Poorer than expected throughput

RouterOS General
1

I have a MT point to point link on 802.11G (24Mbps) and i’m getting between 5-10 megs throughput from one to another. The second of the two has an 802.11b AP on another wireless card. From my computer client to that AP i can get about 3 megs. From my computer to the first point to point link (going back over the point to point link), i can only get about 1.5-1.7 megs per second. Any ideas as to why? (got all numbers from mt’s bandwidth test)

My descriptions might not explain it well, so here’s a textual representation:

[MT RouterBoard] ( --ptp-- ) [MT AP] | ---------- My computer
                 [5 to 10 ]            [ about 3 ]
   [---------------- 1.5 - 1.7 megs ----------------]

–Brad

2

Using 802.11g and 802.11b together on an AP can cause problems if you have not separated the channels.

Have you seen how the CPU load is on the AP when you run traffic through?

3

channels are 2432 g and 2462 b. CPU load is not extreme by any means whatsoever.

Maybe its that the point to point is a WDS link, i’m still not for sure about a WDS point to point link.

4

I have good experience with WDS and this should not be any problem.

Can you change your 802.11g PTP to 802.11b and see how the throughput changes?

Perhaps you can post your config?

Here’s a extract from from an article regarding a combination of 802.11b and .11g

The case of 802.11g is particularly complicated. In a “pure” 802.11g environment, its performance is the same as 802.11a. If any legacy 802.11b devices are present, however, 802.11g must protect its OFDM packets from being stepped on by the 802.11b devices that do not understand OFDM. To preserve the listen-before-talk collision avoidance system, the device sending OFDM packets must precede them by a short 802.11b packet announcing that it will be using the channel. This message is called a clear to send (CTS) packet. Sending a separate packet using the old slow modulation system significantly degrades the performance of the 802.11g devices.

The situation becomes worse when 802.11b devices are transmitting data in the environment. The MAC protocol gives these devices a nearly equal chance of transmitting at any given time. Because the channel is shared among all the devices in time, the long slow packets of the 802.11b devices reduce the total throughput that can pass through the channel. The values shown in Table 1 assume one 802.11g station and one 802.11b station, each trying to transmit as much as possible. The throughput shown is the total net throughput for both devices.

Here’s the link:
http://www.acmqueue.org/modules.php?name=Content&pa=showpage&pid=34&page=4