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Prioritizing handoffs
AJAL . A .J Assistant Professor –Dept of ECE,
Federal Institute of Science And Technology (FISAT) TM MAIL: [email protected]
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bibin v.s
SAR value of my phone
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Priority
Giving handoffs priority over call requests
reduces rate of handoff failure
desirable from user’s point of view
Wireless Cellular System Traffic in a cell
A Cell
New Calls
Handoff Calls From
neighboring cells
CommonChannel
Pool Call completion
Handoff outTo neighboring
cells
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Wireless “ Wireless “ ilities ilities ” besides ” besides performanceperformance
Performability measures of the network’s ability
to perform designated functions
Reliabilityfor a specified operational time
Availabilityat any given instant
SurvivabilityPerformance under
failures
Causes of Service Degradation
Resource full
Resource loss
Long waiting-timeTime-out
Service blocking
Service InterruptionLoss of information
LimitedResources
Equipment failures
Software failures
Planned outages(e.g. upgrade)
Human-errors in operation
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EXECUTION PHASE
DECISION PHASE
INFO GATHERING PHASE
• Using Handover algorithms
1.Mobility management2.Handover management
1.N / W Detection2.Mobile node status3.User preferences1
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Outlines Channel Assignment Strategies Handoff Strategies
► When to handoff► 1G, BS based► 2G or today's, Mobile-Assisted
Prioritizing Handoff
►Guard channels concept►Queuing handoff requests
Practical handoff considerations► Umbrella cell► Cell dragging
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Handoffs Itineraries
Goal is to minimize rate of handoff failure
Handoff SchemesQueuing of HandoffsGiving handoffs priority over call requests
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Quality of a channel
Three measurements are used to determine the quality of a channel:
Word error indicator (WEI) Metric that indicates whether 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).
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Handoff Schemes1. Guard channel method 1. a )Pure guard channel method (GCM)1. b ) GCM with first-in–first-out (GCM-FIFO)
2. Queuing method
2.a) Dynamic Priority Queuing (DPQ)
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1. Guard channel method
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India-Bangladesh border in Kulubari village Tripura
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India pakistan border
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The long orange line – India-Pakistan border from space
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Guard channel
Case 1: FDD
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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.
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Guard channel
Case 2: TDD
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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
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Source : http://www.moonblinkwifi.com/fddvstddwimax.cfm
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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
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Optimization problems
• Optimal Number of Guard Channels
• Optimal Number of Channels
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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 FormulasNew call blocking probability, Pb : Percentage
of new calls rejectedHandoff call dropping probability, Pd :
Percentage of calls forcefully terminated while crossing cells
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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
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Schemes (cont’d)
• Guard Channel Concept
–reserve set of channels for handoffs only–reduces number of blocked handoffs–reduces system capacity
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Advantages / Disadvantages
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Disadvantages of Guard channel • Disadvantage of reducing the total carried traffic,
as fewer channels are allocated to originating calls
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Break Time For queries – 10 minutes
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2. Queuing of handoff requests
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2. Queuing of handoff requests
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►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
2. Queuing of handoff requests
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Queuing
• Queuing of Handoffs
–Put handoff requests in a queue–Serves handoffs on a FCFS basis–reduces number of failed handoffs–reduces system capacity
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Performance Analysis
Network 3
Network 2Network 1
handoff-initial-access channel assignment schemes
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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
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1. the non-prioritized scheme,
General method …..
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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.
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3. Queuing priority scheme
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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.
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QPS
• 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.
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QPS
b) The measured-based priority scheme
(MBPS)
a )FIFO scheme
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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.
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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
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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.
Real time scenario
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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.
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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
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Standard Scope/Application
IEEE 802.3
Physical connections are made between nodes and/or infrastructure devices (hubs, switches, routers) by various types of copper or fiber cable.
IEEE 802.11Provide the basis for wireless network products using the Wi-Fi brand.
IEEE 802.16e
Wireless Broadband standards for Wireless Metropolitan Area Networks (WiMAX). (802.16e-2005 amended version deployed worldwide in 2009)
IEEE 802.21Supports algorithms enabling seamless handoff between inter & intra networks
Some of IEEE Standards
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Concluding RemarksCellular concept is very popularHandoffs are essential in cellular systemsSystem design must take handoff-related
considerations into account.Performance of handoffs is complex and
depends on many factors.
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Summary Channel Assignment Strategies Handoff Strategies
► When to handoff► 1G, BS based► 2G or today's, Mobile-Assisted
Prioritizing Handoff
► Guard channels concept► Queuing handoff requests
Practical handoff considerations► Umbrella cell► Cell dragging
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Further Research should take place in order to minimize the handoff overheads like delays, connection drops and packet losses,
both in case of intra and inter-technology handoff scenario.
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[1] Chapter 1, Handbook of Wireless Networks and Mobile Computing, Edited by Ivan Stojmenovic, John Wiley & Sons, Inc. 2002[2] Sayan Kumar Ray, Krzysztof Pawlikowski, and Harsha Sirisena, Handover in Mobile WiMAX Networks: The State of Art and Research Issues, IEEE[3] D. Hong and S. S. Rappaport, Traffic model and performance analysis for cellular mobile radio telephone systems with prioritized and nonprioritized handoff 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 handoff techniques 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 WiMAX Networks: The State of Art and Research Issues,” IEEE Commun. Surveys & Tutorials , vol.PP, no.99, pp.1-24, 2010
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Thank you!Any Question!
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