1. Discovering Sensor Networks: Applications in Structural Health Monitoring Summary Lecture Wireless Communications

2. Discovering Sensor Networks: Applications in Structural Health Monitoring Distributed networks of wireless sensor nodes –gather critical information about the physical world –communicate the information to remotely located decision makers Example applications: –smart farming –health applications for the elderly –environmental monitoring –monitoring the structural health of a building or bridge

3. Lecture Summary This lecture and corresponding lab introduced key concepts in Electrical and Computer Engineering through the implementation and testing of two random access medium access control protocols for wireless communications. In the background lecture we discussed: –Different uses of the radio spectrum –Basics of radio frequency (RF) communications –Medium access control for sharing a channel In this lecture, we will review what we learned in the lab and focus on the design decisions to be made in designing a random access protocol.

4. Wireless Communications Usually uses radio frequency signals to communicate. Varying frequencies from ~300 kHz to ~300 GHz. Access to the radio spectrum is managed by government agencies for the benefit of society. Uses include public safety, cellular telephones, broadcast television and radio, satellite communications, satellite navigation, and others. Baseband information signals are modulated and demodulated to the desired frequency band through the use of mixers, filters, and antennas. Medium access control protocols are used to share common radio channels. The two major classes of MAC protocols are random and scheduled. The two main types of random access protocols are those with carrier sensing (CSMA) and those without (Aloha).

5. Medium Access Design Goals Efficiency –Is the medium allocated to the maximal benefit of the users? –Is the medium ever wasted (e.g. idle time on the channel) even though some users have data to send? –Is there overhead associated with the sharing itself? Fairness –Is the medium shared equally between the users? –If appropriate, are priorities enforced on the medium?

6. Two Main Approaches to MAC Scheduled Access – Each user is allocated time to use the wireless medium. – Advantages: No contention. Extremely fair. – Disadvantages: Can be extremely inefficient due to idle time and overhead required to maintain and propagate the schedule. Random Access – Users access the medium on an as-needed basis. – Advantages: Extremely simple. Allocations are completely demand-driven. – Disadvantages: Contention can lead to inefficiency.

7. Engineering Design Problem Statement You will be asked to design a random access medium access control protocol for a wireless sensor network to be deployed to monitor the structural health of a bridge. Design decisions include: –Whether or not to employ carrier sensing (Aloha or CSMA) –Whether or not to use time slots –Feedback mechanism to determine the success or failure of packet transmissions –Randomization mechanism to reduce collisions –Adaptation mechanism for responding to network load –Possible mechanisms for avoiding hidden and exposed terminal problems

8. To Sense or Not to Sense? Aloha Less efficient (~18%) Less complex Can be implemented even if carrier sensing is impossible Examples – Satellite Communications – Cellular Registration CSMA More efficient (>80%) More complex Requires a system model that makes sensing possible Examples – WiFi – Bluetooth – Zigbee – Ethernet

9. Laboratory Summary In the lab, we tested the performance of Aloha and CSMA systems. –We emulated both random access strategies. –We tried different data rates and different numbers of transmitters. We saw the higher efficiency of CSMA. Trading complexity and system requirements for better performance. We saw that it was harder to achieve fairness in a heavily loaded system. Trading lower throughput for better fairness.

10. Slotted Time In Aloha systems, performance can be greatly enhanced if time is divided into “slots” of appropriate length for the data packets to be transmitted. Then transmissions are only permitted within a slot. –Why might this improve performance? –Why is this enhancement usually not effective for CSMA? –What would be the challenges associated with using a “slotted” protocol?

11. Feedback Mechanism In the lab, our test protocols had no feedback mechanism. Transmitters had no way to know whether or not their transmissions were successful. Analysis of Aloha and CSMA often assumes perfect, instantaneous feedback. In a real system, how can a transmitter be notified of the success (or failure) of its transmission? –Assume, unlike in the lab, that each transmission is intended for a single receiver. –The receiver needs to provide some kind of acknowledgement to the transmitter, if feedback is required.

12. Randomization Mechanism In the lab, our simple system did not randomize transmissions. –Increases the frequency of collisions, as nodes may collide repeatedly. Simple mechanism: –When a node wants to transmit, it flips an unfair coin. With probability p it transmits. With probability 1-p, it pauses for the amount of time that would be required to send a packet, then flips its coin again. –What should the value of p be? If p is too high, there will be too many collisions. If p is too low, then the channel will be idle. And, the appropriate value of p depends on how many nodes are trying to transmit! Other mechanisms: –Define a “window” of a particular length, then pick a random time within the window. –The appropriate length of the window also depends on the number of nodes trying to transmit.

13. Adaptation Mechanism In the previous slide, we saw that the randomization mechanism needs to adapt to the number of nodes trying to transmit. –But a transmitting node usually has no way to know how many other nodes are contending for the channel at the same time. –Thus, the node needs to adapt its randomization mechanism based on the information that it does have. Lots of Collisions ➔ Use a smaller transmit probability or a larger window. Lots of Idle Time ➔ Use a larger transmit probability or a smaller window.

14. Hidden and Exposed Terminals In wireless systems, performance is also impacted by the hidden terminal problem and the exposed terminal problem. Hidden Terminal Problem: –A is transmitting to B. C is out of range of A, and tries to initiate a transmission to B or to another node, causing a collision at B. Exposed Terminal Problem: –B is transmitting to A. C hears B’s transmission and delays its transmission to D, even though both transmissions could be completed simultaneously. Solutions exist, even in standards like 802.11, but are not widely used. A A B B C C A A B B C C D D

15. Random Access Applications Wireless communications technologies are critical to society. –Sensor Networks for Environmental, Healthcare, and Structural Monitoring –Public Safety Communications including police, fire, ambulance, and air traffic control. –Communications for business and commerce. Random access medium access control is applied in a wide variety of communications applications. –Zigbee (IEEE 802.15.4) Wireless Sensor Networks –WiFi (IEEE 802.11) Wireless Local Area Networks –Some components (location registration and call placement) of Cellular Networks –Satellite communications Random access medium access control design decisions differ by application requirements and operating conditions.