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.
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 TECHNOLOGY
Frequency-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 TECHNOLOGY
Direct-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.
A 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
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.
The following steps have been followed in carrying out the project.
- Study the books on the relevant topic.
- Understand the working of the circuit.
- Prepare the circuit diagram.
- Prepare the list of components along with their specification. Estimate the cost and procure them after carrying out market survey.
- Plan and prepare PCB for mounting all the components.
- Fix the components on the PCB and solder them.
- Test the circuit for the desired performance.
- Trace and rectify faults if any.
- Give good finish to the unit.
- Prepare the project report.
At the transmitter side we are using 4 major components-
- Single ended MAX 232
- General purpose NPN Transistor BC 548
- 555 timer
- 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 %
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.
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.
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