Wireless performance on mixed networks: answers!

[seanadams]

I recently met with Rick Bahr, VP of engineering at Atheros, and was able to get clarity on a few of the questions that frequently come up here about wireless performance.

Q: Does the mere presence of an 802.11B device slow down an otherwise all-G or all-N network?

A: YES This is of course already well known, although the exact implications are often misunderstood. The presence of an 802.11B device on an G or N network causes the newer devices to have to resort to some kludgy behavior to make sure that the B devices don't transmit when the G/N devices are using the airwaves, and to make sure that both the B and G/N devices can see things like beacon packets.

The exact impact on throughput is hard to estimate generally, but it will NOT "slow the whole network to 802.11B" as is often stated. There is however a significant slowdown imposed by the mere presence of a B device, even when it is not active. We (Slim) did some testing of this a couple years ago and found that usually the throughput between the G devices dropped by 30-50% (eg from 20Mbps to 10Mbps), but not nearly as low as the speed of a B-only network (5Mbps in the same environment). The theoretical maximum throughput on 802.11g is 23 Mbps without any B devices associated, and 14Mbps with.

So upgrading any 802.11B devices on your network will most definitely improve the throughput of your wireless SB3 or Transporter, along with any other G or N devices.

The above is not really new information but it was nice to get an authoritative answer that agreed with what we've found in practice. However the next answer is more interesting and contradicts some oft-stated myths that were a holdover from the 802.11B issue.

Q: Will 802.11G devices slow down an all-N network?

A: NO, except insofar as the air-time that they take when active will be at the G throughput level as opposed to the N level. I.e. the devices still each communicate at their optimal rate in each time slice.

Unlike in the 802.11B backward compatibility mode, G devices do not impose any performance-degrading behavior on N devices in order for them to be backward compatible. 802.11g devices are able to recognize the 802.11n preamble, and they play nicely in terms of knowing when one or the other is trying to transmit. The preamble tells which modulation scheme will be used, so the N devices can speak N, while G devices can speak G. They don't have to resort to "Esperanto" as with B in order to cooperate. This means that when the G device is associated but not active, it has no impact at all. When the G devices are active they will consume air time roughly in proportion to the amount of data being transferred. This air time would of course be at the G rate as opposed to the N rate, so in the event that the airwaves are fully saturated (eg by a local file transfer), there would be some reduction in the total Mbps achievable by all devices collectively, but there is no penalty for having the G devices associated.

Q: Is having a (draft) 802.11N access point advantageous, even if most or all clients on the network are 802.11G?

YES, primarily because 802.11N radios have the benefit of more sophisticated multipath reception capability. They can thereby extend the range and throughput available to G devices to some degree.

Q: Why do so few new devices (aside from APs) feature 802.11N?

A: Several reasons:

- Many of these applications would not benefit at all from having higher throughput.
- Since G plays well with N, there would be little benefit from the network's perspective.
- N chips are more power hungry, reducing battery life
- N chips are more expensive
- The N standard is new and is not yet finalized

In conclusion:

- DO: Upgrade to an 802.11N access point
- DO: Phase out any B devices
- DO: Turn off B compatibility in your access point to make sure.
- DONT: worry about G devices on your N network
- DONT: wait for the N version of your favorite gizmo as you could be waiting a long time!

[Eric Seaberg]

Sean, thanks for the info. That's exactly what I started doing with my network 9-10 months ago.

I have TWO wireless networks in my house; 1-802.11N (B/G compatible) and 1-802.11B/G. Only my wife's older Apple iBook accesses the B/G network as it is 802.11B only. All newer MacBook Pros in the house are running 802.11N.

I have a Transporter in the living room, wired, and the SB3 in our master bedroom is accessing the N Airport at G speeds. I can stream lossless to the SB3 without hiccups and syncronizing, even on server 6.55, is FLAWLESS between the TP and SB3.

I can confirm the above recommendations!! It works.

[Sike]

Thanks for that! Just started 'rolling out' N APs in my house and now feel that it's worth it (although there was a hunch...)

[seanadams]

I have TWO wireless networks in my house; 1-802.11N (B/G compatible) and 1-802.11B/G.

Just make sure to put them at least five channel numbers apart.

[Mark Lanctot]

Excellent, I plan to mercilessly dig this thead out next time a newbie wonders at the short-sightedness of Logitech for not making the SBR or next big thing 802.11n. :-)

[Mitch Harding]

I agree, great post. Although for me, support for 802.11n would be
most useful in the wireless bridging case. Hardly a dealbreaker,
though.

On Jan 29, 2008 4:58 PM, Mark Lanctot
<Mark.Lanctot.33yzlz1201647601 (AT) no-mx (DOT) forums.slimdevices.com> wrote:
>
> Excellent, I plan to mercilessly dig this thead out next time a newbie
> wonders at the short-sightedness of Logitech for not making the SBR or
> next big thing 802.11n. :-)
>
>
> --
> Mark Lanctot
>
> Ben Klass: "I won't even eat a pre-7.0 meal. Well, unless it involves
> bacon."
> ------------------------------------------------------------------------
> Mark Lanctot's Profile: http://forums.slimdevices.com/member.php?userid=2071
>
> View this thread: http://forums.slimdevices.com/showthread.php?t=42853
>
>

[Eric Seaberg]

Just make sure to put them at least five channel numbers apart.

The Apple Airports have an 'auto' channel select mode where they sense what else is out there, so they 'defaulted' to 1 and 6. I had always read at least 3 channels apart, but this certainly works.

The range of N is quite incredible, even for the G devices. Speed difference on my MacBook Pro is about 500KB using G and almost 6MB using N when transferring/downloading files. Well worth the cost difference.

[dSly]

Very interesting original post. I am getting unsatisfactory wireless performance from my Squeezebox Boom. I heard that the Boom's wireless was not the greatest but I also have a pretty old microwave that may be interfering. If I understand correctly, it might be advantageous for me to replace my somewhat old WRT54G v8 Linksys router with, let's say, a more modern dual-band router with which I could have the boom by itself on one band and my other stuff on the other band. Would that help or would it be a waste of time (and money)?

[DaveWr]

The big advantage with N networks is they can be at 5 GHz. No interfering neighbours on G networks, and no microwave oven effects. It would really help in many cases if squeezeboxes were upgraded.

Dave

[Siduhe]

dsly, before you shell out for a new router - have you tried the basic troubleshooting steps set out in the wiki? Things like repositioning the router (higher up better, away from other electronics) and changing the channel can make a massive difference, depending on your environment. I have two other wireless networks near me on channels 12 and 13. 1 or 6 should be my best channel, but for some reason, the channel 7 signal is much better.

I also have my router in the middle of my house (i.e. the middle of 3 floors) which works better than having it on the ground floor where most of the electronics are.

http://wiki.slimdevices.com/index.ph...emsSecondGuide