Communication Networks Winter 2019/20
Prof. Jochen Seitz 1
Communication Networks
Chapter 4 – Transmission Technique
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Overview
1. Basic Model of a Transmission System
2. Signal Classes
3. Physical Medium
4. Coding
5. Multiplexing
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1. Basic Model of a Transmission System
Terminology
• Transmission
▪ Transport of physical signals via a transmission medium between two directly connected devices (source and sink)
• Switching
▪ Provision of a complete transmission path between two end systems over several intermediate devices
• End Systems
▪ Devices used by the end user
❖used for input and output of data
❖ responsible for coding/decoding, multiplexing/demultiplexing, modulating/demodulating
❖ send and receive data with error detection and correction functionality
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Source and Sink
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1. Basic Model of a Transmission System
SourceSource SinkSinkPhysical Medium
Information
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Physical Medium
Basic Model of a Transmission System
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1. Basic Model of a Transmission System
SourceSource SinkSinkSCSC LCLC LDCLDC SDCSDCTransmission ChannelTransmission Channel
TransmissionSystem
TransmissionSystem
ue(t) ua(t)
SC = Source Coding SDC = Source DecodingLC = Line Coding LDC = Line Decoding
Signal Classes
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2. Signal Classes
con
tin
uo
us
valu
eco
nti
nu
ou
sva
lue
dis
cret
eva
lue
dis
cret
eva
lue
continuous timecontinuous time discrete timediscrete time
Time t
Value u
Time t
Value u
Time t
Value u
Time t
Value u
analog
digital
Digitalization
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2. Signal Classes
Digitalization of an Analog Signal
1. Sampling
▪ continuous time → discrete time
▪ values are still continuous
▪ Nyquist/Shannon Sampling Theorem:
− determination of minimum sampling frequency
− if fg is the highest frequency of a bandlimited analog signal, the sampling frequency should be at least 2fg to guarantee perfect reconstruction
2. Quantization
▪ continuous values → discrete values
▪ values are mapped onto value intervals → quantization error (noise in D/A conversion)
3. Source Coding
▪ identification of intervals by bit sequence
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2. Signal Classes
Example: Digitalization of Human Speech for ISDN
• Theoretically, voice is not a bandlimited signal
• CCITT telephone channel is limited to 300 Hz – 3,400 Hz
• Sampling
▪ at least with 6,800 Hz, but due to the steepness of the signal‘s edge, 8,000 Hz is chosen
• Quantization
▪ 256 quantization intervals
▪ non-linear quantization to improve signal-to-noise ratio SNR (A-law, m-law)
• Coding
▪ 8 bits per sample
• Resulting data rate
𝐷𝑎𝑡𝑎 𝑅𝑎𝑡𝑒𝑣𝑜𝑖𝑐𝑒 =# 𝑆𝑎𝑚𝑝𝑙𝑒𝑠
𝑆𝑒𝑐𝑜𝑛𝑑
𝐵𝑖𝑡𝑠
𝑆𝑎𝑚𝑝𝑙𝑒= 8 000 ∙ 8
𝑏𝑖𝑡
𝑠= 64 000
𝑏𝑖𝑡
𝑠
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Digital Speech Transmission
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2. Signal Classes
Network
A/D
Line Decoding
D/A
Line Coding
Source Decoding
Source Coding
File Transfer
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2. Signal Classes
Network
Line Decoding
Digital SignalConversion
Line Coding
Digital SignalConversion
File Transfer
Source Decoding
Source Coding
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Physical Medium
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3. Physical Medium
Usage of a physical medium to transfer information
Source Sink
Transmission ChannelTransmission Channel
TransformerTransformer
Medium
Information
x(t) y(t)
x´(t) y´(t)
z´(t)
Primary Signals 𝒙 𝒕 , 𝒚(𝒕): physical parameters related to source/sink
Signals 𝒙′ 𝒕 , 𝒚′ 𝒕 , 𝒛′(𝒕):physical parameters related to channel
Physical Medium:e.g. electrical conductorSource of Interference
TransformerTransformer
≈
𝑦′(𝑡) = 𝐹(𝑥′ 𝑡 , 𝑧′ 𝑡 )
Transmission over PhysicalLayerTaken from F. Halsall (2000)
Repetition from Chapter 2
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3. Physical Medium
Transmitted Data
Transmitted Signal
Sampling Instants
Received Data
0 1 1 110 0 0
0 1 0 110 0 0
+V
-V
Bit Error
TypicalReceived Signal
x(t)
x´(t)
y´(t)
y(t)
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4. Coding
Coding Types (I)
• Source Coding:
▪ Transforming user information so that they can be transferred via the transmission channel as fast as possible and transformed back at the receiver
