How does a Wireless client discover Wireless Router ??
The IEEE 802.11 standards define two methods for a client device to discover wireless networks in the area. Both methods are based on using 802.11 management frames as defined in these standards.
The first method is a passive method. All 802.11 infrastructure devices will send out a beacon frame approximately every 100ms (this is the default for many devices, but often can be configured either higher or lower) for each wireless network that provides service at the time. These beacon frames will contain information about the wireless network such as the SSID (aka network name), encryption details (if any), supported data rates, etc.
Wireless clients that are in the area will be able to "hear" the beacon and know the network is in the area.
The second method is an active method, and is the one most often used by operating systems. The client device can send a probe request frame either generally ("Is anyone out there?") or to a specific network ("Hey, Bill, are you out there?"). An access point (any in the first case, matching ones in the second) will respond to this probe request with a probe response that will contain information similar to a beacon frame.
This probe request/probe response process is also part of the association process by which a station joins a wireless network.
The reason that the active method is generally preferred by most operating systems is obvious when one sits down to think about it. To discover a network passively, the device will have to listen on a channel long enough to stand a good chance of hearing a beacon frame. Since a beacon is only sent every period of time, a device may have to listen (and only listen) for 200ms or more to make sure they hear all the networks. If you only consider 2.4GHz, that is at least 11 channels...do the math.
The active method allows a station to switch to a channel, send a probe request, pause briefly (less than 100ms) and then move to the next channel. This makes this a faster process while still being fairly confident it has found the networks on that channel.
Neelkanth $:iwlist frequency
eth0 no frequency information.Channel 01 : 2.412 GHz
Channel 02 : 2.417 GHz
Channel 03 : 2.422 GHz
Channel 04 : 2.427 GHz
Channel 05 : 2.432 GHz
Channel 06 : 2.437 GHz
Channel 07 : 2.442 GHz
Channel 08 : 2.447 GHz
Channel 09 : 2.452 GHz
Channel 10 : 2.457 GHz
Channel 11 : 2.462 GHz
lo no frequency information.
wlan0 11 channels in total; available frequencies :
Current Frequency:2.427 GHz (Channel 4)
Neelkanth $:iwconfig
eth0 no wireless extensions.
lo no wireless extensions.
wlan0 IEEE 802.11 ESSID:"sparrow"
Mode:Managed Frequency:2.427 GHz Access Point: 1C:67:58:72:5C:04
Bit Rate=72.2 Mb/s Tx-Power=31 dBm
Retry short limit:7 RTS thr:off Fragment thr:off
Power Management:on
Link Quality=44/70 Signal level=-66 dBm
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:0 Invalid misc:0 Missed beacon:0
Usage: iwconfig [interface]
interface essid {NNN|any|on|off}
interface mode {managed|ad-hoc|master|...}
interface freq N.NNN[k|M|G]
interface channel N
interface bit {N[k|M|G]|auto|fixed}
interface rate {N[k|M|G]|auto|fixed}
interface enc {NNNN-NNNN|off}
interface key {NNNN-NNNN|off}
interface power {period N|timeout N|saving N|off}
interface nickname NNN
interface nwid {NN|on|off}
interface ap {N|off|auto}
interface txpower {NmW|NdBm|off|auto}
interface sens N
interface retry {limit N|lifetime N}
interface rts {N|auto|fixed|off}
interface frag {N|auto|fixed|off}
interface modulation {11g|11a|CCK|OFDMg|...}
interface commit
Neelkanth $:iwlist auth
eth0 no authentication information.
lo no authentication information.
wlan0 Authentication capabilities :
WPA
WPA2
CIPHER-TKIP
CIPHER-CCMP
Neelkanth $:iwlist event
eth0 no wireless event capability information.lo no wireless event capability information.
wlan0 Wireless Events supported :
0x8B04 : Set Frequency/Channel (kernel generated)
0x8B06 : Set Mode (kernel generated)
0x8B15 : New Access Point/Cell address - roaming
0x8B19 : Scan request completed
0x8B1A : Set ESSID (kernel generated)
0x8B2A : Set Encoding (kernel generated)
wlan0 Scanning capabilities :
- ESSID
Neelkanth $:iwlist power
eth0 no power management information.
lo no power management information.
wlan0 Current mode:on
Neelkanth $:iwlist txpower
eth0 no transmit-power information.
lo no transmit-power information.
wlan0 unknown transmit-power information.
