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Tech2301: Data Communications Mohammed A. Saleh http://ifm.ac.tz/staff/msaleh/TECH2301.html 1 Multiplexing We have looked at efficient techniques for utilizing a data link under heavy load. Under the simplest conditions a medium can only carry one signal at any moment in time For multiple signals to share one medium the medium must somehow be divided giving each signal a portion of the bandwidth. However, two communicating stations will not utilize the full capacity of a data link. For efficiency, it should be possible to share that capacity This is multiplexing A common application of multiplexing is in long-haul communications Trunks used are high-capacity fiber, coaxial, or microwave links Can carry large numbers of voice and data transmissions simultaneously using multiplexing. 2 Cont … Multiple sources but only one link The multiplexer is connected to the demultiplexer by a single data link The MUX combines data from these ‘n’ input lines and transmits them through the high capacity data link The DEMUX delivers to the appropriate output lines 3 Cont … Widespread use of multiplexing can be explained as follows: Transmission services is very expensive Multiplexing and compression techniques save business money The higher the data rate, the more cost-effective the transmission facility Leased lines, packet switched network networks For a given application and over a given distance, the cost per kbps declines with an increase in the data rate of the transmission facility Similarly, the cost of transmission and receiving equipment, per kbps, declines with increasing data rate. Most individual data-communicating devices require relatively modest data- rate support 4 Cont … Multiplexing techniques can be categorized into the following three types: Frequency-division multiplexing (FDM) Time-division Multiplexing (TDM) Frequency spectrum is divided into several logical channels, giving, each user exclusive possession of a particular frequency band. It is most popular and is used extensively in radio and TV transmission. Each user periodically gets the entire bandwidth for short burst of time. It is also called synchronous TDM, Commonly used for multiplexing digitized voice stream. Statistical TDM This is also called asynchronous TDM, which simply improves 5 on the efficiency of synchronous TDM. Frequency-Division Multiplexing (FDM) Sends signals in several distinct frequency ranges Carry multiple video channels in a single cable Imagine the cost of stringing a wire for each channel to peoples homes Each signal is modulated onto a different carrier and carrier frequencies are separated by guard bands Bandwidth of the transmission medium exceeds required bandwidth of all the signals. Used in radio and cable TV 6 Cont … When channels are very close to one other, it leads to inter-channel cross talk Channels must be separated by strips of unused bandwidth to prevent inter-channel cross talk These unused channels between each successive channel are known as guard bands The range of frequencies have to be non-overlapping 7 Cont … Basic approach: Divide the available bandwidth of a single physical medium into a number of smaller, independent frequency channels. Using modulation, independent message signals are translated into different frequency bands The carriers used to modulate the individual message signals are called sub-carriers, shown as f1, f2, ..., fn Analog signaling is used to transmit the analog signals Broadcast radio and television, cable television FDM is the oldest multiplexing technique. 8 Cont … A multiplexor accepts inputs and assigns frequencies to each device The assignment is for non-overlapping frequency ranges on a medium. All signals are transmitted at the same time using different frequencies The multiplexor is attached to a high-speed communication line At the receiving end: Individual frequencies will add up to greater bandwidths, the line must be able to handle them. The demultiplexor on a high-speed separates the multiplexed signals It is more susceptible to noise because it performs analog signaling 9 Cont … 10 Cont … Frequency distributed to different users 11 Cont … Frequency division multiplexed circuit 12 Cont … Problem with FDM is that is it cannot utilize the full capacity of the cable. Remember: Frequency bands do not overlap There must be a considerable gap between the frequency bands in order to ensure that the signals from band do not affect the signals in another band FDM carries analog signals although modulated digital signals can also be carried using this technique 13 Wavelength-Division Multiplexing (TDM) Same as FDM but applied to fibers The difference is the that the operating frequencies are much higher, i.e. in the optical range Great potential for fiber media since the bandwidth is so huge with different energy bands are passed through a diffraction grating prism. It does this by: Combining at the long distance link Split at the destination Advantages: high reliability and very high capacity 14 Cont … Multiplexing and demultiplexing signals by using a prism. Fiber spectrum Power vs frequency 15 Cont … WDM