Fast Handoff for Seamless wireless mesh Networks

Yair Amir, Clauiu Danilov, Michael Hilsdale

Mobisys’ 06

2006.09.12

Jeon, Seung-woo

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Contents

▣ Introduction

▣ The SMesh architecture

▣ Fast Handoff Protocol

▣ Experimental Results

▣ Conclusion

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Introduction

▣ The handoff Also called ‘handover’ in Europe region A service for mobile clients of one access point which is trying to

move to another coverage of access point Before routing packets through new access point, the handoff

triggering should occur

▣ Requirement Completely transparent to mobile clients No interruption in network connectivity (Seamless) Communication protocols of all mobile clients should not be

unchanged

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Introduction

▣ Handoff in network

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Introduction

▣ Ping-pong problem in handoff

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Introduction

▣ The handoff in cellular network Many techniques are suggested for ensuring connectivity

• Uses embedded signaling, user location for handoff

▣ The handoff in wireless 802.11 mesh network There are no efficient, transparent handoff solutions

• Required specific modified mobile clients for connectivity

• Not sufficient to support real-time application traffic– Voice over IP, video conference

– Delay is most significant factor in real time traffic

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Introduction

▣ The SMesh network Ensures that each clients is served by at least one access point at

any time• Creates wireless mesh topology in which only some of the access points

may be connected to the Internet

Completely transparent wireless mesh network system• The entire mesh network is seen by the mobile clients as a single,

omnipresent access point

Supports peer-to-peer communication between mobile clients • provides automatic routing for clients connected to the mesh

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Introduction

▣ The innovative aspects of the handoff techniques Seamless and fast handoff with supporting real-time applications Novel use of the DHCP protocol

• Monitoring connectivity quality of mobile clients

• Creating a single, virtual access point

Novel use of multicast for robust mesh Internet gateway Novel use of anycast for mobile client to mesh Internet gateway

communication

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SMesh Architecture

Communication

Infrastructure

Interface with

Mobile Clients

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SMesh Architecture

▣ Communication Infrastructure Based on “Spines” messaging system SMash initiates a Spines daemon on each wireless mesh node

• Keeps track of its own direct neighbors by sending periodic messages

Allows to use multicast and anycast functionality in a multi-hop wireless environment

• Multicast group– Automatically builds multicast trees to flood data

• Anycast group

• The groups are defined in the Spines virtual addressing space

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SMesh Architecture

▣ Interface with Mobile clients DHCP server running at each mesh node

• Providing connectivity information to clients through DHCP

Mesh node serves as a default gateway• Packet proxy module as a interceptor and raw socket interfaces

Packet delivery group • Client data group

– Each client is associated with client data group to receive data

• Destination data group– Internet gateways is associated with destination data group to transmit data

Network Address Translation (NAT)

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Fast Handoff Protocol

▣ Handoff in SMesh network Provided by using the default DHCP setting SMesh nodes track their connectivity to the clients

• Forces the clients to change its access point whenever available

Provides clients an illusion of a single global IP• Default gateway and gratuitous ARP messages to force handoff

Access points and mobile clients act in Ad-hoc mode• The access point controls the handoff solely

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SMesh network Handoff

Client control Group

DHCP Packet

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Fast Handoff Protocol

▣ Link Quality Metric Uses DHCP protocol to keep track of the quality of the link to

mobile clients The clients broadcast DHCP requests every 2 seconds Access nodes computes a clients link quality measures

• Based on the observed loss of a client’s DHCP requests

• Checks the number of DHCP requests received

Decay factor (default : 0.85)

Quality Metric If receive packet, set to constant value else, set to 0

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Fast Handoff Protocol

▣ Client Management Each clients has two multicast group Client data group

• Used to forward data packets toward the access points

Client control group• Used to coordinate with other mesh nodes in the client’s vicinity

• A mesh node joins a client’s control group when receives DHCP packets and leaves after not hearing packets for some times

• More than one access nodes which believe it has the best link quality receive and forward packets to the group’s correspondent client

– Duplicated packets are transmitted and received

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Fast Handoff Protocol

▣ Data Group Membership Balances the connectivity and the size of client data group Mesh nodes Join and leave the data group by measures link quality Set threshold in order to decrease oscillations of the Data Group

membership

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Fast Handoff Protocol

▣ Single IP Gateway Mesh nodes advertise a single virtual gateway IP address to all

clients in their DHCP offers and acknowledgements• Mobile clients set their default gateway to this virtual IP address regardless

of access points

• Mobile clients get the illusion of being connected to a single access point

▣ Gratuitous ARP ARP reply that is not sent as a reply to an ARP request, but rather

is broadcasted to the local network voluntarily• Communicates in a client control group by ARP messages

• Mesh nodes advertise to clients which node has the best link quality link by ARP messages

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Fast Handoff Protocol

Client control group

ARP message

DHCP message

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Experimental Results

▣ For stationary clients

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Experimental Results

▣ For stationary clients

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Experimental Results

▣ For moving clients

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Experimental Results

▣ For moving clients

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Experimental Results

▣ For moving clients

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Experimental Results

▣ For moving clients

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Conclusion

▣ Novel technique for wireless mesh network handoff Ensure seamless and fast handoff

▣ Novel use of DHCP combined ARP messages Creates single virtual access point Supports any standard 802.11 devices

▣ What is the problem?