• Channel Coding:
▪ Transforming information to provide transmission on a bandwidth-limited channel over a long distance with as few errors as possible
• Line Coding:
▪ Transforming digital information into a sequence of physical signals to be transmitted over a physical channel
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Coding Types (II)
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4. Coding
CodingCoding
Channel CodingChannel CodingSource CodingSource Coding Line CodingLine Coding
Goal:Reduction of redundancy
Goal:Reduction of redundancy
Goal:Detection and correctionof transmission errors
Goal:Detection and correctionof transmission errors
Goal:Adaptation of code symbols to the physicalchannel
Goal:Adaptation of code symbols to the physicalchannel
Example: Manchester encodinghttp://www.althos.com/Tutorial/images/GSM-tutorial-channel-coding.jpg
01100110
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Source Coding
Lossless Compression
• Eliminating statistical redundancy
▪ No information loss
▪ Examples
❖Huffman Encoding
❖Arithmetic Encoding
Lossy Compression
• Removing unnecessary or less important information
▪ Received information is unequal to original information
▪ Examples
❖MP3 for audio files
❖ JPG for photos
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4. Coding
Channel Coding
Error Detection
• Adding redundancy
▪ Parity bits
▪ Check sums
• Tests at the receiver to prove integrity of transmitted information
▪ Results to be used together with special protocol functions
❖Acknowledgement
❖Retransmission
Forward Error Correction
• Adding even more redundancy
• Detecting and correcting errors
▪ Hamming Distance: the number of bit positions at which two possible symbols are different
• Receiver reconstructs original information
▪ Block Code
▪ Hamming Code
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4. Coding
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4. Coding
Line Coding
• Representing the digital signal to be transported by a waveform that is appropriate for the specific properties of the physical channel
• Requirements:
▪ Eliminate DC component
▪ Facilitate (bit) synchronization
▪ Minimize spectral content
▪ Ease error detection and correction
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Line Coding: Non Return to Zero (NRZ)
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4. Coding
0 1 0 1 1 0 0 0 1 1
NRZunipolar
NRZ-Inverted(NRZI)
bipolar
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4. Coding
Line Coding: Alternate Mark Inversion (AMI)
• AMI-Code:(modified, as used
for ISDN)
▪ Coding Violation (CV):
▪ When „0“ and „1“ are sent at the same time, the receiver will get a „0“
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CV
CV
1 0 0 0 1 1 0 1
Each interval DC-free
„+0” (+750 mV)„1” (0 mV)
„-0” (-750 mV)
1 0 0 0 1 1 0 0 1 0 0 1
Line Coding: Manchester-Code
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4. Coding
0 1 0 1 1 0 0 0 1 1
Manchester
Coding: 1 signal transition +A -A in the middle of the bit interval
0 signal transition -A +A in the middle of the bit interval
Eventually, a signal transition at the beginning of the bit interval is required
+A
-A
0
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5. Multiplexing
Multiplexing
• Focus on a physical (analog) channel.
• Information transferred via physical signals that change over time
• These signals occupy capacity/bandwidth of the transmission channel for a given time (connection time, packet duration, …)
• Requested and offered bandwidth usually not corresponding
▪ wasted resources
▪ not adequate communication
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Requested = Offered Bandwidth
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5. Multiplexing
information codedin physical signals
The application utilizes the physical channelin an optimal way
Cbandwidth capacity
of the physical channel
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Requested > Offered Bandwidth
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5. Multiplexing
information codedin physical signals
In order to transmit the complete amount of information,several channels have to be bundled
bandwidth capacityof the physical channel
bandwidth capacityof the physical channel
Requested < Offered Bandwidth
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5. Multiplexing
information codedin physical signals
Several applications may utilize the physical channelin an optimal way.
