Sensor Network Final Exam

Front 1

Name all requirements for sensor networks.

Back 1

• Energy efficiency
• Memory capacity
• Computing performance
• Bandwidth
• Quality of Service
• Fault tolerant

• Life-time

• Scalability

• Sensor density

• Programmability

• Maintainability

Front 2

What classes of sensor networks are available?

Back 2

• Wired/hybrid type: fieldbus systems
• Wireless sensor networks – WSN

Front 3

What is DCS? / What is CCS?

Back 3

• Distributed Control System
• Centralized Control System

Front 4

Draw picture of CCS/DCS.

Back 4

Front 5

Define sensor network.

Back 5

A sensor network consists of numerous intelligent devices (sensor nodes) that are distributed building a network.

Front 6

What topologies are possible in Fieldbus?

Back 6

• Bus
• Ring
• Tree

Front 7

What kind of types are available in WSN?

Back 7

• Source
• Sink
• Actuator

Front 8

What interaction patterns are possible in WSN?

Back 8

• Event detection
• Periodic measurement
• Function approximation:
• Edge detection
• Tracking

Front 9

How can be sensor nodes deployed in the environment?

Back 9

• Random deployment
• Regular deployment
• Mobile

Front 10

What are the differences of MANET compared to WSN?

Back 10

• Self-organization, energy efficiency, (often) wireless multi-hop
• Applications, equipment: MANETs more powerful (read: expensive) equipment assumed, often “human in the loop”-type applications, higher data rates, more resources
• Application-specific: WSNs depend much stronger on application specifics; MANETs comparably uniform
• Environment interaction: core of WSN, absent in MANET
• Scale: WSN might be much larger (although contestable)
• Energy: WSN tighter requirements, maintenance issues
• Dependability/QoS: in WSN, individual node may be dispensable (network matters), QoS different because of different applications
• Data centric vs. id-centric networking
• Mobility: different mobility patterns (sinks or nodes might be mobile)

Front 11

Name all components of a sensor node.

Back 11

• Controller
• Communication unit Sensors/Actuators

• Memory

• Energy supply

Front 12

What transceiver states exist?

Back 12

• Transmit
• Receive
• Idle
• Sleep

Front 13

What is energy scavenging? Give two examples.

Back 13

• Obtaining energy from the environment
• Batteries
• Vibration
• Solar

Front 14

What is the best to do in order to save energy?

Back 14

• Do not run sensor node at full operation all the time
• Power Save mode

Front 15

What is the problem in switching between modes?

Back 15

Time and power consumption required reaching higher modes not negligible
ESaved < EOverhead

Front 16

What is Dynamic Voltage Scaling?

Back 16

Time and power consumption required reaching higher modes not negligible
ESaved < EOverhead

Front 17

What is Dynamic Voltage Scaling?

Back 17

Low supply voltage & clock

Front 18

Which operation in memory usage is more expensive in terms of energy consumption?

Back 18

Writing / erasing is expensive

Front 19

Transmitter Power:(eqn)

Back 19

Etx = Tstart Pstart + [n / (R x Rcode)] (PtxElec+ αamp + βamp Ptx)

Front 20

Receiver Power:

Back 20

Erx = Tstart Pstart + [n / (R x Rcode)] PrxElec + EdecBits (R)

Front 21

What is multi-hop communication?

Back 21

• Send packets to an intermediate node

• Intermediate node forwards packet to its destination
• Store-and-forward multi-hop network

Front 22

How much energy consumes a single-hop?

Back 22

c*d^α

Front 23

How much energy consumes multi-hop?

Back 23

N*c*(d/N)^ α

Front 24

What sources of mobility do exist?

Back 24

• Node mobility
• Sink mobility

• Event mobility

Front 25

Which Quality of Service goals are defined in WSN?

Back 25

• Throughput/delay/jitter
• Event detection/reporting probability
• Event classification error, detection delay
• Probability of missing a periodic report
• Approximation accuracy
• Tracking accuracy

Front 26

Name all optimization goals in WSN.

