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CS601 - Data Communication - Lecture Handout 29

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  • Whenever the TX capacity of a medium linking 2 devices is greater than the TX needs of the devices, the link can be shared
  • Example: Large Water pipe can carry water to several separate houses at once
  • Multiplexing is the set of techniques that allows simultaneous TX of multiple signals across a single data link
  • As data communication usage increases, traffic also increases
  • We can add a new line each time a new channel is needed
  • Or we can install higher capacity links and use each to carry multiple signals
  • All current TX media i.e. Coax, Optical fiber have high available BWs
  • Each of these has carrying capacity far in excess of that needed for one signal
  • If TX capacity of a link is greater than the TX needs of devices attached to it, the excess capacity is wasted

Set of techniques that allows the simultaneous transmission of multiple signals across a single data link”

In the multiplexed system, ‘n’ devices share the capacity of one link


  • Fig. shows two possible ways of linking 4 pairs of device
  • In fig. (a), each pair has its own link. If full capacity of each link is not utilized, it will be wasted
  • In fig. (b), TX b/w pairs are multiplexed . The same 4 pairs share the capacity of single link
  • Fig. (b) shows the basic format of a Multiplexed system
  • The 4 devices on left direct their TX streams to a MUX , which combines them into a single stream
  • At the receiving end, that stream is fed into a DEMUX, which separates the stream back into its component transmissions and directs them to their intended devices
    • Path: Physical Link
    • Channel: A portion of the path that carries TX b/w a given pair of devices
    • One path can have many channels

Categories of Multiplexing

Categories of Multiplexing


  • An analog technique that can be applied when BW of the link is greater than the combined BW of the signals to be TX
  • Signals generated by each sending device modulate difference carrier frequencies
  • These modulated signals are then combined into a single Composite signal that can be transported by the link
  • Carrier frequencies are separated by enough BW to accommodate the modulated signal
  • These BW ranges are the channels through which the various signals travel

FDM (Guard Bands)

GUARD BANDS: Channels must be separated by strips of unused BW (guard bands) to prevent signals from Overlapping


  • In fig. the TX path is divided into 3 parts, each representing a channel to carry one TX
  • As an analogy, imagine a point where 3 narrow streets merge to form a 3-lane highway
  • Each of these streets correspond to a lane of the highway
  • Each car merging on to the highway from one of these streets still has its own lane and can travel w/o interfering with cars from other lanes


  • Figure shows a Time domain fdm
  • FDM is an analog process and we show it here in using Telephones as I/p & o/p devices
  • Each telephone generates a signal of similar frequency range
  • Inside the MUX, these similar signals are modulated on to different carrier frequencies
  • The resulting modulated signals are then combined into a single composite signal that is sent over a media link that has enough BW to accommodate it

The FDM Process-Freq domain

  • Fig is freq domain representation of FDM process
  • In FDM, signals are modulated onto separate carrier frequencies (f1,f2,f3) using either FM or AM
  • Modulating one signal into the other results in a BW of at least twice the origina
  • In fig, the BW of resulting composite signal is more than 3 times the BW of each input signal
  • Plus extra BW to allow for necessary GUARD BANDS


  • DEMUX uses a series of filters to decompose multiplexed signal into its constituent signals
  • Individual signals are then passed to a demodulator that separates them to the carriers and passes them to the waiting receivers


This figure is the time domain representation of the FDM MUX again using 3 telephones as the communication devices


This figure is the time domain representation of the FDM MUX again using 3 telephones as the communication devices

Wave Division Multiplexing (WDM)

  • It is conceptually the same as FDM except that multiplexing and demultiplexing involves light signals TX through fiber optic channels
  • Idea is the same: We are combining different signals of the different frequencies
  • However in this case frequencies are very high

Wave Division Multiplexing (WDM)

  • Very narrow bands of light from different sources are combined to make a wider band of light
  • At the receiver are separated by DEMUX

Mechanism of WDM

  • Although the technology is very complex, the idea is very simple
  • We want to combine multiple sources into one single light at the the MUX and do the reverse at the DEMUX

Mechanism of WDM


  • Combining and Splitting of light sources is easily handled by a PRISM
  • From Physics, a prism can deflect the light depending upon the angle of incidence and the frequency


