Multiplexing is a technique of transmission of multiple signals over a common channel. A Multiplexed system involves several devices that share the capacity of one link; that’s how a path can have multiple channels.
Frequency Division Multiplexing and Time Division Multiplexing are the two types of multiplexing techniques that allow simultaneous transmission of multiple signals over only one channel.
Although transmitting multiple signals over a common channel is quite difficult, multiplexing makes this easy and possible.
FDM also is known as Frequency Division Multiplexing is an analog technique which is implemented only when the bandwidth of the link is higher than the merged bandwidth of the signals to be transmitted.
TDM also is known as Time Division Multiplexing is a multiplexing technique that works with digital signals likewise to analog signals. It is considered to be a digital procedure which can be employed when the transmission medium data rate quality is higher than the data rate requisite of the transmitting and receiving device.
The difference between FDM and TDM is that in TDM, the various signal gets transmitted in multiple time slots. As against in FDM, multiple signals are transmitted by occupying different frequency slots.
Comparison Table Between FDM and TDM
| Parameter of Comparison | FDM | TDM |
| Definition | FDM can be defined as a technique that allows the transmission of multiple signals using different frequency slots over a common link. | TDM can be defined as a technique that permits the flow of multiple data signals over a communication link in different time domains. |
| Signals | FDM works only with analog types of signals. | TDM works with both analog or digital signals. |
| Cost | The cost incorporated in FDM is very high. | The cost incorporated in TDM is comparatively low. |
| Sharing | Frequency slots are shared in FDM. | In TDM, time slots are shared. |
| Required input | In FDM, the guard band is mandatory. | In TDM, synchronization of the pulse is mandatory. |
| Circuitry | In FDM, the circuitry is very complex. | In TDM, the circuitry is very simple to build. |
| Application | FDM is used in radio and television broadcasting. | TDM is used in pulse modulation. |
What is FDM?
FDM is a networking technique in which multiple data signals are combined for simultaneous transmission through a shared communication medium. FDM is used in both wired and wireless networking for transmitting large amounts of data at high speeds. It is the oldest and simplest form of multiplexing in wireless networking technology.
FDM involves simultaneously transmitting two or more signals on different frequencies. These different frequencies share non-overlapping portions of the total frequency band being used, called a channel.
Bands are joined to form groups and then groups may be joined into larger groups. This process may be considered repeatedly, but such type of technique is common only in large and sophisticated systems and is not a necessary part of FDM.
A guard band is a taper frequency range that separates two ranges of wider frequency. It ensures that simultaneously used communication channels do not interfere so that it can avoid the decrease of both transmissions.
What is TDM?
TDM is a communications process that transmits multiple streaming digital signals over a common channel. In TDM, signals which are received are divided into equal fixed-length time slots. These signals are, after multiplexing, transmitted over a shared medium and reassembled into their original format after de-multiplexing. Time slot selection is directly proportional to the overall system efficiency of the whole process.
TDM is also called a digital circuit switch. TDM comprises two major categories: asynchronous time-division multiplexing (async TDM) and synchronous time-division multiplexing (sync TDM). Synchronous TDM is used for high-speed transmission.
In Synchronous TDM the time slots are pre-assigned and fixed. This slot is given if the source is not ready with data at this time. In that case, the slot is transmitted empty. It is used for multiplexing digitized voice streaming.
In Asynchronous (or statistical) TDM the slots are allocated dynamically depending on the speed of the source or their ready state. It dynamically allocates the time slots according to the input channel’s needs, thus saving the channel capacity.
Following is the way TDM works:
Let fx be the maximum signal frequency and fy is the sampling frequency then
fx>_2fy
Thus, the time duration in between consecutive sample is given as,
Tx=1/fy
Rewriting in terms of fx
Ty<_1/2fx
TDM was developed in early 1870 for large system telegraphy implementation. Packet switching networks use TDM for telecommunication links, i.e., packets are divided into several fixed lengths and assigned fixed time slots for transmission.
Main Differences Between FDM and TDM
- The technique of frequency division multiplexing is used majorly in the case of analog signal transmission. While the time-division multiplexing is a digital signal transmission scheme.
- In TDM multiple signals are transmitted in different time domains, thus the chances of crosstalk between the signals are negligible. However, in FDM the signals are allotted different frequency slots, this leads to the possibility of crosstalk while signal transmission.
- TDM technique requires systematized signal transmission, as the time slot to each signal is pre-assigned, thus the beginning of each pulse requires a synchronization pulse. While in FDM no such synchronization pulse is required.
- The circuit orientation of FDM is quite complex, whereas that of TDM is simple.
- FDM is a comparatively expensive multiplexing technique than the TDM.
- FDM offers low efficiency because the technique is more prone to cause interference while transmission. As against, TDM is highly efficient because the chances of interference are less in its case.
- As the signals are getting transmitted in different time slots in TDM thus it leads to propagation delays. However, FDM does not cause a propagation delay.
Conclusion
Both FDM and TDM are types of multiplexing techniques. The purpose they both services may be similar but they differ from each other in many aspects. The most pronounced difference is in FDM the signal type transmitted is analog and it shares frequencies whereas in TDM the signal type transmitted is digital and it shares time slots.
FDM is used in audio signal transmission such as radio. TDM is engaged in digital signal transmission as is used in computer based communications. They are widely used worldwide. Therefore, in terms of efficiency TDM is considered to be much more efficient than FDM.
