Difference between TDM and FDM

Frequency Division Multiplexing (FDM) is an analog method which is used when the bandwidth of a connection is higher than the linked bandwidth of the signals which are to be communicated

Time Division Multiplexing (TDM) is a digital method where each sender is given the entire possession of the whole bandwidth of the channel for a fixed duration of time. After this, the control is given to the next sender, and the process continues on a round-robin basis.

Read through this article to find out how these two multiplexing techniques work and what are the major differences between TDM and FDM.

What is TDM?

TDM is a technique of transmitting and receiving separate signals across a shared signal path using synchronized switches at either end of the transmission line, with each signal appearing on the line for a fraction of a second in an alternating pattern. This type of signal multiplexing was first devised for telegraphy systems in the late 1800s, but it was most widely used in digital telephony in the second half of the twentieth century.

• TDM uses a shared channel to send two or more digital or analog signals. When the bit rate of the transmission media exceeds the bit rate of the signal to be sent, it can be employed.
• TDM is generally used for digital communications, but it may also be employed in analog multiplexing, in which two or more signals or bitstreams are sent simultaneously as sub-channels in a single communication channel, but take turns on the channel physically.
• Each sub-time channel's domain is split into multiple recurrent time slots of defined length. Sub-channel 1 transmits a sample byte or data block during time slot 1, sub-channel 2 during time slot 2, and so on.
• One TDM frame contains a one-time slot per sub-channel as well as a synchronization channel and, in certain cases, an error correction channel prior to the synchronization.

Types of TDM

There are two types of TDM which are as follows

• Synchronous TDM It is termed "Synchronous" because, each time slot is pre-assigned to a constant source. The time slots are sent irrespective of whether the sources have any records to share or not.
• Statistical TDM A major disadvantage of the TDM method is that some of the time slots in the frame are wasted. If a specific terminal has no information to send at a particular instant of time, then it will share an unfilled time slot. In STDM, the time slots are dynamically allocated to the slots according to demand. The multiplexer checks each input stream in a round-robin manner and allocates a slot to an input line only if data is present there, otherwise, it skips to the next stream and checks it

What is FDM?

FDM is a telecommunications method that divides the overall bandwidth available in a communication channel into a number of non-overlapping frequency bands, each of which carries a distinct signal.

• FDM lets several independent signals share a single transmission channel, such as a cable or optical fiber.
• Separate serial bits or portions of a higher rate transmission can also be carried in parallel.
• Radio and television transmission are the most common examples of FDM, in which several radio signals at various frequencies travel through the air at the same time. Cable television is another example in which many television channels are carried simultaneously on a single cable.

In FDM, different frequencies are combined into a single composite signal and are transmitted on the channel. At the receiver end, the reverse phase is applied to get the individual frequencies back for working. The entire bandwidth of the channel is divided into logical channels, and each sending device is given with the possession of a logical channel.

What is Orthogonal FDM?

In data communications and networking, Orthogonal FDM (OFDM) is a method of digital data modulation, in which a single stream of data is divided into several separate sub-streams for transmission via multiple channels.

OFDM uses the principle of frequency division multiplexing to divide the available bandwidth into a set of sub-streams having separate frequency bands. OFDM was introduced in 1966 by Chang at Bell Labs and was improved by Weinstein and Ebert in 1971.

Disadvantages of Using FDM

There are two major disadvantages in using FDM

• The frequency bands must be separated by guard bands to avoid noise and disruption. This results in bandwidth wastage.
• FDM uses analog signals, which are more prone to noise disruptions than digital signals. So, if there are significant nonlinearities in the transmission link, there could be crosstalk among the different signals, resulting in communication errors.

Difference between TDM and FDM

The following table highlights the major differences between TDM and FDM.

Conclusion

TDM and FDM are both multiplexing techniques. FDM is meant for analog signals, while TDM works well with both analog as well as digital signals. However, TDM is mostly used in digital communications only. In TDM, syncronization pulse is important; whereas in FDM, Guard Band is required.