Post on 31-Dec-2015
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Wireless Networking Using Grid
Douglas S. J. De Couto
http://www.pdos.lcs.mit.edu/grid
(revised)
Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
Goal: Build Networks from Chaos
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Examples:1. Rooftop networks2. Sensor networks3. Rapid deployment
Constraints:1. All wireless2. No centralized
infrastructure3. Mobile4. Scalable
Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
Grid Research Problems
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“A to J: Hello!”
Challenges:1. Routing
i. Forwarding ii. Path selection
2. Capacity3. Power
Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
Finding Good Routes
• Shortest path routing finds bad links
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HA’s maxrange
Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
Link Quality isn’t Bimodal
17 node indoor networkBroadcast 4-byte UDP packets
Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
Indoor Testbed
• 17 static nodes on 5th/6th floors
• iPaq handhelds
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wiredgateway
Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
Effects of Bad Links
• Problem: lossy links slow throughput and reduce capacity– 802.11 features positive ACKs & link-level retransmissions
– Lost packet transmissions waste time and spectrum
• Solution: choose routes other than shortest path– A longer route may have better links
– Use different metric than hopcount
Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
Proposed Route Metric: Transmission Count
• Tradeoff: longer route with fewer retransmits vs. shorter route with more retransmits
• Quantify tradeoff by estimated transmission count metric– Per-packet tx count = number of failed tx + number of successful tx
– Normally exactly 1 successful tx per packet at each hop
• Metric features– Route metric is sum of link metrics
– Directly measures spectrum use of route
– Estimate as tx_count = 1/(fwd_rate * rev_rate)
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Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
Geographic Forwarding
• Packets addressed to idG,locationG
• Next hop is chosen from neighbors to move packet geographically closer to destination location
• Per-node routing overhead constant as network size (nodes, area) grows
• Requires location service, which adds overhead
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Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
Grid Location Service (GLS)
• Each node has a few servers that know its location– 1. Node D sends location updates to its servers (B, H, K)
– 2. Node J sends a query for D to one of D’s close servers
• Spatial hierarchy makes GLS space and communications overhead O(log n)
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Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
The GLS Spatial Hierarchy
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All nodes agree on the global origin of the grid hierarchyAll nodes agree on square sizes
Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
Understanding Network Capacity
• Measure with “packet-hops”: number of simultaneous packet transmissions
• Total capacity scales with number of nodes– Spatial reuse allows capacity to scale with area
– Maximum node density adding nodes adds area
• Simulation result: 802.11 ad hoc total capacity can scale
• Per-node capacity depends on communications patterns– Global communication won’t scale
– Local communication will (e.g. GLS)
Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
Understanding Per-node Capacity
• Network provides O(n) packet-hops, n nodes
• “Random” communication won’t scale– Expected path length = O(sqrt n) each packet uses O(sqrt n) packet-
hops
– Per-node packet rate = n / (n * sqrt n) = O(1 / sqrt n)
• Local communication scales– Expected path length = O(1)
– Per-node packet rate = n / (n * 1) = O(1)
• GLS – Expected path length = O(log n)
– Per-node packet rate = n / (n * log n) = O(1 / log n)
Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
Grid Monitor
Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
Rooftop Testbed
• Omnidirectional antennas
• LCS/AI node has directional (yagi) antenna
LCS/AI
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Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
Grid Summary
• Grid protocols are– Self-configuring
– Easy to deploy
– Scalable
– Adaptable
• http://www.pdos.lcs.mit.edu/grid
Oxygen Alliance Annual Meeting — June 12 - 13, 2002Douglas S. J. De Couto — MIT Lab for Computer Science
SPAN: Reducing Power Consumption
• Reduce network power consumption by turning off radios
• Select coordinators to stay powered on– Maintain network connectivity
– Preserve capacity
• Routes composed of coordinator nodes
• Distributed election algorithm elects, rotates, and withdraws coordinators
• Simulation result: network lifetime doubled