WiFi Rates

[tcviper]

Hello all,

What Im missing here is an article explaining details about the Legacy Rate Sets and what is best to choose as basic rates and supported rates.

For 2.4Ghz you usually have the option of 1,2,5.5,6,9,11,12,18,24,36,48,54.

Most wifi access points allow to set some or all as basic supported and as supported rates.

To skip very slow devices I myself have set the following:
Supported: (ALL) 1,2,5.5,6,9,11,12,18,24,36,48,54
Basic: 5.5 and 11

What is best here? (I dont want very old devices to connect on 1 or 2 so thats why i disabled those). In my opinion All can/should be supported but only the basic rates should be enabled as basic, in my case 5.5 and 11. Correct?

Love to hear from more people about this.

 

[stevech]

Any WiFi device should choose the best rate that the received signal quality can support. This changes moment to moment. So all 802.11 hardware does dynamic modulation mode selection.
You should not have to/need to try to disable some rates in this automated process.

 

[thiggins]

Did your settings help, i.e. did you test that 11b devices can't connect?

 

[tcviper]

Actually i enabled all rates now for supported and basic i set to 1,2,5.5 and 11. And that seems to be working fine. (Cisco WAP371)

 

[azazel1024]

What is the use case? generally you want all supported rates, because you want the client to be able to change the modulation rate to what serves it best based on SINR.

That said, sometimes it can be useful to set "newer" requirements on 802.11 supported specifications. You can set, for example, 11n only for 2.4GHz. It will allow lower modulation rates, but it won't allow non-11n clients to connect.

If it is a home network, it really shouldn't matter one whit as you should be able to control what connects. Even if running a guest network, unless totally unprotected, you should never have to worry about 11b clients. With maybe an exception of a tiny handful or odd feature phones, I don't think there have been any "mobile" clients that had anything slower than an 11g chipset in them.

I guess if your friend brings their 10 year old laptop over with its 11b card in it, it might cause problems.

In general I try to "max" what I can in terms of minimum specifications with my basestations as often times it can provide minor increases, like ensuring short guard interval is enabled, 11n only mode (or n/ac for 5GHz), short preamble, etc. Usually you aren't talking about much, but it is enough to generally squeeze another few percent out of your wireless bandwidth and I have no legacy devices, so I don't have to worry about support.

 

[tcviper]

Unfortunately I still have 11g devices in our home (mostly airplay speakers like the B&W series), so i cannot switch to pure 2.4ghz N unfortunately yet.

The lowest I still need at this point is 11g support on 2.4ghz.
Though on the 5Ghz AC band I have it on A/N which works fine.

 

[sfx2000]

Some good discussion here, a couple of things to consider:

802.11 has a few places to indicated supported rate sets:

1) 11b/g/a - supported rates - this is mostly for pre-G, but this is your basic 1,2,5.5,6,9,11,12,18 (some will show all the way to 54) - there is a limitation on how long that particular item is, so you'll also see...

2) Extended Support Rates - again this is 11 a/b/g - this is for rates that cannot be included in supported rates epigram - 24, 36, 48, 54

3) Short Preamble - this is 11g - 11b doesn't support short preambles typically, so if you have an 11b STA attaching, you might see Barker Preamble Mode in use if "Use Protection" is not set.

If you're doing some packet analysis in Wireshark - you'll see this in the ERP section. For data analysis purposes, if you see ERP (and not HT/VHT), you're looking at an 11b/g Access Point as a general rule - if you see ERP and Short GI in use in the 11b/g sections, it is 11g, and there's only 11g STA's attached to it.

NOTE - having an 11b STA attached on a multiple mode AP (b/g/n) will slow everyone attached to that AP, as they all have to then use the long preamble

NOTE2 - 11b these days are typically only used for older Printers (HP) and some handheld game devices (Nintendo DS as an example).

4) 11n - rate set support in 11n is in the HT Capabilities section for supported MCS rates - you'll also find if the AP is single, two, three or more spatial streams here

5) 11ac - this is all in the VHT capabilities section - there you'll find it in the support Tx/Rx MCS rates (0-9)

It's all good stuff...

