The Scanning Table

Let's look at the scanning table in a bit more detail. This table is primarily a list of access point addresses (BSSIDs), and the parameters that the access point advertises. The 802.11 standard lists at least some parameters that may be useful to hold in the client's scanning table, as in Table 1.
Table 1: Scanning table contents from 802.11 
The Ethernet address of the access point's service for this SSID
The SSID text string
BSS Type
Whether the access point is a real access point, or an ad hoc device
Beacon Period
Number of microseconds between beacons
DTIM Period
How many beacons must go by before broadcast/multicast frames are sent
The time the last beacon or probe response was scanned for this client
Local Time
The value of the access point's time counter
Physical Parameters
What type of radio the access point is using, and how it is configured
The channel of the access point
The capabilities the access point advertises in the Capabilities field
Basic Rate/MCS Set
The minimum rates (and MCS for 802.11 n) that this client must support to gain entry
Operational Rate/MCS Set
The allowed rates (and MCS for 802.11n) that this client can use once it associates
The country and regional information for the radio
Security Information
The required security algorithms
How loaded the access point reports itself to be
WMM Parameters
The WMM parameters that the client must use once it associates
Other Information
Depends on the standards that the client and access point supports
This table contains the fields taken from the access point's beacons and probe responses. Most of the information is necessary for the client to possess before it can associate, because this information contains parameters that the client needs to adopt upon association. By looking at this table, clients can easily see which access points have the right SSID, but will not allow the client to associate. Examples are for access points that require a higher grade of security than the client is configured for, or require a more advanced radio (such as 802.1 In) than the client supports. Most of the time, however, a properly configured network will not advertise anything that would prevent a properly configured client from entering.
In addition to all of this mostly static, configuration information that the access point reports, clients may collect other information that they may themselves find useful when deciding to which access point they should associate. This information is unique to the client, based on environmental factors. Generally, this information (not that in Table 1) is far more important in determining how a client chooses where to hand off or associate to. Table 2 contains some more frequent examples of information that different clients may choose to collect. Again, there is no standard here; clients may collect whatever information they want. Roughly, the information they collect is divided into two types: information observed about the access point, and information observed about the channel the access point is on. This split is necessary, because clients have to choose which channel to use as a part of choosing which access point to associate to. Properties like noise floor or observed over-the-air activity belong to the channel at the point in place and time that the client is in. On the other hand, some properties belong directly to the access point without regard to channel, such as the power level at which the client sees the access point's beacon frames. Furthermore, some of the per-access-point information may have been collected from previous periods when the client had been associated to that access point, and measured the quality of the connection.
Table 2: Other possible scanning table contents 
Signal Strength
The power level of the beacon or probe response from the access point
Channel Noise
The measured noise floor value on the channel the access point is on
Channel Activity
How often the channel the access point is on is busy
Number of Observed Clients
How many clients are on the channel the access point is on
Beacon Loss Rate
How often beacons are missed on that channel, even though they are expected
Probe Request Loss Rate
How many times probe requests had to be sent to get a probe response
Previous Data Loss Rate
If associated earlier, how much loss was present between the access point and client
Probe Request Needed
Whether the client needed to send a probe request
The scanning table is something that the client maintains over time, as a fluid, "living" menu of options. One of the challenges the client has is in determining how old, or stale, the information may be—especially the performance information—and whether it has observed that channel or access point long enough to have some confidence in what it has seen. This is a constant struggle, and different clients (even different software versions from the same client vendor) can have widely different ways of judging how much of the table to trust and whether it needs to get new information. This is one of the sources of the variability present in Wi-Fi.

1 comment:

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