Certificate

Certified that this is a bona fide record of the seminar work entitled

“RONJA ”

Done by

ANKIT SHARMA

Of the VI semester, ELECTRICAL & ELECTRONICS ENGINEERING in the year 2010-11 in the partial fulfilment of the

requirements to the award of degree of Bachelor of Technology in

ELECTRICAL & ELECTRONICS ENGINEERING of

PRANVEER SINGH INSTITUTE OF TECHNOLOGY.

MR. ALOK KUMAR PANDEY MR. ALOK KUMAR PANDEY

SEMINAR INCHARGE SEMINAR GUIDE

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A SEMINAR REORT ON

RONJA

SUBMITTED IN PARTIAL FULLFILMENT FOR

AWARD OF THE DEGREE OF

BACHELOR OF TECHNOLOGY

IN

ELECTRICAL AND ELECTRONICS ENGINEERING

SUBMITTED BY:-

ANKIT SHARMA

ROLL NO: 0816421404

DEPARTMENT OF

ELECTRICAL &

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ELECTRONICS ENGINEERING

PRANVEER SINGH INSTITUTE OF TECHNOLOGY

I would like to express my sense of gratitude and my sincere thanks to the following persons who have made the completion of this Seminar Report possible:

Mr.Alok Kumar PandeyMr.Alok Kumar Pandey, Seminar Incharge, for his vital encouragement, support, constant reminders and much needed motivation.

Mr. D.S.Kushwaha Mr. D.S.Kushwaha , HOD, Department of Electrical & Electronics Engineering for his understanding and assistance.

My sincere thanks to all the Faculty Members of the Department of Electrical & Electronics Engineering for assisting in the collection of the topics for my Seminar Report.

Most especially to my family and friends without their co-operation, this training as well as my project work would not have been a success.

ANKIT SHARMA,

0816421404,

PRE-FINAL YEAR, EN DEPTT.

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Contents

1. Overview 5

2. Free Space vs. Radio 7

3. Optics 9

4. The RONJA Device 10

5. Electronics 13

6. Signals 15

a. NRZ Encoding…………………………………………………………………15b. Manchester Encoding………………………………………………………… 16c. Manchester Decoding………………………………………………………… 18d. MLT-3…………………………………………………………………………20e. BiPhase……………………………………………………………………… 21

7. Modulation 22

8. LED vs Laser 24

9. Improving the RONJA 26

10. Conclusion 28

11. References 29

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Overview

RONJA (Reasonable Optical Near Joint Access) – Allows one to make a free space 10Mbps full-duplex1 Ethernet bridge between two points up to 1.4 km away using visible incoherent light.

The transmitter sends a signal with a Light Emitting Diode (LED), the light rays are collimated (paralleled) by a lens. On the other side of the bridge the receiver uses another lens to focus light onto a photo diode. The Twister is the electronics that cleans up the signal, adds a 1Mhz pulse when no data is being communicated, and adds the link integrity pulse back to the Ethernet cable. The 1Mhz pulse is used to keep the RX knowing what a signal is over noise.

RONJA (Reasonable Optical Near Joint Access) is a Free Space Optics device originating in the Czech Republic. It transmits data wirelessly using beams of light. Ronja can be used to create a 10 Mbit/s full duplex Ethernet point-to-point link.

The range of the basic configuration is 1.4 km (0.9 miles).

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Figure 1:Overview Diagram

Building instructions, blueprints, and schematics are published under the GNU Free Documentation Licence. Only free software tools are used in the development. The author calls this level of freedom "User Controlled Technology". Ronja is a project of Twibright Labs.

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Free Space vs. Radio

Radio (Such as IEEE 802.11) can also be used to create a network bridge. Free Space Optics have some advantages and disadvantages compared to Radio.

1. Eavesdropping

Because of the nature of radio waves, it is harder to contain where the radio waves go, even in directional point to point networks a signal can still be eavesdropped over a large area. For example a typical parabolic antenna has a beam width of 16⁰, at 1.4km one would still be able to receive the signal 194 meters on either side of where it is pointing. The signal can also be listened to behind the intended location. With a free space network you must intercept the light beam, this is much more difficult and can be detected.

2. Interference

Radio waves can interfere with each other. This interference is one of the reasons the FCC licenses spectrum. There are blocks of spectrum free to use, but other devices are also using them. As an example if you used the public block in the 2.4GHz range, your signal can get interference from portable phones, wireless access points, microwave ovens, car alarms, and security cameras. This can make your signal less dependable.

Free space optic networks are free from these types of interference.

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3. Distance

Radio waves have improved distance over free space optics. Free space optics is limited to how far it can travel through the atmosphere because of absorption.

4. Fresnel Zones

The Fresnel zone of light is very small compared to radio waves. If there is an obstruction in the first Fresnel zone it will produce interference.

