02 - multiplexingcontents.kocw.net/kocw/document/2014/pusan/kimjongdeok/2.pdf · 2016-09-09 ·...
TRANSCRIPT
강의의 목표
Multiplexing 기술이 해결하려는 문제를 이해한다.
기본 Multiplexing 기술인 FDM과 Synchronous TDM 기술의 주요
원리 및 특징을 이해한다.
Synchronous TDM과 Statistical/Asynchronous TDM을 비교하여 이
해한다.
CDM, SDM 등 무선 통신에 널리 쓰이는 Multiplexing 기술을 이해
한다.
용어들을 기억한다.
교재 – Chapter 8. Multiplexing, Chapter 9. Spread Spectrum
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용어 (Terminologies)
전송 매체 (Transmission Medium) 분류
전송 매체 : 송신자는 정보를 물리적 신호의 형태로 바꿔 송신자와 수신자 사이
의 전송 매체를 통해 수신자로 전달
예) 신호 – 전자기파, 매체 – 동축선, 신호 – 빛, 매체 – 광섬유
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Guided Media (Wired) Unguided Media (Wireless)Twisted Pair, Coaxial Cable, Optical Fiber Air, Sea Water
Point-to-Point Media Multi-Point/Shared Media점대점 매체, 양끝의 두 장치만 매체를 사용 공유매체, 여러 장치가 매체를 함께 사용
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
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Multiplexing
Multiplexing
점대점 (Point-to-Point) 매체의 공유
Cf) MAC (Medium Access Control) : 방송 매체의 공유
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
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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
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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
현재의 초고속 인터넷을 가능하게 한 기술
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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
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TDM Example: Digital Carrier Systems
통신 사업자들이 디지털 기간 망 구축에 사용한 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.
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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
t4 t3 t2 t1
A
B
C
D
D2 C2 B2 A2 D1 C1 B1 A1
C2 B2 B1 A1
HeaderData
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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
Bus의 평균 Inter-arrival Time이 10분인 경우 (Poisson Arrival) 평균 대기 시간은?
Bus의 Inter-arrival Time이 10분인 경우 평균 대기 시간은?
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
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CDM (Code Division Multiplexing)
같은 주파수를 같은 시간에 사용할 수 있을까?
Direct Sequence Spread Spectrum (DSSS) 기술에 기초
전송하여야 할 Symbol에 Code를 곱함
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0
1010
CDM (Code Division Multiplexing)
Orthogonal Code
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Transmitter
CODE AReceiver #1
Receiver#2
CODE A
CODE BOrthogonal to Code A
동일 코드 사용 시원래 정보 복원
송신자가 사용한 것과 다른 (직교) 코드 사용시 해당 신호는 0(제거됨. 마치 존재하지 않는 것과 같음)
CDM (Code Division Multiplexing)
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time
1 1 1 1
1-11-1 1-11-1 11-1-1
1 1 1 1
-111-1 -1-111 -11-11
1-11-1 1-11-1 11-1-1 -111-1 -1-111 -11-11
-1 -1
-1 -1
11-1-1 11-1-1 1-11-1 11-1-1 1-11-1 -1-111
1 1 1 1
1-11-1 1-11-1 11-1-1 -111-1 -1-111 -11-11
-1 -1
+=0
Orthogonal Code
Transmitter
Receiver #1
+=4 +=-4
time
time
time
time
time
(1-11-1) (11-1-1)=
Receiver #2
CDM (Code Division Multiplexing)
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1 1
1-11-1 1-11-1 11-1-1
-1
Transmitter
1
-1
1
Data for A
11-1-1 11-1-1 1-11-1
1-11-1 1-11-1 11-1-1
Receiver A
+=4 +=4
+=-4
Receiver B
11-1-1 11-1-1 1-11-1
+=4 +=4
+=-4
With orthogonal codes, we can sharea channel with others at same time
without interference.
+
Combined signal
Data for B
SDM (Spatial Division Multiplexing)
동일한 시간에 동일한 주파수를 사용할 수 있을까? 공간이 다르다면.
Example) Cell Sectoring
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