What is a multiplexer?
A multiplexer, sometimes referred to as a multiplexor or simply a MUX, is an electronic device that selects from several input signals and transmits one or more output signals. In its simplest form, a multiplexer will have two signal inputs, one control input and one output. One example of an analog multiplexer is the source control on a home stereo unit that allows the user to choose between the audio from a compact disc (CD) player, digital versatile disc (DVD) player and cable television line.
Multiplexers also are used in building digital semiconductors such as central processing units (CPUs) and graphics controllers. In these applications, the number of inputs is generally a multiple of two, the number of outputs is either one or relatively small multiple of two, and the number of control signals is related to the combined number of inputs and outputs. For example, a two-input, one-output multiplexer requires only one control signal to select the input, and a 16-input, four-output multiplexer requires four control signals to select the input and two to select the output.
Type of Multiplexer
Time Division Multiplexer (TDM)
Short for Time Division Multiplexing, a type of multiplexing that combines data streams by assigning each stream a different time slot in a set. 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.
Wavelength Division Multiplexing (WDM)
WDM system is a technology which multiplexes a number of optical carrier signals onto a single optical fibre by using different wavelengths (i.e. colours) of laser light. This technique enables bidirectional communications over one strand of fibre, as well as multiplication of capacity.
In early WDM systems, there were two IR channels per fibre. At the destination, the IR channels were demultiplexed by a dichroic (two-wavelength) filter with a cutoff wavelength approximately midway between the wavelengths of the two channels. It soon became clear that more than two multiplexed IR channels could be demultiplexed using cascaded dichroic filters, giving rise to coarse wavelength-division multiplexing (CWDM Multiplexer) and dense wavelength-division multiplexing (DWDM).
1. WDM is the analog multiplexing technique. WDM is conceptually similar to FDM, in the sense that it combines different signals of different frequencies into single composite signal and transmit it on a single link.
2. In WDM the different signals are optical or light signals that are transmitted through optical fibre. Wavelength of a wave is reciprocal of its frequency. Therefore, if wavelength goes up, the frequency goes down and Vice-versa.
3. Thus in WDM, various light waves from different sources are combined to form a composite light signal that is transmitted across the channel to the receiver.
4. At the receiver side, this composite light signal is broken into different light waves by demultiplexer.
5. This combining and the splitting of light waves is done by using a prism.
6. One prism is used at the sender side to perform multiplexing and another prism is used at receiver side that performs demultiplexing as shown in fig.
7. The basic principle behind the usage of prisms is that, the prism bends a beam of light based on the angle of incidence and the frequency of light wave.
Applications of WDM
WDM is used in SONET (Synchronous Optical Network). It makes use of multiple optical fibre lines which are multiplexed & demultiplexed.
Dense Wavelength Division Multiplexer (DWDM)
Dense Wavelength Division Multiplexing (DWDM) is a technology that puts data from different sources together on an optical fibre, with each signal carried at the same time on its own separate light wavelength.
DWDM multiplexer works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fibre. In effect, one fibre is transformed into multiple virtual fibres. So, if you were to multiplex eight OC -48 signals into one fibre, you would increase the carrying capacity of that fibre from 2.5 Gb/s to 20 Gb/s. Currently, because of DWDM, single fibres have been able to transmit data at speeds up to 400Gb/s.
A key advantage to DWDM is that it’s protocol- and bit-rate-independent. DWDM-based networks can transmit data in IP, ATM, SONET /SDH, and Ethernet, and handle bit rates between 100 Mb/s and 2.5 Gb/s. Therefore, DWDM-based networks can carry different types of traffic at different speeds over an optical channel.
Frequency Division Multiplexing (FDM)
Frequency-division multiplexing (FDM) is a scheme in which numerous signals are combined for transmission on a single communications line or channel. Each signal is assigned a different frequency (subchannel) within the main channel.
Statistical multiplexers make it possible for multiple RS-232 devices to share a single data line. They also perform error correction to insure error-free transmissions. The term “statistical” refers to their ability to take advantage of the intermittent usage statics of most RS-232 devices (and all PC and terminal users). Because keyboards are idle a large part of each second with no one typing and no data being sent from the computer, each PC or terminal often averages less than 5% of its potential data rate. Statistical multiplexers allow the sum of the PC and terminal rates to exceed the composite link speed between the multiplexers.