The DSSS (Direct Sequence Spread Spectrum) is a technique spread spectrum, but unlike the FHSS, no frequency hopping is place: DSSS causes very rapid state transitions (chipping) which tend to spread the spectrum of the signal: in fact, we have seen that with the modulations FSK, PSK and QAM width of the spectrum was twice the rate of the source. Causing “artificially” very high throughput, spectrum is spread.

To do this, the transmitter sends a sequence of several bits, called chips, for each information bit to be transmitted. For example, we may choose to send 11101 instead of 0 and its inverse (00010) instead of one: in this case, if one wants to transmit information 010, then we will issue the following chips: 11101 00010 11101. In this example, the 11101 sequence is called the “spreading code”. Over this code, the longer the flow is artificially multiplied, so more spectrum is spread. For example, if the flow of data to be sent is 1 Mb / s, but we uses a spreading code of 11 chips, then the chips flow will of course be equal to 11 Mb / s: suddenly, the frequency band occupied by the signal will have a width equal to 22 MHz because the width of the band occupied by the signal is equal to twice the speed of source. Without chipping the occupied band would have only a width of 2 MHz (two After 1 Mb/s).

DSSS has two important interests:

• First, as we have said, the signal spectrum is spread, with all the advantages (and disadvantages) that this brings, especially better resistance to noise;
• in that it emits several chips for each bit of information means that can have a significant redundancy that corrects errors transmission. For example, in the previous example, since the receiver knows the spreading code used (11101) then he knows he should receive than 11101 (for the information bit 0) or 00010 (for bit 1). If it receives 00110, it can easily correct the error in considering that the nearest is 00010 (corresponding to bit 1).

802.11 identified fourteen channels of 22 MHz wide, in the same strip of frequencies in the 2.4 GHz FHSS. To communicate, the transmitter and receiver must agree on a fixed channel to use. A flow rate of 1 Mb / s, the 802.11 DSSS modulation based on 2DPSK we saw the 2 Mbit / s DSSS simply uses the 4DPSK modulation.

In both cases, the spreading code has a length of 11 bits and it is always equal to 10110111000. This code is part of a family of codes to mathematical properties Similar defined in 1953 by the mathematician Barker: they promote a good spread spectrum (as would not do, for example, the code 11111111111) and format makes them well suited to synchronize the transmitter and the receiver, which limits problems due to multipath.

CCK modulation

To achieve data rates of 5.5 Mb / s or 11 Mb / s, the further improved the 802.11b process using the modulation Complementary Code Keying (CCK) to reach the so-called high-speed or high-rate DSSS DSSS (HR-DSSS). This one still based on the same basic principle spreading by chipping with 4DPSK modulation. However, instead of always using the same code for Barker spread the signal, it uses up to 64 different codes, which can be transported 6 information bits (because 26 = 64) in addition to the two bits allowed by the modulation 4DPSK. These codes of 8 bits in length each, are “complementary codes” that is to say, their mathematical properties allow receivers not confused, even if there is some transmission errors and even an offset in the reception due to multipath. Since there is significantly less redundancy, it gets more flow, at least as good reception (so short distance). Since the resistance to multipath is better, HR-DSSS is best suited for indoor and short distances on the DSSS Barker.

Unfortunately, when the FHSS can jump congested channels of noise or interference, DSSS can not: If there are other wireless networks operator near the same channel, DSSS will suffer greatly. knowing that Bluetooth technology is based on the FHSS on the same frequencies to 2.4 GHz we understand why the 802.11 DSSS suffers from the presence of Bluetooth devices near. However, Bluetooth support about the presence of equipment 802.11 DSSS. To summarize, the DSSS supports more homogeneous noise (noise “White”) that the FHSS and vice versa, the FHSS better support focused noise on a particular frequency as the DSSS.

