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Infokommunikációs rendszerek – Infocommunication Systems

Lecture 4. előadás

Kódolás, nyalábolás, kapcsolásCoding, multiplexing, switching

Takács György

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Where we are now in study (tele-, info-) communications systems?

• Networks are working systems of terminals, nodes and links

• The basic technologies in links (wireline and wireless) have been discussed

• Node functions (multiplexing, switching, signalling, demultiplexing) will be discussed today

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Analog modulation systems- (AM)

• Amplitude modulation• The momentary amplitude of the carrier is

proportional to the momentary amplitude of the modulating signal

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Analog modulation systems- (AM)

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The spectrum of the AM in the case of discrete fm modulation frequency

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Frequency modulation systems- (FM)

• The momentary frequency of the carrier is proportional to the momentary amplitude of the modulating signal

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Frequency modulation systems- (FM)

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The spectrum of FM modulated signal in the case of discrete fm modulation frequency

•

 

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Digital modulation methods – Amplitude Shift Keying (ASK)

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Digital modulation methods – Binary Phase Shift Keying (BPSK)

Generation of BPSK

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x

~

Carriersignal

BPSK

t

1 0 0 1 +1

-1

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Digital modulation methods – Constellation Diagram / BPSK

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Eye diagram as a basis for demodulation of BPSK signal

Received BPSK

signal

x

Carrier

signal

comparator1,0,1,0

∫

Eye diagram

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Digital modulation methods –Qadrature Phase Shift Keying (QPSK)

• Two carriers: sine wave (Q) and cosine wave (I)

• The modulated signal is the sum of the two components

• One symbol is two bits

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Digital modulation methods –Qadrature Phase Shift Keying (QPSK)

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Digital modulation methods –Qadrature Phase Shift Keying (QPSK)

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Digital modulation methods –Qadrature Phase Shift Keying (QPSK)

1

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Digital modulation methods –Qadrature Amplitude Modulation (QAM)

• Two carriers: sine wave (Q) and cosine wave (I)

• The modulated signal is the sum of the two components

• Different amplitude and differnt phase values for one symbol

• 16QAM means: one symbol is four bits

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Digital modulation methods –Qadrature Amplitude Modulation (16QAM)

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Digital modulation methods –Qadrature Amplitude Modulation (16QAM)

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Digital modulation methods –Qadrature Amplitude Modulation with channel noise

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Why to use sophisticated modulations -- like QAM?

• To put more bits into the standard medium – twisted pair cable –ADSL, Gigabit Ethernet, – coaxial cable – digital TV, HDTV, INTERNET, – Radio – GSM, satellite TV and radio program

broadcasting

• Efficient use of spectum (the radio spectrum is a limited resource)

Bit error rate as a function of signal to noise ratio using BPSK modulation

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Channel capacity as a function of signal to ratio at different modulation system. The reference is the BPSK

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Multiplexing vs. switching

City A City B

105105

103

Trunks for active calls only

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Multiplexing principles

• To reduce transmission costs• To utilize higher bandwidth• „Framing” and „packing” of information• TDM -- Time Division Multiplexing• FDM -- Frequency Division Multiplexing• CDMA -- Code Division Multiple Access• WDM -- Wavelength Division Multiplexing• Mixed

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TDM principles I. PCM frame

4.50 ábra

(Pulse Code Modulation)

125 µs

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TDM principles II. PDH hierarchy

4.51 ábra

Plesiochronous Digital Hierarchy

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TDM principles III. PDH hierarchy

4.51 ábra

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SDH hierarchy

• SDH – Synchronous Digital Hierarchy• VC – Virtual Container (multiplexing level)• STM-N Synchronous Transport Modules

(line signal level)• POH – path overhead (control and

supervisory information)• POH+Payload=VC• A number of VCs can packaged into a

larger VC

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Transport modules

• RSOH – Regenerator Section Overhead

• MSOH – Multiplexer Section Overhead

• AU Pointer – Administrative Unit Pointer (specifies where the payload starts)

• Duration of STM-1 module is 125 µs

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General Transport Module

4.56. ábra

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SDH Network elementsDXC – Digital Cross ConnectADM – Add-drop MultiplexerTM – Terminal multiplexer

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Example of a physical network

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FDM principles

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TDM/FDM channel architecture as used in GSM

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FDM in Cable TV network (US Standard)

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Variable bit-rate data transfer within TDM time-slots

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The Spread Spectrum Concept

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General Model of Spread Spectrum Digital Communication System

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Frequency_Hopping Spread Spectrum FHSS

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FHSS

• A number of channels are allocated for FH• The transmitter operates in one channel at a time for

fixed time interval (Tc)• During that interval, some number of bits or a fraction of

a bit are transmitted (signal elements)• The time interval of signal elements Ts• The sequence of the channels used is dictated by

spreading code• Both transmitter and receiver use the same code to tune

into a sequence of channels in synchronisation

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Transmitter of the FHSS System

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Receiver of the FHSS System

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Slow FHSS using Multi Frequency Shift Keying Tc>Ts(in this case 4 subfrequencies for 2 bits)

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Fast FHSS using Multi Frequency Shift Keying Tc<Ts(in this case 4 subfrequencies for 2 bits)

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Example of Direct Sequence Spread Spectrum DSSS

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DSSS system Transmitter

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DSSS system Transmitter

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Comparison of FDM –TDM -- CDM

TDMA FDMA

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Principle of WDM

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Principle of WDM

Wavelenght Regions

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Prism refraction demultiplexing

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DWDM Ring Arhcitecure

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Switching techniques in public networks

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The Group Switch interconnects incoming and outgoing time slots

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Time and space switches

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The principle of TST switching

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Group switch with 512 multiple position

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Connections to the local exchange

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Node for packet switching

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Packet node structure

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Connection oriented transfer phases:Connection setup (setup packet with complete address, Logical Channel Number stored in each nodeData transmission (only LCN in the header)Release

Connectionless transport:Destination address in the headerPath selection in the nodesDifferent packets have different delayThe order of received packets has no guarantee

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ATM cell switching principleFixed cell (packet) length – 53 bytes

5 octets header, 48 octet payload

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Why connection oriented packet switching?

• Connectionless – only best effort quality (www = world wide waiting)

• Connection oriented – QoS guarantee is possible

• Quality measures: delay, jitter, packet loss.

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Content of ATM cell header

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Segmentation and multiplexing of differentServices in cell based systems

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The principle of ATM switching

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VP (Virtual Path „coarse level addressing”) and VC (Virtual Channel „fine level addressing”)

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The structure of the ATM switch

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Signalling principles in circuit switching

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Signalling flow in a telephone call

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Signalling for distributed supplementary services or a mobile telephone call

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Signalling in packet switched networks

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Principles of Common Channel Signalling CCS

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Circuit or packet switching???

• A rule of thumb:– Band is cheap: circuit switching– Processing is cheap: packet switching

• Distributed vs. centralized intelligence in the network• Packet processing by 1016 b/s ????

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Photonic Fibre Switches

• In free-space devices, the light is focused from the input fibre, deflected by a micro-mirror (typically several times) and finally launched into the output fibre. The Micro-Electro-Mechanical Systems (MEMS) technology is mature and can produce switch matrices with up to hundreds of input and output ports (128x128 or 256x256), low insertion loss (IL), very low cross talk, low power consumption, millisecond switching speed, and broadband operation. The mirrors can typically be controlled electro-magnetically, electro-statically or by piezoelectric actuators.

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Operational principle and example of 3D MEMS array

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collimator

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