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LECTURE
Introduction
o Whenever the TX capacity of a medium linking 2 devices is greater than the TX needs of the devices, the link can be shared o Example: Large Water pipe can carry water to several separate houses at once o Multiplexing is the set of techniques that allows simultaneous TX of multiple signals across a single data link o As data communication usage increases, traffic also increases o We can add a new line each time a new channel is needed o Or we can install higher capacity links and use each to carry multiple signals o All current TX media i.e. Coax, Optical fiber have high available BWs o Each of these has carrying capacity far in excess of that needed for one signal o If TX capacity of a link is greater than the TX needs of devices attached to it, the excess capacity is wasted Multiplexing 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
o Fig. shows two possible ways of linking 4 pairs of device o In fig. (a), each pair has its own link. If full capacity of each link is not utilized, it will be wasted o In fig. (b), TX b/w pairs are multiplexed. The same 4 pairs share the capacity of single link o Fig. (b) shows the basic format of a Multiplexed system o The 4 devices on left direct their TX streams to a MUX , which combines them into a single stream o 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
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Categories of Multiplexing
FDM
o An analog technique that can be applied when BW of the link is greater than the combined BW of the signals to be TX o Signals generated by each sending device modulate difference carrier frequencies o These modulated signals are then combined into a single Composite signal that can be transported by the link o Carrier frequencies are separated by enough BW to accommodate the modulated signal o 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
o In fig. the TX path is divided into 3 parts, each representing a channel to carry one TX o As an analogy, imagine a point where 3 narrow streets merge to form a 3-lane highway
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o In fig, the BW of resulting composite signal is more than 3 times the BW of each input signal o Plus extra BW to allow for necessary GUARD BANDS
DEMULTIPLEXING o DEMUX uses a series of filters to decompose multiplexed signal into its constituent signals o Individual signals are then passed to a demodulator that separates them to the carriers and passes them to the waiting receivers
DEMULTIPLEXING (Time Domain)
This figure is the time domain representation of the FDM MUX again using 3 telephones as the communication devices
DEMULTIPLEXING (Freq Domain)
This figure is the time domain representation of the FDM MUX again using 3 telephones as the communication devices
Wave Division Multiplexing (WDM) o It is conceptually the same as FDM except that multiplexing and demultiplexing involves light signals TX through fiber optic channels o Idea is the same: We are combining different signals of the different frequencies o However in this case frequencies are very high
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o WDM MUX and DEMUX o Very narrow bands of light from different sources are combined to make a wider band of light o At the receiver are separated by DEMUX
Mechanism of WDM o Although the technology is very complex, the idea is very simple o We want to combine multiple sources into one single light at the the MUX and do the reverse at the DEMUX
PRISM
o Combining and Splitting of light sources is easily handled by a PRISM o From Physics, a prism can deflect the light depending upon the angle of incidence and the frequency
o o 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 o The DEMUX can also be made to reverse the process
TDM o 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
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o The time slots dedicated to a given device occupy the same location in each frame and constitute that device’s channel
o In figure, we have 5 I/p lines Multiplexed onto a single path using synchronous TDM
o 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
Interleaving
o Synchronous TDM can be compared to a very fast rotating switch o 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 o The switch moves from device to device at a constant rate and in a fixed order o This process is called INTERLEAVING o Interleaving can be done by BITS, BYTES or by any other DATA UNIT o In other words MUX can take one byte from each device, then another byte from each device and so on o In a given system interleaved units will always be of the same size
o Fig,, shows interleaving and frame building o 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 o Each device is sending a different message o The MUX interleaves the different and forms them into FRAMES before putting them onto the link o At the receiver the DEMUX decomposes each frame by extracting each character
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o As a character is removed from a frame, it is passed to the appropriate receiving device
Weakness of Synchronous TDM Figure
o Both figures point out major weakness in Synchronous TDM o By assigning each timeslot to a specific I/p line, we end up with empty slots whenever not all the lines are active o In figure only the first three frames are completely filled, The last 3 frames have a collective 6 empty slots o Having 6 empty slots out of 24 means that a quarter of a capacity of the link is wasted o Framing Bits o 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 o The order of receipt tells the DEMUX where to direct each time slot so no ADDRESSING is necessary
Demultiplexing Process
o Demultiplexer decomposes each frame by extracting each data unit in turn o Weakness of synchronous TDM
Framing Bits o Various factor however can cause timing inconsistencies. o For this reason one or more synchronization bits are added to the beginning of each frame o 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 o This synch info consist of one bit /frame alternating b/w 0 and 1.
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