As for general guidance, when setting up AP's - I tend to leave them in b/g/n mode for 2.4GHz (disabling turboQAM for AP's that support that) and a/n/ac for 5GHz - most of these goes back to protection mode and overhead if invoked - 802.11n greenfield will fall into CTS to self in the presence of non-HT neighboring AP's - and this is a 20 percent hit on performance.

last note - authentication - some AP's will allow you to use TKIP for WPA2 - don't do this - if done, TKIP becomes the group Ciphering, even if the AP/STA using AES for the pairwise keying - TKIP is vulnerable, and getting there opens up everything - use WPA2-AES only - most modern g/a/n/ac clients support AES and WPA2 - use it, otherwise, for particular use cases leave it open (coffee shop for example, or a guest network on a VLAN)

 

[sfx2000]

Unfortunately I still have 11g devices in our home (mostly airplay speakers like the B&W series), so i cannot switch to pure 2.4ghz N unfortunately yet.

The lowest I still need at this point is 11g support on 2.4ghz.
Though on the 5Ghz AC band I have it on A/N which works fine.

Leave your 2.4GHz in b/g/n mode - you'll be fine

For 5GHz, a/n/ac is actually preferred - 11ac is designed for backwards compatibility, and works best in this mode, it will schedule the Tx frames in such as way - Seen this first hand with my early 11ac testing in mixed mode with 11a and 11n clients on an 11ac AP with 11ac clients also in use.

11ac is remarkably neighbor friendly, at least from my experience with Broadcom and Marvell chipsets, having seen first hand QCAtheros or Quantenna, but I'm assuming they're similar as mixed mode is part of the spec.

 

[tcviper]

Yep, so basically I have this now (same SSID for both bands (thx to band steer)):

2.4Ghz band (with Band steer enabled (so 5ghz capable devices will auto move)):
802.11b/g/n, 20mhz, lower primary channel, channel 6.
Short Guard: Enabled, Protection: Auto, Beacon Interval: 100, DTIM Period: 2
Legacy rates: supported: all of them from 1.1 upto 54, basic: 1,2,5.5,11

5Ghz band (with Band steer enabled):
801.11a/n/ac, 80Mhz, lower primary channel, channel 36.
DFS support: On, Short Guard: Enabled, Protection: Auto, Beacon Interval: 100, DTIM: 2.
Legacy rates: supported: 6 upto 65, Basic: 6,12,24

That's it.

 

[thiggins]

Leave your 2.4GHz in b/g/n mode - you'll be fine

NOTE - having an 11b STA attached on a multiple mode AP (b/g/n) will slow everyone attached to that AP, as they all have to then use the long preamble

These two statements seem contradictory.

 

[sfx2000]

These two statements seem contradictory.

They're not - you want the AP to be in B/G/N mode - so that 11b STA's can see it - remember, the 11B network might not be your own, might be a neighbor.

For own network - try to avoid having 11b STA's attached to a 11n/11g AP - it's old stuff, but still rings very true. If you have an 11b STA, it will use the long preamble, and this negates much of the advantages offered by the later specs - time to modernize

I don't have any 11B stations (clients) these days, but if I did, I would purpose an older 11g router with a different SSID just for them (and run WPA/TKIP if they support it, otherwise WEP, and VLAN them straight out to the internet and keep them off my primary WLAN and LAN).

 

[thiggins]

They're not - you want the AP to be in B/G/N mode - so that 11b STA's can see it - remember, the 11B network might not be your own, might be a neighbor.

I agree with this. But if you don't want 11b pr 11g STAs associating with an AP, how is that done?

 

[sfx2000]

I agree with this. But if you don't want 11b pr 11g STAs associating with an AP, how is that done?

11g doesn't really hurt much - 11b does - best thing there is to _not_ do it, or use WPA2/AES, which most 11b clients cannot suppport.

