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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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)