  • Using this technique, a MUX can be made to combine several input beams of light each containing a narrow band of frequencies into one o/p beam of a wider band of frequencies
  • The DEMUX can also be made to reverse the process


  • TDM is a digital process that can be applied when the data rate capacity of the TX medium is greater than the data rate required by the sending and receiving devices
  • In such case, multiple transmissions can occupy a single link by subdividing them and Interleaving the portions


In fig, same link is used as in FDM. However, here the link is shown sectioned by time rather than frequency In TDM fig, portions of signals 1, 2, 3 and 4 occupy the link sequentially

Implementation of TDM

TDM can be implemented in two ways:

  • Synchronous TDM
  • Asynchronous TDM

Synchronous TDM

  • The term synchronous has a different from that used in other areas of telecommunication
  • Here synchronous means that MUX allocates exactly the same time slot to each device at all device whether or not the device has any thing to transmit

Synchronous TDM Example

  • Time slot A for example 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 unable to transmit or does not have data to send time slot remains empty

Synchronous TDM Frames

  • Time slots are grouped into Frames
  • A frame consists of one complete cycle of Time slots including one or more slots dedicated to each sending device
  • In a system with ‘n’ I/p lines, each frame has atleast ‘n’ slots with each slot allocated to carrying data from a specific I/p line
  • If all the I/p devices sharing a link are transmitting at the same data rate, each device has 1 timeslot per frame
  • However it is possible to accommodate varying data rates
  • A TX with two slots per frame will arrive twice as quickly as one with 1 slot per frame
  • The time slots dedicated to a given device occupy the same location in each frame and constitute that device’s channel

Synchronous TDM FramesSynchronous TDM Frames

  • In figure, we have 5 I/p lines Multiplexed onto a single path using synchronous TDM
  • In this example all of the I/p’s have the same data rate, so the number of time slots in each frame is equal to the number of I/p lines


  • Synchronous TDM can be compared to a very fast rotating switch
  • As the switch opens in front of a device, the device has the opportunity to send a specifies amount of data on to the path
  • The switch moves from device to device at a constant rate and in a fixed order
  • This process is called INTERLEAVING
  • Interleaving can be done by BITS, BYTES or by any other DATA UNIT
  • In other words MUX can take one byte from each device, then another byte from each device and so on
  • In a given system interleaved units will always be of the same size


  • Fig,, shows interleaving and frame building
  • In the example we interleave the various TXs by character (equal to 1 byte each) but the concept is the same for data units of any length
  • Each device is sending a different message
  • The MUX interleaves the different and forms them into FRAMES before putting them onto the link
  • At the receiver the DEMUX decomposes each frame by extracting each character
  • As a character is removed from a frame, it is passed to the appropriate receiving device

Weakness of Synchronous TDM Figure

  • Both figures point out major weakness in Synchronous TDM
  • By assigning each timeslot to a specific I/p line, we end up with empty slots whenever not all the lines are active
  • In figure only the first three frames are completely filled, The last 3 frames have a collective 6 empty slots
  • Having 6 empty slots out of 24 means that a quarter of a capacity of the link is wasted
  • Framing Bits
  • Because the time slots order in a synchronous TDM does not vary from frame to frame, very little overhead information need to be included in each frame
  • The order of receipt tells the DEMUX where to direct each time slot so no ADDRESSING is necessary

Demultiplexing Process

  • Demultiplexer decomposes each frame by extracting each data unit in turn
  • Weakness of synchronous TDM
    • Waste of empty slots

Demultiplexing Process

Framing Bits

  • Various factor however can cause timing inconsistencies.
  • For this reason one or more synchronization bits are added to the beginning of each frame
  • These bits called Framing bits follow a pattern frame to frame that allows a DEMUX to synchronize with the incoming stream so that it can separate time slots accurately
  • This synch info consist of one bit /frame alternating b/w 0 and 1.

Framing Bits

Synchronous TDM Example

Synchronous TDM Example


  • Multiplexing
  • Frequency division multiplexing
  • Wave division multiplexing
  • Time division multiplexing

Reading Sections

Section 8.1,8.2,8.3,8.4 “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan