cn1-ss23-slides-ch02.pdf

Computer Networks I Physical Layer Prof. Dr.-Ing. Lars Wolf 1 IBR, TU Braunschweig Mühlenpfordtstr. 23, D-38106 Braunschweig, Germany, Email: [email protected] Application Layer L4 Transport Layer L3 Network Layer L2 Data Link Layer L1 Physical Layer Internet: (TCP, UDP,...) Internet: (IP,...) LA...

Computer Networks I Physical Layer Prof. Dr.-Ing. Lars Wolf 1 IBR, TU Braunschweig Mühlenpfordtstr. 23, D-38106 Braunschweig, Germany, Email: [email protected] Application Layer L4 Transport Layer L3 Network Layer L2 Data Link Layer L1 Physical Layer Internet: (TCP, UDP,...) Internet: (IP,...) LAN,..., WAN: (ETH,...) Other lectures of ET/IT,... Introduction 2 Physical Layer... L5 Web Applications Email Complementary Courses Transitions, Address. Computer Networks 1 www.ibr.cs.tu-bs.de Scope Computer Networks 1 www.ibr.cs.tu-bs.de Overview 3 Physical Layer 1 Basics 2 Digital Information – Encoding 3 Multiplexing Techniques www.ibr.cs.tu-bs.de Characteristics MECHANICAL: size of plugs allocation of pins ! Computer Networks 1 ELECTRICAL (or equivalent): voltage levels on wires ! 6 Physical Layer FUNCTIONAL: definition of switching functions; pin allocation (data, control, timing, ground) ! PROCEDURAL: rules for using switching functions e. g. CCITT X.21: protocol between DTE and DCE for synchronized data transfer in public data networks Computer Networks 1 www.ibr.cs.tu-bs.de Mechanical 7 Physical Layer Computer Networks 1 www.ibr.cs.tu-bs.de Electrical 8 Physical Layer e. g... " designed for IC Technology balanced generator differential receiver two conductors per circuit signal rate up to 10 Mbps distance: 1000m (at appr. 100 Kbps) to10m (at 10Mbps) considerably reduced crosstalk interoperable with V.10 / X.26...” Computer Networks 1 www.ibr.cs.tu-bs.de Functional, Procedural Example RS-232-C, functional specification describes connection between pins e.g. "zero modem" computer-computer-connection (Transmit(2) - Receive(3)) meaning of the signals on the lines 9 Physical Layer DTR=1, when the computer is active, DSR=1, modem is active,... Action/reaction pairs specify the permitted sequence per event e. g. when the computer sends an RTS, the modem responds with a CTS when it is ready to receive data www.ibr.cs.tu-bs.de 1.2 Bit Rate and Baud Rate BAUD RATE: measure of number of symbols transmitted per unit of time signal speed, number of signal changes per second changes in amplitude, frequency, phase each symbol normally consist of a number of bits baud rate is equal to the bit rate if there is one bit per symbol BIT RATE: Number of Bits transferred per Second (bps) bit rate may be higher than baud rate ("signal speed") Computer Networks 1 because one signal value may transfer several bits 10 Physical Layer Example: Computer Networks 1 www.ibr.cs.tu-bs.de Basics 11 Physical Layer Bandwidth of a channel: B = fmax - fmin fmax, fmin : maximum resp. minimum frequency Example: traditional phone: min. 3000 Hz How many bits per second can be transmitted on a channel with a certain bandwidth B? Note: Definition of term „bandwidth“ differs from typical use in computer science www.ibr.cs.tu-bs.de Basics: Nyquist Theorem Nyquist theorem: For a noise free channel, the maximum achievable bit rate is max. bitrate = 2 B log2V bps Computer Networks 1 B: V: 12 Physical Layer signal bandwidth (low pass filter) number of discrete levels Example: 3000 Hz channel, binary signal (V=2): max. bitrate = 6000 bps But in reality, we do not have noise free channels... www.ibr.cs.tu-bs.de Basics: Shannon Theorem Shannon theorem: For a noisy channel, the maximum achievable bit rate is max. bitrate = B log2 (1 + S/N ) B S/N signal bandwidth (low pass filter) Signal to Noise ratio Computer Networks 1 10 log10 S/N decibels Example: 3000 Hz channel, S/N = 1 000 (30 dB) maximum bitrate = 30 000 bps Independent of number of levels ! 