Wednesday, February 7, 2018

Chapter 2 - Beacon Frames

Beacon frames are a special type of 802.11 management frames. They contain a number of important information elements regarding the AP or STA's capabilities.

Timestamp - As discussed previously these are used for synchronization

Beacon Interval - Specifies the time between beacon transmissions

Capability Information - This is aptly named since it is used to communication various capabilities. Notably security requirements and if the service set is in an ESS or an IBSS. The book mentions that there are other capabilities advertised here that are specified in the 802.11 standard and its amendments, but does not go into further detail.

SSID - This is the ID or the "name" of the network that the beacon is identifying. I'd say this is a pretty important one.

FH Parameter Set - This is used by old Frequency Hopping STA's

DSSS Parameter Set - An element used by old DSSS PHY methods.

CF Parameter Set - This would be an element used with PCF. However since PCF is not actually used, this isn't used.

IBSS Parameter Set - Only used by STA's participating in an IBSS. Contains the ATIM Window information used for power saving in an IBSS.

TIM - Traffic Indication Map - An Element used by STA's that are using some form of power saving mode. Also the name of the enchanter in Monte Python and the Holy Grail

Supported Rates - This is a list of up to 8 data rates. Some of which would be "Basic" Rates. Defining these Basic rates, in turn defines modulation methods. Which essentially ensures that both the AP and the STA can speak the same language. From those basic rates there are other supported rates, and potentially other modulation methods that the beaconing device supports. But those are not required for a connection. They are just potential modulation methods if both the STA and AP support them.

Extended Supported Rates - This just includes other supported data rates that aren't included in the first 8 supported rates.

ERP Information - This element is only included on the 2.4GHz spectrum. Its used to communicate if a Non-ERP STA is in the cell, a neighboring cell is detected that only allows non-ERP datarates.

RSN - Robust Security Network (RSN) - This element contains information regarding the devices RSN capabilities. Such as their Authentication Cipher, Encryption Cipher, etc.

HT Capabilities - This is used with 802.11n networks, defining maximum MPDU length, short GI, various elements surrounding beamforming, and new to 802.11n (at this point) supported spatial streams.

HT Operation - Further defining some 802.11n capabilities. Such as channels, frequencies, and any protection modes being used.

 VHT Capabilities - Much like HT Capabilities defined 802.11n capabilities, this defines 802.11ac capabilities. Such as max MPDU length, short GI, beamforming options, and supported spatial steams and MCS's

VHT Operation - Defines the 802.11ac channels and frequencies 

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