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Wireless Capacity
A lot of hype Self-organizing sensor networks
reporting on everything everywhere Bluetooth personal networks
connecting devices City wide 802.11 networks run by
individuals and companies No more Cat5 in homes/businesses
Capacity As systems researchers, the most
glaring question is “Does this scale?”
What do we mean by scaling? What is the aggregate network
capacity? What is the per-node capacity for
node-originated data
Observed capacity Das et al. simulation of 100 nodes
2Mbps base throughput 7 simultaneous transmissions Per-node bandwidth few kbps
Others see similar capacity
Physical limit Competition for physical bandwidth
Signal power degrades with distance as 1/r for some
As an order of magnitude, in ns transmission range ~250 meters, interference ~550 meters
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Network capacity Upper bound total capacity,arbitrary
destination
Why? Intuitively, assuming constant density: total area/capacity ~n, diameter/average path length ~n
Global scheduling can achieve:
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nn
What is the limit? As density increases, the number
of nodes a packet interferes with increases Constant power, nodes per unit area
larger Lower power/more hops, total
transmissions increase
802.11 Chain propagation (simulation)
Achieve 1/7 of maximum 1.7Mbps Expected ¼ of maximum 1.7Mbps
MAC inefficiency? 802.11 works
until offered load exceeds capacity
Waste bandwidth at first node
Waste time backed off
Simulation vs. Reality
Solutions? Smaller networks?
Suggested in papers Only helpful if lower overall use
Add extra repeater nodes Requires exorbitant number of nodes Factor of k repeaters, k extra per-node capacity
Local communication patterns? Widespread base stations Local data processing
Be sneaky
Traffic patternPower law traffic pattern
Per-node capacityApproaches constantO(1/log(n)): GLS uses thisO(1/n)
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Be sneaky If we achieve three properties, we
should be able to get scalability All direct communication is local Message paths are short (preferably
O(1)) Squander no opportunities to send
Can we still achieve full connectivity? Maybe: Mobility
Mobility Nodes move randomly
Ergodic (uniform space filling) motion No proof that this is NECESSARY
Persistent communication patterns Random source/destination patterns Unlimited data
Buffering Nodes can buffer data
Mobility To achieve scalability, we want
three properties All direct communication is local
Send messages only to nearest neighbor Distant communication depends on
chance movement Message paths are short (preferably
O(1)) Squander no opportunities
Mobility To achieve scalability, we want
three properties All direct communication is local Message paths are short (preferably
O(1)) Never forward along paths longer than 2
hops Squander no opportunities
Mobility To achieve scalability, we want three
properties All direct communication is local Message paths are short (preferably O(1)) Squander no opportunities Send data
through everyone Whenever you are near any node, give it a (new)
packet for the destination. On average should have data for every possible
destination
Requirements Know closest node/range Schedule local transmissions
They found the standard MAC may be ok
Buffering Scales with radio bandwidth? Scales with expected time to see a
destination node?
Model Is this useful?
Potentially very long time to delivery Potentially wide variance in delivery times Unknown dependence on movement model
Space filling unrealistic(destructive to homes) Another submission claims that travel along random
line segments also works Unclear generalization to multiple hops Static population model/bounded movement model
unrealistic for many random movement models Existing applications seem unlikely
consumers
What next? Radio people
MAC layers tuned to ad hoc mode Wasn’t clear from results presented this
is more than a moderate constant factor Systems/applications people
Communication patterns with good locality
Take advantage of external sources of bandwidth (fiber optics or station wagons of tapes)