Chapter 2: Communication and Internet technologies

There are 3 factors in data transmission:

1. The direction

2. The method of transmission

3. The method of synchronization

Direction:

1.       Simplex – only in 1 direction (example: data from PC to printer)

2.       Half-Duplex – in 2 directions but not simultaneously (example: a phone conversation)

3.       Full-Duplex – in 2 directions simultaneously (example: broadband connection on a phone line)

Method of transmission:

Serial: 1 bit at a time. Less error-prone but slow.

Parallel:  8 bits (1 byte) at a time. Fast but error-prone.

Method of synchronization:

1.       Asynchronous: Data is transferred in an agreed pattern. Data bits are grouped together and sent with control bits. Less error-prone but slow.

2.       Synchronous: Data is transferred as a continuous stream of data. Data is sent by timing signals generated by a clock. Fast but error-prone.

Error-checking:

1.       Parity bits: even and odd parity.

2.       Automatic Request Query (ARQ): Data is sent and the sender waits for a ‘Acknowledgement’ from the receiver stating that the data sent is correct. If it does not come within the ‘Timeout’ limit, the data is sent again.

3.       Checksum: It is a digit representing the sum of the correct digits in a piece of stored or transmitted digital data, against which later comparisons can be made to detect errors in the data. To find checksum, take the sum of all bytes, then divide the sum by 256, then round it off to the nearest whole number, then multiply that whole number by 256 and then subtract the product from the original sum of the bits. That is the checksum for that data packet.

4.       Echo check: The receiver sends the data back to the sender then that data is compared to the original data. If it doesn’t match, then data is sent again to the receiver. There is a high-risk of errors in this because it can’t be determined whether the mistake in the data occurred when the data was being transferred the first time or the second time.

Internet Service Provider (ISP):

These are companies that provide users with access to the Internet. There are plans that decide the amount of bandwidth to be allocated to each user. The users are provided with a username and a password to make the Internet work on their devices.

Bandwidth:

The transmission capacity of a computer network or other telecommunication system.

IP Address:

A unique string of numbers separated by full stops that identifies each computer using the Internet Protocol to communicate over a network.

IP Addresses remain only for one Internet session, and then they are changed, whereas MAC Addresses are not changed during subsequent Internet sessions.

HTTP:

This is the data transfer protocol that must be obeyed while transferring data over the Internet using this protocol.

Web browsers:

This allows a user to display a web page on their device screen. Web browsers translate HTML coding and show the results.

Web servers are translated to IP Addresses, which is part of the URL.