Communication Technologies

Transcription

1 Communication Networks Chapter 4. Transmission Technique o Signal Classes o Physical Medium o Coding o Multiplexing o Multiple Access Communication Technologies Transmission: Transport of physical signals via transmission media between two devices. Switching: Provision of a complete transmission path between two end system by (temporarily) coupling devices for physical transmission. End Systems: Devices at the end user: Information input and output Coding, modulating and multiplexing Sending and receiving data with error detection and correction facilities 130 Prof. Jochen Seitz 1

2 Information Source and Sink Source Information Sink 131 Basic Model of a Transmission System Source SC LC LDC SDC Sink Transmission Channel u e (t) Transmission System u a (t) SC = Source Coding LC = Line Coding Adaptation to the physical medium LDC = Line Decoding SDC = Source Decoding 132 Prof. Jochen Seitz 2

3 discrete value continuous value Signal Classes continuous time discrete time Value u analog Value u Time t Time t Value u Value u digital Time t Time t 133 Digital Speech Transmission Source Source Coding A/D Line Coding Source Decoding Network D/A Line Decoding Sink 134 Prof. Jochen Seitz 3

4 Transmission of Digital Informationen Source File Transfer Sink Source Coding Digital Signal Conversion Source Decoding Digital Signal Conversion Line Coding Line Decoding Network 135 Transmission: 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)) 136 Prof. Jochen Seitz 4

5 Coding Types 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. 137 Coding Types 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 Example: Manchester encoding GSM-tutorial-channel-coding.jpg 138 Prof. Jochen Seitz 5

6 Multiplexing Focus on a physical (analog) channel. Information are transferred via physical signals that change over time. These signals occupy bandwidth of the transmission channel for a given time (connection time, packet duration, ). Requested and offered bandwidth do not have to be corresponding. 139 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. 140 Prof. Jochen Seitz 6

7 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. 141 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 142 Prof. Jochen Seitz 7

8 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) 143 Multiplex Transmission Multiplex: Multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. Different categories of multiplexing: Space Division Multiplexing Time Division Multiplexing Frequency Division Multiplexing Code Division Multiplexing Wavelength Division Multiplexing 144 Prof. Jochen Seitz 8

9 Space Division Multiplex Space is partitioned into different areas. Each user/application is assigned to one area. No interference possible. Examples: Analog telephony Radio cells in mobile telephony 145 Frequency Division Multiplex A separate frequency range is assigned to each user / application. Different users / applications might send concurrently. Receiver can separate the transmissions. Bandwidth (Frequency) Time Application Entity 1 Application Entity Prof. Jochen Seitz 9

10 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 147 Code Division Multiplex On digital channels different streams can be separated based on different codes. The streams occupy the same spectrum. The receiver can separate the streams. Signal Power Bandwidth (Frequency) Time 148 Prof. Jochen Seitz 10

11 Multiplex Transmission Source Modulation Demodulation Sink Source Modulation Demodulation Sink + Source Modulation Demodulation Sink 149 Time and Frequency Division Multiplex Frequency Division Multiplex khz f Time Division Multiplex frequency range, divided into 12 sub-ranges, 4kHz each µs t time period divided into 32 time slots, approx. 3,9 µs each 150 Prof. Jochen Seitz 11

12 Time Multiplexing / Demultiplexing 8 bit code word multiplexing transmission path demultiplexing S 1 S 1 S 2 S 2 S 1 multiplex frame S 1 S 2 S 2 A B S 3 S 3 S 3 S 4 S 4 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 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 152 Prof. Jochen Seitz 12

13 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) Multiple access procedures are responsible for a controlled access to the shared medium, in order to avoid collisions. 153 Literature Krüger, G. and D. Reschke (2004). Lehr- und Übungsbuch Telematik. München; Wien, Fachbuchverlag Leipzig im Carl Hanser Verlag. Seitz, J., M. Debes, et al. (2007). Digitale Sprachund Datenkommunikation. Netze Protokolle Vermittlung. München, Wien, Fachbuchverlag Leipzig im Carl Hanser Verlag. 154 Prof. Jochen Seitz 13