WiFi Flashcards
What are two categories WiFi standards fall under?
- Spread spectrum based: 802.11b
- OFDM based: 802.11 a/g/n/ac/ax/ad
What is multi path?
Signal from transmitter arrives at receiver through many paths
Why OFDM?
- Inter Symbol Interference -> time-domain effect of multi path
- Frequency selectivity -> frequency-domain effect of multi path
What is OFDM based wifi?
- Parallel data transmission on several orthogonal subcarriers with lower rate
- Maximum of one subcarrier frequency appears exactly at a frequency where all other subcarriers equal zero
What is Inter Carrier Interference (ICI)?
Zero ICI achieved if OFDM symbol is sampled exactly at its center frequency.
Why does WiFi consume so much power?
The chip constantly keeps checking for on-air signal
What are the channel selection?
- 20 MHz bandwidth per channel
- 3 non-overlapping channels
What is UNII?
Unlicensed National Information Infrastructure
5 GHz channels
What is Medium Access Control (MAC)?
- Infrastructure network contain wired network and access points
- There are access points outside of the infrastructure network
- Ad hoc networks are outside of that as well
What are MAC requirements?
- Avoid interference among simultaneous transmissions but enable as many non-interfering transmission as possible and maintain fairness among transmission
- Asynchronous - common clock not used
What is Hidden Terminal Problem?
- Interference: If ‘X’ transmits while A and B are in session
What is the Exposed Terminal Problem?
Even though Y is outside the communication zone of A and B, X is, so Y cannot receive anything from X
What is CSMA/CA?
- WiFi uses Carrier-sense multiple access with collision avoidance protocol which listens before talking
- Solves Hidden and Exposed terminal problems
What is RTS/CTS?
- Any node hearing RTS and CTS will defer medium access
- RTS sent from sender, CTS sent from receiver
Who sends Data and who sends ACK?
- Sender sends Data and receiver sends ACK, happens after RTS and CTS
What is NAV?
Network Allocation Vector
- full exchange with ‘virtual’ carrier sense
- NAV (RTS) and NAV (CTS)
What is the WiFi Direct feature?
- International standard established by WiFi alliance
- WiFi connection without a wireless access point
- Using WiFi protected setup
- No extra hardware required
- Supported in android
What is Dual-Band feature?
Simultaneously active in both 2.4 GHz and 5 GHz
What is Tethering Feature?
- Connecting one device to another (Hotspot)
- Allows sharing internet connection of the phone with other devices
How does WiFi based location tracking work?
- WiFi triangulation works similarly to GPS: requires 3+ points of reference to calculate a location
- Each AP records RSSI (Received Signal Strength Indicator) readings from device and transmits RSSI information to WLAN controller which uses a locating algorithm to determine device position
What is Location Fingerprinting?
A technique for location sensing based on WiFi signal.
Also known as scene analysis or pattern matching technique
What is the two-phase process for Location Fingerprinting?
- Radio map of observed Signal Strength (SS) values from different locations are recorded during offline calibration phase
- SS values observed at a user mobile device are compared to radio map values using proximity matching algorithms to infer current user locations
What is RSSI?
Received Signal Strength Indicator
- Signal strength at antenna
- Measured after ADC
- Gain between antenna and ADC has to be compensated
What is RSSI accuracy in real WiFi chip?
Stand alone module
- Check antenna gain
- Check for any other gain elements between antenna and chip
- Check automatic gain control (AGC) algorithm
