Ad hoc and Sensor Networks

Chapter 3: Network architecture

Holger Karl

Computer Networks Group

Universität Paderborn
Goals of this chapter
• Having looked at the individual nodes in the previous
chapter, we look at general principles and architectures
how to put these nodes together to form a meaningful
network
• We will look at design approaches to both the more
conventional ad hoc networks and the non-standard WSNs

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Outline
• Network scenarios
• Optimization goals
• Design principles
• Service interface
• Gateway concepts

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Basic scenarios: Ad hoc networks
• (Mobile) ad hoc scenarios
• Nodes talking to each other
• Nodes talking to “some” node in another network (Web server on
the Internet, e.g.)
• Typically requires some connection to the fixed network
• Applications: Traditional data (http, ftp, collaborative apps, …) &
multimedia (voice, video) ! humans in the loop

o c
h
ad
Access Point

© J. Schiller

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Basic scenarios: sensor networks
• Sensor network scenarios
• Sources: Any entity that provides data/measurements
• Sinks: Nodes where information is required
• Belongs to the sensor network as such
• Is an external entity, e.g., a PDA, but directly connected to the WSN
• Main difference: comes and goes, often moves around, …
• Is part of an external network (e.g., internet), somehow connected to
the WSN
Source
Source Source

Inter
Sink Sink Sink net
• Applications: Usually, machine to machine, often limited amounts
of data, different notions of importance

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Single-hop vs. multi-hop networks
• One common problem: limited range of wireless communication
• Essentially due to limited transmission power, path loss, obstacles
• Option: multi-hop networks
• Send packets to an intermediate node
• Intermediate node forwards packet to its destination
• Store-and-forward multi-hop network
• Basic technique applies to
both WSN and MANET
• Note: Store&forward multi-
hopping NOT the only
possible solution
• E.g., collaborative
networking, network Sink
coding Source Obstacle
• Do not operate on a per-
packet basis

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Energy efficiency of multi-hopping?
• Obvious idea: Multi-hopping is more energy-efficient than
direct communication
• Because of path loss α > 2, energy for distance d is reduced from
cdα to 2c(d/2)α
• c some constant

• However: This is usually wrong, or at least very over-

simplified
• Need to take constant offsets for powering transmitter, receiver into
account
• Details see exercise, chapter 2

! Multi-hopping for energy savings needs careful choice

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WSN: Multiple sinks, multiple sources

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Different sources of mobility
• Node mobility
• A node participating as source/sink (or destination) or a relay node
might move around
• Deliberately, self-propelled or by external force; targeted or at
random
• Happens in both WSN and MANET
• Sink mobility
• In WSN, a sink that is not part of the WSN might move
• Mobile requester
• Event mobility
• In WSN, event that is to be observed moves around (or extends,
shrinks)
• Different WSN nodes become “responsible” for surveillance of
such an event

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WSN sink mobility

Request

Propagation
of answers

Movement
direction

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 WSN event mobility: Track the pink elephant

Here: Frisbee model as example

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Outline
• Network scenarios
• Optimization goals
• Design principles
• Service interface
• Gateway concepts

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Optimization goal: Quality of Service
• In MANET: Usual QoS interpretation
• Throughput/delay/jitter
• High perceived QoS for multimedia applications
• In WSN, more complicated
• Event detection/reporting probability
• Event classification error, detection delay
• Probability of missing a periodic report
• Approximation accuracy (e.g, when WSN constructs a temperature
map)
• Tracking accuracy (e.g., difference between true and conjectured
position of the pink elephant)

• Related goal: robustness

• Network should withstand failure of some nodes

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Optimization goal: Energy efficiency
• Umbrella term!
• Energy per correctly received bit
• Counting all the overheads, in intermediate nodes, etc.
• Energy per reported (unique) event
• After all, information is important, not payload bits!
• Typical for WSN
• Delay/energy tradeoffs
• Network lifetime
• Time to first node failure
• Network half-life (how long until 50% of the nodes died?)
• Time to partition
• Time to loss of coverage
• Time to failure of first event notification

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Optimization goal: Scalability
• Network should be operational regardless of number of
nodes
• At high efficiency
• Typical node numbers difficult to guess
• MANETs: 10s to 100s
• WSNs: 10s to 1000s, maybe more (although few people have seen
such a network before…)

• Requiring to scale to large node numbers has serious

consequences for network architecture
• Might not result in the most efficient solutions for small networks!
• Carefully consider actual application needs before looking for
n ! 1 solutions!

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Outline
• Network scenarios
• Optimization goals
• Design principles
• Service interface
• Gateway concepts

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Distributed organization
• Participants in a MANET/WSN should cooperate in
organizing the network
• E.g., with respect to medium access, routing, …
• Centralistic approach as alternative usually not feasible – hinders
scalability, robustness

• Potential shortcomings
• Not clear whether distributed or centralistic organization achieves
better energy efficiency (when taking all overheads into account)

• Option: “limited centralized” solution

• Elect nodes for local coordination/control
• Perhaps rotate this function over time

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In-network processing
• MANETs are supposed to deliver bits from one end to the
other
• WSNs, on the other end, are expected to provide
information, not necessarily original bits
• Gives addition options
• E.g., manipulate or process the data in the network
• Main example: aggregation
• Apply composable aggregation functions to a convergecast tree in
a network
• Typical functions: minimum, maximum, average, sum, …
• Not amenable functions: median

