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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Helen W and Jochen Giese
Computer Science, Berkeley.
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
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.
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.
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.
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.
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)
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
These were Berkeley’s Specs for the simulated network affect the handoff system
The Simple Mobile IP Architecture follows…
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
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.
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.
Implementation of the Policy Enabled Handoff System – 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.