Prioritizing handoffs


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  • There are three measures of the network’s performance that directly concerns a user’s satisfaction, namely, reliability, availability and survivability.
  • The most important performance indices are New call probability and handoff call dropping probability (called loss formulas). When…
  • Prioritizing handoffs

    1. 1. Prioritizing handoffs AJAL . A .J Assistant Professor –Dept of ECE,Federal Institute of Science And Technology (FISAT) TM    MAIL: 1
    2. 2. bibin v.s SAR value of my phone 2
    3. 3. PriorityGiving handoffs priority over call requests reduces rate of handoff failure desirable from user’s point of view 3
    4. 4. Wireless Cellular System Traffic in a cell Common ChannelNew Calls Pool Call completionHandoff Calls Handoff out From To neighboringneighboring cells A Cell cells
    5. 5. Wireless “ ilities ” besides performance for a specified Reliability operational time Performability measures of the at any given instant Availability network’s ability toperform designated Performance under functions failures Survivability 5
    6. 6. Causes of Service Degradation Limited Long waiting-time Resources Resource full Time-out Service blocking Equipment failuresSoftware failures Resource loss Service Interruption Planned outages Loss of information (e.g. upgrade) Human-errors in operation
    7. 7. 3 EXECUTION PHASE 1. Mobility management 2. Handover management DECISION PHASE • Using Handover algorithms 2 INFO GATHERING PHASE 1. N /W Detection 2. Mobile node status1 3. User preferences 7 7
    8. 8. Outlines Channel Assignment Strategies 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 8
    9. 9. Handoffs ItinerariesGoal is to minimize rate of handoff failure Handoff Schemes Queuing of Handoffs Giving handoffs priority over call requests 9
    10. 10. Quality of a channel Three measurements are used to determine the quality of a channel: Metric that indicates whether  Word error indicator (WEI) the current burst was demodulated properly in the MS.  Received signal strength indication (RSSI) Measure of received signal strength. The RSSI metric has a large useful dynamic range, typically between 80 to 100 dB.  Quality indicator (QI) Estimate of the "eye opening" of a radio signal, which relates to the signal to interference and noise (S/I) ratio, including the effects of dispersion. QI has a narrow range (relating to the range of S/I ratio from 5 dB to perhaps 25 dB). 10
    11. 11. Handoff Schemes1. Guard channel method1. a )Pure guard channel method (GCM)1. b ) GCM with first-in–first-out (GCM-FIFO)2. Queuing method2.a) Dynamic Priority Queuing (DPQ) 11
    12. 12. 1. Guard channel method 12
    13. 13. India-Bangladesh border in Kulubari village Tripura 13
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    16. 16. India pakistan border 16
    17. 17. The long orange line – India-Pakistan border from space 17
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    21. 21. Guard channel Case 1: FDD 21
    22. 22.  A sufficient amount of guard band separates the two  bands so the transmitter and receiver don’t interfere  with one another. Good filtering or duplexers and possibly shielding are  a must to ensure the transmitter does not desensitize  the adjacent receiver.  22
    23. 23. Guard channel Case 2: TDD 23
    24. 24. TDD alternates the transmission and reception of  station data over time. Time slots may be variable in  length. TDD uses a single frequency band for both transmit  and receive. Then it shares that band by assigning alternating time  slots to transmit and receive operations   24
    25. 25. Source : http:/ / 25
    26. 26. Prioritizing Handoffs Two methods of handoff prioritizing ►Guard channel concept •A fraction of available channels is reserved exclusively for handoff requests •Has disadvantage of reducing total carried traffic •Offers efficient spectrum utilization when dynamic channel assignment strategies by minimizing number of required guard channels 26
    27. 27. Optimization problems• Optimal Number of Guard Channels• Optimal Number of Channels 27
