The BInary digiT (AKA BIT) is intrinsic in any reference to Communication. It is the smallest quantum ofinformation that can be represented as one of the two values. The outcome of toss of a coin, heads or tails, can be represented by 0 or 1, and thus be encoded by a bit. The Bit has so certainly transformed our lives, working transparently, making our lives beautiful. The Bit is omnipresent and permeable to anything imaginable. It can climb mountains, cross oceans, travel through thin air, and all in a spur of the moment. If the Bit was tangible, you would find yourself surrounded by bits carrying information. One instance being when you are talking on your cellphone, streams of bits emanating from your cellphone into the air.
Would you like to accompany the Bit as it carries your "Hi" to your friend on the other end of the call on thecellphone. The journey is not a simple one, the Bit has to undergo many transformations and be the subject ofmany algorithms. An effort has been made to keep the description as lucid as possible, brief and away from technical jargon.
Birth of the Bit
The words you speak are captured by the tiny microphone and converted to a voltage. The current as a result ofthis voltage is driven into an electronic circuit which coverts the Analog signal into a Digital Signal. The process converts the continuous value signal into a quantised signal with definite values and encoded into binary which make it much easier to process by the electronic circuits. This is like measuring the wind speed. A sample of thewind speed is taken after every definite interval and approximated to the nearest multiple of 2 and then the value converted to binary. The whole process converts the message spoken into the microphone into a series of Bits. TheBit is born into the Digital world.Getting ready for journey to the outside world
Now is the time to prepare the bit for its journey out of the device and into the Digital Spectrum. And believe me its not at all a friendly place for the Bit to be. It has to be protected from hostile environment, so that it reaches its destination safely and can be reassembled. It is like moving a house. Different articles have to be considered. Disassembled and categorised to help reassembly and occupy less space, packed according to thefragility, labeled so that they can be put together correctly, transported by means which are most economical and reliable.The Bit that was described above is part of a huge stream of bits that constitute the basebandsignal, upon which allthe Signal Processing is applied. The bit becomes the part of a bigger chunk onto which the encoding algorithms are applied. If you have to move a number of books, it is advisable to pack those into boxes, with a few books in each box, so that these are easier to handle while transporting. Similar is the case with our bit.
Intermingling with other communities
The above operations result into data stream from one source which is now ready to be multiplexed with data streams from other sources. These other sources might be a browsing session that was started on the device before making the call. It is similar to the shipping company which collects shipments from many sources and multiplexes them into a single big shipment to the common destination.The multiplexing is done by means of assigning a code to every source that is making a contribution to the final signal to be transmitted. This code for a sender-receiver pair and is known by the receiver. The receiver can applythe same code onto the received signal and single out the data that was meant for it. The process is called Channelisation and the codes, Channelisation. It can be thought of as assigning a specific color for the boxes being sent from a sender A to recipient B. A and B share the same color, so that when the shipment arrives at B, theboxes meant for B can be easily separated.
Combating the threats of the big world
There are beasts lurking in Wireless spectrum, eager to deplete and deform the signal. Noise, Attenuation and Interference are some of the names of these beasts. To protect the signal, it undergoes another transformation. Each bit in the data stream is multiplied with another code which transforms the bit into chips. The process is called Spreading and the codes, Spreading Codes. These chips are much more resilient to Noise and attenuation inthe Wireless Spectrum. The code used for this transformation is known at the receiver, so that the received chips can coalesced into bits.There are other techniques too in addition to the above to reverse the damage caused by the Spectrum. Some ofthese are error detection and error correction bits, Precoding and Signal shaping, etc.
Hopping onto the Train to the City
The resulting amalgamated signal is now ready to explore the outside world. But it needs a carrier go enter intothe Wireless Spectrum. The stream of chips resulting from the above process board the carrier by means ofModulation. The instantaneous property of the carrier, which can be frequency, amplitude, phase or a combinationof these, is effected by individual chips which are boarding the carrier. The modulated carrier is converted into a radio wave and transmitted into Spectrum by the antenna.Losing Identity in the crowd
The original bit is now a part of an analog radio wave. It has lost its digital properties by now. But as result it has increased its chances to reach its destinationGoing through the big junction
The signal transmitted into the air is received by the big antenna at the Base Station. The Base Station and thenetwork beyond this relays the signal back to the receiving cell phone. In this way acts like a big junction for trains.The details of these are omitted here to keep the discussion brief.Touching Destination
The radio waves now hit the antenna of the receiving cellphone. The Signal has touched ground again. Here it undergoes a marathon of electronic circuitry to recover and cleanse it from the effects of the Spectrum. Some ofthe techniques applied are Low pass filters, Signal Shaping, Amplifiers.Rediscovering Self
The resulting signal is then passed through an Analog to Digital Converter, and alas, the signal rejoices after entering familiar Digital Realm again. It has still to go through a lot to attain the form of bits.Unwinding from the journey
All the processes and algorithms applied on the bit stream at the transmitter have to be reversed at the receiver in order to recover the baseband signal, which constitutes the original bits stream in which our bit is hidden. This includes demodulation, despreading, applying all the error detection and error correction techniques.This is like hearing from the packers and movers that you stuff has arrived in your new home. They will arrive with their big truck, bring out the stuff that belongs to you, and start unpacking it. The stuff then goes into place according to the label on it.
Serving its purpose
The bit is happy to find its original form now. It is part a word spoken by you into your cellphone and is now ready to become a voice again. This time emanating from the speaker on your friend's cellphone. The bit might not result into more than a small peep on the speaker, but along with millions of its look alike, has served the purpose well by carrying your message across to your friend.All of the above occurs in a matter of microseconds, without you realising what chain of activities you triggered by making that phone call.
Enjoy reaping the benefits of The Communication Age.
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