chapter 6. bandwidth utilization: multiplexing & spreading

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데이터 통신(Data Communications) 1 Yoon-Seok Nam Dept. of Information and Communication Engineering Dongguk University 707 Sukjang-Dong, Gyeongju-City, Gyeongsangbuk-Do, 780-714, Korea Phone : 054-770-2273(Lab), 054-770-2608(Office), 054-770-2605(fax), 010-7641-5004(CP) Email: [email protected] Chapter 6. Bandwidth Utilization: Multiplexing & Spreading Contents 1. Multiplexing 2. Spread Spectrum

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데이터 통신(Data Communications) 1

Yoon-Seok Nam Dept. of Information and Communication Engineering

Dongguk University 707 Sukjang-Dong, Gyeongju-City, Gyeongsangbuk-Do, 780-714, Korea

Phone : 054-770-2273(Lab), 054-770-2608(Office), 054-770-2605(fax), 010-7641-5004(CP) Email: [email protected]

Chapter 6. Bandwidth Utilization: Multiplexing & Spreading

Contents 1. Multiplexing 2. Spread Spectrum

데이터 통신(Data Communications) 2

정의 : 고용량의 전송설비를 여러 전송이 공유하는 것을 총칭 목적 : 링크를 효율적으로 사용 구성 : MUX(다중화), DEMUX(역다중화), link 동기

데이터전송률이 높을수록 비용-효율이 높아짐 일반적으로 요구되는 데이터전송률은 특정범위 내에 있음

종류 FDM (Frequency Division Multiplexing) : 유선 및 무선에서 사용 WDM(Wavelength Division Multiplexing) : 광케이블에서 사용 TDM (Time Division Multiplexing) : 주로 유선에서 사용 Statistical TDM : 데이터통신망에서 다중화 특성을 향상시키기 위하여 사용 CDM(Code Division Multiplexing) : 유선에서 사용하지 않음

1. Multiplexing

데이터 통신(Data Communications) 3

(1) Frequency Division Multiplexing(FDM)

특성 복수개의 low BW analog signal을 동시에 high BW link 상에 전송 각 Signal은 반송파를 사용한 주파수 변조를 통해 분리(shift)됨 Shifted signals은 주파수가 서로 겹치지 않으므로 동시에 전송이 가능함

수신측에서는 shift된 signal을 filter를 통해 원래 신호로 복조함 Channel allocated even if no data

예: TV, radio KBS, EBS, MBC, SBS 등 서로 다른 주파수 사용 유선 또는 무선

데이터 통신(Data Communications) 4

FDM System

Transmitter Original Signals

m1(t), m

2(t) … m

n(t)

Subscriber Carrier Frequencies f

1,f

2, f

3, …, f

n

Carrier Frequencies f

c

Spectrum of composite baseband modulating signal

Receiver Carrier Frequencies 제거

Bandpass Filter(BPF) Demodulator

데이터 통신(Data Communications) 5

FDM of Three Voiceband Signals

Baseband Signal

DSB(Double Sideband Signal) SSB(Single Sideband Signal)

DSB

SSB

Modulation 하기 전의 원본 신호

데이터 통신(Data Communications) 6

데이터 통신(Data Communications) 7

Analog Carrier Systems

아날로그 반송시스템 (Analog Carrier System) FDM에 기반하여 음성대역신호를 고용량 전송링크로 장거리 전송하기 위함 standardized FDM hierarchy

AT & T, ITU-T AT&T (USA)

Group : 12 voice channels (4kHz each) = 48kHz. Range 60kHz to 108kHz Supergroup : 60 channel. FDM of 5 group signals Mastergroup : 10 supergroups

데이터 통신(Data Communications) 8

(2) WDM

WDM is an analog multiplexing technique to combine optical signals.

Prisms in WDM multiplexing and demultiplexing WDM : other side of FDM. Think of (wave length)*frequency=velocity of light

Practical use : WDM + TDM

(wave length)*frequency =velocity of light

데이터 통신(Data Communications) 9

(3) TDM 음성통신서비스를 기반으로 시작

Time Division Multiplexing 디지털 multiplexing 방법

사용자마다 채널을 사용할 시간 slot을 할당해 다수의 사용자를 지원하는 방법

특정시간에 하나의 사용자만 slot을 사용

데이터 통신(Data Communications) 10

TDM의 전송 과정

데이터 통신(Data Communications) 11

Figure 6.15 Interleaving

In a TDM, the data rate of the link is n times faster, and the unit duration is n times shorter.

