Multiplexing and Demultiplexing Chapter 11 Multiplexing and Demultiplexing © Bobby Hoggard, Department of Computer Science, East Carolina University These slides may not be used or duplicated without permission

Multiplexing / Demultiplexing Multiplexing: combining information from several sources for transmission over a shared medium Demultiplexing: separating combined information from a shared medium back into separate information streams 10010 10010 Source A Dest A 0011 0011 10010 0011 11101 Source B Multiplexer Demultiplexer Dest B shared medium 11101 11101 Source C Dest C

Types of Multiplexing Time Division (TDM) Each sender gets a block of time to send their data Only one sender can transmit at a time Frequency Division (FDM) Each sender transmits data all at the same time, but on different frequencies Wavelength Division (WDM) The same as frequency division multiplexing, but FDM is typically used for radio waves whereas WDM is typically used for light Code Division (CDM) Uses mathematical formulas to combine the data

Asynchronous Time Division Multiplexing Transmits data from each sender, one at a time There are gaps between transmissions if there is no data to send Source A Dest A C B A C B A Source B Multiplexer Demultiplexer Dest B Source C Dest C

Synchronous Time Division Multiplexing Transmits data from each sender, one at a time There are no gaps or delays between transmissions Source A Dest A A C B A C B A C B A C B A C B A C B A Source B Multiplexer Demultiplexer Dest B Source C Dest C

Telephone System Multiplexing The telephone system uses a version of synchronous time division multiplexing An extra "synch" bit is included after each round to ensure sender/receiver stays synchronized The synch bit alternates between 0 and 1 synch bits Source A Dest A 1 1 C B A C B A 1 C B A Source B Multiplexer Demultiplexer Dest B Source C Dest C

Unfilled Slots Synchronous TDM works well if all sources produce data at a uniform rate If a source has no data to produce then time slots are actually filled with idle bits These "idle frames" unnecessarily take up time where another source could be using the network to send data Dest A A3 A2 A1 B2 B1 Dest B A1 C1 B2 A3 D2 A2 D1 B1 A1 Multiplexer Demultiplexer C1 Dest C D2 D1 Dest D

Statistical TDM Skips any source that does not have data ready Dest A B2 B1 Dest B C1 B2 A3 D2 A2 D1 B1 A1 Multiplexer Demultiplexer C1 Dest C D2 D1 Dest D

Statistical TDM Advantage Takes less time to send the same amount of data, and therefore increases the overall data rate of the network Synchronous TDM Takes 11 time units to send all of the data: C1 B2 A3 D2 A2 D1 B1 A1 Statistical TDM Takes 8 time units to send the same data: C1 B2 A3 D2 A2 D1 B1 A1

Statistical TDM Disadvantage Extra bits must be added to identify the receivers to which the data is being sent Receiver X's data is expected in every 4th time block Synchronous TDM D C B A D C B A D C B A 1 2 3 2 2 1 1 1 Statistical TDM Since we don't know when Sender X's data will arrive, extra bits must be added to identify each receiver 1 C 2 B 3 A 2 D 2 A 1 D 1 B 1 A

Frequency Division Multiplexing Each sender transmits data all at the same time Each sender uses a different channel (i.e. a different carrier frequency) The different signals are combined together into a single composite wave Source A Dest A Multiplexer Demultiplexer Source B Dest B

FDM Demultiplexer is a Set of Filters Each filter extracts a range of frequencies around the carrier frequency Dest A Carrier: 440Hz Carrier: 850Hz Carrier: 1275Hz Filter allows 410Hz – 470Hz Filter allows 820Hz – 880Hz Dest B Filter allows 1245Hz – 1305Hz Dest C

FDM Channels Are Independent FDM gives the illusion of each sender/receiver pair having a private communication line on a separate physical medium Any modulation technique can be used on each channel, and each channel can use a different one Source A Dest A Channel A Source B Multiplexer Demultiplexer Dest B Channel B Channel C Source C Dest C

