Computer Networks I Physical Layer Computer Networks 1 Prof. - PowerPoint PPT Presentation

Scope www.ibr.cs.tu-bs.de Computer Networks I Physical Layer Computer Networks 1 Prof. Dr.-Ing. Lars Wolf IBR, TU Braunschweig Mühlenpfordtstr. 23, D-38106 Braunschweig, Germany, 1 Email: wolf@ibr.cs.tu-bs.de 2 Physical Layer Overview 1 Basics www.ibr.cs.tu-bs.de www.ibr.cs.tu-bs.de 1 Basics • Characteristics 1.1 Characteristics 1.2 Bit Rate and Baud Rate • Bit Rate and Baud Rate 1.3 Operating Modes • Operating Modes 2 Analog and Digital Information Encoding and Transmission 3 Multiplexing Techniques Computer Networks 1 Computer Networks 1 3 4 Physical Layer Physical Layer

1.1 Characteristics Physical Layer ISO DEFINITION: the physical layer provides the www.ibr.cs.tu-bs.de www.ibr.cs.tu-bs.de I-Series I-Series I-Series V-Series V-Series V-Series • mechanical, Bell specs. Bell specs. Bell specs. • electrical, Hayes Hayes Hayes • functional and EIA-232 EIA-232 • procedural Host DTE FEATURES DTE DCE DCE Link to initiate, maintain and terminate physical CONNECTIONS BETWEEN • Data Terminal Equipment (DTE) and Terminal Computer Networks 1 • Data Circuit Terminating Equipment (DCE, "postal socket") Computer Networks 1 Interchange Interchange • and/or data switching centers. Host Computer circuits circuits DTE (Data Terminal Equipment = end-system) Using physical connections, the physical layer ensures the transfer of DCE (Data Circuit-Terminating Equipment) a TRANSPARENT BITSTREAM • modem, multiplexer, Digital Service Unit between DATA LINK LAYER-ENTITIES. A PHYSICAL CONNECTION may permit either Phyiscal layer deals with interfaces between • the duplex or • DTE and DCE and • the semi-duplex • DCE and DCE transfer of a bitstream 5 6 Physical Layer Physical Layer Characteristics Mechanical MECHANICAL: size of plugs, allocation of pins, etc. www.ibr.cs.tu-bs.de www.ibr.cs.tu-bs.de • e. g. ISO 4903: • data transfer - 15 pin DTE/DCE connection and pin allocation ELECTRICAL: voltage levels on wires, etc. • e. g. CCITT X.27/V.11: • electrical features for the symmetrical transfer within the area of data communication Computer Networks 1 Computer Networks 1 FUNCTIONAL: definition of switching functions; pin allocation (data, control, timing, ground) • e. g. CCITT X.24: • list of the switching functions between DTE und DCE in public data networks PROCEDURAL: rules for using switching functions • e. g. CCITT X.21: • protocol between DTE and DCE for synchronized data transfer in public data networks 7 8 Physical Layer Physical Layer

Electrical Functional, Procedural www.ibr.cs.tu-bs.de www.ibr.cs.tu-bs.de e. g. .. " Computer Networks 1 Computer Networks 1 • designed for IC Technology Example RS-232-C, functional specification describes • balanced generator • connection between pins • differential receiver • e.g. "zero modem" computer-computer-connection • two conductors per circuit (Transmit(2) - Receive(3)) • signal rate up to 10 Mbps • meaning of the signals on the lines • distance: 1000m (at appr. 100 Kbps) to10m (at 10Mbps) • DTR=1, when the computer is active, DSR=1, modem is active, ... • considerably reduced crosstalk • Action/reaction pairs specify the permitted sequence per event • interoperable with V.10 / X.26 ...” • e. g. when the computer sends an RTS, the modem responds with a CTS when it is ready to receive data 9 10 Physical Layer Physical Layer 1.2 Bit Rate and Baud Rate Basics www.ibr.cs.tu-bs.de www.ibr.cs.tu-bs.de Bandwidth of a channel: B = f max - f min BAUD RATE: measure of number of symbols (characters) transmitted per unit of time f max , f min : maximum resp. minimum frequency • signal speed, number of signal changes per second • changes in amplitude, frequency, phase Examples: • each symbol normally consist of a number of bits • phone: min. 3000 Hz • so the baud rate will only be the same as the bit rate when there is one bit per symbol. • Coax: approx. 300 MHz approx. 10 8 MHz (visable light) • fiber: BIT RATE: Number of Bits transferred per Second (bps) • bit rate may be higher than baud rate ("signal speed") Computer Networks 1 Computer Networks 1 Nyquist theorem (noise free channel) • because one signal value may transfer several bits • max. bitrate = 2 H • log 2 V bps Example: • H ... signal bandwidth (low pass filter) • V ... discrete levels Example: 3000 Hz channel, binary signal ( V =2): • max. bitrate = 6000 bps 11 12 Physical Layer Physical Layer