As with FHSS, the standard defines for the DSSS a coping mechanism automatic flow depending on the distance. Thus, short modulation HR-DSSS will be 11 Mb / s (8-bit information to 8 chips issued). Moreover, further, one automatically to 5.5 Mb / s (4 bits of information for 8 chips issued). Then, down to 2 Mb / s using the DSSS / 4DPSK Barker and then to 1 Mb / s DSSS / Barker and 2DPSK.

The FHSS (Frequency Hopping Spread Spectrum) was invented and patented in 1942 by the actress Hedy Lamarr and pianist George Antheil, which were quite versatile! The principle of FHSS is quite simple: a wide frequency band is divided into multiple channels and communications are jumping (hopping) sequentially from one channel to another in a sequence and a rate agreed to advance between the transmitter and the receiver.

It is difficult to intercept communications if we do not know the sequence chosen, so it was much appreciated by the US military who used for radio guide torpedoes without the enemy to intercept or jam the signal. In the case of 802.11, this function is (unfortunately) not operating because the channel sequences used are not secret.
FHSS also provides significant resistance to interference or even volunteers for interference channels where the noise is too large can simply be avoided. However, 802.11 FHSS does not exploit this ability, unlike Bluetooth and Home RF are two technologies Wireless using FHSS modulation.
A final advantage is that many of the FHSS communications may held simultaneously on the same frequency band as long as they use channel sequences not colliding with each other. For example, communication could use the trivial sequence 1,2,3,1,2,3,1,2,3 while … another communication would have the following sequence: 2,3,1,2,3,1,2,3,1 … so at no time did the two communications using the same channel.
In return, each communication has a relatively low rate since operates only one fairly narrow channel at a time.
In the first version of 802.11, the frequency range from 2400 MHz 2483.5 MHz was cut for the FHSS channels of 1 MHz wide each.

        
In most countries, the channels 2-80 are allowed (from 2401 MHz to 2480 MHz). Within each channel, modulation Gaussian FSK with two states (2GFSK) is used and allows a flow rate of 1 Mb/s. Using 4GFSK modulation (GFSK four states, or 2 bits per symbol) you can reach 2 Mb / s. Using GFSK as a modulation underlying FHSS avoids interference between neighboring channels, allowing multiple users to communicate in FHSS same time without interfering.
802.11 has defined a dynamic rate adaptation mechanism Depending on the signal / noise ratio: When high, the modulation used is 4GFSK to 2 Mb / s, otherwise the 802.11 adapts automatically and “down” in 2GFSK 1 Mb/s.

The basic modulations

No modulation

Take the example of an opera broadcast on a radio station: how music, that is to say, an audio signal, it may be channeled through wave electromagnetic? Audible sound waves have frequencies between 20 Hz to 20 kHz for bass and treble. It would be tempting to simply convert the sound wave in radio waves of the same frequency. Unfortunately, there would be several problems: first, the low radio frequencies also are very difficult to produce and capture; then two simultaneous radios would overlap since they would be issued on the same frequency band (20 Hz to 20 kHz) and would cacophony. [Read more…] about Type of Radio Modulations

This was first created by an English mathematician named George Boole in 1847 and yet, in one of the strange quirks of the computer industry, is now used in everything from circuit design to searching the Internet. In essence Boole developed a system for reducing complex questions into simple yes or no answers by using what are known as truth tables, sometimes called gates.

Although these truth tables or gates can take several forms the three that are relevant to Internet searches are AND, OR and NOT. Each one operates differently.

and

This produces an output only if two specified conditions are met. For example if a search was made on the words ‘Economic’ AND ‘Policy’ the truth table would be as follows:

1st Condition            2nd Condition               Output

(Economic)           (Policy)

NO                            NO                                           NO

YES                          NO                                           NO

NO                            YES                                         NO

YES                          YES                                         YES

Only if both words were found on the same website would a match be returned.

or

This produces an output if either specified condition is met. In the same example as above the truth table would be:

1st Condition        2nd Condition                  Output

(Economic)           (Policy)

