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Transport Methods in Access Networks

and other subsidiary information

Access Networks

lectures

2008 / 09

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data speed (transmission speed) = number of bits pre second - vp [bps]

modulation rate = number of symbols per second - vm [Bd]

relation between them:

mvv mp 2log.

band width fh = around vm

m – number of states of given ‘alphabet’. Each of state (each ‘character’ of code) is expressed by N bits m = 2N N = … (?)

vp = N . vm ….

….. but: because of ingress and other disturbing the m must be less then 2N - there must be some distance (space, gap) to ensure good resolution (of states)

there must be choosen such method of transmission –

- which allows as narrow frequency band as possible

- which ensures at least minimal SNR

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Classification of Methods for Digital Signal Transport

in the baseband : link codes (AMI, HDB3, 2BQ1, …) and PAM

in the modulation band (passband) : modulated signals (PSK, QAM, CAP, DMT)

Utilisation of available bandwidth – by means:

multistate encoding or modulation

numbers of parallel transport paths (so called inverse multiplexing), more carriers

scrambling generation of pseudonoise sequence equalization (or unification) of density of power spectrum

Duplex transport – types... – we have it classified more long time before

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Link codes - - binary data in the frame of physical layer (of OSI model) – lowest level = electric signal coresponding to “0” and “1” or to binary symbols

Fig. 1

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Fig. 2 Ilustration of link coding – voltage levels corresponding to individual binary symbols; transport in the baseband, i.e. low frequences +

eventually also DC [2]

Link codes

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Link codes

Classification

-by number of levels: 2-levels

3-levels (AMI, HDB3, 4B3T)

- multilevels (2B1Q)--unipolar

--bipolar

-Return to Zero (RZ)

- Non Return to Zero (NRZ)

- their specifical properties (advantages and disadvantages – power requirements, freq. spectrum requirements, synchronization problems)

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Fig.3 Illustration of Return-to-Zero Code (RZ)

Link codes

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Fig.4 Illustration of different types of link codes [5]

Non-return–to-Zero L code

bipolar

pseudoternary

Link codes

(polarity alternating)

10Fig .5 AMI, B8ZS and HDB3 link codes [5]

bipolar

B – correct bipolar signalV – violation - ‘artificial’ (for good synchroniz. and detection)

Link codes

(Alternate Mark Inversion)

- for PCM 30/32

(for PDH-E1,E2,E3)

(bipolar with 8 zeros substitution)

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Fig.6

Fig.7

Link codes - NRZ

(for PDH- E4)

- on 10 Mbps LAN interfaces

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Tab.1

[5]Fig.8

1-st bit

(polarity)

2-nd bit

(magnitude)

Quaternary symbol

Voltage

[V]

1 0 +3 2,5

1 1 +1 0.833

0 1 -1 -0.833

0 0 -3 -2.5

2B1Q codeLi

nk c

odes

for U interface of ISDN-BRA, and HDSL

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PAM modulation

original

PAM

time

[Wikipedia]Fig.9 PAM modulation - for transport in baseband

Multistate methods: for transport in baseband

.... they decrease frequence bandwidth requirements...

- there is utilised 8-PAM and 16-PAM – in SDSL lines and others, in Ethernet, there is 5-PAM

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Modulation types in passband

Tab.2 Review of some PSK and QAM modulations

designation name Typical bandwidth for 2Mbps digital stream

BPSK Bipolar PSK 2,8 MHz

QPSK Quadrature PSK 1,4 MHz

DQPSK Differencial PSK 1,4 MHz

8-PSK 8 states PSK 0,8 MHz

4-QAM 4 states quadrature amplitude modulation

1,4 MHz

16-QAM 16 states 0,6 MHz

64-QAM 64 states 0,4 MHz

PSK (Phase Shift Keying)

QAM (Quadrature Amplitude Modulation)

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QPSK - Quadrature Phase-Shift Keying

Fig.10 Principle of BPSK and QAM [Wikipedia]

a) BPSK (Binary PSK)

b) QPSK

φ = 0°φ = 180°

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Fig. 11 QPSK transmitter

Fig. 12 QPSK receiver

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Fig. 13 Vector signal (I-Q) - principle

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QAM

- a modulation scheme which conveys data by changing (modulating) the amplitude of two carrier waves. These two waves, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers—hence the name of the scheme.