Back 26

Quality of Service
Energy efficiency
Scalability

Front 27

What is temporal correlation?

Back 27

Same values don’t change very often send every minute or more

Front 28

What is spatial correlation?

Back 28

In an area neighbors of a node might have same values ask only once

Front 29

Give an example for adaptive fidelity

Back 29

Adapt the effort with which data is exchanged to the currently required accuracy/fidelity
If no important event low messages unless event happens
Ex. When temperature is in acceptable range, only send temperature values at low resolution

Front 30

What means data-centric networking?

Back 30

Focus networking transactions on the data directly instead of their senders and transmitters, ID is not important who sent it

Front 31

What implementation options exist for data-centric networking?

Back 31

• Overlay networks & distributed hash tables
• Publish/subscribe
• Databases

Front 32

Why are ISM bands preferred for WSN?

Back 32

Industrial, scientific, medicine– license-free operation

Front 33

What is the formula for a radio wave? Name the parameters.

Back 33

s(t) = A(t) * sin( 2πf(t) * t ∅(t) )
A(t) : Amplitude
f (t): Frequency
∅(t) : Phase

Front 34

Which options for modulation (keying) do exist?

Back 34

• Amplitude Shift Keying
• Frequency Shift Keying
• Phase Shift Keying

Front 35

Name all ranges for signal propagation.

Back 35

• Transmission range
• Detection range
• Interference range

Front 36

What sources of distortion do exist in wireless transmissions?

Back 36

• Attenuation – energy is distributed to larger areas with increasing distance
• Reflection/refraction – bounce of a surface; enter material
• Diffraction – start “new wave” from a sharp edge
• Scattering – multiple reflections at rough surfaces
• Doppler fading – shift in frequencies (loss of center)

Front 37

What does Friis free-space equation describe?

Back 37

Describes signal strength over a distance d relative to distance do

Front 38

What is noise and interference?

Back 38

• Noise: due to effects in receiver electronics, depends on temperature
• Interference: caused by third party parallel senders
➢ Co-channel interference: another sender uses the same spectrum
➢ Adjacent-channel interference: another sender uses some other part of the radio spectrum, but receiver filters are not good enough to fully suppress it

Front 39

What are the requirements for medium access in wireless networks?

Back 39

• high throughput
• low overhead
• low error rates
• energy-efficient
• handle switched off devices

Front 40

What are the main options for wireless medium access?

Back 40

Front 41

What causes energy problems in MAC protocols?

Back 41

• Collisions – wasted effort when two packets collide
• Overhearing – waste effort in receiving a packet destined for another node
• Idle listening – sitting idly and trying to receive when nobody is sending
• Protocol overhead

Front 42

What is the difference between schedule based and contention based MAC protocols?

Back 42

Front 43

Give two examples for contention based MAC protocols.

Back 43

Sender B asks receiver C whether C is able to receive a transmission: 
Request to Send (RTS). Receiver C agrees sends out a: Clear to Send (CTS). Potential interferers hear either RTS or CTS and know about impending transmission and for how long it will last. This information is stored in a Network Allocation Vector.

Front 44

What are synchronized islands in S-MAC?

Back 44

• Nodes try to pick up schedule synchronization from neighboring nodes
• If no neighbor found, nodes pick some schedule to start with
• If additional nodes join, some node might learn about two different schedules from different nodes. Having to bridge and apply 2 schedules

Front 45

Give two examples for schedule based MAC protocols.

Back 45

• LEACH
• SMACS
• TRAMA

Front 46

Explain main idea of LEACH. What do clusterheads organize?

Back 46

Low-Energy Adaptive Clustering Hierarchy: Group nodes into “clusters”, controlled by clusterhead using hierarchal networks.
Clusterhead organizes CDMA & TDMA

Front 47

What is the goal in SMACS?

