- A multiplexer (or mux; also spelled multiplexor) is a device in electronics that selects between several analogs or digital input signals and forwards the selected input to a single output line. A separate set of digital inputs known as select lines controls the selection. A multiplexer with two inputs has n select lines that determine which input line is sent to the output.
- It is a combinational circuit with multiple data inputs and a single output based on the control or select inputs. Log n (base2) selection lines are required for N input lines, or 2n input lines require n selection lines. Multiplexers are also referred to as “Data n selectors, parallel to serial converters, many to one circuit, and universal logic circuits.” Multiplexers are primarily used to increase the amount of data that can be transmitted over a network in a given amount of time and bandwidth.
What is Multiplexing?
- The process of combining one or more signals and transmitting them on a single channel is known as multiplexing. In analog communication systems, a communication channel is a limited resource that must be used wisely. Multiplexing is a very useful concept for cost-effective and efficient channel use because it allows multiple users to share a single channel logically.
Multiplexing technologies and processes include, but are not limited to, the following.
- TDM – Time-division multiplexing (TDM)- is a communications process that sends two or more streaming digital signals over a single channel. TDM divides incoming signals into equal fixed-length time slots. These signals are multiplexed and then transmitted over a shared medium before being de-multiplexed and reassembled into their original format. The selection of time slots is proportional to the overall system efficiency.
- Multiplexing based on wavelength- WDM is a technology or technique that modulates multiple data streams, i.e. optical carrier signals of varying wavelengths (colors) of laser light, onto a single optical fiber. WDM enables bidirectional communication as well as signal capacity multiplication.
- WDM is frequency division multiplexing (FDM), but it refers to light wavelength rather than frequency. However, because wavelength and frequency are inversely related (a shorter wavelength equals a higher frequency), the terms WDM and FDM refer to the same technology – light in the optical cable used to carry data and communication signals.
- DWDM (Dense Wavelength Division Multiplexing)- Dense wavelength division multiplexing (DWDM) is WDM with typical channel spacings of 100 GHz for 40 channels and 50 GHz for 80 channels. TDM (time division multiplex) signals are present in each channel. And each of the optical fiber’s up to 80 channels can carry 2.5 Gbps for a total of 200 billion bits per second. These signals use the third transmission window, known as the C-Band, with light beam wavelengths ranging from 1530nm to 1565nm. (nm = a nanometer, which is one billionth of a meter).
- FDM (Frequency Division Multiplexing)- FDM is a networking technique in which multiple data signals are combined for simultaneous transmission over a shared communication medium. For each data stream, FDM employs a discrete carrier signal and then combines many modulated signals.
- When FDM is used to allow multiple users to share a single physical communication medium (rather than broadcasting over the air), the technology is referred to as frequency-division multiple access (FDMA).
- OFDM (Orthogonal Frequency Division Multiplexing)- OFDM is a technique, method, or scheme for digital multi-carrier modulation that uses many closely spaced subcarriers – a previously modulated signal modulated into another signal of higher frequency and bandwidth. Each of these subcarriers contains several parallel data streams or channels and is modulated conventionally at a low symbol rate; these are data groups related to (but not the same as) gross bitrate, which is expressed in bits per second.
- This technology is also known as coded OFDM (COFDM) and discrete multi-tone modulation (DMT), and it is used in both wireless and physical communication mediums.
- ADM (Add/Drop Multiplexing)- An optical fiber network relies heavily on add/drop multiplexers (ADMs). It can combine (i.e. multiplex) several low-bandwidth data streams into a single light beam while also dropping or removing other low-bandwidth signals from the data stream and routing them to other network routes. Fabry-Pérot etalon is the filtering technology used with ADM. Optical add/drop multiplexers are another name for add/drop multiplexers.
How Multiplexing Works
- The types of multiplexing most closely associated with telecom today are frequency division multiplexing, time division multiplexing, and wavelength division multiplexing.
- A time division multiplexer in telecommunications and signal processing may select multiple samples of separate analog signals and combine them into one pulse amplitude modulated (PAM) wide-band analog signal. When there are two input signals and one output signal, the MUX is called a 2-to-1 multiplexer; when there are four input signals, it is called a 4-to-1 multiplexer, and so on.
- Several variable bit-rate data streams of input signals (using packet mode communication) may be combined or multiplexed, into one constant bandwidth signal for digital signals in telecommunications on a computer network or with digital video. A limited number of constant bit-rate data streams of input signals can be multiplexed into one higher bit-rate data stream using an alternate method that employs a TDM.
- To complete the process of separating multiplex signals carried by a single shared medium or device, a multiplexer requires a demultiplexer. A multiplexer and a demultiplexer are frequently combined into a single device (also known as a multiplexer) to allow the device to process both incoming and outgoing signals.
- Alternatively, the single output of a multiplexer can be connected to the single input of a demultiplexer over a single channel. Either method is frequently used as a cost-cutting measure. Because most communication systems transmit in both directions, a single combined device (as in the latter example) or two separate devices (as in the former) will be required at both ends of the transmission line.
2×1 Multiplexer:
- In a 2×1 multiplexer, there are only two inputs, i.e., A0 and A1, 1 selection line, i.e., S0, and single outputs, i.e., Y. Based on the combination of inputs that are present at the selection line S0, one of these 2 inputs will be connected to the output. The block diagram and the truth table of the 2×1 multiplexer are given below.
Block Diagram:
Truth Table:
| x | f |
|---|---|
| 0 | 1 |
| 1 | 0 |
The logical expression:
Y=S0′.A0+S0.A1
Logical circuit:
4×1 Multiplexer:
- Four data inputs I3, I2, I1 & I0, two selection lines s1 & s0, and one output Y comprise the 4×1 Multiplexer. The block diagram of a 4×1 Multiplexer is shown in the figure below.
Block Diagram:
Truth Table:
| S1 | S0 | Y |
|---|---|---|
| 0 | 0 | A0 |
| 0 | 1 | A1 |
| 1 | 0 | A2 |
| 1 | 1 | A3 |
The logical expression:
Y=S1′ S0′ A0+S1′ S0 A1+S1 S0′ A2+S1 S0 A3
Logical Circuit:
- Likewise the implementation of the higher-order Multiplexers is done using lower-order Multiplexers.
Applications of Multiplexer
- Multiplexers are used in a variety of fields where multiple data streams must be transmitted over a single line. The following are some examples of multiplexer applications.
- Communication System – A communication system is a collection of systems that allow communication to take places, such as a transmission system, a relay and tributary station, and a communication network. The efficiency of the communication system can be increased considerably using a multiplexer. Multiplexers enable the simultaneous transmission of multiple types of data, such as audio and video, over a single transmission line.
- Telephone Network – Using multiplexers, multiple audio signals are combined on a single line for transmission in a telephone network. Multiple audio signals can be isolated in this manner, and the desired audio signals will eventually reach their intended recipients.
- Multiplexers are used in computers to implement large amounts of memory while reducing the number of copper lines required to connect the memory to other parts of the computer circuit.
- Transmission from a Satellite’s Computer System – A multiplexer can be used to send data signals from a satellite’s or spacecraft’s computer system to the ground system via GPS (Global Positioning System) satellites.
