Transmission efficiency

Transmission efficiency = (useful data/total No bits transmitted)*100
For example for asynchronous transmission of 8-bit characters with one start and one stop bits, we have:
Transmission efficiency = (8/10)*100 = 80%
Let's consider now the synchronous transmission of a block of 480 8-bit characters, assuming 7 synchronisation characters, one start of block character, and one end of block character.
The total number of bits transmitted is 480*8 + 7*8 + 2*8 = 3912 bits. So, we have:
Transmission efficiency = (3840/3912)*100 = 98.16%
Effective Data Rate = (Transmission Efficiency/100)*Capacity

Interfacing (6) - Multiplexing

Another form of interfacing a number of DTEs located together to a resource like a central computer is Multiplexing.
Multiplexing is a data transmission function that permits two or more data sources to share a common transmission medium such that each data source has its own channel. This is achieved by the use of Multiplexers.
A Multiplexer is a communications device that enable a number of lower data rate devices, normally situated in the same location, to share a single higher data rate transmission line. The capacity of the latter must be in excess of the combined rates of the low rate devices.


Interfacing (8) - Protocols

A Protocol is the set of rules that regulate and control the communication between two or more entities.
The communication interfaces define the physical level protocol between communicating devices. They define:
Signalling
Transmission mode
The connectors, and
Control to be used during communication.
In computer based communications a communication interface is accessed and controlled by software. This control software is normally called the interface/device controller/driver