Communication Networks

Transcription

1 Communication Networks Chapter 4 Transmission Technique Communication Networks: 4. Transmission Technique 133 Overview 1. Basic Model of a Transmission System 2. Signal Classes 3. Physical Medium 4. Coding Communication Networks: 4. Transmission Technique 134 Prof. Jochen Seitz 1

2 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 Communication Networks: 4. Transmission Technique Basic Model of a Transmission System Source and Sink Information Source Physical Medium Sink Communication Networks: 4. Transmission Technique 136 Prof. Jochen Seitz 2

3 discrete value continuous value 1. Basic Model of a Transmission System Basic Model of a Transmission System Source Transmission Channel SC LC LDC SDC Physical Medium Sink u e (t) Transmission System u a (t) SC = Source Coding SDC = Source Decoding LC = Line Coding LDC = Line Decoding Communication Networks: 4. Transmission Technique Signal Classes Signal Classes continuous time discrete time Value u analog Value u Digitalization Time t Time t Value u Value u digital Time t Time t Communication Networks: 4. Transmission Technique 138 Prof. Jochen Seitz 3

4 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 f g is the highest frequency of a bandlimited analog signal, the sampling frequency should be at least 2f g 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 Communication Networks: 4. Transmission Technique 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 Rate voice = # Samples Bits bit bit = = Second Sample s s Communication Networks: 4. Transmission Technique 140 Prof. Jochen Seitz 4

5 2. Signal Classes Digital Speech Transmission Source Coding A/D Line Coding Network Line Decoding D/A Source Decoding Communication Networks: 4. Transmission Technique Signal Classes File Transfer Digital Signal Conversion Line Coding File Transfer Source Coding Network Source Decoding Line Decoding Digital Signal Conversion Communication Networks: 4. Transmission Technique 142 Prof. Jochen Seitz 5

6 3. Physical Medium Physical Medium Usage of a physical medium to transfer information Source Information Sink x(t) y(t) Transformer Transformer x (t) y (t) Medium z (t) Source of Interference Transmission Channel 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 conductor y (t) = F(x t, z t ) Communication Networks: 4. Transmission Technique Physical Medium Transmission over Physical Layer x(t) x (t) Transmitted Data +V Transmitted Signal -V Taken from F. Halsall (2000) Repetition from Chapter 2 y (t) Typical Received Signal Sampling Instants y(t) Received Data Bit Error Communication Networks: 4. Transmission Technique 144 Prof. Jochen Seitz 6

7 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 Communication Networks: 4. Transmission Technique Coding Coding Types (II) Coding Source Coding Channel Coding Line Coding Goal: Reduction of redundancy Goal: Detection and correction of transmission errors Goal: Adaptation of code symbols to the physical channel GSM-tutorial-channel-coding.jpg Example: Manchester encoding Communication Networks: 4. Transmission Technique 146 Prof. Jochen Seitz 7

8 4. Coding 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 Communication Networks: 4. Transmission Technique 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 Communication Networks: 4. Transmission Technique 148 Prof. Jochen Seitz 8

9 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 Communication Networks: 4. Transmission Technique Coding Line Coding: Non Return to Zero (NRZ) NRZ unipolar NRZ-Inverted (NRZI) bipolar Communication Networks: 4. Transmission Technique 150 Prof. Jochen Seitz 9

10 4. Coding Line Coding: Alternate Mark Inversion (AMI) AMI-Code: (modified, as used for ISDN) +0 (+750 mv) 1 (0 mv) -0 (-750 mv) Coding Violation (CV): Each interval DC-free CV When 0 and 1 are sent at the same time, the receiver will get a 0 CV Communication Networks: 4. Transmission Technique Coding Line Coding: Manchester-Code +A Manchester 0 -A 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 Communication Networks: 4. Transmission Technique 152 Prof. Jochen Seitz 10

11 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 Communication Networks - 4. Transmission Technique 153 Requested = Offered Bandwidth C information coded in physical signals bandwidth capacity of the physical channel The application utilizes the physical channel in an optimal way Communication Networks - 4. Transmission Technique 154 Prof. Jochen Seitz 11

12 Requested > Offered Bandwidth information coded in physical signals bandwidth capacity of the physical channel bandwidth capacity of the physical channel In order to transmit the complete amount of information, several channels have to be bundled Communication Networks - 4. Transmission Technique 155 Requested < Offered Bandwidth information coded in physical signals information coded in physical signals bandwidth capacity of the physical channel Several applications may utilize the physical channel in an optimal way. Multiplexing Communication Networks - 4. Transmission Technique 156 Prof. Jochen Seitz 12

13 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 Time Application Entity12 Bandwidth (Frequency) Communication Networks - 4. Transmission Technique 157 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 Communication Networks - 4. Transmission Technique 158 Prof. Jochen Seitz 13

14 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 Communication Networks - 4. Transmission Technique 159 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 Time Application Entity 1 Application Entity 2 Bandwidth (Frequency) Communication Networks - 4. Transmission Technique 160 Prof. Jochen Seitz 14

15 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 Bandwidth (Frequency) Time Communication Networks - 4. Transmission Technique 161 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 Signal Power Bandwidth (Frequency) Time Communication Networks - 4. Transmission Technique 162 Prof. Jochen Seitz 15

16 Multiplex Transmission Source Modulation Demodulation Sink Source Modulation Demodulation Sink + Source Modulation Demodulation Sink Communication Networks - 4. Transmission Technique 163 Time and Frequency Division Multiplex Frequency Division Multiplex khz f frequency range, divided into 12 sub-ranges, 4 khz each Time Division Multiplex µs t time period divided into 32 time slots, approx. 3,9 µs each Communication Networks - 4. Transmission Technique 164 Prof. Jochen Seitz 16

17 Time Multiplexing / Demultiplexing 8 bit Code Word S 1 Multiplexing Demultiplexing Transmission Path S 1 S 2 S 2 S 1 Multiplex Frame S 1 S 2 S 2 A B S 3 S 3 S 4 S 4 S 3 S 3 S 4 t 4 t 3 t 2 t 1 t 4 t 3 t 2 t 1 t 4 t 3 t 2 t 1 Time Slot S 4 Communication Networks - 4. Transmission Technique 165 Example of Code Division Multiplex (1) Four senders A, B, C, D Assigned chipping sequences: A ( ) B ( ) C ( ) D ( ) Sender C sends 1 : See Tanenbaum 2011 Sender C sends 0 : Communication Networks: 4. Transmission Technique 166 Prof. Jochen Seitz 17

18 Example of Code Division Multiplex (2) A sends 1, B sends 1, C sends 0, D sends 1 A : B: C: D: Communication Networks: 4. Transmission Technique 167 Example of Code Division Multiplex (2) Receiver wants to find out the value sent by C (knowing C s chipping sequence): combined signal RS ( ) RS C = = = 1 = 0 8 Communication Networks: 4. Transmission Technique 168 Prof. Jochen Seitz 18

19 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 Communication Networks - 4. Transmission Technique 169 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 Communication Networks - 4. Transmission Technique 170 Prof. Jochen Seitz 19

20 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. Communication Networks: 4. Transmission Technique 171 Prof. Jochen Seitz 20