Modularized Two Step Vertical Handoff Scheme In Integrated WWAN and WLAN

Shimin Li

Ying Wang

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What is Vertical Handoff

Vertical handoff is the handoff between different network

Horizontal handoff is the handoff within the same network

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Outline

1. Motivation and Objective

2. Overview of WWAN and WLAN

3. Integration Architecture and Mobile IP

4. Modularized Two Step Handoff Scheme

5. Conclusion

6. Reference

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1. Motivation

WWAN: low rate, high cost and wide coverage

WLAN: high rate, low cost and small coverage usually deployed at hotspots

Combines the 3G WWAN and the IEEE 802.11 WLAN together will benefit to operator and customer

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1. Objective To make the integration effective, vertical handoff

between the two networks is necessary Major problems in Vertical Handoff in integrate

networkDelayLoss packetBilling

In this presentation, we focus on real time application. It needs low delay during the vertical handoff

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1. Related works

In [1] tight coupling, loose coupling and Mobile IP were introduced

[2] and [3] introduce FFT and MAC layer sensing for handoff decision, but the solution need improve because handoff decision does not consider different situation

[4] introduce the multi tunnel idea, but did not get a easy implement solution

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2 Overview of WWAN and WLAN

Architecture of WWAN (CDMA 2000)

Architecture of WLAN

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Architecture of WWAN (CDMA 2000)

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Architecture of WLAN

Router

Internet

Hub/Switch

LAN

Hub/Switch

LAN

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3 Integration Architecture and Mobile IP

Tight coupling Loose coupling Mobile IP

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BS

CDMA2000BSC

InternetCDMA2000Core Network

WLAN Access

Point

WLAN Gateway

WLAN Gateway

TightCoupling Loose

Coupling

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Tight coupling

Basic idea disadvantage

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Loose coupling

Basic idea Advantage

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Mobile IP

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4 Modularized Two Step Handoff Scheme

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Main idea of Modularized Two Step Handoff Scheme 1

Handoff

Pre-HandoffPre-Handoff Message

Handoff Message

MAC Signal

Physical Signal

Decision Module

Action Module

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Main idea of Modularized Two Step Handoff Scheme 2 Decision module

Sensing physical layer and MAC layerSending message to Action module

Action moduleReceiving message from Decision modulePerforming handoff action

Interface between two modulesPre-handoff messageHandoff message

There only two message sequences allowedPre-handoff message followed by pre-handoff messagePre-handoff message followed by handoff message

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Main idea of Modularized Two Step Handoff Scheme 3 Handoff process include two step

Pre handoff step

handoff step When the HA receives the pre handoff message, it

begins to multi-tunnel the packet. It copies the IP packet destined to MN and sends them to the FA in WLAN and WWAN respectively.

When the mobile node meet the handoff condition, it execute handoff.

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4.1 Decision Module

Different with horizontal handoff Handoff from WWAN to WLAN Handoff from WLAN to WWAN

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Different with horizontal handoff

No comparable signal strength When moves from WWAN to WLAN, can

not be triggered by signal decay

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4.1.1 Handoff decision from WWAN to WLAN

Physical-layer sensing MAC-Layer Sensing Handoff decision scheme

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Physical-layer sensing

WLAN air interface will be periodically turned on to scan the signal of WLAN

Once the user moves into WLAN, a valid WLAN service set identifier (SSID) will be detected

If received signal strength index (RSSI) be larger than a preset threshold R1 then send pre-handoff message to Action module

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MAC-Layer Sensing Goal: detect the WLAN network condition, such as

available bandwidth and MAC-layer access delay NAV (Network Allocation Vector) is the main scheme

used in WLAN to avoid collision by setting a busy duration on hearing frame transmissions from other mobile hosts

By listening to and collecting the NAV in MAC layer, we can obtain the available bandwidth and access delay in MAC

Observe the NAV for T times and calculate the average NAV in this observation window. If the average NAV is smaller than a threshold N1, roaming into WLAN is granted; otherwise, the user stays in WWAN

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Relationship between NAV and Available bandwidth

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Relationship between NAV and Mean access delay

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Handoff decision scheme from WWAN to WLAN Detecting the RSSI signal, If RSSI>R1,

then begin collect NAV and send pre-handoff message to Action module.

If the average NAV <N1, then send handoff message to Action module

Else keep the connection with the WWAN

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4.1.2 Handoff decision from WLAN to WWAN The key problem is how to detect the

unavailability and the decay of the WLAN signal

we present an approach that integrate FFT-Based Decay Detection, NAV Occupation and simple physical signal strength for accurately detecting the unavailability of WLAN

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FFT-Based Decay Detection

Regard sin (-(2πn/N)) as a linear filter applied to the sequence x(N)

x (1) is the most smooth metric because sin(-2πn/N)) is the filter with the least high-frequency component

This will reduce the variation of x (1) even x (n) may vary severely

we can set a threshold F1 for x (1)/N. If x(1)/N is smaller than F1, the signal is considered to be decaying

)2

sin()()1(1

0

N

n N

nnxX

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MAC layer sensing

From NAV we can obtain the available bandwidth and access delay in MAC

Set the threshold of average NAV is N2 If the average NAV>N2, it mean that the

delay of WLAN is large, so the user send pre-handoff message to Action module

After short time T, send handoff message to Action module

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Physical Signal Sensing

Some time the signal of WLAN change very quickly. If the level of the RSSI is small then R3, the user will lose the connection

FFT is not very sensitive in this situation we select R4=R3+⊿ where ⊿ is the small amount

as a threshold. When the signal is less R4 send pre-handoff message to Action module, When the signal is less then R3 then send handoff message

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Handoff decision scheme from WLAN to WWAN a)   If RSSI<R4, then send pre-handoff message to

Action module;If RSSI<R3, send handoff messageIn this situation, the user need handoff immediately because the RSSI is too weak.

b)    If RSSI<R2, then begin FFT and send pre-handoff message to Action moduleIf the X (1)/N< F1 then send handoff message

c)    If the NAV occupation > N2 then send pre-handoff message to Action module, after time T send handoff message

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To avoid Ping-Pong effect

R1>R2>R4>R3; N1<N2

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4.2 Action module

Will be introduced by shimin

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5 Conclusion and future work For real time application a Modularized Two Step

Handoff Scheme is proposed for handoff between WWAN and WLAN

In this scheme we design two module and a set of protocols, compare other scheme, it is clear and easy to implement

In this scheme not only physical layer signal but also MAC layer signal be considered

Handoff delay is very little and packet loss can be reduced

Avoids ping-pong effect

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6 References [1]Buddhikot, M.M.; Chandranmenon, G.; Seungjae Han;

Yui-Wah Lee; Miller, S.; Salgarelli, L.; “Design and implementation of a WLAN/cdma2000 interworking architecture” IEEE Commun. Mag.,  Volume: 41 , Issue: 11 , pp. 90–100 Nov. 2003

[2] Q. Zhang, C. Guo, Z. Guo, and W. Zhu, “Efficient mobility management for vertical handoff between WWAN and WLAN,” IEEE Commun. Mag., vol. 41, pp. 102–108, Nov. 2003.

[3]Guo, C.; Guo, Z.; Zhang, Q.; Zhu, W.; “A Seamless and Proactive End-to-End Mobility Solution for Roaming Across Heterogeneous Wireless Networks” Selected Areas in Communications, IEEE Journal on , Volume: 22 , Issue:

5 , pp.834 - 848 June 2004 [4]M.Ye, “The mobile IP handoff between hybrid networks”, IEEE, 2002

Thank you!!!