    28. 28. Performance Measures: Loss formulas or probabilities When a new call (NC) is attempted in an cell covered by a base station  (BS), the NC is connected if an idle channel is available in the cell.  Otherwise, the call is blocked If an idle channel exists in the target cell, the handoff call (HC)  continues nearly transparently to the user. Otherwise, the HC is  droppedLoss Formulas New call blocking probability, Pb   : Percentage of new  calls rejected Handoff call dropping probability, Pd : Percentage of  calls forcefully terminated while crossing cells 28
    29. 29. Guard Channel Scheme Handoff dropping less desirable than new call blocking!Handoff call has Higher Priority: Guard Channel Scheme GCS: g channels are reserved for handoff calls. g trade-off between Pb & Pd New call blocking probability, Pb Handoff call dropping probability, Pd 29
    30. 30. Schemes (cont’d)• Guard Channel Concept –reserve set of channels for handoffs only –reduces number of blocked handoffs –reduces system capacity 30
    31. 31. Advantages / Disadvantages 31
    32. 32. Advantages of Guard channelGuard channel Security 32
    33. 33. Disadvantages of Guard channel• Disadvantage of reducing the total carried traffic, as fewer channels are allocated to originating calls 33
    34. 34. Break Time For queries – 10 minutes 34
    35. 35. 2. Queuing of handoff requests 35
    36. 36. 2. Queuing of handoff requests 36
    37. 37. 2. Queuing of handoff requests►Queuing of handoff requests • Possible due to time interval elapsed when the signal level drops below to threshold until minimum signal level • Decrease probability of forced termination due to lack of available channels • Tradeoff between decrease in probability of forced termination and total traffic • The delay time and queue size is determined from traffic pattern • Queuing does not guarantee zero probability of call termination since large delays will signal level to drop min 37
    38. 38. Queuing• Queuing of Handoffs –Put handoff requests in a queue –Serves handoffs on a FCFS basis –reduces number of failed handoffs –reduces system capacity 38
    39. 39. Performance Analysis handoff-initial-access channel assignment schemes Network 1 Network 2 Network 3 39
    40. 40. handoff-initial-access channel assignment schemes• handoff-initial-access channel assignment schemes 1. the non-prioritized scheme, 2. the reserved channel scheme, 3. the queuing priority scheme, and 4. the sub-rating scheme 40
    41. 41. 1. the non-prioritized scheme, General method ….. 41
    42. 42. 2.Reserved Channel Scheme (RCS) The reserved channel scheme (RCS) is similar to NPS except that a number of channels or transceivers in each BS are reserved for handoffs. In other words, the channels are divided into two groups: ► the normal channels, which serve both new calls and handoff calls, ► the reserved channels, which only serve handoff calls. 42
    43. 43. 3. Queuingpriority scheme 43
    44. 44. 3. Queuing Priority Scheme• The queuing priority scheme (QPS) is based on the fact that adjacent cells in a PCS network overlap.• Thus, there is a considerable area where a call can be handled by either BS of the adjacent cells, called the handoff area.• The time that an MS spends in the overlapped area is referred to as the degradation interval. 44
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    46. 46. • The channel assignment for a QPS new call is the same as that for NPS.• If a channel in the new cell is available for the handoff, the handoff actually occurs.• If no channel is available after the MS moves out of the handoff area-the degradation interval expires-the call is forced to terminate.• In this scheme, when a channel is released, the BS first checks if the waiting queue is empty.• If not, the released channel is assigned to a handoff call in the queue.• The next handoff to be served is selected based on the queuing policy. QPS 46
    47. 47. QPS b) The measured-based priority scheme (MBPS)a )FIFO scheme 47
    48. 48. Scheduling policies for the QPS• a )FIFO scheme : he next handoff call is selected on a first- in-first-out basis.• b) The measured-based priority scheme (MBPS) : uses a non-preemptive dynamic priority policy. The priorities are defined by the power level that the MS receives from the BS of the new cell.• The network dynamically monitors the power levels of the handoff calls in the waiting queue.• We may view a handoff call as having a higher priority if its degradation interval is closer to expiration. 48