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Fig.14 Constellation diagram of 16-QAM (rectangular QAM)

0000

0001

0010

0011

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Fig.15 Block diagram of QAM modulator

equiphase (in-phase) trace

quadrature trace

quadrature signal

flow splitter

data

21[1]

CAP – modulation- (Carrierless Amplitude and Phase)

Instead of modulating the amplitude of two carrier waves, CAP generates QAM signal by combining two PAM signals filtered through two filters designed so that their impulse responses form a Hilbert pair.

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Fig. 16 CAP signal transmitter

Fig.17 CAP signal receiver[1]

CAP – modulation- (Carrierless Amplitude and Phase)

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[1]

Fig.18

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DMT Modulation- discrete multitone techniques

- very efficient technology in comparison with older ones : ...

Fig.19 DMT modulation: a) symbols constellation, b) time running (unreadable, isn’t it? ), c) frequence spectrum

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Fig.20 DMT transmitter

Fig. 21 DMT receiver [1]

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OFDM

It deals with signal transport by means of system of FDM carriers. The neigbouring frequency carriers are orthogonal each to other it is result of Fast Fourier Transform in practice (FFT and IFFT blocks in codecs). The particular carriers are phase- and amplitude- modulated by I-Q modulation, i.e. QAM. That is the basis of DMT technics.

source: http://kupluk2.blogspot.com/2010/03/what-ofdm-means-to-wimax.html

It is the subject-matter for a long lecture, but at same time, it is an application of the known system:

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sources:

http://infowimax.blogspot.com/2008/05/un-panorama-de-ofdm.html

http://www.iss.rwth-aachen.de/Projekte/Theo/OFDM/OFDM_en.html

http://connectedplanetonline.com/wireless/technology/mimo_ofdm_091905/

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-In telecommunication, trellis modulation (also known as trellis coded modulation, or simply TCM) is a modulation scheme which allows highly efficient transmission of information over band-limited channels such as telephone lines. Trellis modulation was invented by Gottfried Ungerboeck in 1982.

-...

[Wikipedia]

Trellis modulation

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Packet and cellular transport modes

Fig.22

ATM

Synchronous transport mode

asynchronous transport mode

Packet transport

5th channel interval (1 Byte) 5th channel interval (1 Byte)

frame (32 Bytes)frame

head (5 Bytes)

cell (53 Bytes)empty cell

packet

no-data

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ATM

Fig. 23 ATM layer model

plane management

layer management

adaptation layer

layer

physical layer

control plane user plane

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Tab.3 Service classes related to ATM adaptation layers

Class A B C D

In time continual noncontinual

Speed constant variable

Communication type

With path creating Without…

Synch. mode emulation (voice)

Compressed video, audio,

voice

Data transport X.25, Frame

RelayData transport IP

Service type and AAL adaptation

1 2 3/4

5 (simpler adaptation)

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Another appendices

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ISDN – line – for rarely connecting to the Internet . (It is only appendix for lecture 3 presentation.)

[2] Fig.24 llustration of ISDN lines

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Tab.4 Ethernet technologies on twisted pairs [2]

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References

[1] http://www.nextep.com.au/upload/DSL_Modulation_Techniques.pdf

[2] T. Anttalainen: Introduction to Telecom. Network Engineering, Norwood (USA - MA), 2003.[3] Vaculík: Prístupové siete. ŽU v Žiline, 2000.[4] J. Vodrážka: Přenosové systémy v přístupové síti. ČVUT, 2003.[5] www.ktl.elf.stuba.sk/~oravec/folie/Linkove%20kody.pdf