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Policy Enabled Handoff Across Heterogeneous Wireless Networks. Helen W and Jochen Giese Computer Science, Berkeley. Outline. Abstract of the Paper Policy Enabled Handoff System - Case and Principles - Operating Environment - Policy Specification/Model

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Policy enabled handoff across heterogeneous wireless networks l.jpg

Policy Enabled Handoff AcrossHeterogeneous Wireless Networks

Helen W and Jochen Giese

Computer Science, Berkeley.

Outline l.jpg

Abstract of the Paper

Policy Enabled Handoff System

- Case and Principles

- Operating Environment

- Policy Specification/Model

- Dealing with the Handoff Synchronization Problem

Implementation of the Policy Enabled handoff System

- Programming Model

- Software Architecture

Conclusions and Future Work

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Integration of Heterogeneous networks

Hand off issues in such networks (includes policy making)

Software Architecture/Support for Heterogeneous Networks

This paper presents a sample description of a heterogeneous

wireless network architecture and focuses on a novel handoff

mechanisms to achieve optimum performance.

The heterogeneous infrastructure experimented was on four types of networks

- Infrared LAN, WaveLAN, Metricomm, GSM Cellular.

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Some terms/ideas of Interest

Vertical Handoff- Intercellular Handoff- Handoff between cells of different


Features of Vertical Handoff-

Vertical handoff is simple and embedded in the system.The mobile host always switches to the smallest coverage area.


Horizontal handoff- Intracellular Handoff- Handoff between cells of the same


Feature of Horizontal Handoff-

Horizontal handoff decision takes place based on received signal strength.

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What was the Problem then……?????

Firstly, The problem was sometimes frequent and unnecessary handoffs.

Secondly, These handoff decisions did not consider system dynamics such as…

Thirdly, the handoff synchronization problem.- The problem of overloading a

network by mobile users simultaneously connecting at the same instant of time.

So, Berkeley came up with some policy deciding when a handoff is necessary.

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Berkeley's Handoff Policy- Case and Principles affect the handoff system

Network technologies differ in terms of bandwidth, latency, power

consumption and potentially their charge model.

The issue was how to integrate these seamlessly.

-Seamless ness was proposed to be achieved

-Handoff latency was proposed to be low enough not to

disrupt the running applications.

The automation of switching from one network to another is based on the

principle of user involvement with minimal user interaction.

User involvement was required for the policy specification.

Handoff decisions and operations are all done at the mobile host. Periodically,

the mobile host collects current dynamic conditions, and consults with a pol-

icy module on which is the best reachable network.

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Policy Specification/Model affect the handoff system

Berkeley embedded its handoff decision policy to one equation

The cost of using a network n is a function of several parameters such as

the bandwidth it can offer(Bn), the power consumption of using the network

access device (Pn) and the cost (Cn) using the network and is represented as...

Costn = f(Bn, Pn, Cn)

  • For a given network….

  • Bandwidth parameter is calculated periodically( Note that available BW

  • is inversely proportional to the number of users)

  • b) Power Consumption are fixed

  • c) Cost parameters are fixed

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Policy Specification/Model (Contd…) affect the handoff system

Costn = f (Bn, Pn, Cn)

Normalization is done….N(x)….. (Why…..)

Costn = f (N(Bn), N(Pn), N(Cn))

The function is further expressed as

Costn = wb*N(Bn), wp* N(Pn), wc*N(Cn))

Where wb, wp and wc are weight values and sum to 1.

The setting of these values are determined by the user

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These were Berkeley’s Specs for the simulated network affect the handoff system

The Simple Mobile IP Architecture follows…

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Stability Period (To avoid frequent handoffs) affect the handoff system

Stability period is a defined as the waiting period before handoffs.

The system calculates the cost function of each reachable network periodically

and hands off to the better network if a network is consistently better for

a stability period.

Only if the network is consistently better than the current one in use for the

stability period does the mobile host perform handoff.

If a mobile user only transiently transfers to a better network, the gain from

using the network may be diminished by the handover overhead and short usage


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Handoff Synchronization problem affect the handoff system

It is possible that several mobile hosts could discover the same better network

and switch to them simultaneously causing it to increase its load dramatically.

The synchronization problem can cause instability for all these mobile hosts

and lead to poor performance.

The problem is solved through randomized stability period.

A random number is generated as the waiting period between handoffs.

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Handoff Synchronization vs. Desynchronization affect the handoff system

The left diagram shows instability due to simultaneous handoffs

whereas that instability is eliminated by incorporating stability period.

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Performance of the Berkeley Prototype Software Architecture

Four networks were included in the prototype:

1) IBM Infrared LAN, 2) Lucent WaveLAN, 3) The Metricom Ricochet network, 4) and the GSM cellular.

Performance was based on handoff latency.

Handoff latency is the amount of time for a mobile host to handoff to a given network.

Handoff latencies were measured from ten trials of manually triggered handoffs.

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Future Work Software Architecture

  • Discovering Parameter Interdependency

  • This is important since varying one parameter can cause changes to the rest of them. It includes understanding and modeling the relationship between these system parameters.

  • 2) Redefining the policy model for various application levels

  • Users can specify priorities or different classes of QOS requirements for each application such as real time and non- real time. This can be easily incorporated into the policy model.