AIT Chapter 5 Sensor Networks

1. Not necessarily correlated with distance

2. Asymmetric

3. Varies with time

1. Connectivity

2. Stretch factor for distance and energy introduced by topology

3. Throughput after modification

4. Robustness to mobility

5. Overhead

1. Remote access to/from WSN

2. Support different radios/protocols

3. General feature of networks

1. Quality of service (QoS) more complicated than in MANET

2. Energy efficiency per correctly received bit or reported event

3. Scalability for any number of nodes

4. Robustness

1. Medium access control (MAC)

2. Sensor-MAC (S-MAC)

3. B-MAC

4. WiseMAC

5. Low energy adaptive clustering hierarchy (LEACH)

6. 802.15.14 with Low rate wireless personal area network (LR-WPAN)

Important due to high energy consumption in idle mode

1970s Xerox PARC with dominant and cheap ethernet

1. Listen to wire before transmission

2. Avoid collision with active transmission

3. Listen while transmitting

4. Assume collision if interference noticed

1. Does not assume that transmission might get stuck in wireless system

2. Difficult because interference at receiver difficult to observe

Hidden station is to far away to sense but creates collisions

Exposed station A can not send (Wrongly assumed!) due to sending B

1. Collisions: Sending of two packets at the same time

2. Overhearing: Receiving packet destined for other node

3. Idle listening: Listening when nobody send

4. Overhead

1. Centralized and distributed

2. Schedule and contention based

1. Simple

2. No collisions

3. Good for few devices

1. Energy consuming

2. OH and delays

1. Fixed OR on demand

2. Requires time synchronization

1. Packets might collide

2. Overall more efficient due to less OH

3. Use same channel before signaling

4. Use separate narrow channel for signaling // Complex HW

1. Request to send (RTS) package before sending

2. Clear to send (CTS) package for sending

3. Other members store usage in network allocation vector

4. Packets contains sender receiver addresses and size/time information

5. Implement in IEEE 802.11 (WLAN)

6. Requires idle listening → synchronized wake-up

1. Turn nodes on simultaneously (So-called rendezvous)

2. Active periods

3. Energy waste in active mode prohibited by timeout-MAC (T-MAC)

4. Synchronized islands

1. SYNCH phase for time information

2. Data exchange phase with RTS and CTS

1. Copy schedules from neighbor or start own schedule

2. If necessary follow multiple schedules

Receiver periodically samples channel with long preambles

1. Combines S-MAC and B-MAC principles

2. Use B-MAC but learn about data exchanges

3. Shorten preambles for frequent communication

1. Dense network with all connected with central sink

2. Nodes are grouped into clusters with clusterheads

3. Rotate role of clusterhead due to energy consumption

1. Setup and 2. steady-state phase

1. Self-election of clusterheads // 5% of nodes

2. Nodes compete for sending slots

Clusterheads organize communication

Standard for WSNs in the IoT

1. Low data rate

2. Low energy consumption

3. Short range operation on several ISM bands

4. Request-able reliability

1. Full and 2. reduced function devices

1. Supports any topology and communication

2. Can act as PAN coordinator

Limited to leafs in topologies and simple communication

1. Piconet // Star

2. Multi-hop peer-to-peer // Mesh

3. Tree

1. Preamble 32bits

2. Start of packet delimiter 8bits

3. PHY header 8bits with data length

4. Phy service data unit (PSDU) 0-1016bits