Multiplexing or multiplexing. It is a form of information transmission in which a communication channel carries several transmissions at the same time . The exact number of simulcasts depends on the type of the communication channel and the information transmission price.
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- 1 Types
- 1 FDM Multiplexing
- 1.1 Advantages
- 1.2 Disadvantages
- 2 TDM Multiplexing
- 2.1 Advantages
- 2.2 Disadvantages
- 3 Statistical multiplexing
- 1 FDM Multiplexing
- 2 External Links
- 3 References
Frequency Division Multiplexing (FDM) is used to transmit several information channels simultaneously on the same communication channel . In this type of multiplexing, the frequency spectrum represented by the available bandwidth of a channel is divided into smaller bandwidth portions according to the number of input channels, which are called subchannels.
The subchannels are separated from each other by a guard band to avoid overlap interference. The FDM multiplexing type modulates each signal for transmission; the signals can be analog or digital, for the analog signals the types of Modulation are used : AM , FM and PM ; in the case of digital ones, they use ASK , FSK , PSK and DPSK.
- The user can be added to the system by adding another pair of modulator and demodulator.
- The FDM system supports the full duplex flow of information that is required by most of the application.
- The noise problem for analog communication has less effect in this type of system.
- The initial cost is high. This may include the cable between the two ends and the associated connectors for the cable.
- Sometimes one user’s problem can affect others.
- Each user requires a precise carrier frequency.
The division multiplexing time (TDM) is a means of transmitting multiple channels of information on the same communication circuit using the time sharing technique, this technique is used today, particularly in digital transmission systems. The total bandwidth of the transmission medium is assigned to each channel for a fraction of the total time (time interval).
The time division multiplexer cyclically samples, or scans, the input signals (input data) from different users, and transmits the frames over a single high-speed communication line. They work at the bit level or at the character level, at the bit level each frame contains one bit from each scanned device; at the character level it sends a character in each channel of the frame. The second is generally more efficient, since it requires fewer control bits than a bit TDM. The sampling operation must be fast enough so that each buffer is flushed before new data arrives. 
- Low cost.
- Small size of terminal equipment.
- Immunity to link amplitude non-linearities.
- Individual phone channels can be inserted and extracted.
- TDM systems cannot be interconnected to FDMs of similar capacity.
- The initial cost is high.
- Greater technical complexity.
- The noise problem for analog communication has the greatest effect.
- They do not allow the transmission of large groups of telephone channels, as very narrow pulses and a wide bandwidth would be needed.
In practical life, no channel transmits continuously, therefore if the time slot is reserved automatically, the moment will come when there will be channels through which no information will be transmitted and other channels will wait unnecessarily. The idea of this specific type of multiplexing is to transmit the data of those channels that are active. For these reasons, TDMA dynamically allocates the time slots between the active terminals and, therefore, the capacity of the line is not wasted during the idle times of the terminals.