데이터 통신(Data Communications) 12

Digital Carrier Systems USA/Canada/Japan use one system Synchronous TDM에 기반하여 고용량의 전송링크로 음성신호를 장거리 전송하기 위하여 고안됨

기본적인 TDM 계층구조는 DS-1/E1 임 DS1 (Digital Signal 1, 24개 채널 1.544Mbps) : 북미, 일본

각 프레임은 채널당 8비트와 한개의 프레임 구성비트로 193를 포함

음성전송규칙 각 채널은 한개의 PCM 단어를 포함하며 8KHz로 sampling됨 8000 (samples/s) *[ 8 (bit/sample) * 24 +1] = 8000 * 193 =

1.544Mbps E1 (32개 채널을 멀티플렉스, 2.048Mbps) : 유럽, 한국

1 for Sync, 1 for signal, 30 for voice 8000 (samples/s) * 8 (bit/sample) * 32 =2.048 Mbps

데이터 통신(Data Communications) 13

DS-1 Frame Format

F

DS1 T1(T-carrier 1) Logical pattern Physical line

데이터 통신(Data Communications) 14

E1 Frame Format 16 Frame 동기가 많이 사용됨(CRC-4)

Signaling Framing

E1 Frame

125usec

E-1 Frame Format

데이터 통신(Data Communications) 15

Figure 6.23: Digital hierarchy

데이터 통신(Data Communications) 16

Table 6.1 DS and T lines rates

데이터 통신(Data Communications) 17

Figure 6.24: T-1 line

데이터 통신(Data Communications) 18

Figure 6.25: T-1 line frame structure

데이터 통신(Data Communications) 19

Table 6.2 E line rates

데이터 통신(Data Communications) 20

DS-1 DS-1C DS-2 DS-3 DS-4

24 48 96

672 4032

1.544 3.152 6.312

44.736 274.176

E-1 E-2 E-3 E-4 E-5

30 120 480

1920 7680

2.048 8.448

34.368 139.264 565.148

Number of voice channels

Frame format

Data rate (Mbps)

Frame format

Number of voice channels

Data rate (Mbps)

North American Europe

TDM carrier standards

데이터 통신(Data Communications) 21

Sonet/SDH

Synchronous Optical Network (ANSI) Synchronous Digital Hierarchy (ITU-T) Compatible Signal Hierarchy

Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1) 51.84Mbps Carry DS-3 or group of lower rate signals (DS1 DS1C DS2)

plus ITU-T rates (e.g. 2.048Mbps) Multiple STS-1 combined into STS-N signal

ITU-T lowest rate is 155.52Mbps (STM-1) STM-1 : 155.52 Mbps STM-4 : 622.08 Mbps STM-16 : 2.48832 Gbps (2.5Gbps) STM-64 : 9.95328 Gbps (10Gbps)

데이터 통신(Data Communications) 22

SONET Frame Format

데이터 통신(Data Communications) 23

T1517950-95

? 1? N

? 3

? 3? 1

? 1

? 3

? 4

? 7? 7

STM-N AUG AU-4 VC-4

AU-3 VC-3

C-4

C-3

C-2

C-12

C-11

VC-3

VC-2

VC-12

VC-11

TU-3

TU-2

TU-12

TU-11

TUG-2

TUG-3

AU-4

139 264 kbit/s(Note)

44 736 kbit/s34 368 kbit/s(Note)

6312 kbit/s(Note)

2048 kbit/s(Note)

Pointer processing

Multiplexing

Aligning

Mapping

NOTE ? G.702 tributaries associated with containers C-x are shown. Other signals, e.g. ATM, can also be accommodated (see 10.2).

C-n Container-n

1544 kbit/s(Note)

Multiplexing structure

x1

x1

x7

x3

x4

x1 x7

x3

x3

xN

데이터 통신(Data Communications) 24

(4) Statistical TDM 패킷 통신이 대표적임.

