Multi Mu tiplexing 01204325 Data Communications and Computer Networks Chaipo Chaiporn J n Jaik aikae aeo De Department of f Computer Engineering Kasetsart Unive versity Based on lecture materials from Data Communications and Networking, 5 th ed., Behrouz A. Forouzan, McGraw Hill, 2012. Revised 2020-08-11
Out Outline line • Multiplexing over frequency • Multiplexing over time • Orthogonal frequency division multiplexing (OFDM) 2
Sha Sharing M ng Medi dium um • A link is divided into channels D 1 link, n channels M E n signals n signals M U (input) (output) X U X Multiplexer Demultiplexer 3
Fr Frequenc equency Divis ivisio ion n Multiple ultiplexing ing (FD FDM) An An analog multiplexing technique to combine signals • Medium BW > Channel BW • Each signal is modulated to a different carrier frequency ◦ E.g., broadcast radio • Channel allocated even if no data 4
Conceptu Con tual View w of of FDM DM Channel 3 Channel 2 Channel 1 f 3 f 2 T i m e f 1 Frequency 5
FD FDM: M : Mul ultipl plexi xing P ng Proce cess 6
FD FDM: D : Demul multipl plexi xing P ng Proce cess 7
Gu Guard Bands • Strips of unused bandwidth to prevent signals from overlapping FDM f f 3 kHz 3 kHz 3 kHz 3 kHz FDM (no guard band) f 3 kHz 3 kHz 3 kHz f 3 kHz FDM (with guard band) f 3 kHz 3 kHz 3 kHz 8
Ex Example – FD FDM Gu Guard B Bands Five 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? 1.5 kHz 1.5 kHz 1.5 kHz 1.5 kHz 3 kHz 3 kHz 3 kHz 3 kHz 3 kHz f 21 kHz 9
Ex Example – FD FDM w with QA QAM Four digital-data channels, 1 Mbps each, use a satellite channel of 1 MHz. Design an appropriate configuration using FDM. (assume 1 Hz/baud) 10
An Analog Hierarchy • Used by AT&T (from 1910s to early 1980s) 4 kHz 48 kHz 12 voice channels 12 voice channels 4 kHz 240 kHz F group 60 voice channels D … 2.52 MHz M F supergroup 600 voice channels 5 groups D 4 kHz M master 10 supergropus 16.984 MHz F group 3600 voice channels D … M Jumbo 6 master groups F group D M 11
Wa Wavelength Division Multiplexing (WDM) An analog An og multi tiplexing technique to o com ombine op opti tical signals • WDM is a special case of FDM l 1 l 1 l 2 l 2 WDM WDM l 1 + l 2 + l 3 l 3 l 3 l 1 l 1 l 1 + l 2 + l 3 l 2 l 2 Fiber-optic cable l 3 l 3 Multiplexer Demultiplexer 12
Time Division Multiplexing (TD Ti (TDM) A digital multiplexing technique to combine data A • Medium Data Rate > Signal Data Rate • Multiple digital signals interleaved in time one unit A Frame T D B M A A A C B C B C B C Time slot • Time slots ◦ are preassigned to sources and fixed ◦ are allocated even if no data ◦ do not have to be evenly distributed among sources 13
14 14 of TDM DM 1 2 3 M D U E X y c w of n e u q 1 e r 2 F tual View 3 Data flow 1 2 3 Channel 1 Channel 2 1 Channel 3 2 Channel 1 Channel 2 3 Time Channel 3 Channel 1 Channel 2 Channel 3 Conceptu M U X 2 3 1 Con
TD TDM Fr Frame mes • A frame consists of one complete cycle of time slots 15
Ex Example – TD TDM F Frame me Four 1-Kbps connections are multiplexed together. A unit is 1 bit. Find: (1) the duration of 1 bit before multiplexing, (2) the duration of a frame, (3) the number of bits in a frame, (4) the duration of a time slot, and (5) the transmission rate of the link. 16
Synchronization Sy 17
Sy Synchronization • Multiplexer and demultiplexer must be synchronized • Framing bits are used to provide synchronization Synchronization pattern … 1 0 1 0 … C B A 1 C B A 0 C B A 1 C B A 0 framing bit 18
Ex Example – Sy Synchronization • Identify all the framing bits from this bit sequence output by a multiplexer ◦ 2 channels ◦ 1 unit = 2 bits 0 0 0 1 1 0 1 1 1 0 0 0 0 1 0 0 1 0 1 1 19
Mu Multi ti-Le Level Mu Multi tiplexing 20
TDM o TD of D Different D Data Ra Rates • Data rate from one source may be faster than the others • More than one time slot can be assigned to certain sources 4 kbps A 8 kbps 16 kbps 1 frame B MUX 4 kbps B C B A C Assume 1 unit = 1 bit 21
Di Digital Signal (DS (DS) ) Hierarchy 64 kbps 1.544 Mbps DS-0 24 DS-0 24 channels 6.312 Mbps T DS-1 4 DS-1 D … 44.376 Mbps M T DS-2 7 DS-2 D DS-0 M 274.176 Mbps T DS-3 6 DS-3 D M T DS-4 D M 1 kbps = 1,000 bps 1 Mbps = 1,000 kbps 22
T L T Line nes a and A nd Ana nalog Si g Signa gnals 23
T-1 1 Frame Stru tructu ture 24
DS DS Servi vices s and T Lines • DS-0, DS-1, etc, are services ◦ T lines are used to implement these services Rate #Voice Service Line (Mbps) Channels DS-1 T-1 1.544 24 DS-2 T-2 6.312 96 DS-3 T-3 44.736 672 DS-4 T-4 274.176 4032 25
E E Lines • European variation of T lines ◦ Also used in Thailand Rate Voice E Line (Mbps) Channels E-1 2.048 30 E-2 8.448 120 E-3 34.368 480 E-4 139.264 1920 26
In Inver erse e Mult ltip iple lexin xing low-speed lines high-speed line high-speed line Inverse Inverse MUX DEMUX 27
OFD OFDM • O rthogonal F requency- D ivision M ultiplexing • Widely used in broadband/wideband digital communications ◦ Digital TV ◦ ADSL ◦ Powerline communications ◦ Wireless networks: Wi-Fi, WiMAX ◦ 4G/5G mobile communications 28
OFD OFDM C Concep ncept • Sub-carriers are orthogonal to each other ◦ No cross-talk; guard bands not necessary Source: http://connectedplanetonline.com 29
Or Ortho hogonalit nality in OFD y in OFDM 20 ms Modulated @500 Hz t Modulated @550 Hz t Modulated @600 Hz t 200 300 400 500 600 700 800 Frequency (Hz) 30
OFD OFDM Implemen Implementatio ion Source: Wikipedia 31
Summa Summary • Multiplexing is a method of combining multiple narrow- band signals into a wider-band signal • FDM multiplexes signals into different frequencies • TDM multiplexes signals into different time slots • OFDM is a FDM method that employs a large number of closely spaced orthogonal sub-carrier signals 32
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