wireless LAN Flashcards
2 categories of wireless LAN & components
Ad hoc WLANs: wireless devices communicate directly with each other.
Infrastructure WLANs: wireless devices communicate with other wireless or wired devices through a wireless router or wireless access point.
Wireless routers combine which features into a single device?
Cable/DSL modem, WAP, DHCP server & NAT, switch, firewall.
Wireless access point does what?
Interconnects a wired LAN with a WLAN. It does NOT interconnect 2 networks. The access point connects to the wired LAN. The wireless devices that connect to the wired LAN via the access point are on the same subnet as the access point.
Factors that affect wireless antenna effectiveness.
Distance, coverage pattern, environment (indoor or outdoor), interference with other access points.
2 types of antenna.
Omnidirectional: radiate power at relatively equal power levels in all directions.
Unidirectional: focus their power in a specific direction. This avoids interference with other wireless devices and reaches greater distances.
Wireless standard (IEEE …?)
802.11
2 bands commonly used for WLAN
2.4-2.5 GHz & 5.75-5.875
Non-overlapping channels
1 (2.412 GHz), 6 (2.437 GHz), 11 (2.462 GHz), 14 (2.484 GHz not supported in US)
What cannot be detected wirelessly?
Collision.
CSMA/CA - what is CA?
Collision avoidance. A transmitting device notifies other wireless devices that it is ready to send and it waits for acknowledgement. If it doesn’t receive acknowledgement, it starts the random backoff timer and tries again later.
FDMA
Frequency division multiple access.
TDMA
Time division multiple access.
CDMA
Code division multiple access. Only one channel occupies the entire bandwidth of the link. All stations can send data at same time (no time sharing). Based on coding theory - each sender is assigned a code which is a sequence of numbers called chips.
3 transmission methods
DSSS, FHSS, OFDM
DSSS
Direct-sequence spread spectrum. Modulates data over an entire range of frequencies. The chip rate is much higher than the information signal bit rate.
FHSS
Frequency-hopping spread spectrum. Devices hop between predetermined frequencies or channels.
OFDM
Orthogonal frequency-division multiplexing. Uses relatively slow modulation rate combined with the simultaneous transmission of data over 52 data streams.
IBSS
Independent basic service set (aka ad hoc WLAN). Contains no access points, just wireless devices.
BSS
Basic service set. Typical WLAN with one AP connected to the network.
ESS
Extended service set. Contains more than one AP working together to provide more coverage than a single AP.
RFI
Radio frequency interference. Caused by other devices using similar frequencies to the WLAN device.
Common sources of RFI
Other WLAN devices, cordless phones, baby monitors, microwave ovens, wireless security systems, physical obstacles, signal strength.
SSID
Service set identifier. Human-readable network name. Beacons are periodically broadcast by access points to advertise the presence of the access point to clients.
Active scanning
When a mobile device wants to join a network or moves around in & out of coverage, it sends a probe message with an SSID. AP’s that receive the probe will respond. Mobile selects one of the AP’s (strongest signal) and sends an association request. The selected AP responds with association response.
Passive scanning.
AP sends out beacons which carry information about the access point, including the SSID. Mobiles hear these beacons, and if they find a better AP they may switch to it.
Why no CSMA/CD in WLAN?
answer
WLAN standards and difference.
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