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Transmission Media: Wires, Cables, Fiber Optics, and Microwaves

Transmission Media: Wires, Cables, Fiber Optics, and Microwaves. Based on Chapter 4 of William Stallings, Data and Computer Communication, 8 th Ed. Kevin Bolding Electrical Engineering Seattle Pacific University. Transmission Media. A signal must be transmitted through some medium.

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Transmission Media: Wires, Cables, Fiber Optics, and Microwaves

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  1. Transmission Media:Wires, Cables, Fiber Optics, and Microwaves Based on Chapter 4 of William Stallings, Data and Computer Communication, 8th Ed. Kevin BoldingElectrical EngineeringSeattle Pacific University

  2. Transmission Media • A signal must be transmitted through some medium • Guided Media determine the path of the signal • Wires (cables, twisted pair, coax) • Fiber Optics • Other things… • Signals Propagate in all directions in Unguided Media • The medium is usually free space (air), but the signal type gets the name • Refers to transmitting signals through passive media that does not change the signal’s direction • Microwaves, broadcast radio waves • Lasers, Infrared

  3. Media Issues • Frequency range • Some media support higher frequencies than others • Impairments • Different media deform signals differently • Some are more susceptible to noise and distortion • Cost • We’re in the real world… • Number of receivers • Broadcast vs. point-to-point

  4. How Fast/How Far can a Signal be Sent? • The question: • Given a source signal with a given power, how far can it go before it is attenuated so much that the SNR is too low to be usable? • As far as media is concerned, the main issue is attenuation • Attenuation increases with distance. Usually expressed in dB/m, dB/100ft, etc. • Attenuation usually increases with frequency.

  5. Attenuation Curves Cat-5: 0.21 mm2RG58: 0.64 mm2 RG6: 1.0 mm2 Attenuation is very dependent on conductor size

  6. Frequency (Hz) 102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 Power/Telephone Radio Microwave Infrared VisibleLight Twisted Pair Coax OpticalFiber AM Radio FM Radio/TV MicrowaveTrans. 106 105 104 103 102 101 100 10-1 10-2 10-3 10-4 10-5 10-6 Wavelength (Meters) Frequency of various signals Source: Stallings, Fig. 4.1

  7. Guided Media • Guided media control the path of the signal wave • Electrical – Signal needs conductor and ground • Differences are in how ground/conductor interact • Twisted pair • Coax • Striplines on PCBs • Optical – Signal is sent using internal reflection • Differences are in light sources and fiber diameter

  8. Differential Signaling Evil Noise(0.6v) Signals gather noise when travelling in a cable Receiver Source 5.6V 5.0V GND If the signal and GND are both sent together, they both experience the same noise. Computing the difference removes the noise. Evil Noise(0.6v) Receiver Source 5.6V 5.0V 5.0V 0.6V GND Differential signaling works best when the two signal conductors are routed as close as possible to each other so they experience the same external noise.

  9. Electromagnetic interference (EMI) Loops make great antennas Antenna strength proportional to the area inside of the loop Worse for shorter wavelengths signal Interference prop. to area return signal Better… return Trace on PCB Ground return Electrical Cables • Keep the two parts of the signal close together • Common ground systems (such as PCBs with ground planes) • Return path directly below signal • Minimizes loop area

  10. Signal Return Twisted Pair Cables Adjacent LoopsOut of phase • Twist the signal and ground together • Both sides experience similar noise effects • Loop size proportional to twist size • Adjacent twists are 180 degrees out of phase • Tend to cancel out • Varying the twist size helps to minimize crosstalk • Data rates • Over long distances, about 1-3 Mbps • Short distances: 1Gbps and higher

  11. (source: Microsoft Networking Essentials) Shielding • Twisted pair usually comes bundled with several pairs in a cable • Unshielded – Just a plastic (teflon) jacket • For distances of around 100m - • Cat-3 UTP: <16Mbps, Cat-5 UTP: 100Mbps, Cat-6 UTP: 1000Mbps • Shielded – Includes a grounded shield

  12. Concentric mesh wire for ground Acts as an excellent shield Very little interference or radiation Coaxial Cables • Center conductor can be large (low resistance), reducing attenuation • Better data rates over long distances than twisted pair • The downside • Expensive to manufacture • More difficult to install

  13. Relies on total internal reflection Light waves bounce of edge of fiber Channels waves to destination Optical Fiber (Source: Stallings, Fig. 4.4) • Varieties • Multi-mode (wide fiber) • Light waves bounce off at different angles • Some have shallow angles (straight path), while others have steeper angles (crooked path) • Results in pulse spreading • Single-mode (narrow fiber) • Only a straight shot down the middle is allowed • Requires a laser source

  14. Fiber has its advantages • Advantages • No electromagnetic interference • Very little attenuation • Extremely high bandwidth (THz) • Small, lightweight • Disadvantages • More expensive transceivers • More difficult to install

  15. Wireless (Unguided) Media • Omnidirectional • Signal radiates in all directions • Good for broadcast • Inexpensive antenna • Directional • Signal radiates in a single direction • Usually requires parabolic (dish) antenna • 2-40 GHz (microwave) • Also works with lasers

  16. Unguided Media Attenuation Transmitted wave spreads out over a spherical surface Power density at receiver: Received power depends on the receiver antenna’s aperture: Thus Free Space Path Loss = f = frequency (Hz) d = distance (m) • = wavelength (m) c = speed of light (m/s)

  17. Ionosphere Terrestrial Radio (All forms) • Ground-wave propagation follows the curvature of the earth • Frequencies below 2MHz • AM radio (550-1600KHz) • Sky-wave propagation relies on the ionosphere and the surface of the earth to refract waves back-and-forth • Frequencies 2MHz-30MHz • Short-wave Radio, HAM radio • Line of site is point-to-point in a nearly straight line • Frequencies 30MHz and up • FM radio, TV, Mobile phones, etc. • Max distance between antennas with height h1 and h2

  18. Requires satellite in geosynchronous orbit 35,784 km Delay of ¼ second (round-trip) Satellites spaced 4 degrees apart http://www.mike-willis.com/Tutorial/gases.htm Satellite Radio • Above 10GHz, signal is attenuated by atmosphere • Higher frequencies use smaller dishes, though

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