Back 47

Set up directional links between neighboring nodes

Front 48

Explain idea in TRAMA.

Back 48

• Nodes are synchronized
• Time divided into cycles, divided into Random & Schedules access periods
• Nodes exchange neighborhood information
• Nodes exchange schedules

Front 49

What approaches are given for error control?

Back 49

• Backward error control – ARQ
• Forward error control – FEC

Front 50

What are standard ARQ protocols?

Back 50

• Alternating bit
• Go-back N
• Selective Repeat

Front 51

How does piggybacking with alternating-bit work?

Back 51

Data is sent in the following sequence (Data, Sequence-No, ACK) and each packet includes the previous packets sequence no in the ACK field if it has arrived or lost.

Front 52

How does Go-back-N work?

Back 52

Send packets P1, P2, P3 one by one in a group and the receiver will acknowledge packet by packet but if a packet is lost it will keep on resending the last correctly received packet in the beginning of the sequence. Sender will have to resend the entire group after the correctly received packet. When timer goes off, retransmit all unacknowledged packets

Front 53

How does selective repeat work?

Back 53

Just resend the missing packet not the entire group

Front 54

When is the best moment to retransmit a packet?

Back 54

• In a BSC channel, any time is as good as any
• In fading channels, try to avoid bad channel states – postpone 
transmissions
• Instead (e.g.): send a packet to another node if in queue (exploit multi-user diversity)

Front 55

What is the main idea in FEC?

Back 55

• Endow symbols in a packet with additional redundancy to withstand a limited amount of random permutations

Front 56

Compare FEC vs. ARQ.

Back 56

Front 57

FEC Code Rate: (Eqn)

Back 57

Front 58

Which roles can a device take in ZigBee?

Back 58

• End Device
• Router
• Coordinator

Front 59

How many maximum nodes can operate in ZigBee?

Back 59

2^16

Front 60

What topologies are possible in IEEE 802.15.4?

Back 60

• Star
• Peer-to-peer
• Cluster Tree

Front 61

How are devices discovered?

Back 61

• Active scanning
• Passive scanning

Front 62

What means GTS and how it is used?

Back 62

• Guaranteed Time Slot: Nodes can choose contention-free communication
• node has to send a request to PAN coordinator
• PAN coordinator accepts request, if enough free time slots are available
• Max. seven GTS possible

Front 63

Explain time sensitive vs. scalability problem.

Back 63

• GTS in IEEE 802.15.4 offers real time guarantee
• GTS is only used for direct communication between node and PAN coordinator
• Only star topology possible -> Not Scalable

Front 64

Explain functionality of Network Layer in ZigBee

Back 64

• Packet routing
• Route management
• Topologies: Mesh, P2P, Cluster-Tree
• Security management: Key distribution

Front 65

Explain functionality of Application Layer in ZigBee.

Back 65

• Endpoint Multiplexing
• Fragmentation of data packets
• Implementation of profiles
• Application objects

Front 66

What are the main reasons for cross layer design?

Back 66

• Limitations: energy, memory, and performance
• High overhead in layer model inefficient
• Features of low-power transceivers and radio channel states 
should be considered in protocol design

Front 67

What can you suggest for interaction of transport layer and physical layer (PHY)?

Back 67

Transmission capacity and sending rate should be controlled conjointly

Front 68

What can you suggest for interaction of routing and physical layer (PHY)?

Back 68

Divide throughput optimization problem into two areas
• 
Multi-hop routing optimization in network layer
• Power optimization in PHY-layer

Front 69

Explain the disadvantage of cross layer design.

Back 69

Loss of exchangeability and conformity of different approaches. Restricts propagation and usage

Front 70

What is middleware? Explain

Back 70

Middleware constitutes a layer in a complex software system, which offers services to other decoupled software components in order to ensure a communication between them.

• Middleware may realize different layers of communication
• Middleware offers interfaces to the outside

Front 71

Which classifications of middleware systems exist? Name all.