NO                              NO                                            NO

YES                            NO                                            YES

NO                              YES                                          YES

YES                            YES                                           YES

If either word is found on a website a match would be returned.

not

This returns an output only if a specified condition is not met. To continue the above example if a search was made on the words ‘Economic’ NOT ‘Policy’ the truth table would be:

1st Condition             2nd Condition               Output

(Economic)                (Policy)

YES                                  YES                                       NO

NO                                    YES                                       NO

YES                                   NO                                        YES

NO                                     NO                                        NO

Only websites which contain the word ‘Economic’ but do not contain the word ‘Policy’ will be returned.

In electronics the output from one set of gates can become the input to others to produce complex switching arrangements which forms the basis of the computer microchip itself. Away from that it is by combining these various logical operators (as AND, OR and NOT are known) that highly precise Internet searches can be conducted, as will soon become clear.

(Metal Oxide Semiconductor Field- Effect Transistor) The fundamental building block of modern VLSI chips such as microprocessors and memory chips. A microscopically small FIELD-EFFECT TRANSISTOR formed on the surface of a prepared silicon wafer by exposing it to a succession of chemical treatments through a sequence of masks.

Each transistor consists of a GATE formed from POLYSILICON separated by a thin layer of silicon oxide insulation from the underlying DIFFUSION LAYER in which the SOURCE and DRAIN are formed by DOPING the silicon surface.

Thin tracks formed in a final metal layer (of aluminium or copper) join all these transistors together to form a complex electronic circuit.

A digital logic circuit that can store a single BIT of information, and is therefore used as the basis for the construction of MEMORY chips, LATCHES and the REGISTERS within processors. A flip-flop can exist in two states, with either a high or low voltage at its output, and flips from one state to the other at each pulse of a CLOCK SIGNAL. Two different implementations of flip-flop are commonly used, called the D flip flop and the j-K flip flop.

DIP switches are those nasty, tiny plastic toggle switches that come mounted together in a row on a little box-like part attached to your computer’s motherboard or on some of your add-in boards. Each individual switch in a DIP switch unit can be set either on or off, allowing you to control some aspect of your computer’s function. For example, on the older IBM pcs and compatibles, you had to set the DIP switches to match the amount of memory installed in your computer, to tell it what kind of monitor you had, and so on. On the add-in modem board I just installed, I had to flip some of the DIP switches so the computer would know where to find the board electronically.

DIP switches are a monumental pain. For one thing, they’re so small it’s hard to switch them. Use the tip of a ballpoint pen, an unfolded paper clip, a baby screwdriver blade, or some other fairly pointy, narrow tool. Worse yet, they’re always mounted in some out-of-the-way place, like on the back of your computer or inside it. Even if you can get to the switches without opening up the computer, you have to pull the machine out from the wall as far as it will go, cram your head back there, and just hope you’ll switch the right switch. And the labels that tell you which direction is on or off are horrible. The writing is too small to see, and usually it says 1 and 0 instead of ON and OFF (yes, 1 [one] means on, 0 [zero] means off).

The point is, if two products are otherwise equal in features and price, choose the one that doesn’t have DIP switches. It will set itself up automatically, or maybe it comes with a software utility that lets you set it up by choosing options on your computer screen. 

DIP is an acronym that stands for dual inline package and refers to the physical layout of most computer chips. The standard chip looks like a bug, or a little rectangular box with legs. Those two rows of metal legs are the “pins” that connect the chip to the computer’s circuits. The whole chip is referred to as a “package,” and it’s a dual inline package because the pins are lined up in two rows. 

A chip is that truly amazing and remarkably tiny piece of silicon that has an entire integrated electronic circuit embedded within it. Chips are what make the computer. Chips are the computer. A tiny chip is one of the biggest pieces of human-made magic on earth. There are different kinds of chips, the most common being the microprocessors which run the whole computer, and memory chips, in which the computer holds and works with your information until you send it to a disk.

