WiFi basic knowledge "turn"

Transferred 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
Operating frequency range	2.4-2.4835ghz	5.150-5.350ghz5.475-5.725ghz5.725-5.850ghz	2.4-2.4835ghz	2.4-2.4835ghz5.150-5.850ghz
Number of non-overlapping channels	3	24	3	15
Physical Rate (Mbps)	11	54	54	600
Actual throughput (Mbps)	6	24	24	100 or more
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. Spectral partitioning

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

1) IEEE 802.11B/G Standard operates in the 2.4G band with a frequency range of 2.400-2.4835ghz and a total of 83.5M bandwidth

2) divided into 14 sub-channels

3) 22MHz per sub-channel width
4) Center frequency interval of adjacent channel 5MHz
5) Multiple adjacent channels exist frequency overlap (such as 1 channels with 2, 3, 4, 5 channels have frequency overlap)
6) only 3 (1, 6, 11) channels are not interfering with each other in the entire frequency band

3. Reception Sensitivity

BER Requirements	Rate	Minimum signal strength
PER (bit error rate) not exceeding 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 operating band at 2.4GHz (2.4ghz-2.4835ghz), with a usable bandwidth of 83.5MHz, China divided into 13 channels, each channel bandwidth 22MHz North America/FCC 2.412-2.461ghz (11 channels) Europe/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
10	2457	2446/2468
11	2462	2451/2473
12	2467	2456/2478
13	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. The STA that can communicate with each other wirelessly can form a BSS (Basic Service Set). If a station moves 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 the STA within an ESS network can communicate with each other, and is a larger virtual BSS formed with multiple BSS with the same SSID. The components that connect BSS are called Distributed Systems (distribution System,ds).
3) SSID

The identity of the service set, all the STA and APS within the same SS must have the same SSID, or they will not be able to communicate.

The SSID is an ESS network identifier (for example:tp_link_1201), Bssid is a BSS identity, 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 is the same within an ESS, but it is not the same for each AP within the ESS as it corresponds to the bssid. If an AP can support multiple SSIDs at the same time, the AP assigns different bssid to correspond to these SSIDs.

BSSID (MAC) <---->SSID

6. AP Type

The FAT AP and fit APs are compared as shown:

7. Three stages of the wireless access process
STA (workstation) initiates initialization, starts to formally use the AP to transmit the data frame, has to pass through three stages to be able to access (the 802.11MAC layer is responsible for the communication between the client and the AP, features including scanning, access, authentication, encryption, roaming and synchronization, etc.):
1) scanning phase (scan)
2) certification phase (authentication)
3) Association (Association)

7.1 Scanning

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

The STA issues the probe request frame in 13 channels in turn, looking for an AP with the same SSID as the STA, and if the AP with the same SSID is not found, the scan continues.

7.2 Authentication

When the STA finds an AP with the same SSID as the SSID, it selects the AP with the strongest signal, and then enters the authentication phase, based on the AP signal strength received. Only sites that are authenticated by authentication can access the wireless access. The AP provides the following authentication methods:
1) Open System identity authentication (Open-system authentication)
2) Shared 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 authentication is passed and the connection phase is entered.
1) STA sends an associated request to the AP
2) The AP returns an associated response to the STA
At this point, the access process is completed, the STA initialization is complete, you can start to transmit data frames to the AP.

7.4 Certification and correlation process

7.5 Roaming process

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WiFi basic knowledge "turn"