Why does client authenticate before associating with the AP. What does open system authentication even achieve. I fail get an accurate reason from my web searches. Best reason I do get is unlike wired medium there is no concept of physical port in wireless medium. This makes sense but still there is one confusion: why don’t we skip authentication and just start with association? The purpose of authentication can be achieved with association itself. No?
Why Authentication Before Association in 802.11 – Explained
ieee 802.11wireless
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This entirely depends on the chipset in your device, as not all of them have equal capabilities on Tx/Rx rates. I personally have found no better resources than wikidevi.com for looking up the capabilities of wireless network adapters.
To understand wireless that is 802.11n or newer, you need to understand the shorthand often used in their technical capabilities, namely AxB:C. A represents the number of Tx radio chains are available to the device, B represents the number of Rx radio chains, and C represents the number of spatial streams.
A device will need to have a number of antennas equal to the highest of either A or B to make use of its capabilities (i.e. I have seen a 3x3:3 adapter in a laptop with only two antennas and in such a case it will operate no better than a 2x2:2 device).
You can never have more spatial streams than you have either Tx or Rx radio chains (i.e. you need at least one radio chain to make use of a spatial stream). You can however have more radio chains than spatial streams, and in these cases they can provide other benefits (resistance to interference, extended range, etc).
To your specific question, I know of several Intel wireless chipsets that do not have equal capabilities on Tx and Rx. The Intel 5100 is an example of a 1x2:2 network adapter. This device can transmit up to 150Mbps and receive data up to 300Mbps.
(Edit: I was mistaken in the second device as I thought it was a 2x3:3, but when I looked at it again, it is a 2x3:2 so correcting that here.) The Intel 4965AGN is an example of a 2x3:2 device. While this device can transmit and receive up to 300Mbps, its actual Rx performance will tend to be better than the Tx due to the added capabilities the extra Rx radio chain provide.
I also believe I have seen other vendors with this type of lopsided Tx/Rx configuration as well, I just don't remember example chipsets off hand.
For your second question, the data rate used by Tx on the device and Tx on the AP can never exceed the capabilities of either device.
The HT Capabilities section provides the MCS values which are supported by the wireless network. These data rates can be used by both the AP and the client to send unicast traffic back and forth.
The HT Information section contains the Basic Data Rates supported (if any). The Basic Data Rates will be a subset of the supported data rates and are used for broadcast, multicast and management traffic.
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This does seem a bit odd. Yes, in modern 802.11 there is an open authentication that takes place before the association request/response. This is also prior to any sort of 802.1X authentication or WPA/WPA2 key exchange.
However, you need to keep in mind that there was an original 802.11 standard from the IEEE. Things like 802.11i (aka WPA2) or 802.11ac are amendments to the existing standard. These amendments for the most part try to extend or enhance the base standard while leaving as much of the existing standard intact to provide backwards compatibility.
So this pre-association authentication takes place because of a legacy feature, namely WEP. This is the point where a WEP client provided it's authentication to the AP. When 802.11i was added to the standard, this was a more involved process for authentication. As such, 802.11i is performed post association, but prior to being given access to the network.
While the new process no longer made use of the pre-association authentication request/response, it was left in the process as an open authentication to provide a consistent experience for clients/APs. It simplifies the process and development if both legacy clients/APs and newer clients/APs can simply expect the authentication/association flow rather than to have to look for two different connection flows.
Note: you can find the full current connection process (with 802.1X) in this Cisco/Meraki document. The initial authentication and association process takes places for all current 802.11 connections.
However, WPA3 proposed by the WFA (which is not an IEEE standard) has for the first time changed this process with the SAE (simultaneous authentication of equals) feature. My understanding is that this changes the 802.11 authentication process from the two frame request/response to a four frame commit/confirm process.
I have yet to see a IEEE working group discussing any changes related to WPA3 at this time.
Update: I did forget to add that the 802.11 authentication is where MAC authentication takes place (mainly forgot since I don't use MAC auth). This is of particular note to modern wireless networking as vendors are adding their own updates for better control of PSKs (Cisco->Identity PSK, Aerohive->Private PSK, Ruckus->Dynamic PSK, MiKroTik->Private PSK, etc).
AFAIK, these wireless platforms all use the 802.11 authentication phase to set the PSK to use for the device in question through some mechanism (varied and non-standardized at this point), all prior to the WPA2 handshake so the WPA2-Personal experience is transparent to the client device.