The chip that runs many pcs, the 80486, has the equivalent electronic power-in its baby-fingernail sized wafer-of one of those room-sized mainframe computers from twenty years ago. The 80486 can be manufactures for just a couple hundred dollars; the mainframes of twenty years ago cost several million dollars. The impact on civilization and humankind of this mass-produced, inexpensive technology is going to be comparable to the impact of the technology of mass-produced, inexpensive books in the fifteenth century.

A breadboard is a thin board, sometimes fiberglass and sometimes plastic, with lots of little holes arranged in a grid. Electronics engineers use breadboards to create prototypes of circuit boards by wiring chips, resistors, and other electronic parts onto the board by hand, with the connecting wires running underneath. The term sometimes refers to the finished prototype itself, but more often it’s applied to the hole-filled board to which the electronic components are attached.

Boolean logic A formal logic system derived from the BOOLEAN ALGEBRA by interpreting its two permissible values 0and 1 as the TRUTH VALUES True and False. It is used in electronics to define the behavior of all the kinds of LOGIC GATE from which computer processors are constructed, and in programming to define operators that work on truth-valued variables.

It’s used to solve the kind of problems you buy a database program to handle: like, “show me the list of men who dance, and who cook or like washing dishes, and who are not married.” (Boolean logic is big on expressions such as and, or, and not.) You use Boolean logic every time you search your database. Boolean logic comes in very handy on a digital computer (which is what you have) because each answer can be a bit that is either on or off, true or false. The computer uses Boolean logic in a great deal of its work.

Analogue-to-digital converter (AID converter, ADC) An important type of electronic circuit that inputs an ANALOGUE signal and outputs a stream of bits that reproduces that input signal in the digital domain. Conversion involves sampling the strength of the input signal at very short intervals and expressing the sampled value as a binary number. ADCs are widely employed in electronic equipment of many kinds, including digital cameras and camcorders, telephones and MODEMS, and the SOUNDCARDS used in personal computers.

The two most important parameters in describing an ADC are the size (in bits) of the samples it takes, and how many samples it takes per second.

For example to digitize an analogue microphone signal at CD quality requires an ADC capable of taking 44 million 16-bit samples per second.

1. A physical object or quantity, for example a moving clock hand or an electrical voltage, that is used to measure or represent some other quantity, and is hence analogous to it.

2 A family of electronic devices that represent other physical quantities by continuously varying voltages rather than the two discrete voltage levels used in DIGITAL devices.

For example the output voltage of an analogue microphone follows exactly the changes in sound pressure of a speaker’s voice. All telephone, radio and television systems prior to the 1990S were based on analogue electronics.

The disadvantage of analogue devices compared to digital ones is that any physical effect that changes the voltage – such as non-linearity in the conductors, stray electrical or magnetic fields, power supply fluctuations – will corrupt the data. The advantage of analogue devices is that they may operate faster than digital devices, or store data more densely, since they avoid the overheads incurred in digitizing the data.

Amplitude Size, magnitude, extent. When used of a WAVEFORM it refers to the height of the wave, measured from top to bottom of the following trough.

Amplitude modulation: The transmission of information by altering the magnitude of the successive cycles of a CARRIER wave, as opposed to FREQUENCY MODULATION where it is the spacing of successive cycles that encodes the information.

An electronic circuit that increases the amplitude of its input signal and outputs the result. The most familiar type is the audio amplifier, as found in domestic hi fi or telephones, but amplifiers are to be found in every sort of device, including silicon chips:

for example there is a microscopic amplifier in each of the thousands of cell rows on a RAM chip.

An electrical current that periodically reverses its direction. The electrical mains supply is an alternating current that varies in a sinusoidal fashion, reversing itself 50 times per second in the UK and Europe, and 60 times per second in the USA.

Digital signals are SQUARE-WAVES that do not actually change direction, but switch between, for example 0 and 5 volts; in their electrical properties, however, they behave more like alternating currents than DIRECT CURRENTS.

