Data Transmission and Networks

1. Data Transmission and Networks Different Types of Data Transmission

2. Learning Objectives • Describe the characteristics and uses of Serial and Parallel transmission of data. • Describe the characteristics of Simplex, Duplex and Half Duplex data transmission. • Explain bit rate/baud rate. • Discuss the use of check sums and parity bits to reduce transmission errors. • Explain network protocols. • Explain the need for both machines involved in the data transmission / reception to be configured to use the same protocols. • Describe the layering of interfaces.

3. Data transmission • Data needs to be transmitted between devices in a computer system. • Data is transmitted in the form of bits. • So an 8 bit byte, which stands for a single character, will be transmitted in 8 parts, one signal for each bit.

4. Modes of Transmission • There are five possible different modes of transmission. • They fall into groups: • Serial / Parallel • Simplex / Half Duplex / Duplex

5. Serial transmission of data • One bit transmitted at a time using a single wire. • Advantage: • Simple and reliable because the next bit is not transmitted until the current one has arrived at its destination. • Disadvantage: • Slow, because only one bit can be transmitted at a time. e.g. 01101101 being transmitted.

6. Parallel Transmission Diagram • Bits sent at the same time using more than one wire. • Usually 8, so a whole byte can be sent at once. • Advantage: • Faster because all the bits are travelling at the same time. • Disadvantage: • Due to fine tolerances (resistances) in the transmission, it is less reliable as the bits can become muddled up (as they may arrive out of order).

7. Simplex mode • Data can only travel in one direction. • Used if only one direction is necessary. • e.g. Teletext information which is passed to a television receiver, but there is no way to send data in the other direction. Transmitter Receiver

8. Half duplex mode • Data can pass in both directions, but only in one direction at a time. • Used if both directions are necessary but not at the same time. • e.g. CB radio system in which each handset can either be set to receive mode or send mode. Device 1 Device 2

9. Duplex mode • Data can pass in both directions at the same time. • Used if both directions are necessary at the same time. • e.g. A telephone conversation as both users can speak and be heard at the same time. Device 1 Device 2

10. Bit / Baud Rate • The number of bits that can be sent in one second is known as the BIT RATE. • The units used to measure the bit rate are BAUD. • 1 baud = 1 bit per second.

11. Example • A picture stored in the memory of the computer needs to be sent down a telephone line to another computer. • High resolution (quality) means more pixels and more colours. • There is a limit to the amount of information that can be transmitted in a given time down the phone line. • Choice 1: • Have as much detail as possible and accept that it will take a long time to transmit it. • Choice 2: • Limit the resolution (quality) of the picture, meaning that there is less information to send so that the message will be sent more quickly.

12. Compression • Speeding up the transmission of the information by reducing the amount of data that is sent. • Text can be sent much more quickly than other forms of information because it needs far fewer bits than other types of data (1 byte per character). • When data other than text is being transmitted, e.g. on the internet, it is important to limit the amount of data that needs to be sent. • Data can be limited by reducing the size of pictures or restrict it to a few colours.

13. Importance of Bit / Baud Rate • More important the more data there is. • Different volumes per second mean that different transfer rates are appropriate.

14. Transmission error checking and correcting • Errors are inevitable when transmitting a large number of bits. • Luckily there are only two possible mistakes that can occur: Either • a 1 is received as a 0 Or • a 0 is received as a 1.

15. Echo Back • Send the data back again. • If the data that is sent back is the same as the data that was sent in the first place then the original data must have reached its destination correctly, if not then it needs to be sent again. • Advantage: • Very effective, • Disadvantage: • Data has to be sent twice: • Slow • Needs duplex (or at least half duplex) transmission mode to allow data transfer in both directions.

16. Parity bit • Used in every byte to force every byte to either have an odd or even number of 1s. • The Number of 1s in a byte must always be either an odd number or an even number. • As long as both devices agree to either even or odd parity then this can be checked by the receiving device after each byte is received.

17. Parity Example • If odd parity is agreed between two devices then if a byte is received that has an even number of 1s, an error must have occurred. • The byte 01011000 has 3 1s in it. • 3 is an odd number, so it fits the rule that it must have an odd number of 1s. • When it is sent there is an error in transmission so that the 1st bit is received as a one. • So, the byte received is 11011000. • This has 4 ones in it, which is an even number, so there must be an error. • The receiving device would ask for it to be sent again.

18. Notes: • If two mistakes are made in the same byte they will cancel each other out and the faulty data will be accepted. • The solution to this is not part of this course. • An ASCII character uses 8 bits in a byte, giving 256 different characters. • This is not true because one of the bits has to be reserved for a parity bit. • This means that there are 128 different characters possible. • Even parity can equally well be used, as long as it has been agreed between the two devices. • Parity is not only used during data transfer between devices, but also when data is transferred between different parts of the CPU.

19. Check Sum • Add all the bytes together that are being sent in a block of data. • Data will normally be sent as a block of bytes rather than as individual bytes. • The carry, out of the byte, is not taken into account, so the answer is an 8 bit number, just like the bytes. • This is calculated before the data is sent, and then calculated again when it is received. • If there are no errors in the transmission, the two answers will match.

20. Protocols • Rules to allow the transfer of data. • Typical rules: • Serial / Parallel • Errors would occur if one device had a serial connection and the other was expecting a parallel connection • Bit Rate • If one device sent data at a particular bit rate and the other device read what it received at a different rate, the message would never be received correctly. • Odd / Even Parity • If one device used even parity and the other device used odd then no correctly sent byte of information would ever be accepted.

21. Plenary • Can you explain the difference between Serial and Parallel transmission of data? • Serial • One bit transmitted at a time using a single wire. • Parallel • Bits sent at the same time using more than one wire.

22. Plenary • Can you explain Simplex, Duplex and HalfDuplex data transmission? • Simplex • Data can only travel in one direction. • Duplex • Data can pass in both directions at the same time. • HalfDuplex • Data can pass in both directions, but only in one direction at a time.

23. Plenary • Can you explain bit rate/baud rate? • The number of bits that can be sent in one second is known as the BIT RATE. • The units used to measure the bit rate are BAUD. • 1 baud = 1 bit per second.

24. Plenary • Can you explain the use of check sums and parity bits to reduce transmission errors? • Parity bits • Used in every byte to force every byte to either have an odd or even number of 1s. • Checksum • Add all the bytes together that are being sent in a block of data.

25. Plenary • What are protocols? • Rules to allow the transfer of data