Wireless Sensor Networks - Advanced Computer Networks - Lecture Slides, Slides of Computer Networks

Download Wireless Sensor Networks - Advanced Computer Networks - Lecture Slides and more Slides Computer Networks in PDF only on Docsity!

WiseMAC: An Ultra Low Power MAC Protocol for the Downlink of Infrastructure Wireless Sensor Networks

Outline

• Introduction
• Infrastructure Network
• WiseMAC
• ZigBee
• Comparison
  • Power-delay characteristics
• Conclusion

Power consumption

• Energy efficiency is important in the sensor nodes
• Power consumption of transceiver in receiver mode is considerable
• Minimize energy waste
  • Idle listening – active listening to idle channel.
  • Overhearing – reception of a packet or part of a packet destined to another node.

Infrastructure WSN

• Composed of a number of access points (AP).
• Each access point serves a number of sensor nodes.
• AP is energy unconstrained
  • Can listen continuously
  • Can send any amount of signaling traffic
  • Exploited by WiseMAC protocol

WiseMAC

• Medium Access Control protocol
• Based on CSMA with preamble sampling
• Sampling minimizes idle listening
• Exploit sensor nodes sampling schedules to minimize length of the wake-up preamble
• Data frames are repeated in long preambles to mitigate overhearing

Sampling

• Sensor nodes regularly sample the medium – listen to the radio channel for a short duration
• If medium found busy listen until frame is received or until idle again
• Sensor node sample with constant period Tw
• Schedule offsets are independent of each other and constant

Sampling schedules

• AP keeps an up-to-date sampling schedule of all sensor nodes
• Sample schedules acquired from every acknowledgment packet
• ACK specifies the remain time until next scheduled sampling

WiseMAC sampling activity

11

Diagram from IEEE Computer Journal feature article, WiseNET: an ultra low-power wireless sensor network solution, published by IEEE Computer Society, August 2004

Drift Compensation

13

• AP may be late, while node may be early, start the preamble 2θL in advance
• Because the sensor node may be late while the AP is early the duration of preamble must be 4θL

Diagram from presentation slides of Real-Time Networking Wireless Sensor Networks by Prof J.-D. Decotignie. http://lamspeople.epfl.ch/decotignie/RTN_WSN.pdf

Drift Compensation (cont’d)

• In cases where L is very large and 4θL is larger than the sampling period Tw, the preamble length of Tw is used.

14

Tp = min (4θL, Tw )

High traffic conditions

• When traffic is high overhearing is mitigated due to the preamble sampling technique and minimized preamble
• Short transmissions are likely to fall in between sampling instants of potential overhearers

Low traffic conditions

• When traffic is low Tp can exceed the length of the data packet
• In which case the wake-up preamble is composed of padding bits and repetitions of the data frame

IEEE 802.15.4 ZigBee

 WiseMAC is compared to the power save MAC protocol in ZigBee

 Uses central coordinator labeled access point (AP) in this document

 AP buffers incoming traffic

 AP sends periodic beacon every Tw

 Beacon contains address of sensor node for which data is buffered

ZigBee Power Save Protocol

 All sensor nodes wake-up regularly to receive beacon  Sensor node polls AP for the buffered data if the beacon contains its address  Also uses frame pending bit in data packet header