In Part 2 of our series, Tim Higgins discovers an embarrassingly low level of interoperability among draft 802.11n products. And they're not very neighborly, either.

i get the idea...

stick to 802.11g

it still rules king

as none of this kit is shipping yet, shouldn't we be hoping that the interoperability issues will be sorted by the time it hits the UK?
54g rules? Uh huh.

You can get up to 108

I'm particularly interested in the interference between draft n and 802.11g equipment. The throughput graphs shown here are quite dramatic with the n network killing the g network my question is what happens if things are done the other way around?

What happens if the n network starts first then the g network tries to transfer?

Also what happens with two g networks, both on channel 6. Do they share the channel or does the newcomer kill the existing transfer as seen with the n networks?

That effect is illustrated in the second half of the graphs.

Two WLANs of any flavor (11b, g, n) on the same channel will battle for bandwidth. Stronger signal tends to win over weaker, but both will suffer high packet loss, which results in lower, more erratic throughput.

That effect is illustrated in the second half of the graphs.
Is it really? I thought that was when the n transmission just stops, not when another g or the same g transmission starts again?

Two WLANs of any flavor (11b, g, n) on the same channel will battle for bandwidth. Stronger signal tends to win over weaker, but both will suffer high packet loss, which results in lower, more erratic throughput.

The thing I'm having difficulty with is whether those graphs show a draft 802.11n problem or just a problem common to all WLAN deployments sharing the same channel.

The chart shows:
- G transmission starts
- N transmission starts 10 seconds later
- G transmission stops shortly after N starts
- N transmission continues by itself
- G transmissions starts again while N is running
- N transmission stops
- G transmission stops

The problem that all draft 11n products have is that no matter what channel they are set to (in this case Channel 1), in 40 MHz mode they will interfere with in-range 11b/g WLANs set to Channel 6.

The problem that all draft 11n products have is that no matter what channel they are set to (in this case Channel 1), in 40 MHz mode they will interfere with in-range 11b/g WLANs set to Channel 6.

Ah okay - so this isn't really a new problem, any WLANs set on the same channel cause problems. In any random deployment though, n equipment is twice as likely to cause a problem since it uses twice the bandwidth and 100% likely to cause a problem with other networks using the middle channels.

I wouldn't say this was a problem with 802.11n just the inevitable outcome of using twice the bandwidth, compensated for by gaining the associated increased throughput.

Excellent articles (Part 1 & 2 and Rangemax 240 review). Unfortunatly I can't take your recommendation of wait on buying any 802.11n equipment! My neighbour has recently upgraded from 802.11g to something newer which has killed my 802.11g router to the point that it won't even work as a wired switch. I've tried all available channels, but about 20 seconds after a reset, the wireless led flashes like mad and it stops functioning.

New wireless routers killing neighbours' 802.11g networks is actually good business for the same companies that are causing the problem in the first place and a big disincentive for them fixing the problem.

I can't change to 802.11a because I have non-upgradeable 802.11g boxes (e.g. squeezeboxs). So I need to upgrade to MIMO. After reading your reviews, I was going to buy a RangeMax 240 Router and a couple of PC Card adaptors for my laptops, but you mentioned that the whole network's speed is brought down if you have legacy 802.11g equipment!

More and more people are going to be in a similar situation to me and be forced to upgrade as neighbours buy 802.11n equipment, so could you please recommend which is the best solution for when you need a pre-802.11n solution that works well next to other pre-802.11n networks, and works well with our existing 802.11g stuff?

Unfortunately, there is no such solution. The draft 11n gear really shouldn't have been allowed on the market, given its ability to interfere with 11b/g gear set to Channel 6.

Just as Atheros was forced to modify its Super-G channel bonding technique so that it would back off in the prescence of neighboring WLANs, 11n will eventually be changed to also play nice with 11b/g. The problem, of course, is that in the meantime, there will be plenty of "bad neighbor" gear sold that will cause problems.

