Multiplexing techniques are techniques with which several signals are transmitted simultaneously or sequentially over existing transmission paths. Multiplex techniques can be used on wireline transmission paths and also in radio technology. They make more efficient use of transmission paths because several transmission channels are transmitted simultaneously over one transmission path. The advantages of multiplex techniques lie in their cost-effectiveness, improved frequency economy and increased data transmission rates.
Multiplex: Time, space, frequency
Multiplex techniques divide the transmission medium in terms of time, space, frequency or wavelength. Time division is time division multiplex(TDM), space division multiplex( SDM), code division multiplex ( CDM) and frequency division multiplex ( FDM) or wavelength division multiplex (WDM). In addition to these, there are variants such as orthogonal frequency division multiplexing( OFDM) or polarization division multiplexing( PDM). As for time division multiplexing, there is synchronous time division multiplexing( STD), asynchronous time division multiplexing( ATD), and statistical time division multiplexing( STDM).
In frequency division multiplexing (FDM), it is possible to use several narrowband data channels simultaneously in one frequency channel. The narrowing of the bandwidth reduces the transmission speed for the individual channel. In time division multiplexing, the available bandwidth is divided into time slots, i.e. several digital input signals are combined into a common output signal. Depending on the type of time division multiplexing, the data is interleaved bit by bit, character by character or block by block.
In each case, a group of bits, characters or blocks is combined to form a data transmission frame, the beginning or end of which is specially marked to facilitate synchronization with the remote station. Basically, one transmits in transmission frames that correspond to data packets on the lower layer. With STD multiplexing, the user is given a fixed time slot within the transmission frame which he can use for transmission/reception. STD-oriented transmission in high-speed networks is also called synchronous transfer mode( STM), and asynchronous transfer mode( ATM).
In wide area networks, wavelength division multiplexing (WDM) is used in optical fiber transmission. This method uses different wavelengths that are assigned to different channels and transmitted simultaneously over the medium. A variant of this technique with considerably more spectral bands is Dense Wavelength Division Multiplexing( DWDM).