EE4512 Analog and Digital Communications Chapter 7Chapter 7Chapter 7Multiplexing TechniquesMultiplexing TechniquesEE4512 Analog and Digital Communications Chapter 7Chapter 7Chapter 7Multiplexing TechniquesMultiplexing Techniques••Time Division MultiplexingTime Division Multiplexing••Pages 364Pages 364--368368EE4512 Analog and Digital Communications Chapter 7••Time division multiplexingTime division multiplexing(TDM) combines several low, (TDM) combines several low, fixed and predefined bit rate sources into a single high fixed and predefined bit rate sources into a single high speed bit stream for transmission over a single digital speed bit stream for transmission over a single digital communication channel:communication channel:DemultiplexerDemultiplexerMultiplexerMultiplexerTime slotsTime slotsEE4512 Analog and Digital Communications Chapter 7••TheTheTDM time slots have to be chosen properly. If the time TDM time slots have to be chosen properly. If the time slots are too small (for example, 1 bit) then the multiplexer slots are too small (for example, 1 bit) then the multiplexer and demultiplexer must switch rapidly. If the time slots are and demultiplexer must switch rapidly. If the time slots are too large (for example, 64 Kb) then the data must be too large (for example, 64 Kb) then the data must be buffered buffered andanddelay delay would be produced. TDM is used for would be produced. TDM is used for baseband (baseband (not not bandpass) data transmission. bandpass) data transmission. DemultiplexerDemultiplexerMultiplexerMultiplexerTime slotsTime slotsEE4512 Analog and Digital Communications Chapter 7••The T1 TDM system for telephone networks uses a 193 bit The T1 TDM system for telephone networks uses a 193 bit frame frame where each frame has 24 8where each frame has 24 8--bit bit slotsslotsand 1and 1--bit in each bit in each frame for signaling. The T1 bit rate rframe for signaling. The T1 bit rate rbb= 1.544 Mb/sec.= 1.544 Mb/sec.The duration of each frame TThe duration of each frame Tff= 193 b / 1.544 Mb/sec = = 193 b / 1.544 Mb/sec = 125 125 µµsec or 8 k samples/sec. Here all the data sources sec or 8 k samples/sec. Here all the data sources have the same data rate.have the same data rate.S&M Figure 7S&M Figure 7--33EE4512 Analog and Digital Communications Chapter 7••If the data sources have different rates a If the data sources have different rates a multiplexer multiplexer schemeschememust reconcile the disparate rates.must reconcile the disparate rates.The data rates are in the lowest possible ratio of 8:16:24 orThe data rates are in the lowest possible ratio of 8:16:24 or1:2:3 for a total of 1 + 2 + 3 = 6 slots. The slots are 1:2:3 for a total of 1 + 2 + 3 = 6 slots. The slots are apportioned to the channels as: apportioned to the channels as: a b b c c c a b b c c c and the channel and the channel data rate is 48 kb/sec.data rate is 48 kb/sec.Another example is data rates of 10, 15, 20, and 30 Kb/sec Another example is data rates of 10, 15, 20, and 30 Kb/sec which reduces to 2:3:4:6 (the LCD is 5) for 2 + 3 + 4 + 6 = which reduces to 2:3:4:6 (the LCD is 5) for 2 + 3 + 4 + 6 = 15 slots15 slotsand the channel data rate isand the channel data rate is45 kb/sec. 45 kb/sec. S&M Figure 7S&M Figure 7--44EE4512 Analog and Digital Communications Chapter 7••If data arrives If data arrives asynchronouslyasynchronouslyfrom variable rate sources, a from variable rate sources, a statistical multiplexer statistical multiplexer with input buffers is used. The design with input buffers is used. The design is performed by observation and tested in simulation.is performed by observation and tested in simulation.S&M Figure 7S&M Figure 7--55EE4512 Analog and Digital Communications Chapter 7••The statistical TDM packet consists of a The statistical TDM packet consists of a start flagstart flag, , address address fieldfield, , control fieldcontrol field, , information bitsinformation bits, , error control, error control, and anand anend end flag.flag.S&M Figure 7S&M Figure 7--66TCP/IP data packetTCP/IP data packetEE4512 Analog and Digital Communications Chapter 7••TDM with four equal data rate sources rTDM with four equal data rate sources rbb= 250 b/sec and a = 250 b/sec and a transmission rate transmission rate rrTDMTDM= 1 kb/sec= 1 kb/sec0110EE4512 Analog and Digital Communications Chapter 7••TDM with unequal data rate sources rTDM with unequal data rate sources rbb= 250, 250 and 500 = 250, 250 and 500 b/sec and a transmission rate b/sec and a transmission rate rrTDMTDM= 1 kb/sec= 1 kb/secrrbb= 500 b/sec (input)= 500 b/sec (input)rTDM= 1 kb/secrrbb= 500 b/sec (output)= 500 b/sec (output)1 msec delay1 msec delayEE4512 Analog and Digital Communications Chapter 7Chapter 7Chapter 7Multiplexing TechniquesMultiplexing Techniques••Frequency Division MultiplexingFrequency Division Multiplexing••Pages 368Pages 368--370370EE4512 Analog and Digital Communications Chapter 7••Frequency division multiplexingFrequency division multiplexing(FDM) divides the total (FDM) divides the total bandwidth available to the system into bandwidth available to the system into nonnon--overlapping overlapping frequency subfrequency sub--bands for transmission over a single digital bands for transmission over a single digital communication channel.communication channel.EE4512 Analog and Digital Communications Chapter 7••FDM usually utilizes FDM usually utilizes guard bandsguard bandsto separate the digital to separate the digital data transmissions. FDM is used for bandpass (data transmissions. FDM is used for bandpass (not not baseband) data transmission baseband) data transmissionEE4512 Analog and Digital Communications Chapter 7••FDM for FDM for FM broadcastingFM broadcastingover a cable system: over a cable system: S&MFigure 7-7aWXTUWXTUWQAN ScrantonWPTSPittsburghEE4512 Analog and Digital Communications Chapter 7••FDM employs a common shared bandwidth but here with FDM employs a common shared bandwidth but here with no apparent guard bands. no apparent guard bands. S&M Figure 7S&M Figure 7--7b7bEE4512 Analog and Digital Communications Chapter 7••Optical FDM is usually called Optical FDM is usually called wavelength division wavelength division multiplexing multiplexing (WDM) and utilizes separate