Physical Layer. Transmission Media. Physical Layer Overview. Physical Layer Trends. MANs WANS LANS. Twisted Pair. Open Line. It is concerned with:

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1 Physical Layer Transmission Medium Signalling Means Timing & s Synchronous Serial Communications Physical Layer Overview The physical layer resides at layer 1 of the OSI-RM. It is concerned with: Mechanical connector Electrical interface Signalling Extraction of timing Control of transmission Transmission Medium (also known as Layer 0) Physical Layer Trends rate Transmission Media 10 Gbps 1 Gbps 100 Mbps MANs Multiple optical fibre WANS Open Wire Co-axial Cable Optical Fibre 10 Mbps 1 Mbps 100kbps 10 kbps LANS unrepeatered copper unrepeatered fibre repeatered fibre 1m 100m 10 km 1000km Length of Circuit route or Aluminium PVC Insulation Optional Braided Shield Open Line Signals & Common Ground Uses: Short-Range Communications ( m, 64 2 m) Connection of printers, modems, terminals PVC Insulation Optional Braided Shield Differential Pair

2 Uses: Near-End and Far-End Cross Talk Capacitative coupling Telephone Subscriber Loop (low grade) Unshielded ( grade or CAT-5) Greater Noise Immunity Long Range Communications 100 Mbps) High Rates ( 100 Mbps) NEXT NEXT Mainly effect of near cable FEXT Increases with cable length FEXT Coaxial Cable Co-axial Cable Impedence 50 Ohm or 75 Ohm Uses: Greater Noise Immunity Dielectric Insulation Braided Outer Optional Solid Shield Long Range Communications 100 Mbps) High Rates ( 100 Mbps) Much Higher Rates using Modulation Single or Multimode Fibre Optical Fibre Input Absorption Dispersion Output Protective Layer Core (h 1 ) Cladding (h 2 ) Wavelengths Used 0.85 mm 1.3 mm 1.5 mm h 2 < h 1 Fibre Capacity Multimode Large Core (e.g. 50 µm) Easier to Install CATV, local connections Single Mode Small Core (8-12 µm) No multi-mode dispersion Distance connections Bandwidth Frequency/bit Capacity 200 nm c = fl Df = c Dl/l b/hz ~ 25 THz

3 Fast Networks => Small Bits RS-442 / V kbps 1 bit at 64 kbps 1 = +V, ÐV 0 = ÐV, +V 1 = +ve 0 = Ð ve 15 ms 15 ms 156 bits at 100 Mbps DTE ÐV Twisted Pair Cable DCE 100 Mbps Signal Ground Termination Resistor Ð V Signal Ground Differential Transmission Balanced v Unbalanced 1 = +V, ÐV 0 = ÐV, +V 1 = +ve 0 = Ð ve Property Elec Interface Unbalanced ± 12V Balanced ± 5V Cable single pair ÐV Distance moderate longer Noise Immunity moderate high Rate 9600 bps (typ) 64 kbps 2Mbps Synchronous Serial Communications Synchronous v Asynchronous Bit Timing Co-axial Cable Transmission Encoding Timing Recovery Transmission Networks Source serial bit stream Synchronous Transmission Transmitter Byte Byte Parallel In Serial Out Shift Register Serial In Parallel Out Shift Register Receiver

4 Synchronous Communications Synchronous Transmission Async Sync NRZ Simple interface (limited rate 19.2 kbps) More complex interface (high rates supported) Transmit Used for connecting: Printer, Terminal, Modem Used for: LANs, WANs, Telephony No clock sent ( & have own clocks) sent with (more configuration options) Requires start/stop which provides byte timing Byte timing from transmission or by link protocol Encoded Signal Harmonics Transmitter Receiver Transmitted square wave Fundamental Encoder Enc Decoder Amplitude 5th Harmonic "What no clock wire?" 3rd Harmonic Received signal (summation) Timing Extraction Properties of a Digital Phase Lock Loop D C D C encoding decoding Provides: Synchronises (regenerated) clock with Minor correction of drift between clock and Requires: + encoded clock signal A stable fast reference clock (e.g. x32) D C D C encoding decoding Sufficient bit transitions in received

5 (1) (2) reconstructed clock in phase with stable 32 shift register phase adjust Digital Phase-Locked Loop to decoder receive clock at 16 X rate edges compared with expected position using regenerated clock clock adjustment correct clock Regenerated Manchester Encoding NRZ Manchester Transmit Properties of Manchester Encoding Manchester and Manchester II 1. 2-signal levels used 2. No d.c. component 3. Timing component at fundamental clock frequency 4. Double bandwidth of NRZ 5. Encoding violation signal possible Transition in centre of each bit 0 encoded: 1 encoded: Manchester II encoding Indifference to decoder start position