Time Division Multiplexing
- 1. TIME DIVISIONTIME DIVISION MULTIPLEXINGMULTIPLEXING •Spandit Kumar Lenka 1401209339 (6) •Subeer Kumar Panda 1401209341 (8) •Spardha Gupta 1401209340 (7) •Subhashree Swain 1401209342 (9) 11
- 2. 2 Multiplexing • Multiplexing to refer to the combination of information streams from multiple sources for transmission over a shared medium • Multiplexor is a mechanism that implements the concept • Demultiplexing to refer to the separation of a combination back into separate information streams • Demultiplexor to refer to a mechanism that implements the concept •Figure illustrates the concept •each sender communicates with a single receiver •all pairs share a single transmission medium •multiplexor combines information from the senders for transmission in such a way that the demultiplexor can separate the information for receivers
- 3. 3 The Concept of Multiplexing
- 4. 4 Need of Multiplexing :- Transmitting two or more signals simultaneously can be accomplished by setting up one transmitter- receiver pair for each channel, but this is an expensive approach. A single cable or radio link can handle multiple signals simultaneously using a technique known as multiplexing. Multiplexing permits hundreds or even thousands of signals to be combined and transmitted over a single medium. Cost savings can be gained by using a single channel to send multiple information signals.
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- 8. 8 Adtran TSU (T1) Multiplexer
- 9. 9 Adtran T3SU 300 (T3) Multiplexer Blackbox Multiplexer
- 10. 10 The Basic Types of Multiplexing There are four basic approaches to multiplexing that each have a set of variations and implementations •Frequency Division Multiplexing (FDM) •Wavelength Division Multiplexing (WDM) •Time Division Multiplexing (TDM) •Code Division Multiplexing (CDM) •TDM and FDM are widely used •WDM is a form of FDM used for optical fiber •CDM is a mathematical approach used in cell phone mechanisms
- 11. 11 Frequency Division MultiplexFrequency Division Multiplex Separation of spectrum into smaller frequency bandsSeparation of spectrum into smaller frequency bands Channel gets band of the spectrum for the whole timeChannel gets band of the spectrum for the whole time Advantages:Advantages: no dynamic coordination neededno dynamic coordination needed works also for analog signalsworks also for analog signals Disadvantages:Disadvantages: waste of bandwidthwaste of bandwidth if traffic distributed unevenlyif traffic distributed unevenly inflexibleinflexible guard spacesguard spaces k3 k4 k5 k6 f t c Channels ki
- 12. Frequency Division MultiplexingFrequency Division Multiplexing (FDM)(FDM) Each signal is allocated a different frequency bandEach signal is allocated a different frequency band Usually used with analog signalsUsually used with analog signals Modulation equipment is needed to move eachModulation equipment is needed to move each signal to the required frequency band (channel)signal to the required frequency band (channel) Multiple carriers are used, each is called sub-Multiple carriers are used, each is called sub- carriercarrier Multiplexing equipment is needed to combineMultiplexing equipment is needed to combine the modulated signalsthe modulated signals 12
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- 14. 14 FDM process
- 16. 16 Time Division Multiplexing Definition: Time Division Multiplexing (TDM) is the time interleaving of samples from several sources so that the information from these sources can be transmitted serially over a single communication channel. At the Transmitter Simultaneous transmission of several signals on a time-sharing basis. Each signal occupies its own distinct time slot, using all frequencies, for the duration of the transmission. Slots may be permanently assigned on demand. At the Receiver Decommutator (sampler) has to be synchronized with the incoming waveform Frame Synchronization Low pass filter ISI – poor channel filtering Feedthrough of one channel's signal into another channel -- Crosstalk Applications of TDM: Digital Telephony, Data communications, Satellite Access, Cellular radio.
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- 18. Simple rotary-switch multiplexerSimple rotary-switch multiplexer 18
- 19. Time Division Multiplexing (TDM)Time Division Multiplexing (TDM) Usually used with digital signals or analogUsually used with digital signals or analog signals carrying digital datasignals carrying digital data Data from various sources are carried inData from various sources are carried in repetitive framesrepetitive frames Each frame consists of of a set of time slotsEach frame consists of of a set of time slots Each source is assigned one or more timeEach source is assigned one or more time slots per frameslots per frame Eeng 360 19
- 20. 20 TDM Composition of one frame of a multiplexed PAM signal incorporating four voice-Composition of one frame of a multiplexed PAM signal incorporating four voice- signals and a synchronizing pulse.signals and a synchronizing pulse.
- 21. 21 Time Division Multiplexing Conceptual diagram of multiplexing-demultiplexing.Conceptual diagram of multiplexing-demultiplexing. PAM TDM System
- 22. Eeng 360 22 Illustrating 4-Channel PAM TDM Multiplexing Four different analog signals can be sampled by a PAM multiplexer. • Signals A and C are continuously varying analog signals. • Signal B is a positive- going linear ramp. • Signal D is a constant DC voltage.
