Wireless Data Transmission between Computers using Sequence Detector

ABSTRACT

Here for the purpose, we have made four distinct stages for the effective implementation .The stages are described in brief as follows. In the first stage, we are constructing the transmitter circuit that transmits data and sequence bits at infrared frequency. Now the second part is to develop receiver circuit that receives infrared signals coming out from transmitter and convert them into bits. The next stage is to compare the incoming sequence with that of receiver computer. If sequence is matched only then it will be able to catch the data sent by transmitting computer. The fourth and final stage is the make software program that gets the data and displays on computer.

INTRODUCTION

Wireless data transmission between computers is a flexible data communication system implemented as an extension to, or as an alternative for, a wired LAN within a building. It can further be extended to wireless LAN with secure data transmission. For the purpose of secure data transmission a unique code is sent before actual data bits. This unique code is known to receiver. In this project sequence detector at receiver side has been used which is set for a particular code. This sequence detector detects binary bit stream coming out from transmitter and if code of incoming bit stream is matched with the code of sequence detector than rest of bits are received in form of data at the receiving computer’s side. To further enhance the concept of security we are using Infrared waves as a channel between transmitter and receiver because security of infrared systems against eavesdropping is better than that of radio systems. The block diagram of whole system is shown below.

Block Diagram

Wireless transmission block diagram

Wireless transmission block diagram

WIRELESS LAN
Wireless LANs use electromagnetic airwaves (radio or infrared) to communicate information from one point to another without relying on any physical connection. Radio waves are often referred to as radio carriers because they simply perform the function of delivering energy to a remote receiver. The data being transmitted is superimposed on the radio carrier so that it can be accurately extracted at the receiving end. This is generally referred to as modulation of the carrier by the information being transmitted. Once data is superimposed (modulated) onto the radio carrier, the radio signal occupies more than a single frequency, since the frequency or bit rate of the modulating information adds to the carrier.

Wireless LAN architecture

Multiple radio carriers can exist in the same space at the same time without interfering with each other if the radio waves are transmitted on different radio frequencies. To extract data, a radio receiver tunes in one radio frequency while rejecting all other frequencies.

In a typical wireless LAN configuration, a transmitter/receiver (transceiver) device, called an access point, connects to the wired network from a fixed location using standard cabling. At a minimum, the access point receives, buffers, and transmits data between the wireless LAN and the wired network infrastructure. A single access point can support a small group of users and can function within a range of less than one hundred to several hundred feet. The access point (or the antenna attached to the access point) is usually mounted high but may be mounted essentially anywhere that is practical as long as the desired radio coverage is obtained.

End users access the wireless LAN through wireless-LAN adapters, which are implemented as PC cards in notebook or palmtop computers, as cards in desktop computers, or integrated within hand-held computers. Wireless LAN adapters provide an interface between the client network operating system (NOS) and the airwaves via an antenna. The nature of the wireless connection is transparent to the NOS.

TECHNOLOGY
Manufacturers of wireless LANs have a range of technologies to choose from when designing a wireless LAN solution. Each technology comes with its own set of advantages and limitations.

NARROWBAND TECHNOLOGY

A narrowband radio system transmits and receives user information on a specific radio frequency. Narrowband radio keeps the radio signal frequency as narrow as possible just to pass the information.

Undesirable crosstalk between communications channels is avoided by carefully coordinating different users on different channel frequencies.

A private telephone line is much like a radio frequency. When each home in a neighborhood has its own private telephone line, people in one home cannot listen to calls made to other homes. In a radio system, privacy and noninterference are accomplished by the use of separate radio frequencies. The radio receiver filters out all radio signals except the ones on its designated frequency.

From a customer standpoint, one drawback of narrowband technology is that the end-user must obtain an FCC license for each site where it is employed.

SPREAD SPECTRUM TECHNOLOGY
Most wireless LAN systems use spread-spectrum technology, a wide band radio frequency technique developed by the military for use in reliable, secure, mission-critical communications systems. Spread-spectrum is designed to trade off bandwidth efficiency for reliability, integrity, and security. In other words, more bandwidth is consumed than in the case of narrowband transmission, but the tradeoff produces a signal that is, in effect, louder and thus easier to detect, provided that the receiver knows the parameters of the spread-spectrum signal being broadcast. If a receiver is not tuned to the right frequency, a spread-spectrum signal looks like background noise. There are two types of spread spectrum radio: frequency hopping and direct sequence.

