802.11 WiFi WLANs
IEEE 802.11b 802.11a 802.11g 802.11n
Year 1999 1999 2003 2009
Frequency Band 2.4 GHz (ISM) 5 GHz (U-NII) 2.4 GHz (ISM) 2.4 or 5
# Channels (#non-overlapping) 11 (3) 23 (12) 11 (3) ??
Max speed (Mbps) 11 54 54 600
Speeds req'd (Mbps) 1,2,5.5,11 6,12,24 6,12,24 many
Optional Speeds (Mbps) 9, 18, 36, 48 9, 18, 36, 48 many
Encoding/Modulation DSSS OFDM DSSS (11Mbps) / OFDM (54Mbps) OFDM MIMO / SDM
notes up to 4 streams @40MHz. frame aggregation. larger range.
ISM: industrial scientific medical

Channels: USA: 11; Europe: 13, Japan 14.
Weaker signals data-rate-shift down to lower speeds. transmission by transmission, per client (per frame?)
Higher frequency-->faster but smaller coverage.
Any b WNIC in a g network causes all transmission to drop down to b speeds. AP of g network can disable b mode.
Matter absorbs, reflects, scatters signal.
Radio propagation is complex: sporadic and usually temporary losses due to fading, shadowing, hand off.
Interference from overlapping channels, other machines (Bluetooth, cordless phones, microwave ovens, baby monitors, motion detectors, fluorescent lighting, radar etc.).
2.4GHz band (slightly) larger range than 5GHz band.

b/g: 2.400-2.487GHz. Channels 20/22 MHz wide every 5 MHz
a: 5GHz
n: channel either 20 or 40 MHZ wide (if 40, then only one nonoverlapping in 2.4GHZ band)

Channels 1, 6 and 11 are "non-overlapping" (or do channels have to differ by five or more, e.g. channels 2 and 7 non-overlapping?)

WLAN APs map


WAP like a shared Ethernet hub (half-duplex and only one transmitter at a time): CSMA/CA
transmitter can't simultaneously receive, so no collision detection (CD) possible, thus CA with explicit ACKs.
WAP: bridge to cabled network.

receiver: if no bit errors (i.e. does a checksum) then send ACK frame immediately.
sender: if don't get ACK then assume failure and resend

optional (configurable in WAP): RTS/CTS (Request To Send / Clear To Send) frames exchange between nodes to solve "hidden node" problem.

ad hoc (Independent BSS [basic service set]). No WAP. Stations transmit directly to [all] other stations. No connectivity to other networks.
infrastructure: all communication via WAP. and to connect to LAN. BSS/cell
ESS [extended service set]: multiple WAPs (different channels) for roaming (with same IP address)
client/station associates with AP.

SSID 32 ASCII character: same for all APs of ESS.
broadcast by AP in periodic beacon (100 ms) or turned off, appears as "unknown network" or nothing depending on client. AP can have multiple SSIDs (each a different MAC) but still one channel.
APs on same channel can work (slowed). APs on overlapping channels will interfere because frames are not clearly received.

site survey for placement of APs: measure signal strengths.

Microsoft Zeroconf (or WNIC software) scans channels, find SSIDs, connects to strongest or preferred or chosen.

WLAN Security
Standard Year Key distribution Device authentication User authentication Encryption
WEP 1997 static yes (weak) none yes (weak crackable 40-bit)
Cisco 2001 dynamic yes yes yes (TKIP)
WPA 2003 both yes yes yes (TKIP)
WPA2 802.11i 2005+ both yes yes yes (AES)
WPA2 not backward compatible with WPA.

Discourage general users:
SSID cloaking: not sent in beacon (or no beaconing?). (but client with null SSID sends Probes.)
MAC filtering: AP has list of allowable MACs. (but hacker can spoof MAC.)

Frames: similar to Ethernet but more complex. 3 types: management, control, data.
Fields: 72/144b Preamble, MAC header, LLC, 2304B Data, 4B FCS.
Header: frame control, duration, Dest MAC, AP MAC, Src MAC, Sequence.

Management frames between AP and client:
Beacon: broadcast by AP (typically every 100ms), MAC of AP, SSID, supported data rates. Can be turned off.
Client listening on all channels for beacons: passive scanning. If no beacon detected or doesn't want to wait, can send Probe Request (active scanning) either to specific SSID or "any". AP can respond with Probe Response unicast with similar info as beacon.
Authentication frames, optionally.
Association Request from client. Response from AP.

Control frames:
ACK 13B contains MAC of intended receiver. Acknowledging successful receipt of previous frame.
RTS and CTS optionally to solve: 1. hidden node problem (manually configured in AP?) and 2. "protection" in mixed b/g networks

2.4GHz, cable replacement, piconet, 802.15 wireless PAN.
low power, short range, low rate.
FHSS (frequency hopping spread spectrum)

WiMax / WirelessMAN 802.16
alternative to cable and DSL for last mile broadband access.
various licensed spectrum. very new: trials underway.

Cellular telephone...
3G 384Kbps ?

[Future] wireless devices: appliances, cameras, transportation, security, pets, plants, humans... Internet of Things (IOT)