A asks MSC B to set up a voice channel. Assuming that a voice channel is available in BS2, MSC B instructs MSC A to start the radio link transfer. 81(#)
  82. 82. Intersystem HandoffStep 3. MSC A sends the MS a handoff order. The MS synchronizes to BS2. After the MS is connected to BS2, MSC B informs MSC A that the handoff is successful. MSC A then connects the call path (trunk) to MSC B and completes the handoff procedure. 82(#)
  83. 83. Seamless Handoff Defined as a handoff scheme that maintains the connectivity of all applications on the mobile device when the handoff occurs. Aims to provide continuous end-to-end data service in the face of any link outages or handoff events. Design Goal:  low latency  Minimal packet loss 83
  84. 84. Universal Seamless Handoff Architecture (USHA)NAT serverNAT server All packets are encapsulated All packets are encapsulated and transmitted using UDP and transmitted using UDP Applications are bound to the tunnel Applications are bound to the tunnel and transparent to the handoff. and transparent to the handoff. 1xRTT 84
  85. 85. Smart Decision Model DM (Device Monitor)  Monitors and reports the status of each network interface:  Signal strength  Link capacity  Power consumption SM (System Monitor)  Monitors and reports system information (e.g. current remaining battery) 85
  86. 86. Handover Performance Metrics Cell blocking probability – probability of a new call being blocked Call dropping probability – probability that a call is terminated due to a handover Call completion probability – probability that an admitted call is not dropped before it terminates Probability of unsuccessful handover – probability that a handover is executed while the reception conditions are inadequate 86
  87. 87. Handover Performance Metrics Handoff blocking probability – probability that a handoff cannot be successfully completed Handoff probability – probability that a handoff occurs before call termination Rate of handoff – number of handoffs per unit time Interruption duration – duration of time during a handoff in which a mobile is not connected to either base station Handoff delay – distance the mobile moves from the point at which the handoff should occur to the point at which it does occur 87
  88. 88. Practical Handoff Consideration• Different type of users – High speed users need frequent handoff during a call. – Low speed users may never need a handoff during a call.• Microcells to provide capacity, the MSC can become burdened if high speed users are constantly being passed between very small cells.• Minimize handoff intervention – handle the simultaneous traffic of high speed and low speed users.• Large and small cells can be located at a single location (umbrella cell) – different antenna height – different power level• Cell dragging problem: pedestrian users provide a very strong signal to the base station – The user may travel deep within a neighboring cell 88
  89. 89. Call drops Call drops ► Excessive delay by MSC due to high traffic load ► ∆ is set too small for handoff time ► No channels are available on any of nearby BS 89 89
  90. 90. Handoff Process Types of Handoff 90
  91. 91. Handoff Process Types of Handoff 91
  92. 92. Horizontal Handoff Process Handoff Schemes 92
  93. 93. Horizontal Handoff Process Handoff Schemes 93
  94. 94. Horizontal Handoff Process Handoff Schemes ► Non-Prioritized Scheme (NPS) ► Prioritization Schemes • Channel Reservation Schemes (CRS) • Handoff Queuing Schemes (HQS) • Channel Transferred Handoff Schemes (CTS) • SubRating Schemes (SBRS) • Genetic Handoff Schemes (GHS) • Hybrid Handoff Schemes (HHS) 94
  95. 95. Horizontal Handoff Process 95
  96. 96. Vertical Handoff Process Handoff Phases ► System discovery ► Handoff decision ► Handoff execution 96
  97. 97. Vertical Handoff Process Handoff Schemes 97
  98. 98. Summary Handoff Strategies ► When to handoff ► 1G, BS based ► 2G or todays, Mobile-Assisted Prioritizing Handoff ( NEXT PRESENTATION) ► Guard channels concept ► Queuing handoff requests Practical handoff considerations( NEXT PRESENTATION) ► Umbrella cell ► Cell dragging 98 98
  99. 99. QuestionWhat happens when mobile moves from one cell to another?Handoff!! The process of transferring a mobile user from one channel or base station to another. 99
  100. 100. THANKS
  101. 101. Q.2. What is Hard Handoff and SoftHandoff?Provide 2 differences between them inrespect of resource allocation and technicalrequirements in mobile device? 101
  102. 102. Answer Q.2.In hard handoff, source channel is released, then target channel isengaged. Connection to source is broken before it is made to thetarget —known as break-before-make. Hard handoff needs to beinstantaneous. It is perceived as an event during the call; requiresthe least processing by the network providing service.In a soft handoff, source channel is retained and used in parallelwith target channel in the target cell. The connection to the target isestablished before the connection to the source is broken – knownas make-before-break. It is perceived as a state of the call, ratherthan a brief event.Hard handoff Soft handoffOnly one channel needs to be Minimum two channels have to beallocated per call. assignedMobile device’s hardware does not Mobile device’s hardware must beneed to be capable of receiving two or capable of receiving two or moremore channels in parallel, which makes channels in parallel, which makes itit cheaper and simpler expensive and difficult 102
  103. 103. Q.3. What is Vertical handoff? In the given scenario, when & how the Vertical Handoff will take place? •Consider Handoff between technologies: 802.3, 802.11, and 802.16e •Docked Laptop with 802.3, 802.11, and 802.16e 103
  104. 104. Answer of Q.3:Vertical handoff refers to a network node changing the type of connectivity it uses to access a supporting infrastructure, usually to support node mobility.Sequence of Events in the given scenario:1. Docked Laptop with 802.3, 802.11, and 802.16e2. Laptop undocks and switches to 802.113. User moves outside the building, laptop switches to 802.16e (Vertical Handoff is completed) 104