1. FREQUENCY HOPPING SPREAD SPECTRUM TECHNOLOGYFrequency-hopping spread-spectrum (FHSS) uses a narrowband carrier that changes frequency in a pattern known to both transmitter and receiver. Properly synchronized, the net effect is to maintain a single logical channel. To an unintended receiver, FHSS appears to be short-duration impulse noise.

2. DIRECT SEQUENCE SPREAD SPECTRUM TECHNOLOGYDirect-sequence spread-spectrum (DSSS) generates a redundant bit pattern for each bit to be transmitted. This bit pattern is called a chip (or chipping code). The longer the chip, the greater the probability that the original data can be recovered (and, of course, the more bandwidth required). Even if one or more bits in the chip are damaged during transmission, statistical techniques embedded in the radio can recover the original data without the need for retransmission. To an unintended receiver, DSSS appears as low-power wideband noise and is rejected (ignored) by most narrowband receivers.

INFRARED TECHNOLOGYA third technology, little used in commercial wireless LANs, is infrared. Infrared (IR) systems use very high frequencies, just below visible light in the electromagnetic spectrum, to carry data. Like light, IR cannot penetrate opaque objects; it is either directed (line-of-sight) or diffuse technology. Inexpensive directed systems provide very limited range (3 ft) and typically are used for personal area networks but occasionally are used in specific wireless LAN applications. High performance directed IR is impractical for mobile users and is therefore used only to implement fixed sub-networks. Diffuse (or reflective) IR wireless LAN systems do not require line-of-sight, but cells are limited to individual rooms.

INFRARED WAVE COMMUNICATION

The electromagnetic spectrum and its uses for communication.

According to Consultative Committee for International Radio (CCIR) frequency band designation, frequency of infrared waves ranges from 300 GHz to 300 THz (1GHz =  Hz and 1THz = 10¹² Hz ). Hence wavelength of infrared waves ranges from 1 mm to 1nm.

Infrared wave communication has following advantages:

• Good for very short range (such as for Wireless LAN)
• Secure communication
• Security against eavesdropping is better than that of radio waves.
• Infrared Diode (Transmitter) and Infrared Transistor (Receiver) are relatively directional, cheap and easy to build.
• No government license is needed to operate an infrared  system, in contrast to radio systems, which must be licensed.

CHRONOLOGY

The following steps have been followed in carrying out the project.

1. Study the books on the relevant topic.
2. Understand the working of the circuit.
3. Prepare the circuit diagram.
4. Prepare the list of components along with their specification. Estimate the cost and procure them after carrying out market survey.
5. Plan and prepare PCB for mounting all the components.
6. Fix the components on the PCB and solder them.
7. Test the circuit for the desired performance.
8. Trace and rectify faults if any.
9. Give good finish to the unit.
10. Prepare the project report.

HARDWARE DESCRIPTION

TRANSMITTER CIRCUIT

At the transmitter side we are using 4 major components-

1. Single ended MAX 232
2. General purpose NPN Transistor BC 548
3. 555 timer
4. Infrared LED

The data and sequence are available at the serial port of motherboard of transmitting computer. Since this port is at RS 232 logic level (logic 1< -3V and logic 0 > +3V), we are to shift its level to TTL logic level (logic 1= +5V and logic 0= 0V). For this purpose we are using MAX 232 device (single ended) where bits from serial port at RS 232 level are applied at pin no.13 and bits at TTL logic level are available at pin no. 12. The pin no. 12 is connected to Base of  general purpose NPN Transistor BC 548 by 10K resistor.

Here the general purpose NPN Transistor BC 548 works as an inverter and we get inverted bits at the Collector of this general purpose NPN Transistor BC 548. The Collector terminal of this general purpose NPN Transistor BC 548 is connected to pin no. 4 (RESET ACTIVE LOW) of 555 timer.

The 555 timer is basically working as an oscillator that generates the square wave at the frequency of 38 KHz and duty cycle of about 25 %.

ton =0.0693 * R1 * C,   (C=0.001 pf, R1=6.8K?)
toff =0.0693 * R2 * C,  (C=0.001 pf, R2=27K?)

