Maximum Ideal Datarate

Saikat Das · Aug 30, 2014

Hi all,
I am working on a project which extends my home wi-fi network over a distance of 1.5 KM.I have 3 points A,B and C.My aim is to carry the network from A to C while B stands in between.I am placing a 300Mbps router at point A with both antennas directed towards B.At B I have planned to use another 300Mbps router whose 1 antenna will be directed towards point A and the other towards point C(Both the antennas are directional antennas made as Biquad and Dish combination).At point C another Directional antenna(same combination) will be aimed to point B which willbe attached to a 300Mbps router.The other antenna will be providing wi-fi coverage in the vicinity of the router at point C with omnidirectional antenna.All will work in WDS bridge.
My question is what will be the ideal connection speed between point A and C for the best case scenario having high SNR and negligible loss?

Thanks

abailey · Aug 30, 2014

I definitely would not create a network like that. I would use dedicated bridge devices and separate AP's to hand out wifi. You could use 4 Ubiquiti Nanostation Loco M5's or 4 Ubiquiti Nanobeam M5-16's. They are about $65 each. Then put a wireless router or AP at location C for Wifi.
Now to your original question. It depends on if your devices have dual radios. If you devices have single radios then you link speed will be 300. Your throughput will be much less, especially if you are using 2.4Ghz. So if your using 2.4Ghz then your throughput is generally 60% of link speed. So 180Mbit/s. Now wifi is half duplex so to compare it to wired throughput you have to cut it in half, so 90Mbit/s. Ok in your scenario, if each of your devices only have one radio, then you are using them to repeat the signal. Each time you repeat the signal your throughput gets cut in at least half. So you repeat it twice so your max throughput on the other end would be 22.5Mbit/s if you have no interference, perfect signal, and one client device using the wifi at a time.
If you do it the way I talked about above you will not suffer the penalty for relaying. And you would use the 5Ghz band which gives you closer to 70% of link rate. So if you did like I proposed then you would get throughput of around 105Mbit/s max for the clients at C. This all assumes a clear line of site between radios.
If the devices you talk about do have dual radios in them you would not suffer the relay penalty but I would need to know more about the devices before I could calculate a throughput.

Saikat Das · Aug 30, 2014

Dear A Bailey,
Thanks for such an elaborate answer.But I have already bought 4 TP-Link w8968 routers and 1 wr841N.I guess that was a bad decision for my purpose.Actually never heard of ubiquiti and I dont think I can get my hands on them here that easily although I must say I really liked your solution.Another thing is I am an electronic hobbyist,so buying them all set could be a let down for me.
Apart from that,I have a related query for you.Since all of them are 300Mbps,the speed everytime,at each repeater oint will necessarily get halved? Yes they are Half-duplex,so the first repeater ,say at first half will receive and rest half will transmit and the connection from this extended network will have 150Mbps.But the second repeater will have its 2nd half free after receiving the 150Mbps from the first repeater at the first half.So the extended network from the second router will not be having the same speed at 150 Mbps? Or it will get further downgraded to 75Mbps as u suggested?
I stay in a rural area with no public wifi hotspots.Only people like me are there who have ther personal home setup.And very less people use microwave.I guess bluetooth will not create any trouble for me.So apparently I shall suffer less interference.Another thing is I can set the channel to 1 or 11 as I guess most routers come with default channel 6 and people here are not that bothered to spend time setting up a fixed channel

.But I agree that those devices you suggested for 5GHz would have wider channels and better speed.
Again thanks for sharing your knowledge.

