2Sharing Medium A link is divided into channels 1 link, n channels n signals (input)M U XD E M U Xn signals (output)MultiplexerDemultiplexer
3Frequency Division Multiplexing (FDM) Medium BW > Channel BWEach signal is modulated to a different carrier frequencyE.g., broadcast radioChannel allocated even if no dataAn analog multiplexing technique to combine signals
4Conceptual View of FDM Channel 3 Channel 2 Channel 1 Time f3 f2 f1 FrequencyTime
7Guard BandsStrips of unused bandwidth to prevent signals from overlapping3 kHzff3 kHzFDMf3 kHzFDM(no guard band)f3 kHzf3 kHzFDM(with guard band)
8FDM: Example 1Five voice channels, each with 3-kHz bandwidth, are to be multiplexed together. If there is a need for a guard band of 1.5 kHz, what is the minimum bandwidth of the link?f3 kHz1.5 kHz21 kHz
9FDM: Example 2Four digital-data channels, 1 Mbps each, use a satellite channel of 1 MHz. Design an appropriate configuration using FDM
10Analog Hierarchy Used by AT&T … … 12 voice channels F D M F D M F D M 4 kHz48 kHz 12 voice channels12 voice channels4 kHz…240 kHz 60 voice channelsgroupFDM2.52 MHz600 voice channels5 groupssupergroupFDM…10 supergropusmaster groupMHz3600 voice channelsFDM6 master groupsJumbo groupFDM
11ExampleThe Advanced Mobile Phone System (AMPS) uses two bands. The first band of 824 to 849 MHz is used for sending, and 869 to 894 MHz is used for receiving. Each user has a bandwidth of 30 kHz in each direction. How many people can use their cellular phones simultaneously?SolutionEach band is 25 MHz. If we divide 25 MHz by 30 kHz, we get In reality, the band is divided into 832 channels. Of these, 42 channels are used for control, which means only 790 channels are available for cellular phone users.
12Wavelength Division Multiplexing (WDM) An analog multiplexing technique to combine optical signalsWDM is a special case of FDMWDM1231+2+311MultiplexerDemultiplexer1+2+3Fiber-optic cable2233
13Time Division Multiplexing (TDM) A digital multiplexing technique to combine dataMedium Data Rate > Signal Data RateMultiple digital signals interleaved in timeTime slotsare preassigned to sources and fixedare allocated even if no datado not have to be evenly distributed among sourcesone unitT D MABCFrameTime slot
14Conceptual View of TDM M U X 1 2 3 1 Data flow 2 D E M U X 3 Time FrequencyChannel 3Channel 2Channel 1
15TDM FramesA frame consists of one complete cycle of time slots
16TDM: ExampleFour 1-Kbps connections are multiplexed together. A unit is 1 bit. Find:the duration of 1 bit before multiplexing,the transmission rate of the link,the duration of a time slot, andthe duration of a frame?
21TDM of Different Data Rates Data rate from one source may be faster than the othersMore than one time slot can be assigned to certain sources4 kbps1 frameBCAAMUX8 kbps16 kbpsB4 kbpsC4 kbps8 kbps16 kbpsMUXABCNote: assume 1 unit = 1 bit
22TDM of Different Data Rates B1 frameAB5 kbpsMUXAA8 kbps3 kbpsBBNote: assume 1 unit = 1 bit
23Bit PaddingDifferent data rates may not be integer multiples of each otherSolution – Bit padding1 ms5 kbps3 kbpsAB6 kbpsABBit PaddingMUX9 kbpsExtra bitAB3000 cycles/sANote: assume 1 unit = 1 bitB