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What is Multiplexing?

Multiplexing is the process of taking multiple signals or streams of information and combining them into one signal for transmission over a medium. The multiplexer (MUX) is a device that performs the multiplexing at the transmitter end by taking inputs from various data sources. Conversely, the demultiplexer (DEMUX) is responsible for separating the multiplexed signals at the receiver end.

Dissecting Multiplexing

In 1853, Julius L. Brenchley, an Englishman, introduced a method to send two telegraph messages simultaneously on the same wire. In 1874, the Quadruplex telegraph was developed by Thomas Edison, which was capable of sending two messages simultaneously in both directions over a single line. This was the first practical example of Time Division Multiplexing (TDM).

The concept was further expanded in the early 20th century with the development of telephony. Frequency Division Multiplexing (FDM) was introduced to allow multiple telephone calls to be carried simultaneously on a single wire. FDM was developed because the existing infrastructure could not keep pace with the growing demand for telephone service. Telephone traffic was expanding faster than new cables could be laid, making it essential to get more use out of the existing cables.

How it Works

The process of data delivery in a multiplexing system involves several stages, and these stages depend on the type of multiplexing used.

  1. Data Generation: The process begins with multiple data streams being generated at the source or sources. These could be various signals coming from different applications or devices.
  2. Channel Allocation: Each data stream is assigned a particular channel or way of distinguishing it from the others. This could be a specific frequency band in FDM, a time slot in TDM, a unique code in CDMA, a particular wavelength in WDM, or some other distinguishing feature depending on the specific multiplexing method being used.
  3. Data Encoding: The data from each stream is transformed or encoded based on the allocated channel. For example, in FDM, each data stream would be used to modulate a carrier wave of a particular frequency. In CDMA, the data would be multiplied by the unique code assigned to it.
  4. Signal Combination: The encoded signals from all the data streams are then combined into a single composite signal. This is typically done by simple addition, though the specifics may vary depending on the multiplexing technique.
  5. Transmission: This composite signal is then transmitted over the shared communication medium. This could be a wire, a fiber-optic cable, a wireless radio frequency band, or any other type of data transmission medium.
  6. Signal Reception: On the receiving end, the composite signal is received and prepared for demultiplexing.
  7. Signal Separation: The received composite signal is then separated or demultiplexed back into its constituent encoded signals. This separation is done based on the distinguishing features assigned to each channel at the source end. For example, in FDM, bandpass filters might be used to isolate signals of different frequencies. In TDM, the timing of the signals would be used to separate them into their original time slots.
  8. Data Decoding: The encoded data in each separated signal is then decoded to retrieve the original data stream. For instance, in CDMA, this would involve correlating the received signal with the expected unique code.
  9. Data Delivery: The decoded data streams are then delivered to their intended destinations. These could be different applications or devices at the receiving end.

Multiplexing Techniques

Navigating the landscape of multiplexing, one encounters an array of methodologies, each offering unique approaches to optimizing data transmission across shared mediums. These techniques are:

  • Frequency Division Multiplexing (FDM): FDM is used primarily in analog systems. Different signals are transmitted at different frequencies within the same channel. Each signal is allocated a specific frequency band within the available bandwidth. FDM is commonly used in radio and television broadcasting.
  • Time Division Multiplexing (TDM): In TDM, signals from multiple sources are divided into separate but equal time slots, and then transmitted in a round-robin manner. TDM is most often used in digital systems, like digital telephony and computer networks.
  • Statistical Time Division Multiplexing (STDM): A variant of TDM, STDM dynamically allocates time based on the input channel requirements. If a source doesn't have data to send, its time slot can be reallocated to another source, leading to more efficient usage of bandwidth.
  • Code Division Multiplexing (CDM): In CDM, also known as Code Division Multiple Access (CDMA), each channel uses a unique code to differentiate itself from others in the same frequency spectrum. This allows all users to share the same bandwidth simultaneously. CDMA is primarily used in some types of cellular networks.
  • Wavelength Division Multiplexing (WDM): This is an analog technology used in fiber-optic cables. Different signals are transmitted at different wavelengths (essentially different colors of light). WDM, and its subtypes DWDM (Dense WDM) and CWDM (Coarse WDM), are widely used in modern optical communication systems.
  • Orthogonal Frequency Division Multiplexing (OFDM): OFDM is a digital multi-carrier modulation method, where a single data stream is split across multiple closely-spaced carriers that are modulated with low rate data. Each carrier is orthogonal to the others to prevent interference. OFDM is used in Wi-Fi networks, digital television and radio broadcasting, and 4G/5G cellular networks.
  • Space Division Multiplexing (SDM): SDM is a technique used in MIMO (Multiple Input Multiple Output) wireless communication, where multiple antennae are used at both the transmitter and receiver ends to improve the communication system’s performance. SDM is also being explored in fiber-optic communication, where multiple spatially separated light paths within the same fiber are used to increase data capacity.
  • Polarization Division Multiplexing (PDM): PDM is commonly used in optical communication systems, where two lightwaves with orthogonal polarizations are used to double the bandwidth of the optical fiber.
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