Question How to Get Good Router Wifi Performance with 2 Devices+

Spielwurfel

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Dec 7, 2005
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Hello all

So I was testing my current wifi setup to get a base line, as I am about to replace it for a hopefully better one. While measuring I got some seemingly weird results, and I would like some insights on what I should look for to avoid these kind of performance issues.

While nobody was at home except me (so nobody else demanding from the wifi connection), I got about 90 Mbit/s on Speed Test multiple times, and with two different devices. Then I tested with two devices at same time with Speed Test (on the same server), and the sum of the speed of both devices was 38.6 Mbps (19.3 Mbps + 19.3 Mbps). Why is the speed with both devices at the same time so much lower than with only one device? Could it be an issue with the config of the access point, or is it maybe a limitation it has with multiple devices? Is there any specific characteristic or capability I should look for in the new AP in order to get better performance with multiple devices?

ISP Band: 100 Mbps
AP: TP-Link TL WA850RE (working as an AP, not an extender)
Router: Intelbras RF 301K

Thanks!
 

kanewolf

Titan
Moderator
Hello all

So I was testing my current wifi setup to get a base line, as I am about to replace it for a hopefully better one. While measuring I got some seemingly weird results, and I would like some insights on what I should look for to avoid these kind of performance issues.

While nobody was at home except me (so nobody else demanding from the wifi connection), I got about 90 Mbit/s on Speed Test multiple times, and with two different devices. Then I tested with two devices at same time with Speed Test (on the same server), and the sum of the speed of both devices was 38.6 Mbps (19.3 Mbps + 19.3 Mbps). Why is the speed with both devices at the same time so much lower than with only one device? Could it be an issue with the config of the access point, or is it maybe a limitation it has with multiple devices? Is there any specific characteristic or capability I should look for in the new AP in order to get better performance with multiple devices?

ISP Band: 100 Mbps
AP: TP-Link TL WA850RE (working as an AP, not an extender)
Router: Intelbras RF 301K

Thanks!
WIFI is a shared radio. The TP-Link can only do 1 thing at a time. It can transmit to device 1 OR transmit to device 2 OR receive from device 1 OR receive from device 2. You are trying to saturate the limited WIFI bandwidth with 2 device. They are conflicting. Device 1 wants to transmit but the TP-Link is busy receiving from device 2. So device 1 has to wait. But the response from speedtest comes back for device 2 and the TP-Link has to transmit it. BUT Device 1 still wants to transmit and conflicts. So BOTH Device 1 and TP-Link have to wait a random time. Finally one of them can continue. This is a classic half-duplex (which WIFI is) problem. Collisions. Collisions hurt performance.

The way to "fix" this is to get WIFI that has WAY more bandwidth than the 2.4Ghz WIFI on the TP-Link. You need a dual band WIFI so that you can use 5Ghz.
 

Spielwurfel

Distinguished
Dec 7, 2005
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WIFI is a shared radio. The TP-Link can only do 1 thing at a time. It can transmit to device 1 OR transmit to device 2 OR receive from device 1 OR receive from device 2. You are trying to saturate the limited WIFI bandwidth with 2 device. They are conflicting. Device 1 wants to transmit but the TP-Link is busy receiving from device 2. So device 1 has to wait. But the response from speedtest comes back for device 2 and the TP-Link has to transmit it. BUT Device 1 still wants to transmit and conflicts. So BOTH Device 1 and TP-Link have to wait a random time. Finally one of them can continue. This is a classic half-duplex (which WIFI is) problem. Collisions. Collisions hurt performance.

The way to "fix" this is to get WIFI that has WAY more bandwidth than the 2.4Ghz WIFI on the TP-Link. You need a dual band WIFI so that you can use 5Ghz.

Thanks for the answer. I was aware about this wifi limitation (half-duplex), but I thought it could do a better job on dedicating time to each individual device. But looks like it spends quite some time just changing between devices before starting to send data. Is it right?

I know the TL WA850RE has two antennas for the 2.4 GHz band. If I get a router with more antennas for this band, would this issue be mitigated?
The new router will surely have a 5 GHz band as well, but various devices in the house will still need to use the 2.4 GHz band because that is what they support, and due to signal reach.

I know that adding more routers would be a better solution, but the house is full of limitations in terms of wiring, and I have a very strict budget limitation as well.

Thanks!
 
This is pretty much a unfixable limitation of wifi.

The higher the percentage of the total wifi bandwidth you are using the worse it is at sharing between different devices.

Now maybe if something like speedtest was a constant download with zero upload it would work better but each device is in effect interrupting the routers ability to send data by sending a confirmation that it received the last group of data every couple packets. The router has to wait each time it detects these transmissions.

What makes wifi usage even worse is when the end device might be able to hear the routers transmission but they can not hear each other. The device will listen to see if any one is using the data channel and if it hears someone it will wait (this is why high utilization causes slowness). If it does not hear anyone it will assume it can transmit. This works fine if everyone can hear each other transmission but if to end devices can not hear each other they can both transmit at the same time. When the data gets to the router, which can hear both, the data overlaps and is damaged.

To a point you can buy a router with lots of radio chips. You can put them all on different channels with different SSID for each device. Problem is there is limited radio bandwidth and to get higher speeds devices are using large numbers of channels. On 2.4g there is only 60mhz total and if you use 40mhz channels only 1 communication path can exist without overlap. This of course ignores that your neighbors are all trying to use wifi and stomp on your signals.
 

Spielwurfel

Distinguished
Dec 7, 2005
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18,685
This is pretty much a unfixable limitation of wifi.

The higher the percentage of the total wifi bandwidth you are using the worse it is at sharing between different devices.

Now maybe if something like speedtest was a constant download with zero upload it would work better but each device is in effect interrupting the routers ability to send data by sending a confirmation that it received the last group of data every couple packets. The router has to wait each time it detects these transmissions.

What makes wifi usage even worse is when the end device might be able to hear the routers transmission but they can not hear each other. The device will listen to see if any one is using the data channel and if it hears someone it will wait (this is why high utilization causes slowness). If it does not hear anyone it will assume it can transmit. This works fine if everyone can hear each other transmission but if to end devices can not hear each other they can both transmit at the same time. When the data gets to the router, which can hear both, the data overlaps and is damaged.

To a point you can buy a router with lots of radio chips. You can put them all on different channels with different SSID for each device. Problem is there is limited radio bandwidth and to get higher speeds devices are using large numbers of channels. On 2.4g there is only 60mhz total and if you use 40mhz channels only 1 communication path can exist without overlap. This of course ignores that your neighbors are all trying to use wifi and stomp on your signals.

Thanks for your comments. And by your comments, looks like the 2.4 GHz band is very limited.
You said it is unfixable, but can it be mitigated somehow / make it less worse?
I would love to wire everything up in here, but I've checked already and it is unpractical, without making tons of holes and leaving lots of wires exposed across the house...
 
If you are willing to live with slower speeds and are willing to buy some more cheap routers/ap you can set up 20mhz radio channels and assign a different SSID to each. Then each device can more or less have dedicated radio bandwidth. This of course ignore that you all your neighbors likely have all their fancy tri-band mesh systems that are using every possible channels more than once.

The only way to fix this is to completely discard the current system and ether use different transmit and receive frequencies and have the router be in control of the radio chips in the end device. Pretty much have to go to a system similar to a cell phone network uses.
But even with all the fancy stuff cell networks have they still have issues with bandwidth utilization. It might share better but it also suffers when there are more users sharing the bandwidth.