Current Tx-Power=31 dBm (1258 mW)
Neelkanth $:sudo iwlist wlan0 scan
wlan0 Scan completed :Cell 01 - Address: 1C:67:58:72:5C:04
Channel:4
Frequency:2.427 GHz (Channel 4)
Quality=46/70 Signal level=-64 dBm
Encryption key:on
ESSID:"sparrow"
Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 18 Mb/s
24 Mb/s; 36 Mb/s; 54 Mb/s
Bit Rates:6 Mb/s; 9 Mb/s; 12 Mb/s; 48 Mb/s
Mode:Master
Extra:tsf=0000000000000000
Extra: Last beacon: 30ms ago
IE: Unknown: 000773706172726F77
IE: Unknown: 010882848B962430486C
IE: Unknown: 030104
IE: Unknown: 050400010000
IE: Unknown: 2A0104
IE: Unknown: 2F0104
IE: IEEE 802.11i/WPA2 Version 1
Group Cipher : CCMP
Pairwise Ciphers (1) : CCMP
Authentication Suites (1) : PSK
IE: Unknown: 32040C121860
IE: Unknown: 2D1AFC181BFFFF000000000000000000000000000000000000000000
IE: Unknown: 3D1604000400000000000000000000000000000000000000
IE: Unknown: 4A0E14000A002C01C800140005001900
IE: Unknown: 7F0101
IE: Unknown: DD180050F204104A00011010440001021049000600372A000120
IE: Unknown: DD090010180206F02C0000
IE: Unknown: DD180050F2020101800003A4000027A4000042435E0062322F00
Cell 02 - Address: 0C:D2:B5:94:6F:1B
Channel:11
Frequency:2.462 GHz (Channel 11)
Quality=25/70 Signal level=-85 dBm
Encryption key:on
ESSID:"nas2013"
Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 6 Mb/s
9 Mb/s; 12 Mb/s; 18 Mb/s
Bit Rates:24 Mb/s; 36 Mb/s; 48 Mb/s; 54 Mb/s
Mode:Master
Extra:tsf=0000000000000000
Extra: Last beacon: 30ms ago
IE: Unknown: 00076E617332303133
IE: Unknown: 010882848B960C121824
IE: Unknown: 03010B
IE: Unknown: 2A0100
IE: Unknown: 32043048606C
IE: Unknown: 2D1AEF191FFFFF000000000000000000000000000000001804810800
IE: Unknown: 3D160B000000000000000000000000000000000000000000
IE: Unknown: 4A0E14000A00B400C800140005001900
IE: WPA Version 1
Group Cipher : CCMP
Pairwise Ciphers (1) : CCMP
Authentication Suites (1) : PSK
IE: Unknown: DD180050F2020101800003A4000027A4000042435E0062322F00
IE: Unknown: DD1E00904C33EF191FFFFF000000000000000000000000000000001804810800
IE: Unknown: DD1A00904C340B000000000000000000000000000000000000000000
IE: Unknown: DD0600E04C020160
IE: Unknown: 7F080100000000000000
Cell 03 - Address: E8:94:F6:4A:24:30
Channel:6
Frequency:2.437 GHz (Channel 6)
Quality=18/70 Signal level=-92 dBm
Encryption key:on
ESSID:"THIRUMALAI"
Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 18 Mb/s
24 Mb/s; 36 Mb/s; 54 Mb/s
Bit Rates:6 Mb/s; 9 Mb/s; 12 Mb/s; 48 Mb/s
Mode:Master
Extra:tsf=0000000000000000
Extra: Last beacon: 23060ms ago
IE: Unknown: 000A54484952554D414C4149
IE: Unknown: 010882848B962430486C
IE: Unknown: 030106
IE: Unknown: 2A0100
IE: Unknown: 2F0100
IE: IEEE 802.11i/WPA2 Version 1
Group Cipher : CCMP
Pairwise Ciphers (1) : CCMP
Authentication Suites (1) : PSK
IE: Unknown: 32040C121860
IE: Unknown: 2D1AFE191BFFFF000001000000000000000000000000000000000000
IE: Unknown: 3D16060D0000000000000000000000000000000000000000
IE: Unknown: DD8E0050F204104A0001101044000102103B000103104700107CCBF78E6D00644555719E97E775ED791021000754502D4C494E4B10230009544C2D57523834304E10240006313233343536104200033030311054000800060050F204000110110019576972656C65737320526F7574657220544C2D57523834304E100800022008103C0001011049000600372A000120
IE: Unknown: DD090010180200F02C0000
IE: Unknown: DD180050F2020101800003A4000027A4000042435E0062322F00
Cell 04 - Address: 18:A6:F7:FE:09:A2
Channel:9
Frequency:2.452 GHz (Channel 9)
Quality=55/70 Signal level=-55 dBm
Encryption key:on
ESSID:"Neel_Ironman_priv"
Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 6 Mb/s
9 Mb/s; 12 Mb/s; 18 Mb/s
Bit Rates:24 Mb/s; 36 Mb/s; 48 Mb/s; 54 Mb/s
Mode:Master
Extra:tsf=0000000000000000
Extra: Last beacon: 30ms ago
IE: Unknown: 00114E65656C5F49726F6E6D616E5F70726976
IE: Unknown: 010882848B960C121824
IE: Unknown: 030109
IE: Unknown: 2A0100
IE: IEEE 802.11i/WPA2 Version 1
Group Cipher : CCMP
Pairwise Ciphers (1) : CCMP
Authentication Suites (1) : PSK
IE: Unknown: 32043048606C
IE: Unknown: 2D1A6F111BFF00000000000000000000000100000000000000000000
IE: Unknown: 3D16090F0000000000000000000000000000000000000000
IE: Unknown: 7F080000000000000040
IE: Unknown: DD180050F2020101800003A4000027A4000042435E0062322F00
IE: Unknown: DD0900037F01010000FF7F
IE: Unknown: DDA90050F204104A0001101044000102103B00010310470010000102030405060708090A0B0C0D0E0F1021000754502D4C494E4B10230014544C2D57523734304E2F544C2D57523734314E4410240003362E3010420003312E301054000800060050F20400011011001E576972656C65737320526F757465722057523734304E2F57523734314E44100800020086103C000101104900140024E26002000101600000020001600100020001