Basic knowledge of WiFi

turn from: http://blog.csdn.net/myarrow/article/details/7930131

1. IE802.11 Introduction

Standard number	IEEE 802.11b	IEEE 802.11a	IEEE 802.11g	IEEE 802.11n
Standard release Time	September 1999	September 1999	June 2003	September 2009
Working frequency range	2.4-2.4835ghz	5.150-5.350ghz 5.475-5.725ghz 5.725-5.850ghz	2.4-2.4835ghz	2.4-2.4835ghz 5.150-5.850ghz
Number of non-overlapping channels	3	24	3	15
Physical Rate (Mbps)	11	54	54	600
Actual throughput (Mbps)	6	24	24	More than 100
Frequency width	20MHz	20MHz	20MHz	20mhz/40mhz
Modulation mode	Cck/dsss	Ofdm	Cck/dsss/ofdm	Mimo-ofdm/dsss/cck
Compatibility	802.11b	802.11a	802.11b/g	802.11a/b/g/n

2. Spectrum Division

WiFi has a total of 14 channels, as shown in the following illustration:

1 IEEE 802.11b/g standard work in 2.4G band, frequency range is 2.400-2.4835ghz, a total of 83.5M bandwidth

2) divided into 14 sub channels

3 width of each sub channel is 22MHz
4 Central frequency interval 5MHz of adjacent channel
5 adjacent channels have overlapping frequency (such as 1 channel and 2, 3, 4, 5 channel frequency overlap)
6 The entire frequency band only 3 (1, 6, 11) Mutual interference channel

3. Receiving sensitivity

BER Requirements	Rate	Minimum signal strength
Per (BER) no more than 8%	6Mbps	-82dbm
9Mbps	-81dbm
12Mbps	-79dbm
18Mbps	-77dbm
24Mbps	-74dbm
36Mbps	-70dbm
48Mbps	-66dbm
54Mbps	-65dbm

4.2.4GHz China Channel Division
802.11B and 802.11g working frequency band in 2.4GHz (2.4ghz-2.4835ghz), its usable bandwidth is 83.5MHz, China is divided into 13 channels, each channel bandwidth is 22MHz North America/FCC 2.412-2.461ghz (11 channel) European/etsi 2.412-2.472ghz (13 channel) Japan/arib 2.412-2.484ghz (14 channel)

2.4GHz band WLAN channel configuration table
Channel	Center Frequency (MHz)	channel low/High frequency
1	2412	2401/2423
2	2417	2406/2428
3	2422	2411/2433
4	2427	2416/2438
5	2432	2421/2443
6	2437	2426/2448
7	2442	2431/2453
8	2447	2426/2448
9	2452	2441/2463
Ten	2457	2446/2468
One	2462	2451/2473
of	2467	2456/2478
of	2472	2461/2483

5. SSID and Bssid

1) Basic Service Set (BSS)

The basic service set is the basic component of the 802.11 LAN. STA, which can communicate wirelessly to each other, can form a BSS (Basic Service Set). If a station is moved out of the BSS coverage, it will no longer be able to communicate with other members of the BSS.

2) Extended Service Set (ESS)

Multiple BSS can form an extended network, called an extended Service set (ESS) network, where an STA within an ESS network can communicate with each other, and is a larger virtual BSS formed by multiple BSS with the same SSID. The component that connects to BSS is called a distributed system (distribution System,ds).
3) SSID

Identity of the service set, all STA and AP within the same SS must have the same SSID, otherwise communication cannot be made.

The SSID is a network identifier of an ESS (e.g.tp_link_1201), Bssid is the identity of a BSS, BSSID is actually the MAC address of the AP, used to identify the BSS managed by the AP, BSSID and SSID one by one mappings within the same AP. The SSID in an ESS is the same, but the corresponding BSSID is not the same for each AP within the ESS. If an AP can support multiple SSID at the same time, the AP allocates different bssid to correspond to these SSID.

BSSID (MAC) <---->SSID

6. AP Types

The FAT AP and fit AP are compared as shown in the following illustration:

7. Wireless access Process three stages
STA (workstation) to start initialization, the official use of the AP to transfer data frames before, after three stages to be able to access (802.11MAC layer is responsible for the client and the AP communication between the functions, including scanning, access, authentication, encryption, roaming and synchronization functions):
1) Scanning stage (SCAN)
2) certification phase (authentication)
3) Association (Association)

7.1 Scanning

802.11 MAC uses scanning to search Ap,sta search and connect an AP, when the STA roams to find a connection a new ap,sta is searched on each available channel.
1) Passive scanning(features: Find a longer time, but STA power saving)
The network is discovered by listening to Beacon frames sent periodically by the AP, which provides information about the AP and the BSS in which it resides: "I am here" ...
2 Active Scanning (features: can be quickly found)

STA in turn issued probe request frames in 13 channels, looking for the same SSID with an AP, if the same SSID can not find AP, the scan continues.

7.2 Authentication

When the STA finds an AP with the same SSID, in the AP that matches the SSID, select one of the most powerful AP signals and enter the authentication phase, depending on the AP signal strength received. Only authenticated sites are allowed to access wireless access. The AP provides the following authentication methods:
1) Open System identity authentication (Open-system authentication)
2) shared secret key authentication (Shared-key authentication)
3 WPA PSK authentication (pre-shared key)
4) 802.1X EAP authentication

7.3 Association

When the AP returns the authentication response information to the STA, the identity authentication is passed and enters the related stage.
1) STA sends associated request to AP
2 AP return associated response to STA
At this point, the access process is complete, STA initialization completed, you can begin to transfer data frames to the AP.

7.4 Authentication and correlation process

7.5 Roaming Process