Physical layer concepts
Distorsion
Effect of attenuation, distortion, and noise on transmitted signal.
Point-to-point communication
Mikrofon Högtalare
Source coding Source decodingDigitalizatingcompression
0110 0110
Error management Error control.
0100010
Bit error
0110010
Flow control Flow control
Modulation Demodulation
0110010NACKACK
Layer6
Layer2
Layer1
Layer7
Digital modulation methods
Binary signal
ASK = Amplitude Shift Keying (AM)
FSK = Frequency Shift Keying (FM)
PSK = Phase Shift Keying (PSK)
0 0.005 0.01-2
0
2000
0 0.005 0.01-2
0
2001
0 0.005 0.01-2
0
2011
0 0.005 0.01-2
0
2010
0 0.005 0.01-2
0
2100
0 0.005 0.01-2
0
2101
0 0.005 0.01-2
0
2111
0 0.005 0.01-2
0
2110
8QAM example:Below you find eight symbols used for a so called 8QAM modem (QAM=Quadrature Amplitude Modulation). The symbols in the first row represent the messages 000, 001, 011 and 010 respectively (from left to right). The second row representents 100, 101, 111 and 110.
a) The signal below is transmitted from the modulator. What bit sequency is transmitted?
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04-2
0
2
Tid [sekunder]
Sp
än
nin
g [V
olt]
Modulatorns utsignal
b) The time axis is graded in seconds. What is the symbol rate in baud or symbols/s?
c) What is the bit rate in bit/s?
Example 2 cont.
Bit rate vs baud rate
Bit rate in bit/s:
Where M is the number of symbols and fs is the symbol rate in baud or symbols/s.
2logb Sf f M
Bit and baud rate comparison
ModulationModulation UnitsUnits BitsBits/symbol/symbol Baud rateBaud rate Bit Rate
ASK, FSK, 2-PSKASK, FSK, 2-PSK Bit 1 N N
4-PSK, 4-QAM4-PSK, 4-QAM Dibit 2 N 2N
8-PSK, 8-QAM8-PSK, 8-QAM Tribit 3 N 3N
16-QAM16-QAM Quadbit 4 N 4N
32-QAM32-QAM Pentabit 5 N 5N
64-QAM64-QAM Hexabit 6 N 6N
128-QAM128-QAM Septabit 7 N 7N
256-QAM256-QAM Octabit 8 N 8N
Sine wave example
I
5 Volt
л/2 radians = 90º
Complex representation
Figure 5.14 The 4-QAM and 8-QAM constellations
Q (Quadrature phase)
I (Inphase)
Q (Quadrature phase)
I (Inphase)
Figure 5.11 The 4-PSK characteristics
Figure 5.12 The 8-PSK characteristics
Figure 5.16 16-QAM constellations
Figure 5.20 The V.32 constellation and bandwidth
Spectrum of ASK, PSK and QAM signal
Figure 9.3 ADSL modem
Figure 9.4 DSLAM
Other DSL Technologies
• SDSL (Symmetric DSL) divides frequencies evenly
• HDSL (High-rate DSL) provides DS1 bit rate in both directions Short distances
Four wires
• VDSL (Very high bit rate DSL) provides up to 52 Mbps Very short distance
Requires Optical Network Unit (ONU) as a relay
