Later part of Chapter 16.

1. WiMAX
   
   1. Line-of-Sight (LOS) links: high-speed between towers.
   2. Radios between towers and client are lower speed, possibly WiFi.
   1. Last-mile alternative to cables
   2. Version for mobile clients.
   3. Also can be used for interconnect.
   4. Do they exist?
      1. Not as ISP service, apparently.
      2. But is apparently an option for 4G phones.
2. Personal Area Network (PAN)
   1. There are a lot of these.
      
   2. Bluetooth.
      1. Replace cables for mouse or headphones, etc. Short distance.
      2. Device is master or slave; master grants permission to slave (a type of polling).
   3. Ultra Wideband (UWB).
      1. Uses a wide bandwidth for high transmission speed than Bluetooth.
      2. Largely overtaken by WiFi.
   4. 802.15.4x. Remote controls. Lots of options.
3. Infrared. Remote controls.
4. Radio Frequency Identification (RFID).
   1. An RFID reader can wirelessly extract an identifier from a tag.
      Uses
      1. Tags in credit cards and all sorts of ID cards.
      2. Tags in items for inventory or purchase.
      3. Sensors.
   2. Passive tags take power from the signal, and need no other source.
   3. Active tags have a battery, which may last years. Better range.
   4. Over 140 standards for different purposes.
5. Cellular
   1. Designed for voice. Now carries data as well.
   2. Pre-Cell Radio Phones
      1. Allocate some number of single-call frequencies.
      2. Central antenna for whole city.
      3. Share those between active mobile phones.
      4. Not enough frequencies: Expensive, waiting lists.
   3. Cells
      
      1. Separate towers use different frequencies.
      2. Each tower is connected to the larger telephone network by wire.
      3. Mobile units use the nearest towers.
      4. As a unit moves from one cell to another, the tower must hand it off to the other.
      5. Towers are assigned frequencies so that no two adjacent ones use the same frequency.
      6. The layout allows any area to be covered using only a fixed number of frequencies.
      7. Real cells are not as regular as the picture.
         1. Smaller cells for denser population areas.
         2. Terrain or other obstructions will make the area covered by the tower irregular.
         3. There will always be margins where some overlap occurs.
   4. Microcells
      1. For particularly small areas. Use low power to avoid interference.
      2. May be for floors of a building.
      3. Or for a home subscriber, limited to listed phones and connected to wired Internet.
   5. Generations. These are tied more to history and marketing than some new technology for each generation. The techniques evolve gradually, and the marketers occasionally change the name.
      1. 1G. 1970s and '80s. Analog modulation of voice signal.
      2. 2G. Use digital signals to carry voice.
         1. Standards fragmented.
         2. Europe specified a TDMA technique called Global System for Mobile Communications (GSM).
         3. US Carriers each came up with their own.
         4. Various improvements brought data rates up to 1 Mbps.
         5. With some 3G features added, sold as 2.5G.
      3. 3G.
         1. Developed using many later techniques from 2G. Combine CDMA and FDM.
         2. Single standard instead of fragmentation.
         3. Still, several combinations in use inside the single standard (so it's a bit of an a la carte menu).
         4. Several speeds depending on details, 2.4 Mbps, 3.1 Mbps
         5. Still based on digital voice service standards, with data as an add-on.
      4. 4G.
         1. Shift to primary data service based on TCP/IP.
         2. The standard specifies a certain speed to advertise as 4G. (The 4G LTE designation isn't actually 4G speed.)
         3. 4G also allows alternate connection over WiFi or satellite.
         4. A 4G phone may use 3G standards for voice calls, and use 4G for data only.
6. Satellite
   1. Sending and receiving from the satellite.
      
   2. Geostationary
      1. Original systems required a 3-meter disk. Not consumer practical.
      2. Very Small Aperture Terminal (VSAT) developed, 1m or less are common now.
      3. Various frequencies with various resistance to rain.
   3. Low-Earth Orbit (LEO).
      1. A Starlink Antenna
         1. A rectangular flat antenna, about a half by third of a meter.
         2. Uses a phased array, which is electronically aimed.
         3. Apparently, earlier ones had an aiming motor also, now you just point it yourself.
      2. Ground stations use a dish to connect satellites to the Internet backbone.
      3. Phased array antenna
         1. Array of small antennas, all sending to form a compound signal.
         2. Successive small phase delay across the surface has the effect of directing the signal.
         3. Result is that the signal can be aimed electronically without moving the device.
   4. Book mentions GPS satellites for no obvious reason.
      1. Twenty-four satellites in six orbital planes. (Actually 31 operational as of July 3, 2023.)
      2. Medium earth orbit at 12,550 miles, about two orbits a day. Orbits, www.gps.gov.
      3. Each satellite has an atomic clock, and sends a time stamp and its position.
      4. Receiver computes distance from the time delay between sending and receipt of message.
      5. Three distances from three known points locates the receiver in one of two possible locations. One is in space, and the other is your location, earthling.
      6. Unless the GPS receiver has its own atomic clock handy to measure the reception time, it needs a fourth satellite connection to get the time.
      7. Doesn't work well when you're too far north, as we learned visiting Fairbanks last summer. (Fairbanks is about 65° north; GPS orbits are inclined 55°.)