A Smart Decision Model for Vertical Handoff

Ling-Jyh Chen*, Tony Sun*, Benny Chen*, Venkatesh Rajendran†, Mario Gerla*

* Department of Computer Science, University of California at Los Angeles, Los Angeles, CA 90095, USA

† Department of Computer Engineering, University of California at Santa Cruz, Santa Cruz, CA 95064, USA

{cclljj, tonysun, cudokido, gerla}@cs.ucla.edu, venkat@soe.ucsc.edu

Presentation Outline Introduction Related Work Smart Decision Model Smart Decision Example Conclusion

Introduction

Problems and Solutions Problem

Mobile devices with multiple network interfaces today cannot perform handoff among devices without losing existing internet connection—due to change of IP addresses.

Solution Universal Seamless Handoff Architecture (USHA) and

Handoff Servers (HS). Problem

Determining when to handoff to another interface is a complex decision.

Solution Smart Decision Model.

Definition of Handoff Horizontal Handoff

Occurs when the user switches between different network access points of the same kind.

e.g. Handoff among 802.11 APs. Vertical Handoff

Involves two different network interfaces which usually represent different technologies.

e.g. Handoff from 802.11 to 1xRTT (CDMA 2000).

Handoff Illustration

Seamless Handoff Defined as a handoff scheme that maintains

the connectivity of all applications on the mobile device when the handoff occurs.

Aims to provide continuous end-to-end data service in the face of any link outages or handoff events.

Design Goal: low latency Minimal packet loss

Related Work

Related Work—Handoff Decision-making

In “Policy-Enabled Handoffs across Heterogeneous Wireless Networks,” Proc. of ACM WMCSA, 1999, by H.J. Wang, R. H. Katz, and J. Giese:

Designed a cost function to decide the best moment and interface for vertical handoff.

Cost functions presented in this paper is very preliminary and not able to handle more sophisticated configurations.

Logarithmic functions used in the cost function will also have difficulty in representing the cost value while the value of the constraint factor is zero (e.g. the connection is free of charge).

Related Work—Handoff Decision-making

In “Cost Metrics For Decision Problem In Wireless Ad Hoc Networking,” IEEE CAS Workshop on Wireless Communications and Networking, 2002, by M. Angermann and J. Kammann:

Modeled handoffs with HTTP traffic. May have problems with other types of traffic, such

as video and audio streaming, where the bandwidth demand is much higher than HTTP traffic.

Smart Decision Model

All packets are encapsulated and transmitted using UDP

All packets are encapsulated and transmitted using UDP

NAT serverNAT server

Applications are bound to the tunnel and transparent to the handoff.

Applications are bound to the tunnel and transparent to the handoff.

Testbed: Universal Seamless Handoff Architecture (USHA)

1xRTT

Smart Decision Model With USHA, mobile hosts are able to select any

network interface for its connection at any time.

However, still need a model that knows which interface to use based on various factors such as

Link Capacity (speed) Cost Power Consumption

Solution: Smart Decision Model

Smart Decision Model HCC—Handoff Control

Center Provides connection

between network interfaces and the upper layer applications.

Composed of DM (Device Monitor) SM (System Monitor) SD (Smart Decision) HE (Handoff Executor)

Smart Decision Model DM (Device Monitor)

Monitors and reports the status of each network interface:

Signal strength Link capacity Power consumption

SM (System Monitor) Monitors and reports

system information (e.g. current remaining battery)

Smart Decision Model SD (Smart Decision)

Integrates user preferences and all other available information provided by DM, SM to achieve a “Smart Decision”

HE (Handoff Executor) Performs a device handoff

if current network interface differs from the one determined by SD.

Smart Decision Process Priority Phase:

1. Add all available interfaces into candidate list.2. Remove user specified devices from the candidate list.3. If candidate list is empty, add back removed devices from

step 1.4. Continue with Normal Phase.

Normal Phase:1. Collect information on every wireless interface in the

candidate list from the DM component.2. Collect current system status from SM component.3. Use the score function to obtain the score of every wireless

interface in the candidate list.4. Handoff all current transmissions to the interface with the

highest score if different from current device.

Priority / Normal Phases Necessary in SD to accommodate user-specific

preferences regarding the usage of network interfaces.

For instance, a user may decide not to use a device when it causes undesirable interferences to other devices (e.g. 802.11b and 2.4GHz cordless phones).

With priority and normal phases in place, the SD module provides flexibility in controlling the desired network interface to the user.

Score Function SD deploys a Score Function to calculate a score for every

wireless interface Handoff target device is the network interface with the

highest score. Score Function:

wj = weight of factor k fj,i = normalized score of interface i of factor j

The equation is thus equivalent to:

where e = Expense, c = Link Capacity, p = Power Consumption.

Score Function Breakdown Expense:

Link Capacity:

Power Consumption:

Note: The coefficients α , β , γ are determined by user preference. Inverse functions are used to bound results from 0 to 1. M = Maximum bandwidth requirement demanded by the user. Link capacity is calculated using CapProbe—because advertised link

speed is seldom achieved due to link congestion or bad link quality.

Smart Decision Example

Smart Decision Example—Scenario A mobile user currently using 1xRTT on his laptop

enters a café. HCC immediately discovers a usable 802.11b

access point inside the café and conducts the following comparisons:

Expense/Cost: 1xRTT: 1 ￠ /min 802.11b: 10 ￠ /min

Link Capacity: 1xRTT: 100 Kbps 802.11b: 5 Mbps

Power Consumption (battery time): 1xRTT: 4 hours 802.11b: 2 hours

Smart Decision Example—Result The mobile user prefers to spend more time in the café and

feels that cost and connection speed are equally important to her, thus

wp=0.4, we = 0.3, wc = 0.3 Coefficient obtained from her preference previously were:

αi = xi / 20 , xi : ￠ /min βi = Min(yi, M)/M , M = 2Mbps γi = 2 / zi , zi : hours

Scores calculated using the score function are: S1xRTT = 0.83 S802.11b = 0.44.

Since S1xRTT > S802.11b , HCC decides to continue using the 1xRTT interface.

Conclusion

Conclusion Smart Decision Model provides a solution for

determining the right time to perform handoffs.

Our proposed model is able to make smart decisions based on

Available network interfaces and properties (e.g. link capacity, power consumption, and link cost).

System information (e.g. remaining battery). User preferences.