1. Prioritizing handoffs
AJAL . A .J
Assistant Professor –Dept of ECE,
Federal Institute of Science And Technology (FISAT) TM
MAIL: ec2reach@gmail.com
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4. Wireless Cellular System
Traffic in a cell
Common
Channel
New Calls Pool
Call completion
Handoff Calls Handoff out
From To neighboring
neighboring cells A Cell cells
5. Wireless “ ilities ” besides
performance
for a specified Reliability
operational time
Performability
measures of the
at any given instant Availability
network’s ability to
perform designated
Performance under
functions
failures Survivability
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6. Causes of Service Degradation
Limited Long waiting-time
Resources Resource full Time-out
Service blocking
Equipment
failures
Software failures Resource loss Service Interruption
Planned outages Loss of information
(e.g. upgrade)
Human-errors in
operation
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 status
1 3. User preferences 7
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9. Handoffs Itineraries
Goal is to minimize rate of handoff
failure
Handoff Schemes
Queuing of Handoffs
Giving handoffs priority over call
requests
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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).
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11. Handoff Schemes
1. 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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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
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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. 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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30. 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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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
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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
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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.
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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.
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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
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47. QPS
b) The measured-based
priority scheme
(MBPS)
a )FIFO scheme
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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.
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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
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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.
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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.
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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
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53. Some of IEEE Standards
Standard Scope/Application
Physical connections are made between nodes
IEEE 802.3 and/or infrastructure devices (hubs, switches,
routers) by various types of copper or fiber cable.
Provide the basis for wireless network products
IEEE 802.11 using the Wi-Fi brand.
Wireless Broadband standards for Wireless
IEEE 802.16e Metropolitan Area Networks (WiMAX).
(802.16e-2005 amended version deployed worldwide in 2009)
Supports algorithms enabling seamless handoff
IEEE 802.21 between inter & intra networks
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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.
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56. 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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57. [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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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…
