The basic building blocks of data transmission are:
- Data (entities that are to be communicated)
- Electromagnetic Signals (entities that are actually transmitted)
- Line Encoder (to convert data into signals)
- Line Decoder (to convert signals back into data)
- Telecommunication Link/Channel (Wired/Wireless Medium)
- Data Exchange Switches (to inter-connect multiple communicating end points)
The diagram given below illustrates a sample data transmission network, where two end nodes are connected through a data exchange.
A brief explanation of the role of each building block is given below:
Data – Any Entity that conveys meaning. Data can be either analog (e.g. raw audio, video) or digital (text, email, images etc.).
Electromagnetic Signals – Electrons when propogated in either in free space or guided media create electromagnetic signals. For transmission over telecommunication links, data has to be first converted into electromagnetic signals. Only signals can travel long distances over wired/wireless media. Just like data, signals too can be either in analog or digital format. The type of signalling used depends mainly on the underlying telecommunication link (wired/wireless). Both analog signalling and digital signalling are widely used in data communication.
Line Encoder – The process of converting the data into electromagnetic signals at the sending node, so that they can be transmitted over wired or wireless links, is called line encoding. Since both data and signals can be in either analog or digital form, there are line encoding strategies available for all the four different combinations of data/signals, namely analog data to analog signalling, digital data to analog signalling, analog data to digital signalling, digital data to digital signalling. Channel multiplexing techniques (that enable one physical wired/wireless media channel to have multiple logical channels/paths) like TDM (Time Division Multiplexing), FDM (Frequency Division Multiplexing) etc. can be considered to be part of the line encoding strategy, as line encoding usually takes care of channel multiplexing too.
Line Decoder – This is present at the receiving node and is used to convert the incoming electomagnetic signal back into data. The line decoding process is usually the reverse procedure of the line encoding process used at the sender.
Telecommunication links / Channels – Just like vehicles need a proper road to travel, Electromagnetic signals travel between communicating end points through either a wired or wireless medium, usually named as the telecommunication link. If a road is big and wide enough, then it can carry lot of vehicles. Similarly, a telecommunication link’s carrying capacity (bandwidth or channel capacity) directly influences the type of electromagnetic signals (and thus the data rate of information) that can pass through it. For e.g. wired fiber optical signals have much higher channel capacity than wired copper links. A telecommunication link is also called as a physical communication channel or path. It is possible to have multiple logical channels/paths (or sub-channels) within a single physical channel through use of channel multiplexing techniques like TDM, FDM etc.
Data Exchange Switches : Just like telephone exchange devices are needed to route telephone calls between different end points, for data communication, data exchange devices like Routers and Switches are needed to route data simultaneously between multiple sender/receiver pairs. These devices have multiple input and output ports/interfaces connected to different telecommunication trunk links. Their duty is to take a unit of data (e.g. a frame or a packet) via. incoming trunk links and transfer/switch the unit of data to the correct outgoing port/interface so that the unit of data moves closer towards the destination node. The devices employ different types of standard switching techniques like circuit switching, packet switching etc.