Read the stickies, but still have a question on mixed mode and 40MHz

[SamS]

Hi,

After some LAN reconfiguration, I've decided to keep my existing access point, the Cisco WAP-4410N. Performance is pretty good. It is 2.4Ghz only, that means it only has a single radio, correct?

The question I have concerns my N-devices (iPads, iPhones, Macbook) and my G-devices (a couple of Logitech Squeezeboxes). I understand that with my WAP, in mixed-mode, my N clients are going to be slowed down to <54Mbps, correct? That is theoretical speed, so in actuality, every device may be only maxing at ~30Mbps?

If the above is all true, I may not be too concerned. My N-devices typically only need internet access, which is 18Mbps download from my ISP. And my G-devices only need to pull down ~500kbps from my NAS.

Also, I have experimented with using 40Mhz on my WAP, and confirm I get 300Mhz via inSSIDer. Although I'm in a residential area, I don't pick up any other SSIDs from my neighbors, inside my house.

Would using the 40Mhz setting offer any improvement, given my situation? I tested internet speed tests with both 20Mhz and 40Mhz configurations, and pings and d/l speeds were identical.

Apologies in advance for these basic questions, but no amount of reading I've done sorts it all out for me

Thanks!

 

[thetoad30]

Hi,

After some LAN reconfiguration, I've decided to keep my existing access point, the Cisco WAP-4410N. Performance is pretty good. It is 2.4Ghz only, that means it only has a single radio, correct?

This is correct.

The question I have concerns my N-devices (iPads, iPhones, Macbook) and my G-devices (a couple of Logitech Squeezeboxes). I understand that with my WAP, in mixed-mode, my N clients are going to be slowed down to <54Mbps, correct? That is theoretical speed, so in actuality, every device may be only maxing at ~30Mbps?

The PHY (link speed) rate will be 54mbps.

The throughput speed you have listed is probably true. However, remember that this is every device on its own. When you have multiple radios (devices) talking at once, like sending from one notebook to another, the bandwidth is shared since wireless is half-duplex. This means that wireless can only send to one device at a time, and receive from one device at a time. So it has to talk to one computer, then send that info to another computer, then back to the first one, etc. until the transfer is done.

If the above is all true, I may not be too concerned. My N-devices typically only need internet access, which is 18Mbps download from my ISP. And my G-devices only need to pull down ~500kbps from my NAS.

Also, I have experimented with using 40Mhz on my WAP, and confirm I get 300Mhz via inSSIDer. Although I'm in a residential area, I don't pick up any other SSIDs from my neighbors, inside my house.

Would using the 40Mhz setting offer any improvement, given my situation? I tested internet speed tests with both 20Mhz and 40Mhz configurations, and pings and d/l speeds were identical.

Pings and DL speeds will not tax the wireless enough to show a difference. A ping is more about latency and not speed. Download speeds, as you said, won't max out your connection. Thus a G or N speed won't be hit, thereby making them look identical.

 

[SamS]

Thanks for your response. And apologies for the typo in the thread title, i.e 40Mhz.

So, it sounds like until I get a dual-band WAP, I'm going to be stuck with slower speeds. In real-world usage, this will probably not impact me very much. Most all my devices are hard-wired. The wireless devices are typically pulling down internet, as opposed to sharing content. The only time when two Wi-Fi devices are communicating is when my wife's Macbook is wirelessly to iPhone/iPad.

I guess I'll stick with 20Mhz, and show some pity on my neighbors

 

[sfx2000]

Mixed mode doesn't automatically mean all devices are slowest rate - most current AP's/Routers can support mixed mode just fine, and 802.11n devices will be a bit slower than greenfield mode, but they will operate at the rated speed.

It's a myth that really needs to get busted, and soon...

 

[sfx2000]

I guess I'll stick with 20Mhz, and show some pity on my neighbors

Going 20MHz is better in 2.4Ghz, not just because it is neighbor friendly (which it is), but also because neighboring AP's can cause the noise factor to be non-linear...

If you're on 1+4 at 40Mhz, and a neighbor is on Ch 1, half your BW is higher noise than the other, just based on co-channel interference... and that impacts your physical layer bandwidth more than a 20MHz channel with a jammer...

 

[StratmanX]

Mixed mode doesn't automatically mean all devices are slowest rate - most current AP's/Routers can support mixed mode just fine, and 802.11n devices will be a bit slower than greenfield mode, but they will operate at the rated speed.

It's a myth that really needs to get busted, and soon...

Please expand on this.

Conventional user wisdom has been that two devices operating simultaneously in mixed mode, one device G-rated and the other N-rated, will operate at the G-rated 54 Mbps theoretical max until the G-rated device stops operations and then the N-rated device returns to full N-rated speed potential.

Some web sites talk about a performance hit negotiating legacy clients but not a full step down to G-rated speeds such as here:

Can 802.11n APs deliver the promised multi-hundred megabits throughput with a mix of 802.11n and legacy (802.11a/b/g) clients?

