1. Communication methods
Synchronous and asynchronous data transmission

2. Synchronous data transmission
Synchronous means occurring at the same time.
Synchronous transmission means that the two devices that are communicating must send and receive the data at the same time.
The system clock is used to time the transmission signals.
The system clocks of the sender and receiver must be synchronised.
Data can then be passed between them without the need for any further control information.
© 2005 Bob Reeves, Dave Fogg/Hodder Murray

3. Asynchronous data transmission
In asynchronous transmission, the transmission is not synchronised between sender and receiver.
Additional bits are added to the data being transmitted, telling the device what to do with the data.
A start bit and a stop bit are added, which tell the receiving device where the data item starts and stops.
A parity bit is usually added as a method of checking whether the data has been received correctly – i.e. it is a data integrity check.
© 2005 Bob Reeves, Dave Fogg/Hodder Murray

4. Example of asynchronous transmission
An 8-bit code is to be transmitted:
The start bit indicates the start of the data.
The data itself is transmitted (along with a parity bit).
The stop bit indicates the end of the data item.
The next piece of data is then transmitted and the process continues.
© 2005 Bob Reeves, Dave Fogg/Hodder Murray

5. Using asynchronous transmission
Asynchronous transmission allows many different computers and devices to communicate where direct synchronisation would not be possible, for example, across the Internet.
It uses more bits – our example used 11 bits to send 8 bits of data.
This reduces the bit rate and explains why a 56kbps modem may only achieve a bit rate of 45kbps.
© 2005 Bob Reeves, Dave Fogg/Hodder Murray

6. Parity bits
The parity bit is used to try to identify any errors that occur during transmission.
All data is a series of zeros and ones and any corruption of the binary codes during transmission could corrupt the data.
A parity bit is an additional bit added to each data item being transmitted.
The parity bit is either a zero or a one depending on how many ones there are in the code being transmitted.
© 2005 Bob Reeves, Dave Fogg/Hodder Murray

7. Even parity
In even parity, the parity bit is set to either 0 or 1 to ensure that the number of ones in the code is an even number.
© 2005 Bob Reeves, Dave Fogg/Hodder Murray

8. Odd Parity
In odd parity, the parity bit is set to either 0 or 1 to ensure that the number of ones in the code is an odd number.
© 2005 Bob Reeves, Dave Fogg/Hodder Murray

9. Using parity
When the code is received, the parity bit is checked to ensure that the number of ones is even (assuming even parity) or odd (assuming odd parity).
However:
corruption of the data could still occur and the number of ones remain the same so the error would not be spotted
the parity bit itself may be corrupt
parity only identifies that corruption of the data has occurred; it does not tell you the specifics of what has been corrupted.
© 2005 Bob Reeves, Dave Fogg/Hodder Murray

10. Handshaking and protocols
Handshaking and protocols are methods used to ensure that different devices can communicate and that there are sets of rules that everyone follows to ensure data is sent and received correctly.
They are needed because:
there are so many different transmissions that take place between different types of hardware and software
there are so many ways of handling and transmitting data.
Handshaking and protocols are particularly important now, because of the widespread use of the Internet.
© 2005 Bob Reeves, Dave Fogg/Hodder Murray

11. Handshaking
Handshaking is the process of two devices sending signals to each other to indicate that they are ready to send and/or receive data.
It is similar to a human handshake in that contact is made and an arrangement is confirmed.
It confirms the speed of the transmission and the protocols that will be used.
© 2005 Bob Reeves, Dave Fogg/Hodder Murray

12. Protocols
Protocols are sets of rules that apply to the transmission of data.
Examples of protocols include:
TCP/IP (Transmission Control Protocol/Internet Protocol): the set of rule that govern the transmission of packets of data around the Internet
HTTP (Hypertext Transfer Protocol): the set of rules governing the way in which web content is transmitted and formatted
FTP (File Transfer Protocol): the set of rules governing the transfer of files on the Internet.
© 2005 Bob Reeves, Dave Fogg/Hodder Murray