3. Cyber Physical Systems

Question 1

What are the characteristics of a cyber-physical system?

Answer 1

• Computing capabilities
• Interconnected
• Interaction with the physical world

Question 2

What are the requirements of cyber-physical systems?

Answer 2

• Energy-efficiency
• Sensor density / efficient communication
• Low bandwidth
• Multi-hop: adressing + routing
• Scalability
• Security / privacy
• Reliability
• Low delay/latency

Question 3

What are the main components in a node regarding energy consumption?

Answer 3

CPU and Radio

Question 4

Name two examples for proactive routing and how does it work?

Answer 4

Optimized link state routing (OLSR):
- Broadcast local links to neighbors
- Broadcast is only forwarded by multipoint relays:
- Minimum set of neighbors, such that each two hop neighbor is connected to at least one multipoint relay

Destination Sequence Distance Vector (DSDV):
- Periodically send full route updates
- On topology change, send incremental route updates
- Unstable route updates are delayed
- Add aging information to route information propagated by distance
vector exchanges; helps to avoid routing loops

Question 5

Name a reactive routing protocol and explain it

Answer 5

Dynamic Source Routing (DSR):
- Broadcast a routing request to all neighbors
- Neighbors add their ID to the route request and continue the broadcast
- First request to reach the destination gets replied with the route of that request

Question 6

How is the Expected Transmission Count calculated?

Answer 6

Forward delivery ratio (FDR): probability that data packet is successfully transmitted to recipient - d_f
Reverse delivery ratio (RDR): The probability an ACK is successfully received - d_r

Expected Transmission Count ETX = 1 / (d_f * d_r)

Question 7

What is Beacon Vector Routing (BVR)?

Answer 7

• Routing based on virtual coordinates relative to beacons
• Beacons are randomly selected nodes
• A tree graph is created for each beacon to determine the distance from the node
• Coordinates are broadcasted to neighbors
• Routing is done by minimizing the distance between source and destination
• 3 different modes of routing: greedy, fallback, scoped flooding

Question 8

How do the different approaches in BVR work?

Answer 8

Greedy: Form the tuple (d+,d-) for each neighbor such that:
d+ = sum of distances to destination for beacons closer to destination than source
d- = sum of distance to destination for beacons closer to source than destination
First try to find a neighbor with a lower d+, then d- and finally proceed with fallback

Fallback: Forward packets towards the next beacon and proceed with greedy routing

Scoped flooding: Flood message with TTL = distance

Question 9

What are the steps to setup and operate a 6LoWPAN device?

Answer 9

1. Commissioning: Setup, link-layer connectivity to other devices
2. Bootstrapping: Network layer address configuration, discovery of neighbors
3. Route initialization: Routing algorithm sets-up path
4. Repeat 2-3

Question 10

Why is the standard IPv6 neighbor discovery not feasible for LoWPAN?

Answer 10

• Heavy use of multicast traffic would flood the network
• Assumption that nodes are always on
• Assumption that each router manages its own IPv6 subnet prefix

Question 11

What is CoAP?

Answer 11

• Constrained Application Protocol
• RESTful
• Messages: CON, ACK, NON, RST

Question 12

What is observation in CoAP?

Answer 12

Only send data on state change