Transcript 6. Media
Transmission Media • Electromagnetic signals representing data propagates in the transmission medium Wavelength (m) Frequency (Hz) > 1 x 10-1 < 3 x 109 Microwave 1 x 10-3 - 1 x 10-1 3 x 109 - 3 x 1011 Infrared 7 x 10-7 - 1 x 10-3 3 x 1011 - 4 x 1014 Optical 4 x 10-7 - 7 x 10-7 4 x 1014 - 7.5 x 1014 UV 1 x 10-8 - 4 x 10-7 7.5 x 1014 - 3 x 1016 X-ray 1 x 10-11 - 1 x 10-8 3 x 1016 - 3 x 1019 < 1 x 10-11 > 3 x 1019 Radio Gamma-ray Winter 2006 ECE ECE 766 Computer Interfacing and Protocols 06 - 1 Twisted-Pair Cable • Unshielded Twisted-Pair (UTP): Two conductors, each wrapped in an insulator, then twisted • Suitable for both voice and data (100Hz-5MHz), very easy to use, cheap Category (by EIA) Properties Winter 2006 ECE 1 Used for voice and low rate data 2 Used for voice and data up to 4Mbps 3 3 twists/foot Up to 10Mbps Current standard for telephone systems ECE 766 Computer Interfacing and Protocols 4 Up to 20 Mbps (16Mbps for token ring) 5 Up to 100Mbps 06 - 2 Twisted-Pair Cable • Shielded Twisted-Pair (STP) A UTP cable covered in a metal sheath which is grounded • Used in environments with high EMI/RFI • STP has the same quality considerations • More expensive and bulky Winter 2006 ECE ECE 766 Computer Interfacing and Protocols 06 - 3 Coaxial Cable • A central conductor enclosed in an insulating sheath, encased in an outer conductor • Outer conductor: – Shield against noise – Completes the circuit • Frequency range: 100kHz-500MHz • Signal is FIELD between conductors Winter 2006 ECE ECE 766 Computer Interfacing and Protocols 06 - 4 Optical Fiber • Guiding light in a solid core Light carries information • Speed of light – In vacuum, c = 3x108 m/s – In other media, v ≤ c v = λf λ ≤ λ0 – Index of refraction n = λ0 / λ • Critical angle and reflection n1 n1 sin θ1 = n2 sin θ2 θ1 n2 n2 θ2 Winter 2006 ECE n1 ECE 766 Computer Interfacing and Protocols θ1 θ2 06 - 5 Optical Fiber • Modes of optical fiber – Multimode • Step-index vs. Graded index – Single mode • Multimode Step-index – Multiple beams travel in the optical fiber n1 n2 Core Cladding Winter 2006 ECE ECE 766 Computer Interfacing and Protocols 06 - 6 Optical Fiber • Multimode Graded-index Core Cladding • Single mode – Step-index fiber and highly focused light source – Very low density (small n), small diameter Critical angle close to 90º Winter 2006 ECE ECE 766 Computer Interfacing and Protocols 06 - 7 Optical Fiber • Light sources: – Light-Emitting Diode (LED) – Injection Laser Diode (ILD) • Advantages – Noise resistant – Less signal attenuation – High bandwidth • Disadvantages – Cost – Installation/maintenance – Fragility Winter 2006 ECE ECE 766 Computer Interfacing and Protocols 06 - 8 Unguided Media: Wireless • Unguided media transport EM waves without using a physical conductor Wireless communication • Three types of propagation Winter 2006 ECE ECE 766 Computer Interfacing and Protocols 06 - 9 Bands Band Range Propagation Application VLF 3–30 KHz Ground Long-range radio navigation LF 30–300 KHz Ground Radio beacons and navigational locators MF 300 KHz–3 MHz Sky AM radio HF 3–30 MHz Sky Citizens band (CB), ship/aircraft communication VHF 30–300 MHz Sky and line-of-sight VHF TV, FM radio UHF 300 MHz–3 GHz Line-of-sight UHF TV, cellular phones, paging, satellite SHF 3–30 GHz Line-of-sight Satellite communication EHF 30–300 GHz Line-of-sight Long-range radio navigation Winter 2006 ECE ECE 766 Computer Interfacing and Protocols 06 - 10 Cellular Telephony • Fixed-wireless communication Wireless last hop • Frequency reuse BS PSTN MSC BS BS BS Winter 2006 ECE ECE 766 Computer Interfacing and Protocols 06 - 11 Satellite Communication • Line-of-sight communication between satellites (and earth stations) • Depending on the type of constellation, single or multiple hops • Number of satellites needed for global coverage Winter 2006 ECE ECE 766 Computer Interfacing and Protocols 06 - 12 Transmission Impairment • Attenuation – Loss of energy – Restored via amplifiers – How about digital signals? • Distortion • Noise What can we do about distortion and noise? Winter 2006 ECE ECE 766 Computer Interfacing and Protocols 06 - 13 Comparison of Transmission Media Medium Cost Speed Attenuation EMI Security UTP Low 1-100Mbps High High Low STP Moderate 1-150Mbps High Moderate Low Coaxial Moderate 1Mbps1Gbps Moderate Moderate Low Optical High Limited by devices Low Low High Satellite High 1-10Gbps Variable High Moderate Cellular High 9.6-200Kbps Low Moderate Low Winter 2006 ECE ECE 766 Computer Interfacing and Protocols 06 - 14