I'm attaching a pic here - this is a good example of what happens with a 11n AP when an 11b (WPA-TKIP) is attached... (MAC address/SSID stripped to protect the innocent)

1) 11n is in non-HT mixed mode - a bit over additional overhead here

2) See in ERP, this indicates that an 11b STA is attached - that you see barker preamble mode set pretty much confirms this

3) RSN information - again, strongly suggests an 11b client in the group ciphers, along with 11g/11n also in use - AES is available for 11g/n, so a bit less impact - the security consideration with TKIP a concern, as all clients will use this for management and multi-cast. This is an edge that can be peeled up to start cracking the AP.

4) It has WPS enabled - another edge to peel up if it's not patched for the WPS bug

5) The AP is in 20/40MHz mode (auto), operating in 20MHz, but again, this is overhead for the chipset, as it needs to switch RF modes if possible, and also clear channel check before it can

End of the day - that 11b STA is slowing down his network and compromising his security.

 

[tcviper]

So if i purely want 11b devices not to be able to connect i simply disable legacy rates 1.1 and 2?

The lowest that should be able to connect is 11g for the speakers, so.

 

[sfx2000]

Oh, and you'll always have the ERP section 11g and later, it's what it inside that element which is the tell...

 

[sfx2000]

So if i purely want 11b devices not to be able to connect i simply disable legacy rates 1.1 and 2?

The lowest that should be able to connect is 11g for the speakers, so.

Leave the legacy rates there - there's no harm in keeping them. Having them there can be helpful, as you want the 11b client to see the AP, just not try to attach.

As I noted earlier - use WPA2/AES, most 11b clients cannot do this.

 

[tcviper]

Leave the legacy rates there - there's no harm in keeping them. Having them there can be helpful, as you want the 11b client to see the AP, just not try to attach.

As I noted earlier - use WPA2/AES, most 11b clients cannot do this.

Cool thank you Got all that now.

 

[thiggins]

As I noted earlier - use WPA2/AES, most 11b clients cannot do this.

Thanks for clarifying the method.

So what are the rate settings used for?

 

[azazel1024]

It depends on the basestation what you can do.

Disabling 1/2Mbps modes won't prevent an 11b client from attaching (remember, they go up to 11Mbps). What will is setting 11g/n mode only, or if you have no 11g clients, 11n mode only. From what I have seen, it'll still advertise all rates, it just won't allow a non-11g/n client from attaching.

This depends heavily on your basestation what it exposes to the configurator on what they can do. I have several TP-Link routers and they almost all have different settings. One of them allows you to set 11b/g/n, 11b/g and 11n only. I have one that'll only do 11b/g or 11n. One that'll do 11b/g/n or 11b/g only. And lastly I have one that'll do 11b/g/n, 11b/g, 11g/n and 11n only. I've tested a few of them with my clients in various modes and they all seem to work appropriately (I've only tested 3 or 4 combos on a single client where I can control all modes it operates in and I tested on the one router that exposed what I would consider all possible combos (other than 11b or 11g only modes). Worked perfectly to prevent attachement, it did tick a couple of "advanced" feature nits when set in higher modes that the management page doesn't expose to the user and it also advertised all rates still).

 

[sfx2000]

Thanks for clarifying the method.

So what are the rate settings used for?

Goes back to 802.11b:

1) We define the broadcast Rate of the beacon - at least one rate is mandatory because of this - all associated STA's and candidate STA's (present but not attached) must monitor the broadcast beacon frames - since most 11b and later STA's support at least 1Mbps, this is why you see it here.

2) Other rates in this information element - any STA that wants to join must support the basic rates advertised in this information element (this is somewhat implied obviously, as 11b doesn't support 18Mbps for example, but it does support 1,2,5.5,11, an 11g/n might support 6,9,12 in addition to the 11b ratesets

Hope that clarifies things...

Now whether to allow at STA to attach to the AP, this can be done within the AP chipset firmware/real time configs - remember that the STA also presents it's capabilities when it attaches to the AP - so for example, if I only want 11g STA's to attach, I can configure the AP chipset to deny 11b - doesn't remove the need to advertise the ratesets in the beacon as discussed above.

sfx