PRACTICAL_9TO STUDY DIFFERENT MULTIPLEXING SCHEMES

MULTIPLEXING:Whenever the Bandwidth of a medium linking two devices is greater than Bandwidth of the device, the link can be shared.Multiplexing is the technique that allows the simultaneous transmission of multiple signals across a single data link or channel.The process of multiplexing increases the bandwidth requirement for the transmissionBandwidth is one of the most precious resources we have in data communication.In a Multiplexed system N lines shares the bandwidth of one link.

Block Diagram of Multiplexing:

Multiplexing is the method of dividing a physical channel into many logical channels so that the independent signals may be simultaneously transmitted to network.It is also known as muxing.A device that performs the multiplexing is called a multiplexer (MUX).A device that performs the demultiplexing is called a demultiplexer (DEMUX).Multiplexing shares an expensive resource that is band width and maximize the utilization of channel.

Advantages of MultiplexingIf no multiplexing is used between the users at two different sites that are distance apart, then separate communication lines would be required as shown in fig.This is not only costly but also become difficult to manage. If multiplexing is used then, only one line is required. This leads to the reduction in the line cost and also it would be easier to keep track of one line than several lines.

APPLICATIONS:1). With advanced systems and more electronic features being added to modern cars, the need arises for a more reliable way to interconnect them. Instead of adding more wiring and more connections that may pose a reliability issue and add cost to the vehicle,manufactures can take advantage of multiplexing technology and send multiple sensor signals over fewer wires. Instead of adding more wiring, multiple sensors will send data to the vehicles computer over the same wire. 2). Modern vehicles also incorperate networking technology where the vehicles computers may share information from a single sensor and use that information to perform a different task. 3). A vehicles power train control module may use the input from a vehicle speed sensor to display the vehicles speed on the instrument panel. The powertrain control module may then share the vehicle speed information with the body controller to activate the door locks when the vehicle reaches a certain speed.DEMULTIPLEXING Demultiplexing is to separate 2 or more channels that have been multiplexed. Signals are typically multiplexed or combined onto one higher speed channel to efficiently use the bandwidth.

Categories of Multiplexing1. Analog Multiplexing:FDM (Frequency Division Multiplexing)WDM (Wave length Division Multiplexing)

2. Digital Multiplexing:TDM ( Time Division Multiplexing)

Frequency-division multiplexing (FDM)is an analog technique that can be applied when the band width of a link (in Hz) is greater than the combined bandwidth signals to be transmitted.In FDM signal generated by each sending device modulates different carrier frequencies.These modulated signals are then combined in to a single composite signal that can be transmitted by the link.

Carrier frequencies are separated by sufficient bandwidth ranges of the channels through which the various signals travel.Channels must be separated by strips of unused bandwidth (guard bands) to prevent signals from overlapping.FDM is an analog multiplexing technique that combines signal.Example: Radio, television broad casting, etc.

Frequency-Division Multiplexing (FDM)is a scheme in whichnumerous signals are combined for transmission on a single communications line or channel.It is analog technique.Each signal is assigned a different frequency (sub channel) within the main channel.

Advantages of FDM:1. A large number of signals (channels) can be transmitted simultaneously.2. FDM does not need synchronization between its transmitter and receiver for proper operation.3. Demodulation of FDM is easy.4. Due to slow narrow band fading only a single channel gets affected.Disadvantages of FDM:1. The communication channel must have a very large bandwidth.2. Intermodulation distortion takes place.3. Large number of modulators and filters are required.4. FDM suffers from the problem of crosstalk.5. All the FDM channels get affected due to wideband fading.Applications of FDM1. FDM is used for FM & AM radio broadcasting. Each AM and FM radio station uses a different carrier frequency. In AM broadcasting, these frequencies use a special band from 530 to 1700 KHz. All these signals/frequencies are multiplexed and are transmitted in air. A receiver receives all these signals but tunes only one which is required. Similarly FM broadcasting uses a bandwidth of 88 to 108 MHz2. FDM is used in television broadcasting.3. First generation cellular telephone also uses FDM.

TIME DIVISION MULTIPLEXING

Time Division Multiplexing (TDM) is a communications process that transmits two or more streaming digital signals over a common channel. In TDM, incoming signals are divided into equal fixed-length time slots. After multiplexing, these signals are transmitted over a shared medium and reassembled into their original format after de-multiplexing. Time slot selection is directly proportional to overall system efficiency.

This term is sometimes referred to as a digital circuit switched.

Types of TDMSynchronous TDMAsynchronous TDM or Statistical TDM

Synchronous Time Division MultiplexingIn Synchronous Time Division Multiplexing, multiplexer allocates exactly the same time slot to each device at all times.For Example: Time slot A, is assigned to device A alone and cannot be used by any other device.Each time its allocated time slot comes up, a device has the opportunity to send a portion of its data.If a device is has to transmit or does not have data to send, its time slot remains empty.

Synchronous Time Division MultiplexingT-1 multiplexing (the classic)ISDN (Integrated Services Digital Networks) multiplexingSONET (Synchronous Optical NETwork)

Advantages of synchronous TDMDigital signalsRelatively simpleCommonly used with ISDN (Integrated Services digital network)

Disadvantages of synchronous TDMWastage of bandwidth

Asynchronous TDM:Asynchronous means something different in multiplexing in the area of data communication.It means flexible or not fixed.Suppose number of inputs =5, then number of slots in each frame = 3Asynchronous Multiplexer transmits only the data from active work stations.If the workstation is not active, no space is wasted on the multiplexed streams.

A Statistical or Asynchronous multiplexers occupy the incoming data streams and creates frames containing only the data to be transmitted.Hence, the Asynchronous Time Division Multiplexing or Statistical TDM is designed to avoid this type of wastage of bandwidth.

Advantages of TDM over FDM

1. TDM provides much better flexibility compared to FDM.2..FDM is older technique of multiplexing whereas TDM is relatively a newer technique of multiplexing.3. TDM advantage over FDM is that it offers bandwidth saving and there is low interference between the signals that are being multiplexed.

Difference between FDM & TDM

Difference between Synchronous TDM and Asynchronous TDM

Synchronous TDM is the standard operation mode of TDM operation where each multiplexed channel has a specific size slot allowed for it, whether data exist to transmit or not. This is very wasteful of bandwidth space if you have channels that only occasionally transmit. Asynchronous TDM is actually called Statistical TDM. STDM does not reserve a time slot for each channel, rather it assigns a slot when the channel is requiring data to be sent or received. This allows for variable bandwidth per channel as required.

WAVE DIVISION MULTIPLEXING

Infiber-optic communications,wavelength-division multiplexing(WDM) is a technology whichmultiplexesa number ofoptical carriersignals onto a singleoptical fiberby using differentwavelengths(colours) oflaserlight. This technique enablesbidirectionalcommunications over one strand of fiber, as well as multiplication of capacity.