1.3 Basics Operating Modes www.ibr.cs.tu-bs.de Shannon theorem (noisy channel) www.ibr.cs.tu-bs.de Transfer directions (temporal parallelism) max bitrate = H • log 2 (1 + S / N ) • simplex communication: • data is always transferred into one direction only • H ... signal bandwidth (low pass filter) • (half-duplex) semi-duplex communication • S /N . . . Signal to Noise ratio • data is transferred into both directions • but never simultaneously • 10 log 10 S / N decibels • full-duplex communication • data may flow simultaneously in both directions Example: Computer Networks 1 Computer Networks 1 • 3000 Hz channel, • S /N = 1 000 (30 dB) • max. bitrate = 30 000 bps independent of number of levels ! This is an upper bound! • real systems rarely achieve it 13 14 Physical Layer Physical Layer Serial and parallel transmission Operating Modes: Synchronous Transmission www.ibr.cs.tu-bs.de www.ibr.cs.tu-bs.de Definition • parallel: • the point in time at which the bit exchange occurs is • signals are transmitted simultaneously over several channels pre-defined by a regular clock pulse (requires synchronization) • serial: • whereby the clock pulse lasts as long as the • signals are transmitted sequentially over one channel transmission of a series of multiple characters takes Implementation Computer Networks 1 Computer Networks 1 • receiving clock pulse • on a separate line (e. g. X.21) or Serial Parallel • gained from the signal • bit synchronous or frame synchronous 0 0 1 0 0 0 0 0 1 0 1 1 (frames in fact on data link level) 0 0 time 0 0 • special characters 0 0 e. g. 0 0 0 0 SOH Start of Header 1 1 Symbol STX Start of Text ETX End of Text 15 16 Physical Layer Physical Layer

Guided Transmission Media: 1.4 Operating Modes: Asynchronous Transmission Twisted Pair and Coax Definition www.ibr.cs.tu-bs.de www.ibr.cs.tu-bs.de UTP: unshielded twisted pair • clock pulse fixed for the duration of a signal Amplifier or Repeater Twisted pair • termination marked by Signal • Stop signal (bit) or • number of bits per signal Implementation • simple: • sender and receiver generate the clock pulse independently from Signal each other • frame size usually approx. 9 bit Computer Networks 1 (of this approx. 70% reference data) Computer Networks 1 example: Coaxial cable 7 Bit ASCII reference data Side View 1 Parity Bit (odd, even, or unused) Front View 1 Start-Bit Coaxial 1 Stop-Bit cable • example: RS-232-C • UART (universal asynchronous receiver and transmitter) IC module Twisted Protective • often between pairs insulation inner cover • computer and printer or conductor Outer (on each cable conductor • computer and modem if not within 17 18 a system cover) Physical Layer Physical Layer Analog and Digital Fiber Optics 2 Information Encoding and Transmission www.ibr.cs.tu-bs.de www.ibr.cs.tu-bs.de Three examples of a light ray from inside a silica fiber impinging Variants and examples: on the air/ silica boundary at different angles Transmission Light trapped by total internal reflection analog digital Total internal Air/silica β 1 boundary β 2 reflection β 3 analog “old” telephone system ISDN (voice service) music) (voice, (POTS) Internet Audio � AM, FM � PCM, DM, … Information Coding α 1 α 2 α 3 Silica Computer Networks 1 Light source Computer Networks 1 modem (modulator demodulator) at analog traditional computer (texts, images) telephone connection networks and Types: digital Radio Data System RDS applications • Multimode � PAM, PPM, PFM, … ISDN (data service) • several rays with different angles (’modes’) � Manchester and • Monomode V.21, V.22 bis, …, V.32 Encoding, … • fiber diameter reduced to few wavelengths of light bis, V.34. • light can propagate in straight line 19 20 Physical Layer Physical Layer