First part of Chapter 16.

1. Classes of Wireless Network
   1. Local Area Networks (LAN), mostly WiFi.
   2. Metropolitan Area Networks (MAN), mostly just theoretical.
   3. Wide Area Networks (WAN), mostly cell phone and satellite.
   4. Personal Area Networks (PAN), headphones and such.
2. Industrial, Scientific and Medical (ISM) frequencies.
   1. Governments regulate the use of radio frequencies to prevent chaos.
   2. Many require licenses, giving a monopoly on a frequency in an area.
   3. The ISM frequencies are available for any user. WiFi goes here.
   4. Some regulation about how devices using the frequencies may behave.
      
3. Spread Spectrum
   1. Techniques generally send data modulating multiple frequencies.
   2. This increases performance and resists noise.
      1. Direct Sequence Spread Spectrum (DSSS).
         1. Before transmission, modulates a known high-frequency signal over the original, and send over a wider channel.
         2. Receiver knows the high-frequency add-on and removes it.
         3. Wide channel limits interference, and can allow multiple signals to succeed on the same frequencies, CDMA-style.
      2. Frequency Hopping Spread Spectrum (FHSS). Send data on a sequence of frequencies, and receiver follows.
      3. Orthogonal Frequency Division Multiplexing (OFDM). Multiple carries arranged so they do not interfere.
4. WiFi Standards.
   1. These are the major ones, and also the original, which is largely gone.
      
5. Frame Format
   
   1. Don't ask hard questions about what all those fields are.
   2. WiFi access points generally receive Ethernet frames from the OS, and encapsulate them in the above frame format for transmission.
   3. The received WiFi frame is rebuilt as Ethernet before delivery to receiving software. That's why Wireshark shows WiFi traffic in Ethernet frames.
   4. The four (MAC) addresses represent Ethernet sender and receiver (ones actually visible to endpoint software) and wireless sender and receiver.
   5. Often, some of these are the same.
6. Wireless Infrastructure mode.
   
   1. Each Access Point (AP) controls a Basic Service Set (BSS) of endpoints
   2. Endpoints must “associate” with a particular endpoint.
   3. The endpoints communicate with the access point only, never each other.
   4. In early WiFi, access points had to coordinate a handoff for a moving station, like a cell tower.
   5. Now, it's pretty much the endpoint's problem which to associate with.
   6. Endpoints share their frequency using CSMA/CA. Early standards had the endpoint assign different frequencies so they don't interfere. This was dropped.
7. Wireless Ad-Hoc
   1. Devices connect and reconnect to each other as the can.
   2. Routing is based on dynamic route discovery as the network changes.
   3. Not often used.
   4. Some printers can connect to computers using the ad hoc mode, where the single link presents no routing issue.