Duty cycle =       (ton/(ton+toff)) * 100 %

=         (6.8/(27+6.8)) * 100 %

~  25%

Hence at pin no. 3 of 555 timer we have bit string at 38 KHz and duty cycle of  25 %.

The pin no. 3 of 555 timer is connected to Infrared LED, that transmits the bit string at the Infrared frequency with switching of 38 KHz.

HARDWARE DESCRIPTION

RECEIVER CIRCUIT

At the receiver side we have Infrared sensor TSOP1738 that intercepts Infrared signal coming out from transmitter and converts them into bit stream as output in ACTIVE LOW form. These ACTIVE LOW bit stream are applied at pin no.1 and pin no.2 of Quad 2–input NAND Schmitt trigger MC14093B and we get ACTIVE HIGH bit stream at pin no.3.

These bit streams are applied to S input of  R S Flip Flop 4043 and we get output at pin no.1 of this Flip Flop depending upon pattern of bit stream.

The output of  R S Flip Flop is again applied to Quad 2–input NAND Schmitt trigger MC14093B to perform AND operation with output of LM567 clock generator.

Hence the output of LM567 clock generator and output of R S Flip Flop are ANDed and applied to pin no.11 of two 4060 divider.

First 4060 divider is used to give clock frequency of 1.2 kHz to Shift Register. The Shift Register holds the incoming sequence coming out from Quad 2–input NAND Schmitt trigger MC14093B and its contents are compared by Comparator with receiver computer’s sequence. If sequence is matched then Comparator sends READ signal to parallel port of computer to read the data from serial port. Hence here combination of Comparator and Shift Register work as Sequence Detector.

Here MAX232 is used to shift the voltage level from TTL logic level (logic 1= +5V and logic 0= 0V) to RS 232 logic level (logic 1< -3V and logic 0 > +3V), since RS 232 is connected to serial port of motherboard of receiving computer.
The RESET signal coming out from Shift Register is used to reset all the devices and hence to reset reception process for next reception.

HARDWARE DESIGN PROCEDURE

STEP TAKEN WHILE PREPARING CIRCUIT
(A)    PCB DESIGNING :-

The main purpose of printed circuit is in the routing of electric currents and signal through a thin copper layer that is bounded firmly to and insulating base material some time called the substrate. This base is manufactured with integral bounded layers of thin copper foil which has to be partly etched of other wise remove to arrive at a pre designed pattern to suite the circuit connections or whatever other application is noted.

The term printed circuit board is derived from the original method where by a printed pattern is used as the mask over wanted areas of copper. The PCB provides an ideal baseboard upon which to assemble and hold firmly most of the small components.

From the constructor’s point of view, the main attraction of using PCB is its role as the mechanical support for small components. There is less need for complicate and time consuming metal work of chassis contraception except perhaps in providing the final enclosure. Most straight forward circuit designs can be easily covered into printed wiring layer, the thought required to carry out the inversion cab footed high light as possible because error that would otherwise be missed in conventional point to point wiring .The finished project is usually neater and truly a work of art.

Actual size PCB layout for the circuit shown is drawn on the copper board. The board is then immersed in FeCl3 solution for 12 hours. In this process only the exposed copper portion that is etched out by the solution.

Now the petrol washes out the paint. Now the copper layout on PCB is rubbed with a smooth sand paper slowly and lightly such that only the oxide layers over the Cu is removed. Now the holes are drilled at the respective places according to component layout.

(B) LAYOUT DESIGN :-

When designing the layout one should observe the minimum size (component body length and weight). Before starting to design the layout we need all the required components in hand so that an accurate assessment of space can be made. Other space consideration might also include from case of mounted components over the printed circuit board or to access path to present components.

It might be necessary to turn some components round to a different angular position so that terminals are closer to the connections of the components. The scale can be checked be positioning the components on the squared paper. If any connection crosses, then one can reroute to avoid such condition.

All common or earth lines should ideally be connected to a common line routed around the perimeter of the layout. This will act as the ground plane. If possible try to route the outer supply line to the ground plane. If possible try to route the other supply lines around the opposite edge of the layout to through the center. The first set is tearing the circuit to eliminate the crossover with out altering the circuit detail in any way.

Plan the layout looking at the topside to this board. First this should be translated inverse later for the etching pattern large areas rate recommended to maintain good copper adhesive it is important to bear in mind always that copper track width must be according to the recommended minimum dimensions and allowance must be made for increased width where termination holes are needed. From this aspect, it can become little tricky to negotiate the route to connect small transistors.

