ExOR: Opportunistic Multi-Hop Routing For Wireless …Gaurav Gupta Motivation Motivation If the...
Transcript of ExOR: Opportunistic Multi-Hop Routing For Wireless …Gaurav Gupta Motivation Motivation If the...
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ExOR: Opportunistic Multi-Hop Routing For Wireless Networks
Presented By:
Gaurav Gupta
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Motivation
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Motivation
If the end-to-end capacity is the most concerned, Is the framework of traditional deterministic
hop-by-hop unicast-based routing the best approach?
If there is another way around to improve the e2e capacity, what would it be?
What about the simple definition? Throughput = number of delivered packets per
second
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Motivation
Intuition is that ... The best path for unicast is determined by
typically the expectation of overall link quality
A physical topology of wireless link may or may not support this assumption
What about using multi-path? Whatever path available at whatever link quality Implications?
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Motivation
Using multiple paths require: Need to know what paths are available at
what expected throughput (quality) Need to schedule packet transmission on
multiple paths Who to reassemble all the sent packets? Reassembly may need per-packet state at
intermediate and the destination nodes
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New Approach
Opportunistic packet transmission! Whoever receives this, please forward it
toward the destination Table driven routing Cooperative transfer Target traffic
Bulk transfer (web traffic?) Interactive traffic is out of the scope (ssh)
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Outline
Basics of ExOR Introduction Functioning Comparison
Design Challenges Packet Buffer Forwarder List Batch Maps Transmission Tracker Forwarding Timer
Evaluation Setup Results
Advantages/Limitations
Conclusion
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Introduction
Its an integrated routing and MAC protocol
Used in multi-hop wireless networks
Delayed Forwarding
Each hop of packet’s route is chosen after transmission for that hop
Operates on batches of packets
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Functioning
Source broadcasts each packet
A subset of nodes receive the packet
Subset runs a protocol to find who all are in the subset
Node closest to destination broadcasts the packet
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Functioning (contd…)
source destination
continues……
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Comparing ExOR
Traditional Routing: One path followed from source to destination All packets sent along that path
Co-operative Diversity: Broadcast of packets by all nodes Destination chooses the best one
ExOR: Broadcast packets to all nodes Only one node forwards the packet Basic idea is delayed forwarding
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ExOR Vs. Traditional Routing
Each transmission may have more independent chances of being received or forwarded
Transmissions may reach unexpectedly far or fall unexpectedly short
source destinationA CB D
X Traditional routing
ExOR
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Design Challenges
Determine which subset received the broadcast
Decide on a forwarder among the subset
In large networks, choosing too many potential forwarders incurs greater cost for agreement
Avoiding simultaneous transmissions (collisions)
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Maintaining State For Each Packet
Packet Buffer Local Forwarder List Batch Map Transmission Tracker Forwarding Timer
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Packet Buffer
Place where each node stores the packets that it receives
Packets are stored according to their batch numbers
If the node is the selected forwarder, all packets for that batch are sent out from the buffer
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Forwarder List (FL)
Prioritized List of Nodes Based on expected cost of packet
delivery to destination
Specified by source For a batch, all nodes use the same list
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Batch Map (BM)
Highest priority node known to have received a copy of the packet
Sent for each packet in the batch
Guessed by the current forwarder
Included in the broadcast message
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Transmission Tracker (TT)
Measures sending rate of current forwarder
Tells the expected number of packets left to be sent by the current forwarder
Used to adjust the forwarding timer
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Forwarding Timer (FT)
Indicates the time at which the node should start sending
Set far enough to account for higher priority nodes
Adjusted when packets from other nodes are heard
Predicted value
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Design Challenges (Revisited)
Determine which subset received the broadcast
Decide on a forwarder among the subset
In large networks, choosing too many potential forwarders incurs greater cost for agreement
Avoiding simultaneous transmissions (collisions)
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Solution
FL is included in each packet
Receiver examines packets that it receives:
If FL includes the recipient, the packet is buffered for the corresponding batch
For each entry in BM of packet, if packet’s entry > corresponding entry in local BM => replace local BM
Highest priority forwarder forwards the packets in its buffer
Forwarder also forwards its own BM for each packet
Remaining forwarders transmit packets not acknowledged in BMs of higher priority nodes, in order
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Evaluation: Setup
Roofnet
Outdoor rooftop 802.11b network
38 nodes distributed over 6 sq. kilometers of Cambridge
Each node a PC
With 802.11b card connected to a roof mounted omni-directional antenna
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Evaluation: Setup (contd…)
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Evaluation: Configuration
Batch size – 100 packets
Each packet – 1024 byte payload
ExOR header – 44 - 114 bytes
Network bit rate – 1Mb/sec.
Experiment is performed between 65 randomly selected node pairs
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Transmission time-line
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Results: Throughput
ExOR – 33 Kbytes/sec. Traditional – 11 Kbytes/sec. ExOR achieves much higher throughput than traditional routing.
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Results: Throughput Vs. Distance25 highest throughput pairs
ExOR performs much better than traditional routing even for shorter routes.
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Results: Throughput Vs. Distance (contd…)
25 lowest throughput pairs
ExOR’s throughput increases as the route length increases.
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Results: Summary
As route length increases, ExOR’s performance increases Likelihood of finding additional forwarding nodes
increases
ExOR is able to use long asymmetric links Deliver many packets in forward direction but few packets
in reverse direction
ExOR batch maps and data packets can follow different paths
Traditional Routing needs the same path for both the data packet and the ACK
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Advantages
Transmits each packet fewer times
Increases total network capacity
Increases individual connection throughput
Each packet is transmitted fewer times, so less interference for other users
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Limitations
Selection of potential forwarders can be tricky
As size of FL grows Size of BM also grows Size of ExOR header also grows
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Conclusion
Outperforms traditional routing by increasing network throughput considerably
Can use long radio links with high loss rates
Uses no more network capacity than traditional routing
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Questions / Comments ?