multiplexes multiple data streams onto a single fiber optic line Different wavelength lasers (lambda) transmits the multiple signals Each signal carried on the on the fiber can be transmitted at a different rate from the other signals Dense WDM combines as many as 30, 40 or 60 channels in one fiber Costly equipments Coarse WDM combines only a few lambdas Channels more widely spaced Low costs 16 Time Division Multiplexing Sharing of the signal is accomplished by dividing available transmission time on medium among users Digital signaling is used exclusively Time is being divided Maybe in a round robin fashion Computer communication Telecommunications It comes into two basic forms: Synchronous TDM Statistical TDM or asynchronous 17 Cont … 18 Cont … Like FDM, TDM saves money by allowing more than one telephone call to use a cable at the same time. Since this is synchronous the clocks have to be synchronized before transmission This way each receiving time slot know to which terminal the data belongs to 19 Cont … Instead of dividing the cable into frequency bands, it splits the cable usage into time slots Each user is given a time slot in which to send a PCM signal TDM can transmit different rates a hierarchy of data streams at 20 Cont … Used for digital signals or analog signals carrying digitals data Data rate of transmission media exceed data rate of signals Uses a frame One slot for each slice of time Number of bits and bytes to form one particular unit at which to send it over Slow device will send lower number of bits during his time High speed device will send larger number of bits Slots normally come periodically One or more slots for each device = channel 21 Cont … Time slots are transmitted whether source has data or not Problem with TDM The receiver does not know the bits coming, except once it receives the frame it will forward it to the appropriate device Suppose Si is sending to Ri, during the turn of Si it happens to have nothing to send, it is not possible for Sj to use that particular time slot to send its frame The slot will have to be transmitted empty. 22 Summary of Synchronous TDM Sender and receiver have to agree on the length of time slot The multiplexor accepts input from attached devices in a round-robin fashion and transmit the data T-1 and ISDN telephone lines are common examples of synchronous time division multiplexing If one device generates data at a faster rate than other devices The multiplexor must sample the incoming data stream more often or buffer the incoming stream If a device has nothing to send the multiplexor must still insert a piece of data and multiplex the stream. 23 Hybrid Multiplexing Schemes Both FDM and TDM can be combined The available channel is broken up into frequency bands In each band, multiple channels are accommodated through TDM Example is like the cell phone communications 24 Statistical Division Multiplexing This is also known as asynchronous TDM As well as TDM, but on demand rather than fixed Addresses the problem of the original TDM Multiplexor cannot send an empty slot rather if a device has nothing and and another one does then it will allow the one with frames to send to transmit at that particular time. Utilizes the channel more efficiently Capability of rescheduling the link on per-packet basis Packets from different sources interleaved on the link The problem now comes at the demultiplexor side There is no relationship that Si uses a Ti slot for it to be received by Ri As a result you have to have an address of the receiving host in the packet 25 Cont … Allows connection of more nodes to the circuit than the capacity of the circuit Works on the premise that not all nodes will transmit at full capacity at all times Must transmit a terminal ID Possible because with busty data there are still idle times need to be utilized to make the efficient Rate of utilization may increase Destination ID May require storage 26 Cont … 27 Cont … Buffer packets that are contending for the link Packet queue may be processed FIFO, but not necessarily Buffer overflow is called congestion Other schemes may apply as well, example high priority queue, If all senders decide to send at the same time Congestion occurs As a result there may be data loss A statistical multiplexor transmits only the data from active workstations If a workstation is not active, no space is wasted on the multiplexed stream 28 Cont … A statistical multiplexor accepts the incoming data streams and creates a frame containing only the data to be transmitted To identify each piece of data, an address is identified. In this case the size is fixed 29 Cont … If the data size is of variable size, a length is included 30 advantages FDM Simple Cheap Popular TDM Digital signals Multiplexing hierarchy At different places you may require different data rates STDM More efficient bandwidth use Frame can contain control information Packet can be of varying sizes 31 Advantages WDM Very high capacity Scalable Low noise sensitivity 32 Disadvantages FDM TDM Wasted bandwidth STDM Susceptible to noise Wasted bandwidth Limited frequency range More complex and expensive WDM More costly than FDM and TDM 33 Questions