Wifi performance is slow only when there are multiple radios in one router

Question

I have a wireless network setup like this:

Laptop \  |     Wifi Router A       |     Wifi Router B        |  Wifi Router C |
        \ |     /             \     |     /              \     |   /       \    |
         \|wlan0              wlan1----wlan0            wlan1-----wlan0   eth0--|
          |hostapd     managed mode | hostapd    managed mode  |  AP    Internet|

So basically a straight line of laptop -> router A -> router B -> router C -> internet.

Routers A and B have two 802.11a radios in them and are embedded linux boards with high-power radios (Ubiquiti 600mW). Router C is a Linksys E2500 and connects to the Internet via ethernet. There is no bridging being used, only simple IP routing.

All wifi connections have great signals (at least -60dBm) and very low noise (-99dBm).

For speed testing I am using iperf with a large window size (3MB) and 10 parallel connections, with varying transmit times of 100-240 seconds.

The laptop consistently gets results of 6-8mbps between it and router B (upload and download). I have tried many different setting changes on the wifi side and nothing seems to impact the performance, I even checked things like hardware receive/checksum offloading on the interfaces and those settings do not make a difference either.

However, if I test between the laptop and router A, I get around 30mbps. Testing between router A and router B is also well over 25mbps. But when the laptop must traverse more than one radio, the performance is very bad (6-8mbps). I tested my theory by replacing the link between router A and router B with an ethernet cable, and then the speed tests from the laptop to router B more than doubled, to 25-28mbps consistently.

Why is the performance so bad when packets are sent across more than one radio in a router?

Updated:

What I'm trying to achieve is the end-user client network performance (the laptop) that I gain when connecting the AP's with ethernet... without doing so.

I realize that wifi is half-duplex and the more radios there are in the chain, the worse performance will be.

Is it possible to use seprate radios for transmit and receive to solve the problem?

I have 4 mini-pci slots in each board.

score 3 · Accepted Answer · answered Aug 06 '13 at 07:20

Of course the performance will be degraded after so many traversals. Radio is a shared medium (half-duplex), collisions can happen and to avoid them Wi-Fi uses CSMA/CA. Your performance will at least be degraded more than in half. Every client adds to the latency

score 0 · Answer 2 · answered Aug 07 '13 at 08:56

I don't think it's feasible to separate one radio board to be Tx and one to be Rx, because that would result in a huge amount of traffic across the PCI backplane.

Similarly, I suspect that for whatever reason, a lot of the problems are caused by lots of traffic across said backplane already, resulting in what you percieve as latency.

If I were designing a similar system (I do wireless networks for a living [kinda]).. I'd separate the link radios, in other words, Inbound and Outbound pairs, per building into separate physical boxes, connected back-to-back by gigabit ethernet.

That would have the effect of allowing the network adapters, and network hardware to handle the inter-radio, intra-AP traffic, which I suspect may be the bottleneck. Without more information, it's hard to actually make a good diagnosis. It's also hard to ask for whichever information, because frankly, I don't know. That old adage of, you don't know what you're looking for, but will when you find it.

That said, I don't know whether these router boards expose statistics on backplane traffic volumes and so on.

Alex has a few salient points too, although I'm unconvinced about CSMA/CA (Shouldn't it be /CD?), but it's always a possibility. Wireless communication is by its very nature, lossy, and repeating a signal like this is not unlike photocopying a photocopy, or cloning a VHS tape (Remember those? ;).

To recap, I generally don't like the idea of having two radios on the same board, and would try separating them into different physical devices, interconnected by ethernet.

YMMV.

Wifi performance is slow only when there are multiple radios in one router

2 Answers2