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In-network processing: Aggregation example
• Reduce number of transmitted bits/packets by applying an
aggregation function in the network

1 1

1
1
3 1
1 1
6 1

1 1

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In-network processing: signal processing
• Depending on application, more sophisticated processing
of data can take place within the network
• Example edge detection: locally exchange raw data with
neighboring nodes, compute edges, only communicate edge
description to far away data sinks
• Example tracking/angle detection of signal source: Conceive of
sensor nodes as a distributed microphone array, use it to compute
the angle of a single source, only communicate this angle, not all
the raw data
• Exploit temporal and spatial correlation
• Observed signals might vary only slowly in time ! no need to
transmit all data at full rate all the time
• Signals of neighboring nodes are often quite similar ! only try to
transmit differences (details a bit complicated, see later)

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Adaptive fidelity
• Adapt the effort with which data is exchanged to the
currently required accuracy/fidelity
• Example event detection
• When there is no event, only very rarely send short “all is well”
messages
• When event occurs, increase rate of message exchanges
• Example temperature
• When temperature is in acceptable range, only send temperature
values at low resolution
• When temperature becomes high, increase resolution and thus
message length

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Data centric networking
• In typical networks (including ad hoc networks), network
transactions are addressed to the identities of specific
nodes
• A “node-centric” or “address-centric” networking paradigm
• In a redundantly deployed sensor networks, specific source
of an event, alarm, etc. might not be important
• Redundancy: e.g., several nodes can observe the same area
• Thus: focus networking transactions on the data directly
instead of their senders and transmitters ! data-centric
networking
• Principal design change

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Implementation options for data-centric networking
• Overlay networks & distributed hash tables (DHT)
• Hash table: content-addressable memory
• Retrieve data from an unknown source, like in peer-to-peer networking –
with efficient implementation
• Some disparities remain
• Static key in DHT, dynamic changes in WSN
• DHTs typically ignore issues like hop count or distance between nodes when
performing a lookup operation
• Publish/subscribe
• Different interaction paradigm
• Nodes can publish data, can subscribe to any particular kind of data
• Once data of a certain type has been published, it is delivered to all
subscribes
• Subscription and publication are decoupled in time; subscriber and
published are agnostic of each other (decoupled in identity)
• Databases

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Further design principles
• Exploit location information
• Required anyways for many applications; can considerably
increase performance
• Exploit activity patterns
• Exploit heterogeneity
• By construction: nodes of different types in the network
• By evolution: some nodes had to perform more tasks and have
less energy left; some nodes received more solar energy than
others; …
• Cross-layer optimization of protocol stacks for WSN
• Goes against grain of standard networking; but promises big
performance gains
• Also applicable to other networks like ad hoc; usually at least
worthwhile to consider for most wireless networks

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Outline
• Network scenarios
• Optimization goals
• Design principles
• Service interface
• Gateway concepts

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Interfaces to protocol stacks
• The world’s all-purpose network interface: sockets
• Good for transmitting data from one sender to one receiver
• Not well matched to WSN needs (ok for ad hoc networks)
• Expressibility requirements
• Support for simple request/response interactions
• Support for asynchronous event notification
• Different ways for identifying addressee of data
• By location, by observed values, implicitly by some other form of group
membership
• By some semantically meaningful form – “room 123”
• Easy accessibility of in-network processing functions
• Formulate complex events – events defined only by several nodes
• Allow to specify accuracy & timeliness requirements
• Access node/network status information (e.g., battery level)
• Security, management functionality, …
• No clear standard has emerged yet – many competing, unclear
proposals

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Outline
• Network scenarios
• Optimization goals
• Design principles
• Service interface
• Gateway concepts

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Gateway concepts for WSN/MANET
• Gateways are necessary to the Internet for remote access
to/from the WSN
• Same is true for ad hoc networks; additional complications due to
mobility (change route to the gateway; use different gateways)
• WSN: Additionally bridge the gap between different interaction
semantics (data vs. address-centric networking) in the gateway
• Gateway needs support for different radios/protocols, …

Internet Remote
users
Gateway
node

Wireless sensor network

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WSN to Internet communication
• Example: Deliver an alarm message to an Internet host
• Issues
• Need to find a gateway (integrates routing & service discovery)
• Choose “best” gateway if several are available
• How to find Alice or Alice’s IP?
Alert Alice
Alice‘s desktop

Internet
Gateway
nodes

Alice‘s PDA
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Internet to WSN communication
• How to find the right WSN to answer a need?
• How to translate from IP protocols to WSN protocols,
semantics?

Remote requester

Internet Gateway
Gateway
nodes

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WSN tunneling
• Use the Internet to “tunnel” WSN packets between two
remote WSNs

Internet

Gateway Gateway
nodes

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Summary
• Network architectures for ad hoc networks are – in principle
– relatively straightforward and similar to standard
networks
• Mobility is compensated for by appropriate protocols, but
interaction paradigms don’t change too much
• WSNs, on the other hand, look quite different on many
levels
• Data-centric paradigm, the need and the possibility to manipulate
data as it travels through the network opens new possibilities for
protocol design

• The following chapters will look at how these ideas are

realized by actual protocols

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