    49. 49. 4. Sub-rating Scheme• The sub-rating scheme (SRS) creates a new channel on a blocked BS for a handoff access attempt by subrating an existing call.• Subrating is the process of temporarily dividing an occupied full-rate channel into two channels at half the original rate, one to serve the existing call and the other to serve the handoff request. Divide & Rule policy 49
    50. 50. Real time scenario Implementation Issues• To implement prioritizing handoff schemes, a radio system must have a physical channel, that is, a system signaling channel, for the MS to request the link transfer even when all traffic channels are in use. This channel should always be available, and, therefore, cannot be used as a traffic channel.• Some PCS radio systems already reserve a channel for other purposes, such as system broadcast channel, which can be shared by the handoff prioritizing procedure.• For systems with conventional handoff procedures, the reserved channel is not necessary because the request is made through the network. 50
    51. 51. Modeling• Several analytical and simulation models have been proposed to evaluate the performance of the handoff- channel assignment schemes.• The results are summarized here. RCS is easy to implement, and it reduces the forced termination probability more effectively than NPS.• The new call-blocking probability for RCS, however, is larger than that of NPS.• Thus, RCS is desirable only when reducing forced termination is much more important than reducing new call blocking.• The queuing priority schemes take advantage of the handoff area to buffer the handoff calls.• The implementation for the measurement-based priority scheme (MBPS) is more complex than that for the FIFO scheme, but the performance is almost identical. 51
    52. 52. Advantages of Queuing priority schemes• Queuing priority schemes effectively reduce forced terminations, at the expense of increased new call blocking.• The probability of incomplete calls for FIFO and MBPS is slightly lower than that for NPS.• Queuing priority schemes add hardware /software complexity for both BSs and MSs to manage the waiting queues.• The subrating scheme has the least forced termination probability and the probability of incomplete calls when compared with the other schemes.• This benefit is gained at the expense of the extra hardware/ software complexity required to subrate a channel.Trade off s 52
    53. 53. Some of IEEE StandardsStandard Scope/Application Physical connections are made between nodesIEEE 802.3 and/or infrastructure devices (hubs, switches, routers) by various types of copper or fiber cable. Provide the basis for wireless network productsIEEE 802.11 using the Wi-Fi brand. Wireless Broadband standards for WirelessIEEE 802.16e Metropolitan Area Networks (WiMAX). (802.16e-2005 amended version deployed worldwide in 2009) Supports algorithms enabling seamless handoffIEEE 802.21 between inter & intra networks 53
    54. 54. Concluding RemarksCellular concept is very popularHandoffs are essential in cellular systemsSystem design must take handoff-related considerations into account.Performance of handoffs is complex and depends on many factors. 54
    55. 55. Summary Channel Assignment Strategies 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 55
    56. 56. Further Research should take place in order to minimizethe handoff overheads like delays, connection drops and packet losses,both in case of intra and inter-technology handoffscenario. 56
    57. 57. [1] Chapter 1, Handbook of Wireless Networks and Mobile Computing, Editedby Ivan Stojmenovic, John Wiley & Sons, Inc. 2002[2] Sayan Kumar Ray, Krzysztof Pawlikowski, and Harsha Sirisena, Handoverin Mobile WiMAX Networks: The State of Art and Research Issues, IEEE[3] D. Hong and S. S. Rappaport, Traffic model and performance analysis forcellular mobile radio telephone systems with prioritized and nonprioritizedhandoff procedures, IEEE Trans. Veh. Technol., Vol. VT-35, No. 3, pp. 448–461, August 1986.[4] N. Ekiz, T. Salih, S. Kucukoner, and K. Fidanboylu, “Overview of handofftechniques in cellular networks,” Int. J. Inf. Technol., vol. 2, no. 3, pp. 132–136,2005.[5] Ray, S; Pawlikowski, K; Sirisena, H; , ”Handover in Mobile WiMAXNetworks: The State of Art and Research Issues,” IEEE Commun. Surveys &Tutorials , vol.PP, no.99, pp.1-24, 2010 57
    58. 58. Thank you!Any Question! 58
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