ExOR : Opportunistic Multi-hop Routing for Wireless Networks

Sanjit Biswas and Robert MorrisM.I.T. Computer Science and Artificial Intelligence Laboratory

Presented by : Arpan Roy

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Acknowledgements

• The results data have been taken from the original slides of Sanjit Biswas and Robert Morris.

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Index

Background

Basic Idea

Proposed Protocol

Results

Discussions

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Routing in wireless networks

• In the initial stages, wireless routing treaded the same path that wired routing took. Extensive research led to proactive protocols like DSDV and later to reactive ones like DSR in this regard.

• Reality Injection - The ETX metric was brought into the picture to take into account the effects of lossy, assymetric links in wireless networks.

• Ultimate aim - Reliability through diversity• Finally ETX and some of the advantages of cooperative

diversity were put together and ExOR evolved – a combined routing and MAC protocol for wireless networks, that was found to increase the throughput.

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Index

Background

Basic Idea

Proposed Protocol

Results

Discussions

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Overview• For every source-target pair, the source

broadcasts the packet. • Some subset of the wireless nodes between the

source and the destination is supposed to receive this packet. These nodes run a protocol to discover and agree as to which nodes are in that sub-set.

• The node in that sub-set that is closest to the destination again broadcasts that packet. Again the nodes that receive this packet agree on the closest receiver and forward that packet. This chain continues until the packet reaches its destination.

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What is opportunistic about ExOR ?

• Can make the best out of a bad situation & also reap benefits from some new found reception luck. Traditional routing can’t do this.

A B C D E

ExORTraditional

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Only Assumption

• Reception of packets from the same broadcast at different nodes in the subset are independent of each other.

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Index

Background

Basic Idea

Proposed Protocol

Results

Discussions

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Issues

Four primary issues choice of a cooperative agreement protocol

that is robust and yet provide negligible communication overhead. (Solution – Batches)

metric that is used to reflect the right choice of the forwarder (Solution – ETX)

in a large dense network how to reduce the overhead in selecting a subset (Solution – Thresholding)

reduce collisions (Solution – Scheduling)

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ETX in ExOR• ETX is a good metric because it takes into

consideration lossy, assymmetric links and interference between successive links.

• If a node has multiple paths to the destination,

the links associated with minimum ETX values is chosen for forwarding.

• By ETX, we prioritize an estimate of the cost to the destination.

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Batches• ExOR protocol operates on batches of

packets in order to reduce the communication cost of agreement.

• Every packet in a batch contains its senders’ snapshot of the network scenario.

• Every receiving node maintains its own snapshot of the network scenario.

• With the reception of each packet this keeps getting updated.

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Node State• Every node maintains a state for each batch of packets. Packet Buffer – It stores successfully received packets in

the current batch. Local Forwarder List – It contains a copy of the prioritized

list of nodes copied from one of the packets in the packet buffer. (*)

Forwarding Timer – It indicates the time at which the node predicts that it should start forwarding packets from its packet buffer. Governed by its priority.

Transmission tracker – It keeps track of the current rate of data sending and the number of packets left to transmit.

Batch map – It indicates for each packet in a batch the highest priority node to have received a copy of that packet.

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The ExOR packet header

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Batches

• Transmission – Source and subsequently every intermediate node gathers a batch of packets headed to the same destination. Then each packet is broadcast.

• Reception - Receiving node check to see if it is on forwarder list. If so, it checks the batch map. If no higher priority node has received the packet yet, it adds the packet to its corresponding packet buffer. It may also update the receiving node’s local batch map if any of the packets indicates reception by a higher-priority node.

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Batch ACK

• Batch map field indicates the highest priority node that is known to have received a copy of that packet.

• The source knows that the batch has been successfully received when 90% of the batch map entries are filled with IDs of higher priority nodes.

• No ACK needed.• After 90% of the packets have been sent, rest

10% of the packets is sent using traditional routing using link level ACKs.

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Thresholding

• The problem in the case of dense networks is that the expected number of the batch’s packets that any node is responsible for forwarding might come down to zero.

• So the source runs an ExOR simulation based on the link loss probabilities and selects only the nodes that transmit at least 10% of the total transmission in a batch.

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Scheduling

• Since ExOR tends to opportunistically select the occasional longer link, virtual carrier sensing might not work that well.

• To avoid collision, ExOR performs scheduling that allows higher priority nodes to send first. This in turn allows for the batch maps of lower priority nodes to get updated.

• The forwarding timer is involved in accomplishing this.

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Index

Background

Basic Idea

Proposed Protocol

Results

Discussions

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Throughput : 2x overall improvement

• Median throughputs: 240 Kbits/sec for ExOR, 121 Kbits/sec for Traditional

Throughput (Kbits/sec)

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ExORTraditional

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Distance per transmission

• ExOR is able to move packets farther.• ExOR average: 422 meters/transmission• Traditional Routing average: 205 meters/tx 21

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0.6 ExORTraditional Routing

0 100 200 300 400 500 600 700 800 900 1000

58% of Traditional Routing transmissions

25% of ExOR transmissions

Distance

Index

Background

Basic Idea

Proposed Protocol

Results

Discussions

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Summary

• ExOR capitalizes on the nature of the wireless medium.

• ExOR was implemented on RoofNet testbed.• ExOR increases throughput by a factor of two

or four times over that of traditional wireless routing.

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ExOR CaveatsThe forwarder list is chosen with the help of the

knowledge of inter-node loss rates over the whole network. This needs a periodic round of link state flooding of per-node measurements.

Candidate selection is tricky (How often is ETX updated)

May require changes to the MAC protocol. How exactly does ExOR compare to cooperative

diversity schemes ?Selection of the best data rate

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Recent Developments

• Sanjit Biswas is currently the CEO of his own start-up, Meraki Networks. Biswas's protocol, combined with commonly available hardware components, allows Meraki to produce Wi-Fi routers that cost as little as $50. (The routers Biswas originally used at MIT, initially cost $1,500.)

• Here's how a Meraki network actually works: a user plugs a router into a broadband Internet connection; that person's neighbors stick routers to their windows, and a mesh network of up to hundreds of people forms automatically. Users can give away or sell Internet access to their neighbors. There are already Meraki-based networks in 95 countries, from Slovakia to Venezuela, serving more than 15,000 users.

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Questions

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