abailey · Aug 30, 2014

The TP-Link w8968 has a single radio in it, so you will be repeating the signal and will suffer at least a 50% decrease in throughput each time you repeat the signal. The reason this happens is because Wifi works just like a CB radio or Walkie Talkie. Meaning, being half duplex it can either send or receive information, but not both at the same time (like a CB radio you can talk or listen but not both at the same time). Also only one device can talk at a time (like on a CB Radio, only one person can talk at a time). So since you have only one radio per AP, it takes twice as long to relay the information as it would if you were communicating directly to the AP in destination A. This is because it has to talk to the device first, then stop talking to the device and relay that exact information to the next AP, thus taking twice as long to send the information. Each time you add another relay it does the same thing, thus cutting the throughput in half again. If your AP had multiple radios in it, or if you used multiple AP's per site, then you could talk to the next Ap or device at the same time you are relaying. This would allow you to avoid the 50% decrease each time. Also, since you are relaying with single channel radio's they all have to be on the same channel. This means when any of the radios or the devices on wifi at point C or talking, nothing else in the chain can talk or communicate. This means it can slow things down even more than the 50% because of the relay. At the very least you may want to purchase one more AP for the devices at location C and put it on a different channel from the wireless bridge you have (you will need to connect it to the "bridge AP" at site C with an ethernet cable. That would help the latency problem with everything being on the same channel. Make sure you only use channels 1, 6, or 11 on the 2.4ghz band. All other channels overlap one of those and won't help you.
In fact to get the 300Mbit/s on those AP's it looks like you will have to choose 40Mhz wide channels. This effectivly takes up 2/3 of the available 2.4ghz spectrum. If this is true then you cannot use channel 6, only 1 and 11. So maybe make the wireless bridge channel 1, at 40Mhz wide. If you do put another AP at location C make sure to put i on channel 11 at 20Mhz wide. You cannot run it at 40Mhz wide or it will interfere with the bridge AP's and you will be no better off. This means the maximum link rate to the client will be 150Mbit but that should be fine as it is still way above the bridge speed after you take the penalty for repeating.

Saikat Das · Aug 31, 2014

abailey :

The TP-Link w8968 has a single radio in it, so you will be repeating the signal and will suffer at least a 50% decrease in throughput each time you repeat the signal. The reason this happens is because Wifi works just like a CB radio or Walkie Talkie. Meaning, being half duplex it can either send or receive information, but not both at the same time (like a CB radio you can talk or listen but not both at the same time). Also only one device can talk at a time (like on a CB Radio, only one person can talk at a time). So since you have only one radio per AP, it takes twice as long to relay the information as it would if you were communicating directly to the AP in destination A. This is because it has to talk to the device first, then stop talking to the device and relay that exact information to the next AP, thus taking twice as long to send the information. Each time you add another relay it does the same thing, thus cutting the throughput in half again. If your AP had multiple radios in it, or if you used multiple AP's per site, then you could talk to the next Ap or device at the same time you are relaying. This would allow you to avoid the 50% decrease each time. Also, since you are relaying with single channel radio's they all have to be on the same channel. This means when any of the radios or the devices on wifi at point C or talking, nothing else in the chain can talk or communicate. This means it can slow things down even more than the 50% because of the relay. At the very least you may want to purchase one more AP for the devices at location C and put it on a different channel from the wireless bridge you have (you will need to connect it to the "bridge AP" at site C with an ethernet cable. That would help the latency problem with everything being on the same channel. Make sure you only use channels 1, 6, or 11 on the 2.4ghz band. All other channels overlap one of those and won't help you.
In fact to get the 300Mbit/s on those AP's it looks like you will have to choose 40Mhz wide channels. This effectivly takes up 2/3 of the available 2.4ghz spectrum. If this is true then you cannot use channel 6, only 1 and 11. So maybe make the wireless bridge channel 1, at 40Mhz wide. If you do put another AP at location C make sure to put i on channel 11 at 20Mhz wide. You cannot run it at 40Mhz wide or it will interfere with the bridge AP's and you will be no better off. This means the maximum link rate to the client will be 150Mbit but that should be fine as it is still way above the bridge speed after you take the penalty for repeating.

Again many thanks for spending so much time to clear my doubts.I will surely follow your solution and put another router to locally broadcast at point C.\
But I have another question to ask.You said if I use multiple APs then 50% reduction in speed at each repeating point can be overcome.My question is even if I want to use two 300Mbps routers at point B,how both will be connected together?They are not Gigabit router so if I connect them together by ethernet cable,the speed will be limited to ethernet's max speed of 100Mbps right?

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Maximum Ideal Datarate

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