1 Basics of Data Communications Discrete-time Communication vs Continuous-time Communication Is human communication continuous-time or discrete-time ? Voice vs. words Any piece of information can be represented in discrete-time signals ! What is Data Communications ? The exchange of digital information between two devices using an electronic transmission system. Using Binary number system to represent digital information

2 Basics of Data Communications Question: How to send the word “Yes” from Bob to Alice? Solution-1: Bob writes the letters “Y”, “e”, “s” on a piece of paper. (a 26-symbol alphabet is utilised in this solution) Solution-2: Bob nodes his head down once, and shakes his head to say “No”. (a 2-symbol alphabet is utilised in this solution) Question: Is it a coincidence that “Yes” and “No” are very short words in most human languages? Question: How to send the word “Yes” from computer-1 to computer-2?

3 Character Codes Much of the data communication discipline deals with representing information in terms of 0’s and 1’s. (Not as easy as it sounds !) Fact: Computers are digital logic systems that operate on a 2-symbol alphabet : {0, 1}. – Binary Number System There are many ways to represent characters and these different representation s are known as Character Codes. Mostly formed by ‘Zero’ and ‘One’

4 Morse Code: A variable-lenght character code “Yes”: -.-- |. | … Examples: silence interval to separate characters Character Codes

5 Baudot Code: A fixed-length character code Character Codes Letter code: switch to letters Figure code: switch to figures

6 Extended Binary Coded Decimal Interchange Code (EBCDIC): A fixed-length, 8-bit, character code. 256 different combinations of 1’s and 0’s does not utilise all possible combinations only a subset is displayed here no parity Character Codes

7 American National Standard Code for Information Exchange (ASCII): A fixed-length, 7-bit, character code. 128 different combinations of 1’s and 0’s only a subset is displayed here can be used with a one-bit parity check 1+7=8 bits =1 byte there is also extended-ASCII that uses 8 bits instead of 7 bits.

8 1 byte = 8 bits bit-8 = parity check bit bit-1 bit-7 bit-1 to 7 = data bit previous byte … Character Codes

9 Parity Check Odd-Parity = number of bit ‘1’ must be odd bit-1bit bit bit bit or bit-1

10 Parity Check Even-Parity = number of bit ‘1’ must be even bit-1bit bit bit bit or bit-1

11 WCB/McGraw-Hill

12 Parallel Transmission WCB/McGraw-Hill

13 Serial Transmission WCB/McGraw-Hill

14 Parallel vs. Serial Transmission Parallel Transmission Serial Transmission Faster Expensive Suitable for short distances Slower Cheaper Better for long distance

15 Synchronous vs. Asynchronous Transmission Synchronous Transmission However the block must be preceded by SYNC bytes or a separate clock line must be provided !

16 Synchronous vs. Asynchronous Transmission Asynchronous Transmission

17 Synchronous vs. Asynchronous Transmission Total bits sent = data-bits + control-bits Transmission or Throughput

18 If a Synchronous transmission involves sending 480 bits plus an overhead of 20 control bits for synchronisation, what is the efficiency of the transmission? If 480 characters are sent using Asynchronous transmission, what is the transmission efficiency? Example Synchronous transmission: Efficiency = 480/(480+20) = 96% Asynchronous transmission: Efficiency = 80% (if 8 bits is used for data bits)

19 Transmission Modes: Simplex, Half-Duplex, Full-Duplex Simplex Transmission

20 Transmission Modes: Simplex, Half-Duplex, Full-Duplex Half-Duplex Transmission

21 Transmission Modes: Simplex, Half-Duplex, Full-Duplex Full-Duplex Transmission

22 Tutorial 2 What is a major difference between Morse code and ASCII code? How many characters can Baudot code represent? How many characters can ASCII code represent? What is a parity bit? What is parallel transmission? When do we use parallel transmission?

23 What is serial transmission? When do we use serial transmission? What is asynchrorous transmission? What is synchrorous transmission? Why asynchronous transmission always has 80% efficiency? If a synchronous transmission involves sending 1000 characters and an overhead of 10 characters are needed for handshake, what is the efficiency of the transmission?

24 What is simplex communication? Give examples. What is half duplex communication? Give examples. What is full duplex communication? Give examples.