Digital Communication Lecture 8
What is Digital Communication? • In the design of large and complex digital systems, it is often necessary to have one device communicating to and from other devices. • One advantage of digital information is that it tends to be far more resistant to errors than analog information. • This accounts for the clarity of digitally-encoded telephone connections, digital CD’s, Digital TV’s and so on. • However, digital communication has its own unique pitfalls.
One Example.. • You speak to your friend, he listens to you and understand what you are saying –How? • As you speak, a wave is generated that travels though the medium(air) and reaches to your friend’s ear drums. • This is called analog communication • So in analog communication a wave is used to carry the message from one place to other • But suppose, we ask you to tell something to your friend, but, without speaking then? • See the next slide..
You can use a torch(light source), and you can settle with your friend in advance that - • If you see torch light ON-OFF once, that means ‘H’ • If you see torch light ON-OFF and ON-OFF twice, that means ‘I’. • (ON-OFF) -> ‘H’ • (ON-OFF) and (ON-OFF) ->’I’ • Now, suppose you want to say ‘HI’ to him, then following sequence will do the trick • (ON-OFF) WAIT (ON-OFF) (ON-OFF) • This is called digital communication-----NO WAVE
Why Digital Communication Better • An analog wave may get distorted or may become weak and you may not understand what message has been sent. • For example can you speak something to your friend who is half kilometer away from you? • You sought..he may listen to you but not clearly • But again if you use a torch? • Your torch light may look dull to him, but if he could see ON-OFF (even dull) that means ‘H’…….. not ‘P’. • Here, weakness of signal can’t change his understanding • That means digital signals are easy to reproduce
Computer’s Language • That is how computers communicate • Using a long sequence of ON-OFF. • Sensor in the computer’s circuit detect availability/Non-availability of the electricity in the circuit • Using this sequence information is encoded, decoded and sent from one place to other • (ON-OFF), (ON-OFF, ON-OFF), (ON-OFF) • ON can be represented by 1, and OFF by 0. • That’s why we say that computer understands only 1 and 0 (binary language) • 10101010
Examples of digital communication • E-mailing • - Computers • Texting • - Cell Phones • Online games • - Web Kinz • -Club -Penguin
E-Mail • Electronic mail is often abbreviated to e-mail • Email is a store-and-forward method of writing, sending, receiving and saving messages through computers
Texting • The common term for the sending of "short“ messages from mobile phones or portable devices is called texting • Texting works using the Short Message Service (SMS)
Why Digital Communication? • Fast and Inexpensive. • Privacy by using data encryption. • Greater dynamic range of power. • Less immune to noise • Reproduction of message is extremely reliable. • Less prone to errors. • Large storage.
A communication Model • Source - generates data to be transmitted, examples are telephones and computers • Transmitter - converts data into transmittable signals • Transmission System - carries data from source to destination • Receiver - converts received signal into data • Destination - takes incoming data
Cellular telephone system • The cellular mobile telephone system consists of: • Mobile Stations (MS), Base Stations (BS) and Mobile Switching Center (MSC), connected to the Public Switching Telephone Network (PSTN)
Satellite Communication system • The information-bearing signal is transmitted from the earth terminal to the satellite via the uplink, amplified by the transponder (electronic circuitry on board the satellite), and then retransmitted from the satellite via the downlink to the other earth terminal
Computer Networks and the Internet • Figure in the next slide illustrates the network communication • Key elements of network communications are server, hosts, switches, routers , cables etc • Most hosts that use the Internet are connected to a network, such as a local area network (LAN) or a wide area network (WAN). These networks are in turn connected by routers.
Digital Communication: Some Historical Background • 1844– The Telegraph was invented by Samuel Morse • 1875 – The Telephone was invented by Alexander Graham Bell • 1887 – Heinrich Hertz confirmed the existence of radio waves • 1901 – Marconi received a radio signal, 1700 miles across the Atlantic • 1918 – Edwin Armstrong invented the radio receiver • 1928 – The Television system was demonstrated by Philo Farnsworth • 1933 – – The first computer, ENIAC, was built at Pennsylvania university • 1962 – The Telstar satellite, built by Bell Laboratories, was lunched • 1971 – The first computer network, called the ARPANET, was built • 1985 – The ARPANET was renamed the Internet • 1983 - Advanced Mobile Phone System (AMPS) was lunched in US • 1991 - Global System for Mobile (GSM) was lunched in Europe