Multiplexing

What is multiplexing?

• Multiplexing or (muxing) - It is a processes combining multiple signals (analog or digital) for transmission over a single line or media. 
• A common type of multiplexing combines several low-speed signals for transmission over a single high-speed connection.

Concept of Multiplexing

• As shown in figure multiplexer takes 4 input lines and diverts them to a single output line. 
• The signal from 4 different devices is combined and carried by this single line. 
• At the receiving side, a demultiplexer takes this signal from a single line and breaks it into the original signals and passes them to the 4 different receivers.

• Communication is possible over the air (radio frequency), using a physical media (cable), and light (optical fiber). All mediums are capable of multiplexing.
• When multiple senders try to send over a single medium, a device called Multiplexer divides the physical channel and allocates one to each. 
• On the other end of communication, a De-multiplexer receives data from a single medium, identifies each, and sends to different receivers.

Frequency Division Multiplexing

• When the carrier is frequency, FDM is used. FDM is an analog technology. FDM divides the spectrum or carrier bandwidth in logical channels and allocates one user to each channel. 
• Each user can use the channel frequency independently and has exclusive access of it. All channels are divided in such a way that they do not overlap with each other. 
• It is used in the analogue signal, a multiplexing technique that uses different frequencies to combine multiple streams of data for transmission over a communications medium. 
• FDM assigns a different carrier frequency to each data stream and then combines many modulated carrier frequencies for transmission. 
• The carrier frequencies are separated by a sufficient bandwidth to accommodate the modulated signal. 
• These ranges of bandwidth are the channels through which the different signals travel.
• Channels are separated by guard bands. Guard band is a frequency which is not used by either channel.

Time Division Multiplexing

• TDM is applied primarily on digital signals but can be applied on analog signals as well. 
• In TDM the shared channel is divided among its user by means of time slot. Each user can transmit data within the provided time slot only. 
• Digital signals are divided in frames, equivalent to time slot i.e. frame of an optimal size which can be transmitted in given time slot.
• A type of multiplexing that combines data streams by assigning each stream a different time slot in a set. 
• TDM is designed for digital signals, which combining several low-rate channels into high-rate one. TDM repeatedly transmits a fixed sequence of time slots over a single transmission channel. 
• Within T-Carrier systems, such as T-1 and T-3, TDM combines Pulse Code Modulated (PCM) streams created for each conversation or data stream. 
• TDM, slots are further divided into Frames. In order to separate channels AND gates are used in a TDM receiver.
• TDM works in synchronized mode. Both ends, i.e. 
• Multiplexer and De-multiplexer are timely synchronized and both switch to next channel simultaneously.

• When channel A transmits its frame at one end,the De-multiplexer provides media to channel A on the other end.
• As soon as the channel A’s time slot expires, this side switches to channel B. 
• On the other end, the De-multiplexer works in a synchronized manner and provides media to channel B. 
• Signals from different channels travel the path in interleaved manner.

Wavelength Division Multiplexing

• Light has different wavelength (colors). In fiber optic mode, multiple optical carrier signals are multiplexed into an optical fiber by using different wavelengths. 
• It used in the analog signal, a type of multiplexing developed for use on optical fibre. 
• The idea is the same: different signals on different frequencies are combined. 
• However, the difference is that the frequencies are very high. WDM is the optical equivalent of FDM.
• The figure gives a conceptual view of a WSM multiplexer and demultiplexer. Very narrow bands of light from different sources combine to achieve a wider band of light. In the receiver, the signals separated by the demultiplexer.
• This is an analog multiplexing technique and is done conceptually in the same manner as FDM but uses light as signals.

• Further, on each wavelength time division multiplexing can be incorporated to accommodate more data signals.

Code Division Multiplexing

• Multiple data signals can be transmitted over a single frequency by using Code Division Multiplexing. 
• FDM divides the frequency in smaller channels but CDM allows its users to full bandwidth and transmit signals all the time using a unique code. 
• CDM uses orthogonal codes to spread signals.
• Each station is assigned with a unique code, called chip. Signals travel with these codes independently, inside the whole bandwidth.
• The receiver knows in advance the chip code signal it has to receive.