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

2. 1.Sensor read-out electronics and data conversion 2.Sensor devices, MEMS, and Microsystems 3.Radio-frequency (RF) wireless data communications 4.Wireless sensor networks This lecture/lab module is the third part in a four part sequence that will introduce you to key concepts in Electrical and Computer Engineering through the implementation and analysis of a wireless sensor network. The four parts in this sequence are:

3. The I-35W Bridge Disaster On Aug.1, 2007 the I-35 W bridge collapsed in Minneapolis during evening rush hour. –13 deaths and ~100 injuries The need for additional Structural Health Monitoring (SHM) of the bridge had been previously prescribed. –But with human inspectors it is impossible to do this on a continuous basis.

4. A Solution: A Network of Wireless Sensors Wireless networks of stress/strain/vibration sensors can be deployed to continuously monitor stresses on bridges and other civil infrastructure Distributed networks of wireless sensor nodes –gather critical information about the physical world –communicate the information to remotely located decision makers

5. 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

6. Discovering Sensor Networks

7. Lecture Objectives This lecture (and corresponding lab) introduces key concepts in Electrical and Computer Engineering through the implementation and testing of two random access medium access control protocols for wireless communications. In this lecture we will discuss: –Different uses of the radio spectrum –Basics of radio frequency (RF) communications –Medium access control for sharing a channel

8. Wireless Communications “ The wireless telegraph is not difficult to understand. The ordinary telegraph is like a very long cat. You pull the tail in New York, and it meows in Los Angeles. The wireless is the same, only without the cat.” - Albert Einstein (attributed)

9. Why Wireless? Wired sensors could provide high bandwidth and wires could also be used to provide power to sensors. So, in some ways a wired sensor network might be preferred to a wireless one. But, stringing wires around an existing structure can be difficult, expensive, and unsightly. The ability to attach sensor nodes to a structure quickly may be critical. Example: In September 2009, a crack was discovered in an eyebar on the Bay Bridge in San Francisco. A temporary repair was enacted, and the repair failed suddenly in October 2009. A sensor network monitoring the long term health of the bridge might have discovered the problem before the initial crack. But deploying a wireless sensor network at the time of repair might have enabled detection of the impending failure of the repair.

10. What Are Radio Waves? Electromagnetic radiation with a frequency below about 300 GHz are known as radio waves.

11. Spectrum Access Access to the radio spectrum in the United States is regulated jointly by the Federal Communications Commission (FCC), an independent agency of the U.S. government, and the National Telecommunications and Information Administration (NTIA), an agency of the Department of Commerce. The FCC has a mandate to be sure that the radio spectrum is used in ways that are good for society. Specifically, the FCC issues licenses to use the radio spectrum for purposes deemed to be for the “public interest, convenience and necessity.” Other countries have similar agencies, and there is international coordination, too.

12. Spectrum Access What are some examples of radio frequency communications systems that benefit society and support the “public interest, convenience and necessity?” (Name the communication system and explain how it benefits society.) 1. 5. 2. 6. 3.7. 4.8.

13. Baseband Signals Information to be communicated over radio frequencies must first be encoded as an electrical signal, known as a baseband signal. The baseband signal may be either –analog (like a voice signal coming from a microphone) or –digital (like bits coming from a computer) In modern communications systems, analog signals are converted to digital signals prior to transmission. Analog Signal Digital Signal

14. Baseband Signals A baseband signal can be analyzed to assess its frequency content through the use of a Fourier transform. The most important characteristic of a baseband signal is its bandwidth. The bandwidth is the highest frequency contained in it. –An analog voice signal has a bandwidth of about 4kHz. –An analog music signal has a bandwidth of about 30 kHz. –An analog television signal has a bandwidth of about 6 MHz. –For digital signals, the bandwidth is generally proportional to the bit rate.

15. Modulation Since an RF system can only use assigned frequencies, the baseband signal must be up converted to the desired carrier frequency. This process is knows as modulation. Up conversion is accomplished by mixing the signal with a sinusoid at the desired carrier frequency. The baseband and RF signals are also filtered to insure that they do not contain unwanted frequency components. The RF signal is then amplified and passed to an antenna.

16. Demodulation A similar process is used to convert the RF signal back to baseband. The desired RF signal is first separated from surrounding signals using a filter. Then, a mixer down converts the signal back to baseband. A filter is used to remove (undesired) replicas that appear at higher frequencies.

17. Wireless Sensor Node Transceiver A/D Sensor Physical Parameter Sensor Readout Microprocessor Transmit Amplifier Antenna Wireless Channel TRANSMITING NODE Sensor Information Display/Storage Antenna RECEIVING NODE USB Zigbee Transceiver DIGITALANALOG/RFANALOG Zigbee Transceiver Microprocessor Receive Amplifier

18. Sharing: A Key Issue in Wireless Communications Globally: How do we divide the radio spectrum for the maximum benefit of society? Locally: How do multiple users on a single channel share that channel? The problem of how multiple users share a single radio channel is called medium access control (MAC). “A wireless medium is more like a cocktail party than a telephone call.” - P. R. Kumar

19. 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?

20. Two Main Approaches to MAC Scheduled Access – Each user is allocated a portion of the wireless medium. – Advantages: No contention. Extremely fair. – Disadvantages: Can be extremely inefficient due to idle time and overhead required to maintain and share 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.

21. Scheduled Access: Examples Time Division Multiple Access (TDMA) – Time is divided into frames and each frame is divided into a slot for each user. – Used in the GSM cellular system. – Advantages: No contention. Extremely fair. Energy efficient (transceiver can be switched off or perform other tasks during inactive time slots). – Disadvantages: All users must be precisely synchronized in time and must know the slot schedule. Idle users waste system resources. Token Passing – A special packet, known as the token, is passed from user to user in sequence. Only the user possessing the token can transmit. – Advantages: No contention. Extremely fair. Responsive to system load (idle users can pass the token on immediately, taking minimal resources.) – Disadvantages: The ordering of users must be determined and maintained when users enter/leave the system. There must be a mechanism to recover the token if it is lost. – Used in some fiber optic and older local area network systems. Rarely used in wireless.

22. Random Access Widely used in data communications because traffic is bursty. – Bursty traffic can make scheduled access inefficient and challenging to manage. Two main techniques: – Aloha – Carrier Sense Medium Access (CSMA)

23. Aloha The simplest, random access protocol. Invented at the University of Hawaii in 1970. The rules: – If you have data to send, send it. – If the message collides with another message, try resending it later.

24. Aloha Conversation Analogy: – Aloha is used in conversation by people who always blurt out whatever occurs to them the instant it occurs to them. After a collision, you must wait a random amount of time before trying to resend your message. – Why? Best throughput performance for Aloha is about 18.4% – This means that 81.6% of the total available channel resource is wasted by collisions. – On a 11 Mbps channel, this means that the actual data rate will be less than 2 Mbps.

25. Aloha Performance

26. Carrier Sense Multiple Access (CSMA) The rules: – If you have data to send, listen to the channel. If it is free, send the data. If not, try listening again later. – If your message collides with another message, try again later. Conversational Analogy: – CSMA is used in polite conversation. You don’t interrupt, but wait for an opening in the conversation to interject.

27. Carrier Sense Multiple Access (CSMA) If the channel is sensed busy, you still should randomize your retry timing. – Why? Throughput performance for CSMA is variable: – Depends on the time required to sense the medium and start transmission (  ) and the time required to transmit a packet (T). – Why? – For simple CSMA, typical throughput performance is 88 - 96%.

28. CSMA Performance

29. Aloha vs. CSMA Obviously, CSMA is much more efficient than Aloha. – Hence, CSMA is used in WiFi as well as in wired computer data networks. – But, CSMA requires the ability to sense the channel and detect if anyone else is using it, which may not be available in some systems. – It is generally impossible for ground stations in satellite systems to hear each other. Hence, use of Aloha-like protocols is common (e.g. for credit card transaction processing). – It is often difficult for cellular telephones to hear each other even within a cell. (The base station antenna is usually mounted on a tower, from which it can hear all the terminals in the cell.) – In some modern software radio systems, slow reaction time (to switch from sensing to transmitting) makes CSMA performance poor and thus leads to a preference for Aloha. – Thus, both are widely used, with the choice between them depending upon the tradeoffs in a particular scenario.

30. Hybrid Medium Access Control Hybrid MAC refers to systems that use a mixture of random access and scheduled access. The best example of Hybrid MAC is used in most cellular systems. When you press the “Send” button to place a call or when you first turn on your phone, random access is used, during a special time interval, to get attention from the base station. Once a call is established, though, scheduled access is used to insure the quality of the ongoing call.

31. In the Lab… In the lab for this part, you will measure the performance of a wireless system with medium access protocols approximating Aloha and CSMA. This lab will require that everyone in your lab section cooperate. – Some teams will transmit packets while others receive packets. – The transmitting and receiving teams will count the number of packets sent and received. Over the course of the lab, we will vary: – The number of transmitters. – The protocol (Aloha or CSMA) that the transmitters are using. – The rate at which the transmitters send packets. The packets carry sensor values measured by your strain gauge, if you completed labs for the previous parts. Freescale ZigBee-ready RF Transceiver attached to Freescale Project board

32. Lecture Summary We routinely use radio frequency communications, and this technology is critical to the functioning of society. –Applications include public safety, commerce, and entertainment. –The allocation of radio spectrum is thus an important public policy issue. Transmitting and receiving signals at radio frequencies requires circuits consisting of oscillators, mixers, filters, and amplifiers. Medium access control techniques allow multiple transmitters to share a radio channel. Random access techniques are often used for data communications. –Aloha and CSMA are major types of medium access control.