Infokommunikációs rendszerek –
Infocommunication Systems
Lecture 4. előadás
Kódolás, nyalábolás, kapcsolás
Coding, 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)
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Generation of BPSK
Carrier
signal
~
1
0
0
x
BPSK
1
+1
-1
t
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Digital modulation methods –
Constellation Diagram / BPSK
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Eye diagram as a basis for demodulation of
BPSK signal
Eye diagram
Received BPSK
signal
x
∫
comparator
1,0,1,0
Carrier
signal
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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)
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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
Trunks for active calls only
City B
103
105
105
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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
(Pulse Code Modulation)
4.50 ábra
125 µs
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TDM principles II. PDH hierarchy
Plesiochronous Digital Hierarchy
4.51 ábra
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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 elements
DXC – Digital Cross Connect
ADM – Add-drop Multiplexer
TM – 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 timeslots
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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
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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 node
Data transmission (only LCN in the header)
Release
Connectionless transport:
Destination address in the header
Path selection in the nodes
Different packets have different delay
The order of received packets has no guarantee
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ATM cell switching principle
Fixed 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 different
Services 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 MicroElectro-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
collimator
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