Example 200KHz channel assignments with 20KHz guard bands Interference can occur if the frequencies of two channels are too close together Carrier frequencies on a network are chosen with a gap (guard band) between them to reduce the possibility of interference Likewise, the FCC regulates radio stations to make sure there's enough spacing between frequencies to reduce interference between stations Example 200KHz channel assignments with 20KHz guard bands Channel Frequencies Used 1 100KHz – 300KHz 2 320KHz – 520KHz 3 540KHz – 740KHz 4 760KHz – 960KHz 5 980KHz – 1180KHz 6 1200KHz – 1400KHz 20 KHz guard band 1400 1200 1000 800 600 400 200 6 KHz 5 4 3 2 1

Why Allocate a Range of Frequencies? Frequency Modulation FM modulates between multiple frequencies Note: Amplitude and Phase modulation technically only need one frequency Increase Data Rate Use the extra frequencies in the range to transmit individual bits of data Increase Immunity to Interference Use the extra frequencies in the range to transmit copies of the same data

Increased Data Rate Use the extra frequencies in the range to transmit individual bits of data Example: Divide the frequency range into 8 smaller frequency blocks Transmit one bit of data in each of the smaller frequency blocks Result: parallel transmission of 1 byte of data Channel 1: 100KHz to 300KHz 105KHz – 120KHz 280KHz – 295KHz 130KHz – 145KHz 155KHz – 170KHz 180KHz – 195KHz 205KHz – 220KHz 230KHz – 245KHz 255KHz – 270KHz 1 8 bits of data transmitted simultaneously One bit on each sub-channel It's using frequency division multiplexing for subsections of a larger frequency division multiplexed system

Increased Immunity to Interference Use the extra frequencies in the range to transmit copies of the same data Example: Divide the frequency range into 3 smaller frequency blocks Transmit the same data in each of the frequency blocks Result: a high probability that one of the 3 will make it to the receiver with no interference Channel 1: 100KHz to 300KHz 100KHz – 150KHz 250KHz – 300KHz 175KHz – 225KHz 1 1 0 1 1 1 0 1 error Interference 203KHz – 267KHz

Hierarchial FDM Because FDM requires each source to have a different frequency range, if sources have the same frequencies, they can be shifted into a different ranges so they can be properly multiplexed 0 – 4KHz 0 – 4KHz 2 telephone channels 0 – 8KHz 0 – 4KHz 4KHz – 8KHz 0 – 8KHz 0 – 16KHz 0 – 4KHz 4KHz – 8KHz 0 – 4KHz 8KHz – 16KHz 2 telephone channels 0 – 8KHz 0 – 4KHz

1 jumbo group of 3600 phone channels Hierarchial FDM FDM hierarchy used in the telephone system 10 super groups of 60 phone channels per group 240 KHz each 5 groups of 12 phone channels per group 48 KHz each 6 master groups of 600 phone channels per group 2.40 MHz each 12 phone channels 4 KHz each 1 jumbo group of 3600 phone channels 14.4 MHz total bandwidth

Wavelength Division Multiplexing The application of frequency division multiplexing to optical fiber The inputs/outputs of this multiplexing are wavelengths of light A prism can demultiplex by taking white light and spread it out into separate colors A prism can also multiplex by taking separate colors of light and focus them into a single beam of white light Source A Dest A optical fiber beam of light Source B Dest B Source C Dest C

Inverse Multiplexing Using several low speed connections in parallel to achieve a single high speed connection Ten, 10 Gbps connections 100 Gbps connection 100 Gbps connection Demultiplexer Multiplexer 10 Gbps connection

Code Division Multiplexing Does not rely on the physical properties of time or frequency, but instead uses mathematical formulas to combine and separate data Each sender is assigned a chip sequence, which is a binary value used to encode their data The encoded values are added together and transmitted as a single data value The receiver uses the chip sequence to help split the data and retrieve the portions that it wants The chip sequence acts like mathematical filter, to filter out the unwanted data and keep the desired data Used in the cellular telephone system and in some satellite communications The specific variation used by the cell phone system is called CDMA (Code Division Multi-Access)

