Wi-Fi (802.11abgn) - 1

Introduction:

Many will be using Wi-Fi products at home. Routers, Gateways, Stations, Mobile phones etc. Here I am going to share my knowledge on 802.11 WLAN technology. Whatever I have learnt in my 1 year experience, I will try to share with the world, in a simple lucid way.

Some useful links:

http://www.wi-fi.org/

http://www.wifinotes.com/

P.S. Here whatever I am going to share, they are purely based on ieee standards, my testing experience and learnings.

In Short:

To whomever, they are good in wired networks,

Literally there is no difference between wired and wireless networks on the top layer (OSI layers above Data Link Layer). The wireless networks differs only from the data link layer. In Wireless network, almost all the controls and managements are implemented in MAC sublayer. The MAC sublayer is fully responsible for establishing and maintaining the wireless connection. It is the only software layer which differs the wired and wireless networks.

802.11:

The wireless networks over small range is often referred as BSS (Basic Service Set). If you have a Wi-Fi Router or Access Point, to which one or more Laptops are connected, then they together form a Basic Service Set.

There are two types of BSS,

a. Independent BSS (IBSS), it lacks any master device such as Routers, Access Points(AP) or Gateways. It is a simple connection established between two or wireless stations (STA). The Ad-hoc network established between two or more laptops is best example for IBSS.

b. Infrastructure BSS (By default BSS refers to Infrastructure BSS), in infrastructure BSS there is an infrastructure element (e.g. AP, router) to which many STAs are connected and the infrastructure element is responsible for the connectivity and management of the WLAN (Wireless LAN).

For Initial articles, I will be dealing more with BSS than IBSS. Because it is easy to understand the protocols in BSS and later it will be so easy to map the same for IBSS.

BSS:

How Connection is Established:

As told earlier the BSS will have infrastructure elements, which will take care of the establishing and maintenance of the connectivity between different elements of the WLAN.

(i) Every infrastructure element should send beacons periodically to the air. "BEACON" is a Management Frame, which will tell clearly about the configurations of the wireless router such as SSID, channel, security mode etc.

(ii) STA will be sending as "Probe Request" which will have the STAs configuration.

The Beacons and Probe Requests are Broadcasted (Sent for everyone in the Air. The Destination MAC address will be FF:FF:FF:FF:FF:FF).

(iii) If a Network Infrastructure receives a matching Probe Req, i.e. the SSID, security mode and the other wireless parameters should be same, then it needs to response with a "Probe Response". From the Probe Response, the Packets will be unicasted (Sent to proper destination).

(iv) "Auth" Frames will be sent by the STA (will not be sent for OPEN Authentication).

(v) "Association Request" will be sent by the STA. In this stage, the STA would have get tuned to AP's parameters with reference to Probe Response.

(vi) "Association Response" will be sent by the Infrastructure Element to the STA.

(vii) The Key handshakes will take place for establishing the secure connection (This step will not be there for "No Security" connection).

All these connection frames can be analysed with wireless sniffer. Wireshark, Omnipeek, airmagnet are some of the widely used sniffer softwares. Your simple Wi-Fi capable laptop can act as sniffer with this softwares.

In the next article, we will learn about the wireless channels.

Article 2