You can try some of the techniques in this article
http://www.tomsnetworking.com/2004 [...] n_collide/
but if the 11b/g spectrum is crowded, your only real choice (aside from putting up aluminum foil wallpaper) is to move to 802.11a and find some other way (HomePlug, perhaps) to connect in the non-upgradable gear.

That's interesting, are you sure it's interference from your neighbour? Have you tried your router somewhere else or asked your neighbour to turn their equipment off for a test?

Are your problems intermittent or 24/7? If it was an interference problem surely it would only be seen when the neighbours equipment was transmitting which is wouldn't be doing all the time.

He used to use an 802.11g router because I used to see his SSID transmitted, then my wife saw him receive a delivery of computer kit and the SSID disappeared soon after. That's when the problems started. I should talk to him, but don't want to introduce myself with a complaint!

My network had worked perfectly for years, then it mostly stopped working, then the last couple of times I've tried to sort it out, the complete shutdown. The shutdown only ocurrs with wireless enabled.

What doesn't make sense is that the problems ocurr accross all channels!

I don't want to buy a replacement 802.11g router incase that has the same problem, so I'm looking at pre-802.11n or the RangeMax 240 even with their problems.

I had considered HomePlug to link the two floors of my house and then wire everything up, but I hadn't thought of combining that with 802.11a which would still allow me to use my laptops wirelessly. But if I have to upgrade anyway, I'd like to try and get a speed boost.

I'd also thought of aluminium wallpaper or the radio blocking paint you can get, but surely these new routers aren't so bad?

Trying to "out-shout" or jam the other guy's signal is a lose-lose. You might get temporary relief, but operation won't be reliable enough for streaming.

The best way is to either find a way that both of you can be happy sharing the 2.4GHz band, or implement the previous suggestions.

Your article talked about n and g interoperability, but not n and n. I was hoping that two neighbouring recent MIMO technology networks would play nicely with each other.

Thanks for taking the time to answer my questions by the way!

The article described both "draft n and draft n" AND draft n /g interoperability and also covered draft 11n and Airgo MIMO interoperability.

The "n and n" interoperability is described in the tables on Page 3:
http://www.tomsnetworking.com/2006 [...] page3.html

Sorry, I didn't make myself clear.

As I understand the tables on page three, they demonstrate interoperability between different pre-n and Airgo MIMO router and wireless card combinations. I can't find a reference to competing pre-n / Airo MIMO network interoperability.

I was hoping two different pre-n networks in close proximity would settle down so that both networks worked (by channel hopping away from each other?), as opposed to competing pre-n and g networks where the pre-n effectively kills the g network.

The scenario that you're looking for isn't directly shown, but can be inferred from the tests that were done.

All products tested so far that use a 40MHz bandwidth will interfere with each other if set to use pretty much anything other than one on channel 1 and the other on Channel 11. This is because they default to using 40MHz of bandwidth and don't shift back to 20 MHz mode when detecting any other active wireless LAN.

Hi all,

I have some old D-Link kit from way back when they first introduced A/B dual-band products... I find my D-Link A/B/G tri-band WAP is killed by 2 neighbouring G access points (I live in terrace) in the 2.4Ghz spectrum.

However when I switch to A I am cruising at 54Mbps as nobody uses the 5Ghz band... The range is not great but at least the signal is not wiped out!!

It is worth considering changing to the 5Ghz band if you are getting lots of interference in 2.4Ghz band.

I'll stick to my mainly wired home gigabit ethernet network... At least it works 24/7, gives much better thru-put and doesn't drop out every so often!! Wireless networking is like Windows flakey and insecure!!

Bob Wya

Tim,

Excellent job!

Ultimately, there should be some civil or regulatory liability of these vendors that jam the other lawful 2.4 GHz users off the air.

It would also be interesting to have WECA comment on whether they are going to put these same interoperability tests in the eventual 802.11n certification.

Robb