- 23. 23 Block diagram of TDM systemBlock diagram of TDM system PAM TDM System A Typical Framing Structure for TDM
- 24. 24 Time Division Multiplexing Frame structure of a certain TDM signal Composite Signal Format
- 26. 26 Time Division Multiplexing Pulse width of TDM PAM: s s f T 3 1 3 = n Ts 3 Pulse width of TDM PCM: s 1 satisfies Nyquist rates s f f T =
- 27. 27 A time-division multiplexer used to produce pulse-amplitude modulation
- 28. 28 Waveforms for a PAM multiplexer
- 29. Eeng 360 29 A PAM demultiplexer
- 30. 30 Synchronous TDM •TDM is a broad concept that appears in many forms •It is widely used throughout the Internet •Figure shows items being sent in a round-robin fashion •Most TDMs work this way •No gap occurs between bits if a communication system uses synchronous transmission •When TDM is applied to synchronous networks, no gap occurs between items; the result is known as Synchronous TDM •Figure illustrates how synchronous TDM works for a system of four senders
- 32. 32 •Data rate of medium exceeds data rate of digital signal to be transmitted •Multiple digital signals interleaved in time •May be at bit level of blocks •Time slots preassigned to sources and fixed •Time slots allocated even if no data •Time slots do not have to be evenly distributed amongst sources Synchronous Time Division Multiplexing
- 33. 33 •Telephone systems use synchronous TDM to multiplex digital streams from multiple phone calls •they use the acronym TDM to refer to the specific form of TDM used to multiplex digital telephone calls •The phone system TDM includes an interesting technique •to insure that a demultiplexer stays synchronized with the multiplexer •Why is synchronization needed? •observe that a synchronous TDM sends one slot after another without any indication of the output to which a given slot occurs •A demultiplexer cannot tell where a slot begins– a slight difference in the clocks used to time bits can cause a demultiplexer to misinterpret the bit stream
- 34. 34 •To prevent misinterpretation, the version of TDM used in the phone system includes an extra framing channel as input •Instead of taking a complete slot, framing inserts a single bit in the stream on each round •A demultiplexor extracts data from the framing channel and checks for alternating 0 and 1 bits •If an error causes a demultiplexor to lose a bit •it is highly likely that the framing check will detect the error and allow the transmission to be restarted •Figure illustrates the use of framing bits
- 35. 35 Framing Used in the Telephone SystemFraming Used in the Telephone System Version of TDMVersion of TDM
- 36. 36 The Problem with Synchronous TDM:The Problem with Synchronous TDM: Unfilled SlotsUnfilled Slots Synchronous TDM works well if each source produces data at aSynchronous TDM works well if each source produces data at a uniformuniform, fixed rate equal to, fixed rate equal to 1/N1/N of the capacity of the shared mediumof the capacity of the shared medium Many sources generate data in bursts, with idle time between burstsMany sources generate data in bursts, with idle time between bursts To understand why, consider the example in FigureTo understand why, consider the example in Figure sources on the left produce data items at randomsources on the left produce data items at random the synchronous multiplexor leaves a slot unfilledthe synchronous multiplexor leaves a slot unfilled • if the corresponding source has not produced an item by theif the corresponding source has not produced an item by the time the slot must be senttime the slot must be sent In practice, a slot cannot beIn practice, a slot cannot be emptyempty because the underlying systembecause the underlying system must continue to transmit datamust continue to transmit data the slot is assigned a value (such as zero)the slot is assigned a value (such as zero) and an extra bit is set to indicate that the value is invalidand an extra bit is set to indicate that the value is invalid
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- 39. 39 How can a multiplexing system make better use of a shared medium? •One technique to increase the overall data rate is known as statistical TDM or statistical multiplexing or Asynchronous TDM •some literature uses the term asynchronous TDM •The technique is straightforward: •select items for transmission in a round-robin fashion •but instead of leaving a slot unfilled, skip any source that does not have data ready •By eliminating unused slots •statistical TDM takes less time to send the same amount of data •Figure illustrates how a statistical TDM system sends the data from Figure in only 8 slots instead of 12 Asynchronous TDM
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- 41. 41 •Statistical multiplexing incurs extra overhead shown below: •Consider demultiplexing: •In a synchronous TDM system a demultiplexor knows that every N slot corresponds to a given receiver •In a statistical multiplexing system, the data in a given slot can correspond to any receiver •Each slot must contain the identification of the receiver to which the data is being sent •Output data rate less than aggregate input rates •May cause problems during peak periods •Buffer inputs •Keep buffer size to minimum to reduce delay
- 42. 42 42 Synchronous TDM vs. Statistical TDMSynchronous TDM vs. Statistical TDM
- 43. 43 T-1 Carrier System •A T-1 carrier is a dedicated telephone connection or a time- division-multiplexed digital transmission service that supports a data rate of 1.544 Mbps. A T-1 line generally includes 24 separate channels, each of which is able to support 64 Kbps. Every 64 Kbps channel is often customized to transport voice or data traffic. The majority of telephone companies only allow the purchase of a few of the individual channels, referred to as fractional T-1 access. •The T-carrier systems are totally digital and use time-division multiplexing (TDM) and pulse code modulation (PCM). The original T carrier system was developed by Bell Labs in the early 1960s.
- 44. 44 Left: A 66 block; center and right: Cabinets containing Smartjack network interface devices for T-1 circuits.
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- 48. 48 •T-1 is the method that is conventionally used by the telephone companies for transporting digitized telephone communication among central offices. Since the 1960s, a single T-1 channel was able to carry 24 high-quality voice conversations. •Because T-1 is fully digital, it eradicates the chance of crosstalk, which is typical among analog carrier networks where the copper pairs tend to pick up transmissions from neighbouring pairs.
- 49. 49 Advantages of TDM : 1.Full available channel bandwidth can be utilized for each channel. 2. lntermodulation distortion is absent. 3. TDM circuitry is not very complex. 4. The problem of crosstalk is not severe. 5. Only one carrier in the medium at any time 6. Throughput high even for many users
- 50. 50 Disadvantages of TDM : 1.Synchronization is essential for proper operation. 2.Due to slow narrowband fading, all the TDM channels may get wiped out. 3.Requires reliable ‘Synchronozation’. 4.Requires A/D conversions at high rate. 5.Requires larger bandwidth. 6.Probability of error or Bit Error Rate