IEEE 802 Protocol Family

802.3: Ethernet, L1 <-> L2 transfer
802.1q: VLANs
802.11: Wireless standards
General standard defining the main characteristics, such as frame format
802.11a/b/g subspecifications for specifics

Transmission Range

Send and receive data using antennas
Specific protocol to modulate data
Choose a frequency/wavelength to transmit in
Visible spectrum, used in e.g. fibre optics
Radio spectrum used in e.g. WiFi

Basics of Transmission

Antennas can both radiate and pick up electromagnetic waves
There are directional and omni-directional antennas
The waves can be reflected, diffracted, absorbed, polarised, refracted, ... by obstacles

Noise
Unwanted signal, either from other devices or naturally occurring
White noise
Signal to noise ratio (SNR)
Interference
Signals on roughly the same frequency
Signal to Interference Noise Ratio (SINR)
Fading
Strength of signal degrades over distance

Wireless Spectrum

The higher the frequency, the higher the data rate
Wireless is usually on 2.4/5 GHz
Unlicensed frequencies -> anyone can transmit on it

Wireless Networks

Wireless personal are network (WPAN)
Range of ~10 meters
Master device communicates with up to 7 clients
Wireless LANs (WLAN)
Range of ~100 meters
All stations communicate through AP or Ad Hoc
Wireless MANs (WMAN)
Range of <50 KM
All stations communicate through AP or mesh
Cellular radio networks (WRAN)
Large coverage, i.e. entire countries
Network is divided into smaller areas (cells)

Transmission always uses infrastructure
The whole path being wireless is very rare

Bluetooth

Replacement for cables
Operates in 2.4 GHz
Short range
No need for infra, ad hoc
The more range and data transfer -> the more energy used

WiFi (802.11)

Distribution system: e.g. LAN/Ethernet
Station: device
Medium
Access point / Base Station

Basic Service Set: basically a local network
Can also be run in ad hoc mode: devices connect directly to one another without need for an AP

Channels

In 2.4 GHz there are 14 channels, each 5 MHz apart
Protocols require 25 MHz of channel separation to function
Channel can be chosen in AP settings

Scanning

Passive scanning
Beacon frames sent from AP
Host listens on each channel periodically
Association request frame sent from host to AP, response from AP
Active scanning
Probe request frame broadcast from host
Probe responses sent from AP
Association request sent by Host, responded by AP

Contention for the Medium

If A and B both want to transmit to C, a collision will occur
Collision detection is hard in wireless networks
It is possible that a device outside your range is transmitting, so only the destination hears both, messing up both frames
Not everyone can hear everyone: hidden terminals/hosts

Hidden terminal: a transmitting terminal out of your range
A <> B <> C
A sends to B, C cannot sense this
Visible terminal: a transmitting terminal in your range, which is not part of your network
Ideally you would not wait for this, but you have to
A <> B <> C

Distributed Coordinated Function (DCF)

Part of the 802.11 standard
A method for accessing the medium: when can you send a frame?

DIFS: DCF Inter-Frame Space
The amount of time to listen and wait before sending
If the medium seems to be free, send data
SIFS: Short Inter-Frame Spacing
Time required for station to sense end of frame and start transmitting response

If the medium was not free, we back off
Choose a random $r$ in range $[0,CW]$ where $CW$ is the Contention Window
While $r>0$
Sense the medium for one slot
If medium free through slot: $r--$
If $r=0$: transmit frame

Collision Avoidance: Request to Send & Clear to Send (RTS-CTS)

Host sends request to send, hosts respond with clear to send
Guarantee that no collision will happen

Use cases:
Hidden terminals
Critical/large frames

802.11 Frames

Fields are called address 1-4, the actual meaning depends on the application. For frames to/from APs:

1. Destination: MAC address of the AP
2. Source: MAC address of sender
3. Forward: MAC address of actual end device
   If local subnet use the actual MAC
   Otherwise use the MAC of your default gateway

AP generates a new frame with itself as the source

Fourth address is used in the case of frames being passed between multiple radios

802.16 WiMax

Technology created to bridge larger distances
However, this lost to cellular
Almost the entirety of metropolitan areas is served by cell towers