index.md (2974B)
1 +++ 2 title = "Physical: Transmission Media" 3 +++ 4 5 # Physical: Transmission Media 6 - A van full of tapes 7 - Wires 8 - Twisted pair 9 - used in LAN, telephones 10 - twists reduce radiation (and interference) 11 - shielded (STP) or unshielded (UTP, like Cat 3/5/5e/6/6A/7) 12 - connectors: RJ11 (4 wires), RJ45 (8 wires, modern buildings) 13 - link types 14 - simplex — only one fixed direction at all times 15 - half-duplex — both directions, but not at the same time, senders take turns 16 - full-duplex — both directions at once (so use different twisted pairs for each direction) 17 - coaxial 18 - better shielding, more bandwidth, longer distances, higher rates 19 - copper core surrounded by insulation and covering 20 - power lines 21 - e.g. household electrical wiring 22 - convenient to use, bad for data transmission 23 - cheap and easy, but short-distance with moderate bandwidth and low security 24 - Fibers 25 - utilise total internal reflection in a thin strand of glass (silica fiber) 26 - high rates and long distances, enormous bandwidth in THz with little loss 27 - types 28 - single-mode — narrow, 10 µm core where light can’t even bounce. used with lasers for long distances 29 - multi-mode — light can bounce in 50 µm core, used with LEDs 30 - an example is TAT-14 TransAtlantic, with two main and two backup fiber pairs on the ocean floor (3 Tbps capacity) 31 - very secure and fast, but less convenient and more expensive 32 - Wireless 33 - compared to wires, deployment is easy and inexpensive, with natural support for broadcasting 34 - however, transmissions interfere and data rates vary with signal strength 35 - Electromagnetic spectrum — carefully divided and regulated, except for WiFi ISM bands 36 - radio waves 37 - λf = c in vacuum (freq f, wavelength λ) 38 - radio/light travels at around 1 foot/nsec 39 - penetrate buildings, propagate long distances 40 - Very Low Frequency, LF, MF bands — waves follow curvature of earth 41 - HF band — waves bounce off ionosphere 42 - microwaves 43 - high bandwidth, used in WiFi and 3G/satellites 44 - strength varies with mobility due to multipath fading (delayed waves arriving out of phase with direct waves & cancelling out) 45 - light transmission 46 - line-of-sight can be used for links 47 - highly directional, lots of bandwidth 48 - use LEDs/cameras and lasers/photodetectors 49 - Satellites 50 - Geostationary (GEO) 51 - 36,000 km, delay ~250msec up/down 52 - does not need tracking 53 - VSAT can use it to communicate through a hub 54 - Low (LEO) 55 - good for coverage, e.g. Iridium’s 66 satellite network for comms routing 56 - can relay to each other, or on ground using a bent pipe 57 58 