5.2 Types of Multiplexing
Key forms of of multiplexing include:
- Time division multiplexing (TDM)
- Frequency division multiplexing (FDM)
- Wavelength-division multiplexing (WDM
- Code division multiplexing (CDM)
Time Division Multiplexing
Time Division Multiplexing works by the multiplexor collecting and storing the incoming transmissions from all of the slow lines connected to it and allocating a time slice on the fast link to each in turn. The messages are sent down the high speed link one after the other. Each transmission when received can be separated according to the time slice allocated.
Theoretically, the available speed of the fast link should at least be equal to the total of all of the slow speeds coming into the multiplexor so that its maximum capacity is not exceeded.
Two ways of implementing TDM are:
- Synchronous TDM
- Asynchronous TDM
Synchronous TDM works by the muliplexor giving exactly the same amount of time to each device connected to it. This time slice is allocated even if a device has nothing to transmit. This is wasteful in that there will be many times when allocated time slots are not being used. Therefore, the use of Synchronous TDM does not guarantee maximum line usage and efficiency.
Synchronous TDM is used in T1 and E1 connections.
Asynchronous TDM is a more flexible method of TDM. With Asynchronous TDM the length of time allocated is not fixed for each device but time is given to devices that have data to transmit.
This version of TDM works by tagging each frame with an identification number to note which device it belongs to. This may require more processing by the multiplexor and take longer, however, the time saved by efficient and effective bandwidth utilization makes it worthwhile.
Asynchronous TDM allows more devices than there is physical bandwidth for.
This type of TDM is used in Asynchronous Transfer Mode (ATM) networks.
Frequency Division Multiplexing
Frequency Division Multiplexing (FDM) works by transmitting all of the signals along the same high speed link simultaneously with each signal set at a different frequency. For FDM to work properly frequency overlap must be avoided. Therefore, the link must have sufficient bandwidth to be able to carry the wide range of frequencies required. The demultiplexor at the receiving end works by dividing the signals by tuning into the appropriate frequency.
FDM operates in a similar way to radio broadcasting where a number of different stations will broadcast simultaneously but on different frequencies. Listeners can then “tune” their radio so that it captures the frequency or station they want.
FDM gives a total bandwidth greater than the combined bandwidth of the signals to be transmitted. In order to prevent signal overlap there are strips of frequency that separate the signals. These are called guard bands.
Click on this link to review a website explaining each of the types of multiplexing using diagrams.
Uses of FDM
A common example of FDM use is Cable television (CATV). This can be achieved with coaxial cable or fibre-optic cable.
A multiplexor is used to combine many channels to maximize the use of the available bandwidth and a demultiplexor built into the television or set top box will separate the channel that the viewer wants to watch.
There may be instances when the use of analogue communication links do not meet requirements. The alternative is to use a Digital Data Service (DDS). These are digital point to point full duplex links that provide a permanent connection.
There are different types of DDS including T1 and T3. These are used in the US and are a digital transmission technology that uses two wire pairs to transmit and receive data. T1 links can carry voice, data and video traffic and allows for a transfer rate of 1.544Mbps. Due to the expensive nature of T1 and T3, subscribers could choose to use only a fraction. These fractions are available in 64kpbs channels and this is known as Fractional T1.
In Europe the equivalent of the US T1 is E1. The data transfer rate of E1 is 2.048Mbps