A-to-D conversion (analog-to-digital conversion) is the process of converting analog information from life into digital information that a computer can understand. Most information in nature is in analog form, covering a continuous range of values (such as the passing of time, as exemplified by the hands of a regular clock).

Digital information is in discrete, or separate, values, such as the countable chunks of seconds and minutes in a digital clock. You might want to use a computer to record analog information from nature by hooking it up to some kind of sensor like a light or pressure meter. But to store this information in the computer, the analog readings from these meters must be passed through an A-to-D converter to turn them into a digital form.

8088

The 8088 is the chip that launched the PC revolution-IBM designed the original IBMPC around the 8088 microprocessor. The 8088 was also used in the PC/XT. Even by the standards of its day, the 8088 was slow, and it suffered from technical shortcomings that have bedeviled PC software developers ever since.

8086

The 8086 is identical to the 8088 except that it can access twice as much data in one gulp, and that makes it faster. (In technical terms, the 8086 has a 16-bit data bus, while the 8088 has an 8-bit bus.) IBM chose to use the 8088 instead of the faster 8086 because in those days the 8-bit support chips for the 8088 were substantially cheaper than the 16-bit support chips required for the 8086. Hence IBM could build a less expensive machine, which was crucial to their marketing plans. A few other manufacturers such as Epson did go with the 8086, making their PCs inherently a little faster.

80186

The 80186 is just an 8086 with some extra circuits added so that fewer “helper” chips are needed. Very few pcs used this chip.

Practically speaking, the 80486’s main advantage over the 80386 is just that the 486 is faster. Even if you compare the two chips running at the same clock speed, the 486 will generally finish its calculations sooner, sometimes much sooner, than the 386.

There are at least three versions of the 80486 that you may run across. The standard model, called the DX, includes a built-in math coprocessor, whereas the sx doesn’t. pcs based on the sx are a little cheaper, but if you decide to add a math coprocessor later (it’s easy to do), you’ll spend more than if you bought a DX machine to start with. And there’s the DX2.This model does its internal computations twice as fast as a standard DX running at the same clock speed, slowing back down when it’s time to move data to or from memory. The DX2is decidedly faster than a standard 486DX,and not much more expensive.

The 80386 chip has one big advantage over the 80286: it includes fully functional, built-in circuits for multi-tasking, or running two or more programs at the same time. Although software like Windows, DesQview, or Unix will let you “multi-task” using lowlier chips, the 80386 does it more reliably and with less hassle. Windows, for example, running in “386 Enhanced Mode,” lets you use multiple standard DOS programs at the same time, whereas you’re limited to only one DOS program at a time with an 80286. Another improvement in the 80386 is its ability to access even larger amounts of memory than the 80286, and to do so with less trouble. Again, DOS is oblivious to the extra memory, but you can buy software that bypasses this limitation through a DOS extender.

The 80386 comes in two versions: the standard model, called the ox, and the cheaper, slightly slower SX. The only difference is that the DX accesses data 32 bits at a time (it has a 32-bit bus), while the sx can only move 16 bits at once. That makes a PC based on the 386DXsomewhat faster than a PC that uses a 386sx- but nowhere near twice as fast, since both chips do everything else with equal speed.

The 80286 was the microprocessor used in IBM’SPC/AT. The 80286 calculates faster than the 8088. It also corrected one of the biggest problems with the 8088-its inability to access more than a small amount(1 megabyte) of memory. But-and this is a big catch-DOS couldn’t takeadvantage of the extra memory. Why, you ask? Well, because DOS had torun on 8088-based computers too, so it couldn’t be changed to accommodatethe new chip. Windows, however, does utilize the special talents ofthe 80286 in “standard mode.” In fact, you can’t run Windows 3.1 unlessyour PC has an 80286 or an even newer microprocessor. However, there isa way to create software that uses the extra memory but still manages torun in DOS.