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