Post on 27-Dec-2015
transcript
Sensor Networks
Pete Perlegos
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Outline
Background Ad-hoc Wireless Networks Smart Dust – TinyOS PicoRadio
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How are they possible? Moore’s Law:
The historical trend…
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Lets look at the other side
Moore’s Law is also pushing a given functionality into a smaller, cheaper, lower-power unit.
486DX 1989 486DX 2001
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Other Trends
Complete systems on a chip (SoC) Integrated low-power communication
RF, optical Integrated low-power transducers
power capacitor, solar cell, battery Integrated sensors
Detect light, heat, position, movement, chemical presence, etc.
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Other Trends
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Outline
Background Ad-hoc Wireless Networks Smart Dust – TinyOS PicoRadio
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Ad-hoc Wireless Networks
No base stations or infrastructure required
Multi-hop wireless networks Each node can talk with a neighbor
Applications Sensor networks Intelligent control applications
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Ad-hoc Wireless Networks
MAC schemes Addressing Routing
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Geographical Routing Algorithm
Geographicalnetwork
Assumptions: Each node knows its own position and its neighbors’
position Nodes don’t know the global topology Destination address is a geographical position to
which the packet is to be delivered
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A Simple Routing Algorithm
Routing Decision: Route to the neighbor which is nearest to the packet destination
Source
Destination
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Problem with Simple Routing
Source
DestinationWall
Simple routing does not always work The Geographical routing algorithm is an
extension of the simple routing algorithm
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Route Discovery
Packet gets “stuck” when a node does not have a neighbor to which it can forward the packet
When a packet is stuck, a Route Discovery is started to destination D
A path is found to D Entry [position(D), s(i+1)]
is added to the routing table of s(i) Source
DestinationWall
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Routing Tables
Routing Table for Station n:
(x,y) position Neighbor
d(8,6)
b
Position of n -
Position of neighbor a a Routing Algorithm:
• Packet arrives for position p at node n• Node n finds the position to which p is closest and . forwards to the . corresponding neighbor
Position of neighbor b
Routing Tables: Routing tables contain some additional entries beside . neighbors
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Example
Pos(A) = (1,1)Pos(B) = (2,2)Pos(C) = (3,1)
Links:A ---- BB ---- C
A
B
C
Pos(A) ---
Pos(B) B
Pos(B) ---
Pos(A) APos(C) C
Pos(C) ---
Pos(B) B
A gets a packet for Pos(C) A forwards it to B because pos(B) is closer to pos(C) B forwards it to C because pos(C) is closer to pos(C)
Pos(C)
Pos(C)
Pos(C)
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Route Discovery
Pos(A) = (1,1)Pos(B) = (2,2)Pos(C) = (3,1)Pos(D) = (2.5,0)Links:A ---- BB ---- CC ---- D
B
C
A gets a packet for Pos(D) Packet gets stuck at A because Pos(A) is closest to Pos(D) Initiate route discovery for D from A Update the routing tables and forward the packet
Pos(D)
Pos(D)
A
D
Pos(A) ---
Pos(B) B
Pos(D) ---
Pos(C) C
Pos(B) ---
Pos(A) A
Pos(C) C
Pos(C) ---
Pos(B) B
Pos(D) D
Pos(D) B
Pos(D) C
Pos(D)
Pos(D)
Pos(D)
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Desirable Properties of Location Service
Spread load evenly over all nodes. Degrade gracefully as nodes fail. Queries for nearby nodes stay
local. Per-node storage and
communication costs grow slowly as the network size grows
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Grid Location Service (GLS)
n
s
ss
s
s
s
s
s s
s is n’s successor in that square. (Successor is the node with “least ID greater than” n )
sibling level-0squares
sibling level-1squares
sibling level-2squares
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GLS Updates
... 1
...
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23, 2
11, 2
6
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location table content
location update
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query from 23 for 1
GLS Query
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Outline
Background Ad-hoc Wireless Networks Smart Dust – TinyOS PicoRadio
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Active Messages - Benefits
Event based model: Avoids busy waiting for data to arrive Allows the system overlap communication
with computation
Lightweight architecture: Balances the need for extensible
communication network while maintaining efficiency and agility
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Active Messages – What are they?
Each Active Message contains: the name of a user-level handler to be
invoked on a target node upon arrival a data payload to pass in as arguments
The handler function serves a dual purpose: extracting the message from the network either integrating the data into the
computation or sending a response message
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Active Messages – What are they?
The network is modeled as a pipeline with minimal buffering for messages. This eliminates many of the buffering
difficulties faced by communication schemes that use blocking protocols or special send/receive buffers.
To prevent network congestion and ensure adequate performance, message handlers must be able to execute quickly and asynchronously.
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Event Based Programming Event handlers are invoked to deal with
hardware events (directly or indirectly). The lowest level components have handlers
connected directly to hardware interrupts: external interrupts, timer events, or counter
events Events propagate up through the component
hierarchy. To perform long-running computation,
components request to have tasks executed on their behalf.
Once executed by scheduler, tasks run to completion and execute autonomously with other tasks.
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Event Based Programming
Events propagate up through the component hierarchy.
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Processor Utilization
The vast majority of the power consumption occurs in the active state, with very little power used in the idle state
The system should embrace the philosophy of getting work done as quickly as possible and going to sleep
This is a great benefit of the event based model
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Outline
Background Ad-hoc Wireless Networks Smart Dust – TinyOS PicoRadio
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PicoRadio The ever-evolving scaling of the semiconductor
technology is enabling the co-integration of the interfacing, computation, position location, and communication functions into a single silicon circuit.
Benefits of the system-on-a-chip approach: Maximally reduces the size of the sensor node Allows the use of advanced circuit architectures
which provide the optimal trade-off between flexibility and energy-efficiency
The tight integration of communication and computation functions into a single chip will provide the desired functionality at the lowest possible cost and energy.
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PicoRadio A range of technologies are still necessary for the
realization of ultra-low energy wireless sensor networks: The study of multi-hop networks, and MAC layers that
support low (but variable)-rate data transmission, while ensuring low energy-consumption.
Chip architectures that enable the implementation of these advanced algorithms. (A heterogeneous combination of programmable, configurable, and fixed components.)
Mapping the advanced networking and communication algorithms onto such an architecture is a real design methodology problem.
Ensuring and verifying that these distributed and embedded systems will behave correctly is especially hard.
An RF front-end that meets the demands of variable bit-rates and energy-efficiency.
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Protocol Stack
Specification of protocol stack derived from: System requirements (top-down) Wireless channel properties (bottom-up)
UI
MAC
Transmit Receive
Synchronization
Filter
Tx_data Rx_data
Mulaw Mulaw
Transport
User Interface Layer
Transport Layer
Mac Layer
Data Link Layer
Voice samples
Tx/Rx
Service Requests
UI
MAC
Transmit - CRC Receive - CRC
Synchronization
Filter
Tx_data Rx_data
Mulaw Mulaw
Transport
User Interface Layer
Transport Layer
Mac Layer
Logical Link Layer
Voice samples
Tx/Rx
Service Requests
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Energy Consumption
Multi-hop Networks and Low-Energy Consumption
Using several short hops to send a bit is more energy efficient than using one longer hop
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Energy Consumption
Multi-hop Networks and Low-Energy Consumption
Energy optimal number of hops as a function of distance
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Architecture
Conceptual PicoNode chip architecture
Allows for flexibility Trade-off between flexibility and energy
efficiency must be managed
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Conclusion
We seem to be getting close to realizing networks of sensors. Lightweight, event-based
communication Shrinking die size (lower power and
cost) Advancement of system on a chip
development