동기 시분할 멀티플렉싱에서는 고정 슬롯 할당에 의해 많은 채널 용량이 낭비될 여지가 있음 : 음성통화 효율은 25% 미만(낭비)

Statistical TDM (asynchronous TDM, intelligent TDM이라고도 불림)에서는 슬롯을 고정적으로 할당하지 않고 실제 데이터가 있을 경우에만 동적으로 할당하도록 하여 채널 용량을 효율적으로 사용함 연결된 장치들이 항상 계속해서 데이터를 전송하지 않는 성질을 이용 Overbook이 기본임 : 동기TDM 대역보다 사용자들에게 더 많이 할당

구성 다수의 I/O회선과 하나의 고속멀티플렉스 회선 각 I/O 회선은 각각 버퍼를 가짐 n개의 I/O회선 중 k개의 회선만 TDM 프레임을 위해 사용됨

동작 입력버퍼를 조사하여 데이터가 있으면 프레임을 채워서 전송

멀티플렉스된 회선의 데이터 전송률은 접속 디바이스들의 데이터 전송률의 합보다 적을 수 있음

데이터 통신(Data Communications) 25

데이터 통신(Data Communications) 26

평균 Delay 대 점유율

Performance

Output data rate less than aggregate input rates

May cause problems during peak periods Buffer inputs Keep buffer size to

minimum to reduce delay

평균 Buffer Size 대 점유율

데이터 통신(Data Communications) 27

2. Spread Spectrum

정의 Spread data over wide bandwidth

목적 Makes jamming(방해) and interception(도청) harder

방식 Frequency hoping

Signal broadcast over seemingly random series of frequencies

Direct Sequence Each bit is represented by multiple bits in transmitted

signal Chipping code

데이터 통신(Data Communications) 28

(1) Spread Spectrum Concept

Input fed into channel encoder Produces narrow bandwidth analog signal around central frequency

Signal modulated using sequence of digits Spreading code/sequence Typically generated by pseudonoise/pseudorandom number

generator Increases bandwidth significantly

Spreads spectrum Receiver uses same sequence to demodulate signal Demodulated signal fed into channel decoder

1 0 0 1 0 ….. 1 0 0 1 0 …..

데이터 통신(Data Communications) 29

Spread Spectrum 방식의 Gains Immunity from various noise and multipath distortion

Including jamming Can hide/encrypt signals

Only receiver who knows spreading code can retrieve signal Several users can share same higher bandwidth with little interference

Cellular telephones Code division multiplexing (CDM) Code division multiple access (CDMA) WLAN, WPAN(Bluetooth)

Pseudorandom Numbers Generated by algorithm using initial seed Deterministic algorithm

Not actually random If algorithm good, results pass reasonable tests of randomness

Need to know algorithm and seed to predict sequence 따라서 Transmitter와 정보를 공유하고 있는 Receiver만 데이터 수신이 가능

데이터 통신(Data Communications) 30

Figure 6.27: Spread spectrum

데이터 통신(Data Communications) 31

(2) Frequency Hopping Spread Spectrum (FHSS)

Signal broadcast over seemingly random series of frequencies Receiver hops between frequencies in sync with transmitter 도청

Eavesdroppers hear unintelligible blips 방해

Jamming on one frequency affects only a few bits

Basic Operation Typically 2k carriers frequencies forming 2k channels Channel spacing corresponds with bandwidth of input Each channel used for fixed interval

300 ms in IEEE 802.11 Some number of bits transmitted using some encoding scheme

May be fractions of bit (see later) Sequence dictated by spreading code

데이터 통신(Data Communications) 32

데이터 통신(Data Communications) 33

Frequency Hopping Example

Figure 6.31: Bandwidth sharing

데이터 통신(Data Communications) 34

Frequency Hopping Spread Spectrum System (Transmitter)

확산코드

sd(t)=A cos(2π(f

0+0.5(b

i+1)∆f)t) for iT<t<(i+1)T

A=Amplitude f

0=reference frequency

bi=i_th data bit (binary 1=+1, binary 0=-1)

∆f=frequency interval T=bit duration

p(t)= sd(t) c(t)