There are basically two ways of copper interconnections pattern in the under side to the board. The first is the removal of only the amount of copper necessary to isolate the  junction to the components to each other resulting in the large areas of copper. The second is to make the interconnection pattern looking more like conventional point wiring by routing uniform width of copper from component to component.

(C)     ETCHING PROCESS :-

Etching process requires the use of chemicals acid resistant dishes and running water supply. Ferric chloride is mostly used solution but other etching materials such as Ammonium per Sulphate can be used. Nitric acid can be used but in general it is not used due to poisonous fumes.

The pattern prepared is glued to the copper surface of the board using a latex type of adhesive that can be cubed after use. The pattern is laid firmly on the copper using a very sharp knife to cut round the pattern carefully to remove the paper corresponding to the required copper pattern areas. Then apply the resist solution, which can be a kind of ink proportion for the purpose of maintaining smooth clean outlines as far as possible. While the board is drying, test all the components.

Before going to next stage, check the whole pattern and cross check against the circuit diagram. Check for any free metal on the copper. The etching bath should be in a glass or enamel disc. If using crystal of ferric- chloride these should be thoroughly dissolved in water to the proportional suggested. There should be 0.5 lt. of water for 125 gm of crystal.
Waste liquid should be thoroughly deflated and dried in water land. Never pour down the drain. To prevent particles of copper hindering further etching, agitate the solutions carefully by gently twisting or rocking the tray.

The board should not be left in the bath a moment longer than is needed to remove just the right amount of copper. Inspite of there being a resistive coating there is no protection against etching away through exposed copper edges. This leads to over etching. Have running water ready so that etched board can be removed properly and rinsed. This will halt etching immediately.

Drilling is one of those operations that call for great care. For most purposes a 1mm drill is used. Drill all holes with this size first those that need to be larger can be easily drilled again with the appropriate larger size.

(D) COMPONENT ASSEMBLY : -

From the greatest variety of electronic components available, which runs into thousands of different types it, is often a perplexing task to know which is right for a given job.

There could be damage such as hairline crack on PCB. If there are, then they can be repaired by soldering a short link of bare copper wire over the affected part.

The most popular method of holding all the items is to bring the wires far apart after they have been inserted in the appropriate holes. This will hold the component in position ready for soldering. Some components will be considerably larger. So it is best to start mounting the smallest first and progressing through to the largest. Before starting, be certain that no further drilling is likely to be necessary because access may be impossible later.

Next will probably be the resistor, small signal diodes or other similar size components. Some capacitors are also very small but it would be best to fit these after wards. When fitting each group of components mark off each one on the components as it is fitted and if we have to leave the job we know where to recommence.

Although transistors and integrated circuits are small items there are good reasons for leaving the soldering of these until the last step. The main point is that these components are very sensitive to heat and if subjected to prolonged application of the soldering iron, they could be internally damaged.

All the components before mounting are rubbed with sand paper so that oxide layer is removed from the tips. Now they are mounted according to the component layout.

(E) SOLDERING : -

Flux is applied to the tips and then the components are soldered. Now the board is prepared. Antenna and condenser mike are connected at respective place and 9v dc supply is given.

LIMITATIONS

Project has following limitations:

• Infrared waves can not pass through solid objects because of their long wavelength.
• LOS and restricted communication range.
• There is no feedback arrangement, after the data transmission transmitter can not know whether data is received by receiver or not.

CONCLUSION

The objective of project is to make high speed wireless LAN using sequence detector for secure data transmission. The principle and technique used in project can be used to make wireless Local Area Network (LAN) in an office where several computers are connected to each other besides providing provision for security.

Hence, our project will be very useful for wireless networking for short range secure data communication.

REFERENCES

[1]    Computer Networks by Andrew S. Tanenbaum (3rd Edition)
[2]    Microwave and Radar Engineering by M. Kulkarni
[3]    http://authors.phptr.com/tanenbaumcn4/

Project By,

1. ABHAY BHISIKAR
2. AVINASH GAJBHIYE
3. KABINDRA YADAV
4. NILESH MORE
5. VIJENDRA KATARE
6. VIKAS SHARMA
7. VIKAS WARUDKAR