Back 71

• Classic
• Data-Centric
• Virtual Machines
• Adaptive

Front 72

What is the difference between physical and logical location?

Back 72

Physical Location: with respect to coordinate system ex. 48° 44‘ 42.85‘‘ N
Logical Location: your own interpretation of the place ex. Room 230

Front 73

What main approaches for location estimation exist? Name and explain.

Back 73

• Proximity: Exploit finite range of wireless communication
• (Tri-/Multi-) lateration and & angulation: Use distance or angle estimates, simple geometry to compute position estimates
• Scene analysis: Radio environment has characteristic 
“signatures”. Can be measured beforehand, stored, compared with current situation.

Front 74

What is ToA? Explain functionality.

Back 74

Time of arrival: Use time of transmission, propagation speed, time of arrival to compute distance

Front 75

What is TDoA? Explain functionality.

Back 75

Time Difference of Arrival: Use two different signals with different propagation speeds. Compute difference between arrival times to compute distance

Front 76

What is overlapping connectivity?

Back 76

Position is estimated in the center of area where circles from which signal is heard/not heard overlap

Front 77

How does approximate point in triangle work?

Back 77

• Determine triangles of anchor 
nodes where node is inside, 
overlap them
• Check whether inside a given 
triangle – move node or simulate 
movement by asking neighbors

Front 78

What are the options for topology control?

Back 78

Front 79

What is Relative Neighborhood Graph (RNG) and how does it work?

Back 79

Edge between nodes u and v if and only if there is no other node w that is closer to either u or v

Front 80

What is Gabriel graph and how does it work?

Back 80

Smallest circle with nodes u and v on its circumference must only contain node u and v for u and v to be connected

Front 81

How does centralized power control algorithm work?

Back 81

• Use a centralized, greedy algorithm
• Initially, all nodes have transmission power 0 
 Connect those two components with the shortest distance between them (raise transmission power accordingly)
• Second phase: Remove links (= reduce transmission power) not needed for connectivity

Front 82

What are the key concepts of security?

Back 82

• Loss of Confidentiality
• Loss of Integrity
• Loss of Availability

Front 83

What happens if you lose confidentiality? /integrity? /availability?

Back 83

• Loss of Confidentiality: Unauthorized disclosure of information
• Loss of Integrity: Unauthorized modification or destruction of information
• Loss of Availability: Disruption of access to use of information or an information system

Front 84

What types of attacks are possible?

Back 84

• Passive attacks: gaining information
• Active attacks: modify/fake data

Front 85

Explain Eavesdropping. What kind of attack is it?

Back 85

When someone listens to an ongoing conversation between two parties without modifying the data being transferred. It is a passive attack.

Front 86

What happens in masquerading attacks? / replay attacks? / denial of service attacks?

Back 86

• Masquerading attack: Attacker pretends to be a legitimate user and steals the identity
• Replay attack: Attacker records a message from an ongoing conversation between two parties and then replays this message.
• Denial of service attack: Attacker stops Alice from reaching Bob and pretends to be Alice.

Front 87

What defensive measures are available in WSN?

Back 87

• Key establishment & management
• Secure routing
• Verification of sensor data

Front 88

Explain functionality of symmetric encryption

Back 88

A mechanism to provide confidentiality using symmetric keys. That is same key has to be shared between sender and receiver and it is used to decrypt/ encrypt messages.

Front 89

Explain functionality of asymmetric encryption

Back 89

A mechanism to provide confidentiality using asymmetric keys (public / private keys). The sender uses the public key of the person it wants to send to which is found publicly to everyone and uses it to encrypt the message, then the receiver uses his private key which is only available to him to decrypt the message.

Front 90

How does message authentication code (MAC) work?

Back 90

Sender and receiver share symmetric keys.
Sender produces a hash of the message to be sent using the key and sends it along with the message. Message is sent in plain text.
Receiver receives message and the hash. He then computes the hash of the message and compares it to hash received to check integrity.