Protocols define the format, timing sequence and error checking used on network.



Two computers must run compatible protocol stacks before they can communicate, because each layer in one computer’s protocol stack must interact with a corresponding layer in the other computer’s protocol stack. For example, refer to Figure 2.0. It shows the path of a message that starts in the Transport layer. The message travels down the protocol stack, through the network medium, and up the protocol stack of the receiving computer. If any layer in the receiving computer cannot understand or is not compatible with the corresponding

layer of the sending computer, the message cannot be delivered.













Figure 2.0











To place this concept into perspective, imagine two people wishing

to communicate. If one is blind and the other is deaf, there will be a

communication problem. Both people need to convey the thought

through some form of media. However, the blind person uses voice

to transmit, which requires the receiving person to use hearing, while

the deaf person uses sign language to transmit, which requires the

receiving person to use sight.











B1. Describe, with the aid of diagrams, various techniques in which digital signals can be transmitted.



Standard telephone lines can transmit only analog signal. Computers however store and transmit data digitally. Modems can transmit digital computer signals over telephone lines by converting them to analog form. Converting one signal to another is called modulation. Recovering the original signal is called demodulation. The word modem derives from the terms modulation and demodulation. Modems are classified according to the transmission method they use for sending and receiving data. The two basic types of modems are as follow.



Synchronous (parallel)




synchronous transmission A transmission

method that uses a clock signal to

regulate data flow.

In synchronous transmissions, frames are

separated by equal-sized time intervals.

Timing must be controlled precisely on the

sending and the receiving computers.