A Cross-Layer (Layer 2 + 3)Handoff Management Protocol forNext-Generation Wireless SystemsByShantidev Mohanty and Ian F. Akyildiz, Fellow, IEEE
Presentation ByMuhammed Syyid
NGWS
Next Generation Wireless System Multiple kinds of wireless systems
deployed UMTS (WAN) 802.11 (WLAN) Bluetooth (PAN) Satellite (Global)
Unification of systems to provide optimal data availability
NGWS
NGWS Design Goals Support for the “best” network selection
Mechanism to ensure high-quality and security
Seamless inter-system mobility
Scalable architecture (any # of wireless systems)
QoS provisioning
Mobility Management
Location Management Track Location of users between
consecutive communications Handoff Management
Keep connections active while moving between base stations
Handoff in NGWS
Handoff in NGWS
Horizontal Handoff Link Layer Handoff IntraSystem Handoff
Vertical Handoff InterSystem Handoff
Current Status
Link Layer Handoff Efficient algorithms available in literature
InterSystems and IntraSystems Handoff Signaling Delay Packet Loss
Goals for Seamless Handoff
Minimize Handoff Latency Minimize Packet Loss Limit Handoff Failure Minimize False Handoff Initiation
Handoff Protocols By TCP/IP Layer
Network Layer Mobile IP
Transport Layer TCP-Migrate MSOCKS (Split proxy & TCP SPLICE) Modification of SCTP (Stream Control
Transmission Protocol) SIP
Mobile IP Issues
Triangular Routing High Global Signaling Load High Handoff Latency
Reasons for handoff latency Handoff Requirement Detection Registration at New Foreign Agent (NFA)
Proposed Solutions in Literature Triangular Routing
Route Optimization High Global Signaling Load / Registration at NFA
Hierarchical Mobile IP (HMIP) Cellular IP IDMP HAWAII
Solution to Handoff Latency due to Requirement Detection Use Link Layer Information Calculate probability of Handoff
Factors affecting handoff signaling delay Traffic Load on the network Wireless Link Quality Distance between user and home network User’s Speed
Analysis of Current Systems RSS: Received Signal Strength BS: Base Station MT: Mobile Terminal OBS: Old Base Station NBS: New Base Station FA: Foreign Agent OFA: Old Foreign Agent NFA: New Foreign Agent Sth: The threshold value of RSS to initiate handover Sath: The Adaptive threshold value of RSS to initiate handover Smin: The minimum value of RSS required for successful
communication a: Cell Size d: Cell Boundary v: Speed of MT’s movement : Handoff signaling delay
Movement during Handoff
Handoff Scenario
1) MT moves with speed v2) RSS of the OBS decreases continuously3) RSS drops below Sth at the point P marking
cell boundary d.4) RSS < Sth triggers registration for NFA.5) Pre-registration messages sent through OBS to
NFA (must be completed before signal drops below Smin)
False Handoff
At point p, it can move in any direction with equal probability F()=1/2 where - < <
Handoff possible only when [(- 1, 1)]
Where 1=arctan(a/2)/(d)=arctan(a/2d)
Probability of False Handoff Initiation is
Using S=Vt (Distance=Velocity X Time) i.e. t=S/V or t=d/v The largest possible distance to cover while
travelling to NBS is (a/2)2+(d)2
As velocity increases the time to cover distance will decrease
When the time to leave the cell falls below the handoff signaling delay, handoff will fail
Therefore Pf = 1
Using S=Vt (Distance=Velocity X Time) i.e. t=S/V or t=d/v As velocity increases the time to cover distance will
decrease While the time to leave the cell is greater then
handoff signaling delay, handoff will succed When d/v > the handoff will succeed Therefore Pf = 0
False Handoff Initiation As cell boundary d is increased, the probability
of false handoff initiation increases (keeping cell size a constant)
As cell size a is decreased the probability of false handoff increases (keeping d constant)
Cell sizes are currently trending towards smaller size to cope with capacity and improve data rates.
Hence, value of d must be carefully selected.
Handoff Failure and Speed
From the above, handoff failure depends on speed (keeping a,d,Sth fixed).
As speed increases the probability of failure increases
For intersystem handoffs the handoff latency is higher making it more susceptible to failure
Increasing the value of d/Sth reduces the probability of failure
Handoff Failure & Signaling Delay
The higher the signaling delay the greater the probability of failure (over a constant d)
The higher the value of d the lower the probability of failure for a single value for signaling delay
Therefore to optimize and minimize handoff failure , the distance (and therefore Sth) must be adaptive to signaling delay.
Analysis Summary
For Fixed value of d(and Sth) handoff failure probability increases as MT’s speed increases
For Fixed value of d(and Sth) handoff failure probability increases as handoff signaling delay increases
Large values of d(and Sth) increase the probability of false handoff initiation
CHMP
Derive information from link layer (2) and network layer(3) to create adaptive architecture
Titled proposed solution as Cross-Layer (Layer 2+3) Handoff Management Protocol or CHMP
CHMP Modules
Neighbor Discovery Unit Determines BS’s neighboring the MT’s current
BS
Uses network discovery protocols
Speed Estimation Unit Uses VEPSD (Velocity Estimation using the Power Spectral
Density of RSS) to estimate speed. The doppler frequency is used to determine speed v
V=(c/fc)fm
where c= speed of light in free spacefc = carrier frequency of RSSfm = maximum doppler frequency
Handoff Signaling Delay Estimation Unit Estimates delays associated with intra/intersystem handoffs
Handoff Trigger Unit Collects previously collected and
calculated information to determine the appropriate time to initiate handoff
Handoff Execution Unit Triggers the Actual handoff at the
appropriate time calculated by the Handoff Trigger Unit
Operation
Neighborhood Discovery
Determine neighbors using the neighbor discovery unit.
