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How fast? by Mike Everest

Q: I am finally done with the configuration. However, when I test file transfer through the backhaul in door (2 711G on the same table), the speed is only around 40Mbps (i.e. 4-5MB/s). What is the max speed of 711G? 300Mbps (802.11n), 150Mbps, or 54Mbps(802.11g)? We are using it to transfer video camera data, and need up to 100Mbps. Thank you very much!


Answer:

300mbps is only the ‘air rate’ of the link, sometimes also referred to as ‘carrier’ or ‘oscillation’ rate.  The true data rate of actual payload is always significantly lower.

At optimum throughput, routerOS 802.11n using dual channels (40MHz in total) and dual chains (2x2 mimo) the maximum data rate for UDP traffic is about 220mbps.

Maximum TCP throughput is about 100mbsp – in one direction only.

Note that due to the nature of wireless, these data rates are one-way only (i.e. not full duplex)  transmitting data in both directions at once, you will achieve much less than half of the total one-way capacity.

For example, on a perfect link that can support 200 meqabits of udp, loading up the link in both directions will generally result in 75mbps or less each way.  Running TCP in both directions at once will deliver 40 megabits or less.

In order to achieve maximum throughput, you need a few conditions to be just right:

1.    Very low background noise: sgnal to noise ratio must be at least 40dB ideally more.
2.    Dual channel 802.11n – i.e. a 20 MHz wide primary chanel with a second 20MHz extension channel.  This is sometimes referred to as ‘40MHz channel width’ (though technically it is really two independent channels) In routerOS, it is called ‘40MHz HT above’ (or ‘40 MHz ht below’) – choose the option that does not cause you to operate beyond the allowed spectrum rules.
3.    Dual chain (MIMO) operation – MIMO systems have two antennas.  To get maximum transmission rates, the system needs to use both antennas at the same time.  In routerOS, you need to enable ht0 *and* ht1 for both tx and rx (find it on the ‘ht’ tab of wireless interface properties)
4.    Dual antennas must have maximum isolation (i.e ‘cross polarised’) typically, a 2x2 mimo antenna has both horizontal and vertical antennas built-in to the antenna design.
5.    Antenna alighnment must be close to perfect.  At each end of the link, the antennas must be aligned EXACTLY – even if the remote antenna is rotated just 1 degree, this can often have very significant impact on the performance capacity.

I hope it all makes some sense to you – depending on your current setup, 40mbps may be good or not good – if you are unsure, please describe your setup in detail, and we can offer further comment.

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