CDM Send Example Step #1: Convert the values to vectors of bits 2 Senders / 2 Receivers Sender Chip Sequence Data To Send A 1 0 1 0 1 0 B 1 1 0 1 1 0 Step #1: Convert the values to vectors of bits Use -1 to represent binary 0 and use 1 to represent binary 1 1 0 Chip Sequence A ( 1, -1 ) 1 0 1 0 Data A ( 1, -1, 1, -1 ) 1 1 Chip Sequence B ( 1, 1 ) 0 1 1 0 Data B ( -1, 1, 1, -1 )

CDM Send Example Step #2: Multiply the chip sequence by the data 2 Senders / 2 Receivers Sender Chip Sequence Data To Send A 1 0 1 0 1 0 B 1 1 0 1 1 0 Step #2: Multiply the chip sequence by the data Chip Sequence A ( 1, -1 ) ( 1, -1 ) ( -1, 1 ) ( 1, -1 ) ( -1, 1 ) Data A ( 1, -1, 1, -1 ) Chip Sequence B ( 1, 1 ) ( -1, -1 ) ( 1, 1 ) ( 1, 1 ) ( -1, -1 ) Data B ( -1, 1, 1, -1 )

Data to be transmitted--- i.e. a sequence of signal strengths CDM Send Example 2 Senders / 2 Receivers Sender Chip Sequence Data To Send A 1 0 1 0 1 0 B 1 1 0 1 1 0 Step #3: Add the values together ( 1, -1 ) ( -1, 1 ) ( 1, -1 ) ( -1, 1 ) 1 -1 -1 1 1 -1 -1 1 + -1 -1 1 1 1 1 -1 -1 0 -2 0 2 2 0 -2 0 ( -1, -1 ) ( 1, 1 ) ( 1, 1 ) ( -1, -1 ) Data to be transmitted--- i.e. a sequence of signal strengths

CDM Receive Example 2 Senders / 2 Receivers Sender Chip Sequence Data To Send A 1 0 1 0 1 0 B 1 1 0 1 1 0 Step #1: Treat the received signal strengths as vectors 0 -2 0 2 2 0 -2 0 Receiver A Receiver B ( 0, -2 ) ( 0, 2 ) ( 2, 0 ) ( -2, 0 ) ( 0, -2 ) ( 0, 2 ) ( 2, 0 ) ( -2, 0 )

CDM Receive Example ( ) ( * ) * 2 Senders / 2 Receivers Sender Chip Sequence Data To Send A 1 0 1 0 1 0 B 1 1 0 1 1 0 Step #2: Multiply the vectors by the chip sequence of the desired sender Receiver A Receiver B ( ) ( ( 1, -1 ) * ( 0, -2 ) ( 0, 2 ) ( 2, 0 ) ( -2, 0 ) ( 1, 1 ) * ) ( 0, -2 ) ( 0, 2 ) ( 2, 0 ) ( -2, 0 ) ( 0, 2 ) ( 0, -2 ) ( 2, 0 ) ( -2, 0 ) ( 0, -2 ) ( 0, 2 ) ( 2, 0 ) ( -2, 0 )

CDM Receive Example Step #3: Add each group values Receiver A 2 Senders / 2 Receivers Sender Chip Sequence Data To Send A 1 0 1 0 1 0 B 1 1 0 1 1 0 Step #3: Add each group values Receiver A Receiver B ( 0, 2 ) ( 0, -2 ) ( 2, 0 ) ( -2, 0 ) ( 0, -2 ) ( 0, 2 ) ( 2, 0 ) ( -2, 0 ) 0 + 2 0 + -2 2 + 0 -2 + 0 0 + -2 0 + 2 2 + 0 -2 + 0 2 -2 2 -2 -2 2 2 -2

CDM Receive Example 1 1 1 1 Step #4: Convert to binary Receiver A 2 Senders / 2 Receivers Sender Chip Sequence Data To Send A 1 0 1 0 1 0 B 1 1 0 1 1 0 Step #4: Convert to binary Negative values represent binary 0 and positive values represent binary 1 Receiver A Receiver B 2 -2 2 -2 -2 2 2 -2 1 1 1 1