1. Transmission media
   1. Guided: Wire or other conduit.
   2. Unguided: Not.
2. Taxonomy.
   1. Electricity
      1. Twisted pair wire.
      2. Coaxial cable.
   2. Light
      1. Optical fiber.
      2. Infrared.
      3. Laser.
   3. Radio
      1. Terrestrial
      2. Satellite
3. Wire
   1. Must form a circuit, so you need two.
   2. Metal wires absorb energy from stray random electromagnetic radiation on the environment: noise.
   3. Wire pair subject to interference. Different effects on each wire.
      
   4. Twisting them can reduce the damage. The effect adds up to the same in each direction.
      
      Ethernet uses twisted pair.
   5. Coaxial uses the ground as a shield for the positive. Effect on the shield is balanced.
      
      Cable TV and cable internet.
   6. Twisted pair may also be shielded.
   7. Shielding makes the cable less flexible.
4. Electromagnetic spectrum. Radio and light are electromagnetic radiation on a spectrum of frequencies.
   
   1. Different frequencies behave differently.
   2. Lower frequencies travel farther and better penetrate objects.
      1. Short-wave radio signals travel around the world.
      2. AM radio is higher frequency, but still can travel a great distance.
      3. FM radio and TV signals usually cover a city.
      4. Microwaves will penetrate walls, but infrared and light do not.
5. Optical cable.
   1. A physical cable which guides a visible light signal.
   2. Transparent materials have different optical densities.
   3. When light travels between materials of different type, it is refracted or reflected, depending on the densities and angle.
      
      

      

      
   4. Optical fiber uses a central core and a cladding of lower density.
      
      1. Relies on the internal reflection to keep the light inside.
      2. Sharp bends will let the light escape.
      3. Light started at different angles will have different travel times. Called “diffusion.”
         
         
   5. Types
      1. Multimode, step index. Cheapest. Sharp density change at the cladding produces sharp reflection and more diffusion.
      2. Multimode, graded index. Medium cost. Gradual density change reduces diffusion.
      3. Single mode. Most expensive. Very narrow core to minimize diffusion.
6. Wiring v. Optical fiber
   1. Wire is cheaper.
   2. Optical does not suffer from interference.
   3. Optical supports higher bandwidth.
   4. Optical is easier to break and more difficult to repair.
7. Infrared
   1. Same frequencies as used for remote controls.
   2. Line-of-sight: Won't go around corners or through walls, but produces a wide beam.
   3. Used for nearby peripherals, such as a printer.
   4. Has been used to synchronize a PDA.
   5. Don't see it much now.
8. Unguided Laser
   1. Creates a narrow point-to-point, line-of-sight link.
   2. Endpoint devices must be carefully aligned.
   3. Used out-of-doors, such as between adjacent buildings.
   4. Can also be used to link a mountaintop to nearby city, or cross a body of water.
   5. Rain, fog and smoke tend to block the channel.
9. Terrestrial Radio
   1. Radio and TV frequencies are not usually used with computer networks, but may carry digital media.
   2. There is an old practice of using short wave for computer networking. Seems mainly a hobby thing.
   3. Most common is WiFi, which is in the microwave range.
   4. As we will see later, higher frequencies are can support higher bandwidth. (That's probably why we don't just run the Internet over shortwave, and save a bunch on infrastructure.)
10. Satellites
    1. Radio signals in the microwave band are sent to a satellite, which relays them back to another location on earth.
    2. Some physics.
       1. A satellite must move, otherwise it falls to earth.
       2. In general, an orbit is an ellipse, but we usually use circular ones.
       3. Speed depends on altitude. The higher, the slower.
       4. God chooses the combinations of speed and altitude; you don't get to write your own natural laws.
    3. Geostationary (aka geosynchronous) satellites. Satellites won't stay in one place, but this gets pretty close.
       1. Use a circular orbit over the equator.
       2. Set the speed so that the orbital period is 1 day.
       3. Now the (moving) satellite is synchronized with the rotation of the earth, so it appears fixed from any observation point on the (rotating) earth.
       4. The required height is 22,236 miles.
    4. Problems with geostationary satellites.
       1. The round-trip time to the sat and back is more than 0.2 sec.
       2. Ground stations need considerable power to send a signal that distance.
       3. Since the signal must be sent to just one satellite, some separation is required. There's only room for 45 to 90.
    5. Low-Earth Orbit
       1. Orbits above 500 kilometers (about 300 miles).
          1. Starlink satellites use 550 km.
          2. The International Space Station orbits at 408km. Watch your head.
       2. Advantages over geostationary.
          1. Much cheaper to launch.
          2. Eliminates the long signal delay.
          3. Ground terminals need much less power.
          4. Avoids much of the crowding.
       3. But satellites are seen to move from a point on earth.
       4. Usual approach:
          1. Use polar orbits as the earth turns below.
          2. Have enough satellites on each orbit, and enough orbits, that there's always at least one in view from any point on earth.
          3. Base stations switch the between satellites, much as cell phones in a moving car switch between towers.
          4. Satellites can forward between themselves until reaching one in view of a desired base station.
11. Trade-offs
    1. Major performance parameters:
       1. Channel capacity: Maximum data rate that can be sustained.
       2. Propagation delay: Time for a signal to traverse the medium.
    2. Cost: materials, installation, operation and maintenance.
    3. Quality of transmission: attenuation and distortion.
    4. Susceptibility to interference and noise.
    5. Security: susceptibility to eavesdropping.
12. Maximum data rate.
    1. Receiving digital data from an analog signal is really a form of sampling (more on this later). So, the highest available frequency will produce two levels per cycle.
    2. If there are K data levels, the maximum data rate D for a signal of bandwidth B is given as D=2Blog2K
    3. This is the “Nyquist Theorem,” which encourages fancy encoding schemes.
13. Limits on the data rate.
    1. Any transmission medium is subject to noise.
       1. May be interference.
       2. If you manage to clear that up, there are still quantum effects.
    2. The “loudness” of the signal and of the noise are measured in power (energy over time). The quantity S/N is called the (wait for it) “signal-to-noise ratio.”
    3. Shannon showed that noise limits transmission to
       C=Blog2(1+S/N)
       You can increase the signal strength, but:
       1. There are limits to how much energy you can pump in and transmit.
       2. The log means you don't get much improvement for a lot of increased power.
       3. Creates a hard limit on the bandwidth of a particular channel.
    4. S and N are not usually given separately, but signal-to-noise ratio is usually given as a single quantity, expressed in decibels.
    5. A ratio of powers in decibels is given
       dB=10log10

       (

       P2

       
       

       P1

       )
    6. Example from text. A voice phone line.
       1. Analog bandwidth 3000 Hz.
       2. S-to-N of 30 dB, which gives S/N=1030/10=1000.
       3. C=3000×log2(1+1000)≈30,000
       4. This is a limit on the speed of a dial-up modem.
14. Nyquest encourages fancy encodings. Shannon puts a limit on what you can do with them.