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Optics

Geometric

The RONJA uses a double convex spherical lens, which is typical found in magnifying glasses. As shown in Figure 2, the RONJA uses the lens to take light from a LED and collimate it, as well as take incoming light into a point.

The Transmitter side takes light from a LED and it will collimate it towards the Receiver side.

The Receiver will take incoming parallel light and will focus it into a point using a double Convex Lens.

Figure 2:RAY DIAGRAM

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The Device

One Ronja device is a single long-distance optical transceiver that is capable of running against the same or compatible device on the other side of the link. The topology is point-to-point.

Building a Ronja is rather lenghty job (this will hopefully change in future) that however pays off in a reliable and performant device that is capable of delivering steady connectivity with little maintenance and can be run freely without a need of authorities' approval. Also a possibility of interference and eavesdropping is negligible. Dropouts are infrequent and determined solely by weather and are thus foreseeable.

He who wants to enjoy the adrenaline sport of driving primitive retail parts into flawless cooperation to provide the uncurbed full duplex connectivity experience must withstand these nuisances:

Ronja is somewhat labor expensive. Things have been made much easier by putting the most complicated electronics on a PCB. The cost of parts is negligible in comparison with the labor, for example the components for the whole Ronja 10M Metropolis cost just 1500CZK. Further reduction in labor demands is planned by putting RX and TX on PCB too.

The user must possess certain basic manual skills as soldering, drilling, painting, and technical drawing/schematic reading. But people without any previous experience with soldering have built a piece that worked on the first try!

The user must not cut the corners during the building

but there are also certain conveniences:

The parts were chosen to be of the widest availability possible and equivalents are provided where applicable

Innovative approach is used to speed up the work and make it convenient. Sector codes are present to make the population

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easy. Drilling is simplified by drilling templates - just print and no measurement is necessary in the workshop!

The device is based on the KISS rule (Keep It Simple, Stupid) which makes the device plug-and-play immediately after the building provided the user hasn't botched anything.

The design is rugged and overdimensioned to withstand variations in the components. As a consequence, the resulting device is rock solid in steady performance and provides outstanding electromagnetic intereference immunity and electromagnetic compatibility. Withstands -20°C as well as direct sunlight and heat with obvious margin. During a lightning storm, lightning strike in proximity usually doesn't generate even a single lost bit.

In case of device failure (direct lightning strike), the measuring points can be inspected and bad components replaced without a need to throw the whole device out.

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Figure 3

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Electronics

Light Emitting Diode (LED)

An LED is a semiconductor diode that emits incoherent narrow-spectrum light when an electric current is applied. Entire papers can be written on LEDs alone.

The RONJA makes use of the HPWT-BD00-F4000 LED by LUMILEDS. This LED has a peak wavelength of 640nm with FWHM of 28nm. A response time of 20ns (50MHz) and Luminous flux from 3 to 7.3 lm.

Figure 4: LED

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Photodiode

A photo diode is a photo detector that can converts light into a change in current or voltage. The photo diode is used to receive the signal emitted by the LED from the transmitter located at the other side of the bridge.

The RONJA uses the VISHAY BPW43 photo diode which has a response time of 8ns (125MHz), and range of spectral bandwidth 550-1000nm with peak sensitivity at 900nm. The relative spectral sensitivity at the peak wavelength of the LED used is approx 0.65.

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Signals

Information in a computer is stored as a set of hi/lo values or binary. For example, an ASCII ’a’ is represented as 01100001. This can be communicated as two sets of different voltages, or like in the RONJA blink on and blink off. There are many ways to encode the information to make it more resistant to noise or to make sure that timings are synchronized. The next subsections go over some of the encodings used in Ethernet 10BASE-T and 100BASE-TX. The RONJA currently supports 10BASE-T.

NRZ Encoding

NRZ (Non-Return to Zero) is a simple encoding, hi for 1 and lo for 0, each bit delimited by time (See Figure 6). In NRZ, clock timing is important and is often used internally when everything is operating off the same clock. Imagine a long stream of 1’s there would not be any indication of how to set your clock between bits. If this information is sent to another system that has a slightly different clock, it could easily become out of sync.

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Manchester Encoding

An Ethernet 10Base-T network sends it’s data in a Manchester encoding [7]. A prop-erty of the Manchester encoding is that it needs twice the bandwidth of the data com-pared to NRZ, thus a response capable of 10Mhz is needed (1 Hz/bit). Our LED and photo diode are capable of handling these response times.

Encoding a Data Byte:->

Encoding is the process of adding the correct transitions to the message signal in relation to the data that is to be sent over the communication system. The first step is to establish thedata rate that is going to be used. Once this is fixed, then the mid-bit time can be determined as ½ of the data rate period. In our example we are going to use a data rate of 4 kHz.