information codedin physical signals
➔Multiplexing
bandwidth capacityof the physical channel
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5. Multiplexing
Conjoined Transmission of Information Streams
• Different streams of information of concurrently active application entities that use the same frequency range interfere with each other
• They cannot be separated at the receiver
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Bandwidth(Frequency)
Time
ApplicationEntity1
ApplicationEntity 2
5. Multiplexing
Multiplex Transmission
• Multiplex:
▪ Multiple analog message signals or digital data streams combined into one signal over a shared medium
▪ Sharing an expensive resource
• Different categories of multiplexing:
▪ Space Division Multiplexing
▪ Time Division Multiplexing
▪ Frequency Division Multiplexing
▪ Code Division Multiplexing
▪ Wavelength Division Multiplexing
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5. Multiplexing
Space Division Multiplex
• Space partitioned into different „areas“
• Each user/application is assigned to one area
• No interference possible
• Examples:
▪ analog telephony
▪ radio cells in mobile telephony
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5. Multiplexing
Frequency Division Multiplex
• Separate frequency range assigned to each user / application
• Different users / applications might send concurrently
• Receiver can separate the transmissions by tuning to according frequency
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Bandwidth(Frequency)
Time
ApplicationEntity 1
ApplicationEntity 2
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5. Multiplexing
Time Division Multiplex
• Each user / application has exclusive access to the complete bandwidth of the channel, but only at certain time periods
• Assignment can be done statically or on demand
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Bandwidth(Frequency)
Time
5. Multiplexing
Code Division Multiplex
• On digital channels, different streams can be separated based on different (mutually orthogonal) codes (chipping sequences)
• The streams occupy the same spectrum
• The receiver can separate the streams
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Bandwidth(Frequency)
Time
Signal Power
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Multiplex Transmission
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5. Multiplexing
Source
+
Modulation Demodulation Sink
Source Modulation Demodulation Sink
Source Modulation Demodulation Sink
Time and Frequency Division Multiplex
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5. Multiplexing
1 2 3 4 5 6 7 8 9 10 11 12
60 108 kHz f
Frequency Division Multiplex
frequency range,divided into 12 sub-ranges, 4 kHz each
1 2 3 4 30 31 32
0 125 µs t
Time Division Multiplex
time perioddivided into 32 time slots, approx. 3,9 µs each
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Time Multiplexing / Demultiplexing
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5. Multiplexing
A B
S1
S2
S3
S4
S1S2
S3 S4
S1
S4
S2
S3
MultiplexFrame
S1
S2
S3
S4
Time Slott4 t3 t2 t1 t4 t3 t2 t1
t4 t3 t2 t1
8 bit Code WordMultiplexing
Transmission PathDemultiplexing
5. Multiplexing
Example of Code Division Multiplex (1)
• Four senders A, B, C, D
• Assigned chipping sequences:
▪ A → (-1 -1 -1 +1 +1 -1+ 1 +1)
▪ B → (-1 -1 +1 -1 +1 +1 +1 -1)
▪ C → (-1 +1 -1 +1 +1 +1 -1 -1)
▪ D → (-1 +1 -1 -1 -1 -1 +1 -1)
• Sender C sends “1”:
• Sender C sends “0”:
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See Tanenbaum 2011
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5. Multiplexing
Example of Code Division Multiplex (2)
• A sends “1”, B sends “1”, C sends “0”, D sends “1”
▪ A :
▪ B:
▪ C:
▪ D:
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5. Multiplexing
Example of Code Division Multiplex (2)
• Receiver wants to find out the value sent by C (knowing C‘s chipping sequence):
▪ combined signal RS (-2 -2 0 -2 0 -2 +4 0)
▪ 𝑅𝑆 ∘ 𝐶 =−2 ∗−1 + −2∗+1 + 0∗−1 + −2∗+1 + 0∗+1 + −2∗+1 + +4∗−1 + 0∗−1
8
=2−2+0−2+0.2.4+0
8= −1 =„0“
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5. Multiplexing
Multiplexing - Recapitulation
• Multiplexing allows the transmission of several streams (→ logical channels) over one physical channel
• Multiplexing might support
▪ synchronous communication→ fix data rate and transmission time
▪ asynchronous communication→ variable data rate and transmission time
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5. Multiplexing
Multiple Access
• According to the multiplexing techniques, several multiple access procedures can be defined for shared media:
▪ Space Division Multiple Access (SDMA)
▪ Time Division Multiple Access (TDMA)
▪ Frequency Division Multiple Access (FDMA)
▪ Code Division Multiple Access (CDMA)
• Controlled access to the shared medium → avoid collisions
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References
• Comer, Douglas E. (2015): Computer Networks and Internets. Sixth Edition. Boston, Columbus, Indianapolis: Pearson.
• Proakis, John G.; Salehi, Masoud (2015): Fundamentals of Communication Systems. Second Edition. Boston: Pearson.
• Seitz, Jochen; Debes, Maik (2016): Kommunikationsnetze. Eine umfassende Einführung. Anwendungen – Dienste – Protokolle. Ilmenau: Unicopy Campus Edition.
• Stallings, William (2014): Data and Computer Communications. 10th edition. Upper Saddle River, N.J.: Pearson.
• Tanenbaum, Andrew S.; Wetherall, David J. (2011): Computer Networks. 5th edition. Boston: Pearson Prentice Hall.
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