Multiplexing 부산대학교정보컴퓨터공학부 김종덕 (kimjd@pusan.ac.kr)
강의의목표 Multiplexing 기술이해결하려는문제를이해한다. 기본 Multiplexing 기술인 FDM과 Synchronous TDM 기술의주요원리및특징을이해한다. Synchronous TDM과 Statistical/Asynchronous TDM을비교하여이해한다. CDM, SDM 등무선통신에널리쓰이는 Multiplexing 기술을이해한다. 용어들을기억한다. 교재 Chapter 8. Multiplexing, Chapter 9. Spread Spectrum 2
용어 (Terminologies) 전송매체 (Transmission Medium) 분류 전송매체 : 송신자는정보를물리적신호의형태로바꿔송신자와수신자사이의전송매체를통해수신자로전달 예 ) 신호 전자기파, 매체 동축선, 신호 빛, 매체 광섬유 Guided Media (Wired) Twisted Pair, Coaxial Cable, Optical Fiber Unguided Media (Wireless) Air, Sea Water Point-to-Point Media 점대점매체, 양끝의두장치만매체를사용 Multi-Point/Shared Media 공유매체, 여러장치가매체를함께사용 3
용어 Simplex signals transmitted in one direction eg. Television Half Duplex both stations transmit, but only one at a time eg. police radio Full Duplex simultaneous transmissions eg. telephone 4
전송매체공유의필요성 통신을원하는모든단말이직접연결 (Direct Link) 되어야한다면? 개의통신단말들이 Full Mesh 로연결될경우필요한연결의수? 5
Multiplexing Multiplexing 점대점 (Point-to-Point) 매체의공유 Cf) MAC (Medium Access Control) : 방송매체의공유 Multiplexing Type FDM (Frequency Division Multiplexing) WDM (Wavelength Division Multiplexing) Synchronous TDM (Time Division Multiplexing) Statistical (Asynchronous) TDM CDM (Code Division Multiplexing) Based on Spread Spectrum Technology 6
FDM (Frequency Division Multiplexing) Each signal is modulated to a different carrier frequency Carrier frequencies separated so signals do not overlap, Guard Bands Ex) broadcast radio Fixed allocation Channel allocated even if no data 7
WDM (Wavelength Division Multiplexing) Multiple beams of light at different frequency Carried by Optical Fiber A form of FDM, Each color of light (wavelength) carries separate dat a channel 현재의초고속인터넷을가능하게한기술 8
Synchronous TDM (Time Division Multiplexing) Multiple digital signals interleaved in time Time slots pre-assigned to sources and fixed May be assigned at block level Fixed Allocation Time slots allocated even if no data 9
Synchronous TDM System Overview 10
TDM Example: Digital Carrier Systems 통신사업자들이디지털기간망구축에사용한 TDM 체계 USA/Canada/Japan use one system, ITU-T use a similar (but different) system US system based on DS-1 format (T1 carrier) Multiplexes 24 channels, Each frame has 8 bits per channel plus one framing bit, 193 bits per frame For Voice Communication each channel contains one word of digitized data (PCM, 8000 Hz) Data rate 8000x193 = 1.544Mbps Same format to provide digital data service. 11
Statistical/Asynchronous TDM Allocates time slots dynamically based On Demand In Synchronous TDM many slots are wasted Multiplexer scans input lines and collects data until frame full Data rate on line lower than aggregate rates of input lines Statistical Gain A D2 C2 B2 A2 D1 C1 B1 A1 B C C2 B2 B1 A1 D t4 t3 t2 t1 Data Header 12
Performance Output data rate less than aggregate input rates May cause problems during peak periods Buffer inputs; Queueing; Queueing Delay Keep buffer size to minimum to reduce delay? Queueing Theory Bus의평균 Inter-arrival Time이 10분인경우 (Poisson Arrival) 평균대기시간은? Bus의 Inter-arrival Time이 10분인경우평균대기시간은? STDM as M/D/1 Queueing Model Mean Queue Length N= ρ + ρ²/[2(1-ρ)] ; Means Average Buffer Size Mean Waiting Time Tr= (2 - ρ)/2m(1-ρ); Means Mean delay 13
CDM (Code Division Multiplexing) 같은주파수를같은시간에사용할수있을까? Direct Sequence Spread Spectrum (DSSS) 기술에기초 전송하여야할 Symbol 에 Code 를곱함 0 1010 14
CDM (Code Division Multiplexing) Orthogonal Code 동일코드사용시원래정보복원 CODE A Trans mitter Recei ver #1 Recei ver #2 CODE A CODE B Orthogonal to Code A 송신자가사용한것과다른 ( 직교 ) 코드사용시해당신호는 0 ( 제거됨. 마치존재하지않는것과같음 ) 15
CDM (Code Division Multiplexing) Transmitter 1 1 1 1 1 1 1 1-1 -1 time -1-1 time 1-11-1 1-11-1 11-1-1-111-1-1-111-11-11 1-11-1 1-11-1 11-1-1-111-1-1-111-11-11 time time 1-11-1 1-11-1 11-1-1-111-1-1-111-11-11 11-1-1 11-1-1 1-11-1 11-1-1 1-11-1-1-111 1 1 1 1 Receiver #1-1 -1 time time +=4 +=-4 +=0 Receiver #2 (1-11-1) (11-1-1)= Orthogonal Code 16
CDM (Code Division Multiplexing) 1 1-1 Data for A Receiver A 1-11-1 1-11-1 11-1-1 +=4 +=4 1-11-1 1-11-1 11-1-1 Combined signal +=-4 Transmitter 1 1-1 Data for B + 11-1-1 11-1-1 1-11-1 +=4 +=4 11-1-1 11-1-1 1-11-1 Receiver B +=-4 With orthogonal codes, we can share a channel with others at same time without interference. 17
SDM (Spatial Division Multiplexing) 동일한시간에동일한주파수를사용할수있을까? 공간이다르다면. Example) Cell Sectoring 18
SDM (Spatial Division Multiplexing) 다중안테나와진보적인신호처리기술로다양한경로로전달되는공간의무선스트림들을개별적으로처리가능해짐 MIMO (Multi-Input Multi-Output) / Beamforming 19