1. Transmission Media No. 1  Seattle Pacific University Transmission Media: Wires, Cables, Fiber Optics, and Microwaves Based on Chapter 4 of William Stallings, Data and Computer Communication Kevin Bolding Electrical Engineering Seattle Pacific University

2. Transmission Media No. 2  Seattle Pacific University 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. Transmission Media No. 3  Seattle Pacific University 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. Transmission Media No. 4  Seattle Pacific University 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. Transmission Media No. 5  Seattle Pacific University Attenuation Curves Attenuation is very dependent on conductor size Cat-5: 0.21 mm 2 RG58: 0.64 mm 2 RG6: 1.0 mm 2

6. Transmission Media No. 6  Seattle Pacific University Frequency of various signals Power/ Telephone RadioMicrowaveInfrared Visible Light Twisted Pair Coax AM RadioFM Radio/TV Microwave Trans. Optical Fiber 10 6 10 5 10 4 10 3 10 2 10 1 10 0 10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 910 10 11 10 12 10 13 10 14 10 15 Frequency (Hz) Wavelength (Meters) Source: Stallings, Fig. 4.1

7. Transmission Media No. 7  Seattle Pacific University 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. Transmission Media No. 8  Seattle Pacific University Differential Signaling Evil Noise (0.6v) Source Signals gather noise when travelling in a cable 5.0V 5.6V Receiver GND If the signal and GND are both sent together, they both experience the same noise. Computing the difference removes the noise. 5.0V Evil Noise (0.6v) 5.6V Source Receiver GND 0.6V 5.0V 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. Transmission Media No. 9  Seattle Pacific University Electrical Cables Electromagnetic interference (EMI) Loops make great antennas Antenna strength proportional to the area inside of the loop Worse for shorter wavelengths signal return Interference prop. to area signal return Better… Common ground systems (such as PCBs with ground planes) Return path directly below signal Minimizes loop area Trace on PCB Ground return Keep the two parts of the signal close together

10. Transmission Media No. 10  Seattle Pacific University Twisted Pair Cables 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 Signal Return Adjacent Loops Out of phase Data rates Over long distances, about 1-3 Mbps Short distances: 1Gbps and higher

11. Transmission Media No. 11  Seattle Pacific University 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 (source: Microsoft Networking Essentials)

12. Transmission Media No. 12  Seattle Pacific University Coaxial Cables Concentric mesh wire for ground Acts as an excellent shield Very little interference or radiation 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. Transmission Media No. 13  Seattle Pacific University Optical Fiber Relies on total internal reflection Light waves bounce of edge of fiber Channels waves to destination 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 (Source: Stallings, Fig. 4.4)

14. Transmission Media No. 14  Seattle Pacific University 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. Transmission Media No. 15  Seattle Pacific University 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. Transmission Media No. 16  Seattle Pacific University Unguided Media Attenuation f = frequency (Hz) d = distance (m) = wavelength (m) c = speed of light (m/s) 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 =

17. Transmission Media No. 17  Seattle Pacific University Terrestrial Radio (All forms) Ionosphere 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 h 1 and h 2

18. Transmission Media No. 18  Seattle Pacific University Satellite Radio Requires satellite in geosynchronous orbit 35,784 km Delay of ¼ second (round-trip) Satellites spaced 4 degrees apart Above 10GHz, signal is attenuated by atmosphere Higher frequencies use smaller dishes, though http://www.mike-willis.com/Tutorial/gases.htm