13 Physical Layer This is an upper bound! Real systems rarely achieve it! www.ibr.cs.tu-bs.de 1.3 Operating Modes Transfer directions (temporal parallelism) simplex communication: data is always transferred into one direction only (half-duplex) semi-duplex communication Computer Networks 1 data is transferred into both directions but never simultaneously 14 Physical Layer full-duplex communication data may flow simultaneously in both directions www.ibr.cs.tu-bs.de Serial and parallel transmission serial: signals are transmitted sequentially over one channel parallel: 15 Physical Layer Serial 0 1 0 0 0 0 0 1 10000010 time Parallel 0 1 0 0 0 0 0 1 Symbol Computer Networks 1 signals are transmitted simultaneously over several channels www.ibr.cs.tu-bs.de Operating Modes: Synchronous Transmission Definition exact time when a bit exchange occurs is pre-defined by a regular clock pulse (requires synchronization) Implementation receiving clock pulse Computer Networks 1 on a separate line or gained from the signal 16 Physical Layer bit synchronous or frame synchronous (in fact, frames are on data link level) www.ibr.cs.tu-bs.de Operating Modes: Asynchronous Transmission Definition clock pulse fixed for the duration of a signal termination marked by stop signal (bit) or number of bits per signal Implementation simple: Computer Networks 1 sender & receiver generate clock pulse independent from each other 17 Physical Layer classical example: RS-232-C UART (universal asynchronous receiver and transmitter) IC module In former times often used between computer & printer or computer & modem Computer Networks 1 www.ibr.cs.tu-bs.de 2 Digital Information – Encoding 21 Physical Layer Digital information at end system E.g., TTL-Logic ("1" : 3V, "0" : 0V) Digital transmission sender/receiver synchronization signal levels around 0V (lower power) ! Conversion Coding techniques binary encoding, non-return to zero-level (NRZ-L) 1: high level 0: low level return to zero (RZ) 1: clock pulse (double frequency) during interval 0: low level... Manchester Encoding Differential Manchester Encoding... Computer Networks 1 www.ibr.cs.tu-bs.de Binary Encoding 22 Physical Layer Binary encoding (Non-return to zero): "1": voltage on high "0": voltage on low i. e. + simple, cheap + good utilization of the bandwidth (1 bit per Baud) - no "self-clocking" feature Computer Networks 1 www.ibr.cs.tu-bs.de Manchester Encoding 23 Physical Layer Bit interval is divided into two partial intervals: I1, I2 "1": I1: high, I2: low "0": I1: low, I2: high + good "self-clocking" feature - 0,5 bit per Baud Application: 802.3 (CSMA/CD) Computer Networks 1 www.ibr.cs.tu-bs.de Differential Manchester Encoding 24 Physical Layer Differential Manchester Encoding: bit interval divided into two partial intervals: "1": no change in the level at the beginning of the interval "0": change in the level + good "self-clocking" feature + low susceptibility to noise because only the signal’s polarity is recorded. Absolute values are irrelevant. - 0,5 bit per Baud - complex Multiplexing Techniques The cost for implementing and maintaining either a narrowband or a wideband cable are almost the same ! multiplexing many conversations onto one cable FDM (FREQUENCY DIVISION MULTIPLEXING) Channel 1 Channel 2 Channel 3 Channel 4 Time Channel 2 Channel 1 Channel 4 Channel 3 Channel 2 Channel 1 TDM (TIME DIVISION MULTIPLEXING) Frequency Computer Networks 1 Frequency www.ibr.cs.tu-bs.de 3 25 Time Physical Layer... Computer Networks 1 www.ibr.cs.tu-bs.de Frequency Multiplexing 26 Physical Layer Principle: frequency band is split between the users each user is allocated one frequency band Application example: multiplexing of voice telephone channels filters limit voice channel to 3 000 Hz bandwidth each voice channel: 4 000 Hz bandwidth (2 x 500 Hz gap (guard band)) Quality of filters and guard bands important: ! Otherwise adjacent channels overlap ! noise Computer Networks 1 www.ibr.cs.tu-bs.de Time Division Multiplexing 27 Physical Layer Principle: user receives a time slot during this time slot user has the full bandwidth Application: multiplexing of end systems, but also in transmission systems