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This provides a bit period of 1/f = 1/4000 = 0.00025s or 250 μs. Dividing by two gives us the mid-bit time (which we will label “T”) of 125 μs. Now let's look at how we use this to encode a data byte of 0xC5 (11000101b). The easiest method to do this is to use a timer set to expire or interrupt at the T interval. We also need to set up a method to track which ½ bit period we are currently sending. Once we do this, we can easily encode the data and output the message signal.

1. Begin with the output signal high.2. Check if all bits have been sent, If yes, then go to step 73. Check the next logical bit to be coded4. If the bit equals “1”, then call ManchesterOne(T)5. Else call ManchesterZero(T)6. Return to step 27. Set output signal high and return

Implementation of ManchesterOne(T)

1. Set the output signal low2. Wait for mid-bit time (T)3. Set the output signal high4. Wait for mid-bit time (T)5. Return

Implementation of ManchesterZero(T)

6. Set the output signal high7. Wait for mid-bit time (T)8. Set the output signal low9. Wait for mid-bit time (T)10. Return

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These easy routines will provide an output at the microcontroller pin that exactly encodes the data into a Manchester message signal at the desired data rate. The accuracy of the data rate and duty cycle depends on the accuracy of the clock source and the method used to create the wait times.

Manchester Decoding

Decoding is where most people attempting to work with Manchester have questions. There are several ways to approach this and each has unique benefits. This section will describe how to implement two different methods. To start we will look at the steps that are needed for either methodology.1. The data rate clock must be either known or discovered (we will assume a known value)2. We must synchronize to the clock (distinguish a bit edge from a mid-bit transition)3. Process the incoming stream and recover the data using the previous two steps.4. Buffer or store this data for further processing.This provides the basic outline for how we will perform Manchester decoding. All that remains is to implement this in software. As mentioned, we have two different options for consideration.One is based on timing while the other utilizes sampling.

Timing Based Manchester Decode

In this approach we will capture the time between each transition coming from the demodulation circuit. The Input Capture function on a micro-controller is very useful for this because it will generate an interrupt, precise time measurements, and allow decision processing based on the elapsed counter value.

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1. Set up timer to interrupt on every edge (may require changing edge trigger in the ISR)

2. ISR routine should flag the edge occurred and store count value

3. Start timer, capture first edge and discard this.

4. Capture next edge and check if stored count value equal 2T (T = ½ data rate)5. Repeat step 4 until count value = 2T (This is now synchronized with the data clock)

6. Read current logic level of the incoming pin and save as current bit value (1 or 0)

7. Capture next edgea. Compare stored count value with Tb. If value = T

i. Capture next edge and make sure this value also = T (else error)ii. Next bit = current bitiii. Return next bit

c. Else if value = 2Ti. Next bit = opposite of current bitii. Return next bit

d. Else i. Return error8. Store next bit in buffer

9. If desired number of bits are decoded; exit to continue further processing

10. Else set current bit to next bit and loop to step 7

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MLT-3

MLT-3 is a encoding used in Ethernet 100BASE-TX. It changes from it’s current state to the next every time there is a high. It has 3 states: typically positive, zero, and negative. A reason to use this encoding is to reduce the frequency by four (one cycle: hi-med-lo-med) compared to something like Manchester. This makes it easy to be transferred in copper cable. It reduces the frequency traveling through copper cable for the 100BASE-TX to 31.25 MHz. This is of no help in reducing the frequency when you have only two states on and off. Creating a third state with the LED alone (such as half intensity), would reduce the range.

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BiPhase

BiPhase adds a level of complexity to the coding process but in return includes a way to transfer the bit frame data clock that can be used in the decoding to increase accuracy. BiPhase coding says that there will be a state transition in the message signal at the end of every bit frame. In addition, a logical “1” will have an additional transition at the mid-bit.

Figure 5

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Modulation

Modulation refers to the act of adding the message signal to some form of carrier. The carrier, by definition, is a higher frequency signal whose phase, frequency, amplitude, or some combination thereof, is varied proportionally to the message. This change can be detected and recovered (demodulated) at the other end of the communication channel. There are a number of ways this can be done but for simplicity we will only look at Amplitude Modulation (AM), On-Off Keying (a variation on AM), and Frequency Modulation (FM). Modulation is typically carried out in hardware.

Amplitude Modulation

In amplitude modulation, the amplitude of the carrier is changed to follow the message signal. In this case we can see a “ripple” on the carrier, its envelope contains the message.This can be demodulated using an extremely simple envelope detector that captures this ripple as a low frequency response.

Figure 6

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On-Off Keying

This form of modulation takes the amplitude modulation as described above to the extreme. In this instance, we have only two states: Carrier and No Carrier. This approach lends itself nicely to the transmission of digital data because the carrier can be simply switched “on” or “off” depending on the state of the data being sent. The demodulated output is either high or low depending on the presence of the carrier.

Frequency Modulation

Frequency modulation is more complicated but provides the benefit of constant output power independent of the message being sent. With this approach, the frequency of the carrier is not constant but varies in relation to the message. This requires a much more complicated demodulation circuit typically implemented using a Phase Lock Loop (PLL).

Figure 7: Frequency Modulation

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LED vs LASER

A common response to those introduced to the RONJA is that a LASER should be used instead of a LED. This section will try to compare a LASER vs LED.

1 Response Time

The LASER has an advantage of shorter response times than the LED. The LASER diode is stimulated to emit instead of spontaneous like the LED. LASER diodes with less than 1ns (1Ghz) response times are available. It is hard to find diodes with less than 20ns (50Mhz) response times. The LED response time works fine for the RONJA(10Mhz), if we wanted to make a faster link, this is where the LASER could have an advantage.

2 Monochromatic

A LASER tends to emit a beam more monochromatic (<5nm FWHM, compared to > 24nm FWHM), it makes it easier to add filters to the optics to only allow the light from the LASER wavelength reducing ambient noise, and possibility of photo diode saturation from outside sources.

There are also disadvantages, because of the narrow wavelengths emitted by the laser there could be conditions where that wavelength is absorbed, such as certain ice crystals in the air that absorb certain narrow bands of wavelength. With the LED you are emitting a much broader set of wavelengths. If some of it gets absorbed, you still have the rest to fall back on.

3 Scintillation

Scintillation can be far worse for a LASER than the LED because the LASER is more coherent than the LED. As the beam travels through the air, it will hit packets of air of different temperature which have a different index of refraction causing constructive and destructive

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interference which can ruin your signal. This effect can be especially bad depending on the terrain it travels over. Asphalt, ponds, fountains, all create temperature differences that will contribute to scintillation.

4 Safety

LASERs can be dangerous. Devices sold to the market place that contain lasers must go through the FDA. Even low powered lasers should have extra safety precautions such as auto shutoff if there is a beam break. For lasers rated higher than IIIa/3R protective eye goggles are needed when working with them. Even light reflected after it has hit a object can be enough to damage the eye depending on the power and wavelength of the LASER.

The LED emits a more wider spectrum and is much safer.

Improving the RONJA

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1. Mount Considerations

The distances involved requires that the mounts be very stable. If the wind causes an angle change, the end point will be displaced s = d tan θ, where d is the distance between points and θ is the angle change caused by the vibration. An end point displacement of 20cm when d = 1.4km is only an angle change of θ = 0.008◦. If mounted on tall buildings, the swaying of the building can also be a problem as the current design does not compensate for that.

At large distances, aiming is difficult. The original RONJA design has a mechanism to make fine adjustments. Improvements to the RONJA design might be a finder scope and auto aiming mounts.

2 Fresnel Lenses

Fresnel lenses give you large apertures and shorter focal lengths, while still being light weight. It may be possible to use a large Fresnel lens to get a better signal from the transmitter, or to make the optical tube shorter (smaller focal length). When shortening the focal length extra ambient light may get to the photo diode and that would need to be considered in the design.

3 Shorter and Longer Distances

There may be instances were one needs to create a shorter or longer network bridge. The use of smaller lenses at short distance maybe also be beneficial at times. If a link of smaller than 90m is needed a few of the electronics (a capacitor and resistor) need to be changed in the receiver. To get a longer signal you can also use two or more transmitters pointed to one receiver.

4 Faster Data Transfer

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Our LED has a max response of 20ns (50Mhz). So we would run into a problem if we wanted to increase the speed to 100Mbits/s by transmitting in a similar manner with the LED.

There are a few solutions. We could use a LASER which has much faster response rates than LEDs and read in the MLT-3 and transmit NRZ at max frequency of 62.5 Mhz, with the receiver converting NRZ back to MLT-3. We can also communicate in parallel instead of serial, we would require several transmitters and receivers. For example if we had three transmitters/receivers then each transmitter could handle a maximum frequency of 21Mhz. Using multiple transmitters/receivers brings up several issues: noise from other transmitters, 4B5B would not guarantee a hi-lo transition for each detector, mounting space issues. One thing is certain making a 100Mbit/s network would be much more complex.

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Conclusion

The Twibright Ronja datalink can network neighbouring houses with cross-street ethernet access, solve the last mile problem for ISP’s, or provide a link layer for fast neighbourhood mesh networks.

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References

[1] Twibright Labs. Ronja. <http://ronja.twibrightlabs.org>.

[2] Kenneth Krane. Modern Physics Second Edition. John Wiley & Sons, Inc.

www.wikipedia.org

[3] Twibright Labs. Making Ronja on short tracks. <http://ronja.twibright.com/faq/short.php>.

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