If OBS and NBS have common FA link-layer handoff occurs (CHMP is not used)
IF OBS and NBS have different FA (intrasystem) or belong to different systems (intersystem) CHMP is used.
Handoff Signaling Delay Estimation Unknown which BS the MT will move to Using the neighborhood discovery step, compile list of
possible BS/FA’s. Send an invalid Authentication Extension message to the
GFA (for intrasystem) or HA (intersystem). GFA/HA respond with an HMIP Registration Reply indicating
registration failure. The round trip response time is used to estimate the
handoff signaling delay. Uses existing HMIP protocol without any extra
implementation Causes extra signaling overhead but solution still improves
performance significantly. Alternative delay estimation algorithms available in
literature if signaling overhead is not tolerable.
Handoff Anticipation
When the RSS continuously decreases, a handoff is anticipated
Existing movement prediction techniques used to estimate the next BS
Retrieve estimated signaling delay from the Handoff Delay Estimation Unit.
Handoff Initiation Estimate optimal moment to initiate
handoff Estimate Sath using speed and
handoff signaling delay estimates. Trigger handoff when RSS < Sath Sath for intrasystems is referenced as
Sath1 Sath for intersystems is referenced as
Sath2
Pr(x) Received power at point x Pr depends on various factors, including
frequency, antenna heights, antenna gains etc
d0 is known as reference distance Typical values for d0 are
1 km for macrocells 100 m for outdoor microcells 1m for indoor picocells
is the path loss exponent Depends on cell size and local terrain
characteristics Typical values range between 3-4 for macro and
2-8 for microcelluar environments is a random variable representing
variation in Pr(x) due to shadowing Typical value is 8dB
Pr(x)=Pr(d0)(d0/x) + Sath=10log10[Pr(a-d)]
Handoff Execution HMIP registration started when the handoff trigger is
received. After registration the MT is switched to the NBS Simultaneous Binding preserved for a limited time by
binding CoA of both OFA and NFA to the GFA for intrasystem and HA for intersystem, this avoids the ping-pong effect.
Packets are forwarded to both CoA’s If the MT returns to the old BS there is no need to
carry out HMIP handoff again. If the MT does not return to the old BS, it deregisters
from the old BS
CHMP Location Implemented at MT referred to as Mobile Assisted network
controlled Hand Off (MAHO) MT implemented components
Speed Estimation RSS Measurement Handoff Signaling Delay Estimation
Network implemented components Handoff Trigger Unit Handoff Execution Unit
Implemented at Network referred to as Network Assisted mobile controlled Hand Off (NAHO) Network implemented components
Speed Estimation RSS Measurement Handoff Signaling Delay Estimation
MT implemented components Handoff Trigger Unit Handoff Execution Unit
Types of Handoffs Intersystem
Macro-Inter: Between a macro-cellular system and another macro-cellular system (Inter_MA_HO)
Micro-Inter: Between a microcellular system and another microcellular system (Inter_MI_HO)
Macro-Micro-Inter: Between a macro-cellular system and a micro-cellular system (Inter_MAMI_HO)
Micro-Macro-Inter: Between a micro-cellular system and a macro-cellular system (Inter_MIMA_HO)
Usually Microcellular systems are overlapped by macrocellular systems. Therefore for Inter_MAMI_HO there is no handoff failure
Intrasystem Macro-Intra: Between two cells of a macro-
cellular system (Intra_MA_HO) Micro-Intra: Between two cells of a
microcellular system (Intra_MI_HO)
Performance Evaluation
Relationship between Sath and Speed
Sath increases as speed increases That is for a high speed MT handoff should be
initiated early Sath increases as increases
When is large the handoff must start earlier to allow time for registration/handoff to complete
In order to compensate for Shadow fading and errors in estimation, Sath was increased by 10 percent
Relationship between Handoff Failure Probability and Speed When MT’s speed is known, there is a 70-80
percent reduction in Handoff Failure Probability with CHMP
With CHMP in use probability of failure becomes independent of speed
Comparing the figures for fixed RSS thresholds, failure probabilities are different for intra and intersystem handoffs
This further enhances the case for adaptive thresholds
Relationship between Handoff Failure Probability of CHMP and Handoff Signaling Delay There is a 70-80 percent reduction in Handoff
Failure Probability with CHMP compared to fixed RSS schemes.
With CHMP in use probability of failure becomes independent of
Probability of failure is limited to desired values irrespective of speed and variation of handoff signaling delay
Relationship between False Handoff Initiation Probability of CHMP and Speed Fixed value of RSS Threshold Sth is calculated
such that a user with highest speed is guaranteed the desired value of handoff failure probability.
Comparatively the adaptive CHMP reduces the false handoff initiation probability by 5-15 percent
CHMP initiates handoff while preventing an early handoff (minimizing false handoff initiation) and late handoff (minimize probability of failure)
Conclusions When a fixed value of RSS threshold (Sth) is
used handoff failure probability increases with an increase in speed or handoff signaling delays
The adaptive CHMP Protocol estimates speed and handoff signaling delay of possible handoffs creating a dynamic RSS threshold (Sath)
CHMP significantly enhances the performance of both intra and intersystem handoffs
CHMP reduces the cost associated with false handoff initiation