= A cos(2π(f0+0.5(b

i+1)∆f)t) cos(2πf

it)

= 0.5A [cos(2π(f0+0.5(b

i+1)∆f+f

i)t) + cos(2π(f

0+0.5(b

i+1)∆f-f

i)t) ]

s(t)= 0.5A cos(2π(f

0+0.5(b

i+1)∆f+f

i)t)

데이터 통신(Data Communications) 35

Frequency Hopping Spread Spectrum System (Receiver)

확산코드

s(t)= 0.5A cos(2π(f0+0.5(b

i+1)∆f+f

i)t)

p(t)= s(t) c(t) = 0.5A cos(2π(f

0+0.5(b

i+1)∆f+f

i)t) cos(2πf

it)

= 0.25A [cos(2π(f0+0.5(b

i+1)∆f+f

i+f

i)t) + cos(2π(f

0+0.5(b

i+1)∆f)t) ]

sd(t)= 0.25A cos(2π(f

0+0.5(b

i+1)∆f)t)

데이터 통신(Data Communications) 36

Slow and Fast FHSS

Frequency shifted every Tc seconds Duration of signal element is Ts seconds Slow FHSS has Tc ≥ Ts

여러비트가 동일 주파수대로 전송 Fast FHSS has Tc < Ts

한 비트가 여러 주파수대로 전송 Generally fast FHSS gives improved performance in noise (or jamming)

si(t)= A cos(2πf

it)

f

i=f

c+(2i-1-M) f

d f

c=Carrier frequency

fd

= Difference of frequency M= number of signal element =2L

L= number of bits for signal element

데이터 통신(Data Communications) 37

Slow Frequency Hop Spread Spectrum Using MFSK (M=4, k=2)

Ws= 2kW

d

데이터 통신(Data Communications) 38

Fast Frequency Hop Spread Spectrum Using MFSK (M=4, k=2)

Ws= 2kW

d

데이터 통신(Data Communications) 39

(3) Direct Sequence Spread Spectrum (DSSS)

Each bit represented by multiple bits using spreading code Spreading code spreads signal across wider frequency band

In proportion to number of bits used 10 bit spreading code spreads signal across 10 times

bandwidth of 1 bit code One method:

Combine input with spreading code using XOR Input bit 1 inverts spreading code bit Input zero bit doesn’t alter spreading code bit Data rate equal to original spreading code

Performance similar to FHSS

데이터 통신(Data Communications) 40

+

+

Direct Sequence Spread Spectrum Example

0 0 | 0 0 1 | 1 1 0 | 1 1 1 | 0

데이터 통신(Data Communications) 41

Direct Sequence Spread Spectrum Transmitter

sd(t)=A d(t) cos(2πf

ct)

A=Amplitude f

c=Carrier frequency

d(t)=data bit (binary 1=+1, binary 0=-1)

s(t)=A d(t) c(t) cos(2πfct)

c(t)=PN sequence

d(t)

데이터 통신(Data Communications) 42

Direct Sequence Spread Spectrum Receiver

s(t) c(t)=A d(t) c(t) c(t) cos(2πfct) = s

d(t)

c(t) x c(t) = 1

d(t)

데이터 통신(Data Communications) 43

Direct Sequence Spread Spectrum Using BPSK Example

가능한 방법 1. d(t)x c(t) 한 다음, BPSK 2. d(t), BPSK, c(t)를 곱함

데이터 통신(Data Communications) 44

Time Domain에서의 곱 Frequency Domain에서 Convolution

SNR G

p= T/ T

c = R

c/ R ~ W

s/ W

d

R : Data Rate

Rc : Spread Rate

Wd : Bandwidth of Signal

Ws : Bandwidth of SS Signal

Approximate Spectrum of DSSS Signal

데이터 통신(Data Communications) 45

데이터 통신(Data Communications) 46

참고문헌 1. 데이타통신과 네트워킹 (Data Communications and Networking),

Behrouz A. Forouzan 저, 이재광, 김중규, 박동선, 홍충선 공역, 교보문고, 2013

2. 데이터통신 및 컴퓨터통신, Stallings, 김종상 역), 사이텍미디어, 2013