4 ms·

For those curious about the "Wi-Fi spam" comment: even though nothing is connected to the network and it's a hidden SSID, it still has to broadcast beacons ever

by r1ch 5y ago

For those curious about the "Wi-Fi spam" comment: even though nothing is connected to the network and it's a hidden SSID, it still has to broadcast beacons every 100ms. The 802.11 standard says beacons must be sent at the lowest rate the AP supports, so your ~350 byte beacon at 1mbps (2.4 GHz) uses around 5% of the frequency. It doesn't take many SSIDs on the same 2.4 GHz channel to make the throughput fall through the floor. Beacon spam is one of the reasons why 2.4 GHz is practically unusable in dense environments these days. Thankfully for 5 GHz this isn't as bad since the lowest rate is 6mbps and the signal penetrates less. Some routers have the option to disable 802.11b which raises the minimum 2.4 GHz beacon to 6mbps as well, but unless everyone does this it won't make much difference.

londons_explore 5y ago
Would there have been a better way to do design beacons? The general problem is a device needing to advertise itself for other devices to connect to, other devices which might not be capable of any faster rate... Beacons being 350 bytes is pretty stupid though... They could have been 20 bytes long, saying "I'm a device with mac address XXX, plz contact me to know what services I have on offer".
- r1ch 5y ago
  In retrospect, definitely. But a huge part of why Wi-Fi is so successful is the wide compatibility, so we're stuck with a design that is 20+ years old. Beacons also serve to do power saving wake-ups and various other things these days, and there's room to optimize them if the Wi-Fi alliance were stricter - some vendors are broadcasting their manufacturer and model (and sometimes serial number!) every 100ms as well as lots of other unnecessary information elements.
- tradertef 5y ago
  There is out-of-band discovery for devices that support multiple bands. That should help in 6 GHz band but it looks more and more that 2.4 GHz will be only for discovery in the future.. https://www.extremenetworks.com/extreme-networks-blog/the-off-road-trail-to-6-ghz-ap-discovery/ https://www.extremenetworks.com/extreme-networks-blog/the-of...
- SavantIdiot 5y ago
  Beacons also have to broadcast the DTIM which allows stations to come out of standby if there is data waiting for them. This can be up to 128 bytes. But I agree, beacons are stupidly fat. The basic beacon is quite small except for the additional tags. If you fire up wireshark you'll see the bulk of the bytes in broadcasting all kinds of feature nonsense that IMHO should only be broadcast during a request to join. e.g the beacon frame is 24B on my Netgear, but the tagged paramters are 300 bytes. Sure, the SSID and DTIM are in there, but then there's a bunch of extended support rates, RSN info, HT info, and vendor specific stuff (Microsoft WMM/WME parameters, 3Com stuff... huh??)
tradertef 5y ago
Another, maybe bigger, problem is probe requests. A client device, e.g. smartphone, uses "active scanning" where it sends probe packets and asks "is there any AP in this channel?" instead of "passive scanning" where it would wait for beacons. Active scanning has the advantege of being much faster. In passive scanning, client needs to stay 200-300 ms in each channel and that ads up quickly when you consider 2.4 GHz and 5 GHz channels. So clients prefer to use active scanning to quickly discover APs. Some clients send periodic probe requests even when they are connected to an AP in case they need to switch to another AP. All these probe requests together with beacons pollute 2.4 GHz.

by r1ch 5y ago

londons_explore 5y ago

Would there have been a better way to do design beacons? The general problem is a device needing to advertise itself for other devices to connect to, other devices which might not be capable of any faster rate... Beacons being 350 bytes is pretty stupid though... They could have been 20 bytes long, saying "I'm a device with mac address XXX, plz contact me to know what services I have on offer".

r1ch 5y ago
In retrospect, definitely. But a huge part of why Wi-Fi is so successful is the wide compatibility, so we're stuck with a design that is 20+ years old. Beacons also serve to do power saving wake-ups and various other things these days, and there's room to optimize them if the Wi-Fi alliance were stricter - some vendors are broadcasting their manufacturer and model (and sometimes serial number!) every 100ms as well as lots of other unnecessary information elements.
tradertef 5y ago
There is out-of-band discovery for devices that support multiple bands. That should help in 6 GHz band but it looks more and more that 2.4 GHz will be only for discovery in the future.. https://www.extremenetworks.com/extreme-networks-blog/the-off-road-trail-to-6-ghz-ap-discovery/ https://www.extremenetworks.com/extreme-networks-blog/the-of...
SavantIdiot 5y ago
Beacons also have to broadcast the DTIM which allows stations to come out of standby if there is data waiting for them. This can be up to 128 bytes. But I agree, beacons are stupidly fat. The basic beacon is quite small except for the additional tags. If you fire up wireshark you'll see the bulk of the bytes in broadcasting all kinds of feature nonsense that IMHO should only be broadcast during a request to join. e.g the beacon frame is 24B on my Netgear, but the tagged paramters are 300 bytes. Sure, the SSID and DTIM are in there, but then there's a bunch of extended support rates, RSN info, HT info, and vendor specific stuff (Microsoft WMM/WME parameters, 3Com stuff... huh??)

tradertef 5y ago

Another, maybe bigger, problem is probe requests. A client device, e.g. smartphone, uses "active scanning" where it sends probe packets and asks "is there any AP in this channel?" instead of "passive scanning" where it would wait for beacons. Active scanning has the advantege of being much faster. In passive scanning, client needs to stay 200-300 ms in each channel and that ads up quickly when you consider 2.4 GHz and 5 GHz channels. So clients prefer to use active scanning to quickly discover APs. Some clients send periodic probe requests even when they are connected to an AP in case they need to switch to another AP. All these probe requests together with beacons pollute 2.4 GHz.