Last Post: July 11:
is there any type of beacon that advertises possible data rate based on the location of a client and environment conditions such as SNR and RSSI?
I am using Ekahau Site Survey and by looking into user manual I read that measured data rate is obtained from beacons.
However, visualized data rate suggests different values on different spots of surveyed area. I was wondering how is that possible because I know beacons advertise only basic and supported data rates which are constant.
Unless some weird RRM is involved, I would say the answer would be NO.
Even though a Beacon advertises a list of rates, it does not mean that either the client or AP is selecting a particular rate for transmission. The rates actually used depend on the "secret sauces" in both the AP and the client. This is especially true when it comes to rate adaptation algorithms during roaming - some are very simplistic.
Mismatched power levels, Hidden Nodes, and poor antenna connections (in the client device) are always suspects in my book.
I have read in Cisco documentation that "The Data Rate used in a Wi-Fi conversation is generally chosen by the devices based on the quality of the connection in terms of SNR.". Does this mean that when station sends its supported data rates to AP in an association frame, these data rates are determined based on the SNR?
Or in other words, does the client take into account SNR to decide if it can achieve, for example, min. of 24 Mbps that was advertised from the AP as a mandatory data rate?
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Back to your question, the answer is no. But before we get into a further discussion, lets assume the everything is OK on the AP side to allow a connection, and that has enough resources available for clients to connect.
The AP always transmits all its available rates in its beacons (depending on how it was configured). The client will eventually respond with a list of all its possible rates (usually defined by its chip set, and not configurable by the user). Basically, if the two lists overlap, and the client attempts to connect, with the proper protocol packets, the AP will allow the connection.
The rates chosen by the AP also depend on several factors, SNR is one of the most important, but definitely not the only one. think errors, and/or noise etc..
The main question is, at what speed to they have to use at to successfully communicate. Some devices may change rates faster and stabie than others to negotiate to that point, but they will both try to communicate at the best rate they think possible.
They exact ways they perform rate adaptation will depend on the "secret sauce", built into each device on each side of the link. Just remember some devices, including AP's, have better "sauces" than others.
Hope this answers your question.
I am confused because I read that cell size can be defined by choosing mandatory data rates - the greater these rates are, the smaller cell size.
I understand that there must be overlapping between client's and AP's data rates list. However, I am curious about client's list. Is this list determined only with respect to supported 802.11x standards by the client?
For example, there is a 802.11g client and AP with minimum mandatory data rate of 24 Mbps. We all know that 802.11g nominally supports such data rate so the client's and AP's data rate lists overlap. Does this mean that the client will associate with the AP despite the fact that in that very moment it can achieve data rates only lower than 24 Mbps because he is far from the AP? If this is the case then I don't get the purpose of mandatory data rates because even if the client is nominally supporting that data rate, in reality it may achieve significantly lower data rates equal to those of 802.11 standards we wanted to disable.
Or, in that very moment, does the client knows it cannot achieve such data rate so the list of supported data rates sent to AP will not include rates greater then 24 Mbps?
The rates advertised by both ends of the link, except perhaps some peculiar RRM setups, do not depend on the distances between the endpoints.
It's not really that the cell size is "defined" by the lowest mandatory rate. It's more like it's limited by the lowest mandatory rate.
For the sake of this discussion, lets ignore frequency band, output power, receive sensitivity, multi-path interference, and MIMO radios. Also remember that cell shapes are not really perfect circles.
If we assume that the IEEE sensitivity requirements mimic receive sensitivity performance figures (and they do), the SNR requirement for 1 Mbps is 4 dB, but for 24 Mbps it is 12 dB. What this implies is that is easier to receive 1 Mbps signals than it is to receive at 24 Mbps. Likewise, at 54 Mbps the requirement is 21 dB SNR, which has even a shorter range (at the same output power of course).
The difference for 24 Mbps is (12-4) = 8 dB, which ends up creating a range difference of between one-half to one quarter the range for 24 Mbps signals as compared to the 1 Mbps signals.
One rule of thumb is that 6 dB accounts for about twice, or half, the range, depending on sign.
Since Beacons are sent at the lowest Mandatory rate, a 24 Mbps beacon from an AP would be un-recognized at long distances.
That is what is limiting the cell size.
Howard, your answers have been really helpful. Thank you for the effort.
What will happen to a previously associated client to a AP with a mandatory data rate of 24 Mbps if it goes outside the 12 dB of SNR perimeter?
In addition, perhaps slightly off-topic, how the client indicates its presence to the associated AP to prevent being dissociated? What frame type do they use for this?
It will depend on what the "supported" rates are. There are 3 types of rates : Mandatory, Supported, and by implication, Un-supported rates.
Assume for this example that only 1 AP with the correct SSID exists.
There is really no such a thing as the un-supported rate list, but obviously if its not supported or mandatory, it has to be un-supported.
If the rates lower than 24 Mbps are included in the supported list, then for the most part, the device will stay connected - until it gets to the point that a Mandatory rate is required. For example, if the client tries to perform a passive scan, and can't see any Beacons, and/or then tries an active scan and can't find the correct SSID.
A client does not typically disconnect itself immediately, and will try to "hang around" to see if it re-acquires its connection, i.e. it starts communicating again. Although newer clients can and do send out de-authenticates packets themselves when they are powered off. Older client devices didn't do this, which often meant that an AP would run out of resources (i.e. available client slots in its internal tables).
If the lower rates are NOT supported then after a short delay, the AP will probably send out a de-authenticate command to the client, and remove that device from its internal associated-device list.
Just about any kind of data packet, and some others will keep a link active. In addition, there are also so-called "keep alive frames, usually at an application level, that will insure the link stays up. I haven't had to program those myself, so I can't tell you the details of them. Many commercial printers have a "keep alive" option buried somewhere in their firmware.
Let's assume that AP requires data rates 24 Mbps or greater and that the client gets associated with that AP. Then, client walks away from the AP in the area of lower SNR.
Does this mean that the client will eventually get disassociated because it won't be able to properly receive frames anymore from the AP at a minimum data rate of 24 Mbps?
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