Presence of legacy clients will limit the performance improvement for 802.11n WLANs. Legacy clients will continue to operate at legacy speeds occupying the medium for longer intervals per frame transmission. Further, legacy clients associating with an 802.11n AP will trigger the HT-protection mode in this AP leading to additional overhead. Both these factors will reduce the net throughput of the 802.11n devices associated with the same AP.

What will the performance decrease be when operating in mixed mode A/B/G plus N?

 

[thiggins]

Read Add, Don't Replace When Upgrading to 802.11n

N devices only drop to 54 Mbps link rates if WEP is used or WMM is disabled.
When associated to the same AP, all devices share that AP's bandwidth. Faster devices (N) must wait for slower devices (G).

N slowdown is most evident when both types of devices are simultaneously active for long periods of time (streaming video, transferring files). For short periods of activity like web browsing, email, etc. you won't see the effect.

 

[sfx2000]

@stratmamx

3 modes for 802.11n High-Throughput MAC layer

Greenfield (HT) - the AP does not expect to connect to any legacy mode clients (STA's) and assumes that none are operating in the area. The beacon frame is the same, and some control frames are the same (and transmitted at 20MHz, by the way). It's enough 802.11 that legacy devices can at least see the network if not attach

Legacy Mode - all frames are in 802.11g/a format, so that legacy g/a clients can understand and decode the frames at the PHY/MAC level. This basically locks the AP/STA into the legacy formats - wide channels are not found here...

HT Mixed-Mode - Legacy Preamble on the frames, but the option of using HT or legacy format afterwords. The legacy preamble allows the older STA's to detect transmissions, acquire timing sync, and the L-SIG field then allows them to estimate the length of the transmission and set their Network Availability Vector (NAV). Having the NAV is the first step towards true Phased Coexistence Operation (PCO). Wide Channels can exist here, all STA's, both HT and Legacy, can operate, and hit the best rate possible - goes without saying though that a very active legacy client can impact HT clients around it...

Most earlier 802.11n Draft 1.0 (and Draft 2.0) either ran in Legacy or Greenfield mode, so the mix-mode guidance rang true - in my experience, most gear from Tier 1's these days, they support HT Mixed Mode for B/G/N (or A/N at 5GHz), so here, the HT STA's take a bit of a hit, not near as much as the older Legacy Mode...

Also consider that 802.11n, along with 802.11e, also have PHY layer enhancements that do allow HT STA's to hit better performance targets, even in Legacy Mode. Turning off 11e (WMM in Wifi Alliance terms) negates even that...

just my thoughts...

 

[SamS]

Great info guys.

Since I'm mostly wired, the worst case scenario for my house would be one or two G-clients pulling down ~300Kbps from a music streamer, and someone on the internet via laptop/iPad (18Mbps/1.5Mbps).

 

[StratmanX]

Thank you for the additional information. Do I have the following correct?

• An N rated wireless router will force G rated speeds on all clients if WEP security is chosen or WMM is turned off.
  
• Older Draft-N wireless routers did not operate in HT Mixed Mode and therefore slowed N rated clients more than new N rated routers when legacy and N rated clients are running simultaneously. The amount of slowing is not forced to G rated speed maximum in either instance.

Interesting that Greenfield Mode uses 20 Mhz while HT Mixed Mode uses 40 MHz. Would think it would be the reverse.

 

[thiggins]

Thank you for the additional information. Do I have the following correct?
- An N rated wireless router will force G rated speeds on all clients if WEP security is chosen or WMM is turned off.

Correct

Older Draft-N wireless routers did not operate in HT Mixed Mode and therefore slowed N rated clients more than new N rated routers when legacy and N rated clients are running simultaneously. The amount of slowing is not forced to G rated speed maximum in either instance.

I would have to run a test to see if mixed STA bandwidth sharing has improved. Note that most consumer products provide only simple mode controls, i.e. N-only, Mixed G/N, etc.

 

[sfx2000]

Thank you for the additional information. Do I have the following correct?

• An N rated wireless router will force G rated speeds on all clients if WEP security is chosen or WMM is turned off.
  
• Older Draft-N wireless routers did not operate in HT Mixed Mode and therefore slowed N rated clients more than new N rated routers when legacy and N rated clients are running simultaneously. The amount of slowing is not forced to G rated speed maximum in either instance.

Interesting that Greenfield Mode uses 20 Mhz while HT Mixed Mode uses 40 MHz. Would think it would be the reverse.

802.11n Beacon's are always in 20Mhz in the primary channel... whether you're in 20MHz or 40MHz bandwidth modes - this is so that legacy clients can see the network. That's another reason why 40MHz channels in 2.4Ghz are just a bad idea - the bonded channel is a jammer to adjacent 802.11n 20MHz and 802.11b/g AP's (along with Bluetooth and other RF using the ISM band)

802.11n requires WPA2 and WMM to be enabled... some AP's may look aside at this, but that would be contrary to the specs.

 

[sfx2000]

When associated to the same AP, all devices share that AP's bandwidth. Faster devices (N) must wait for slower devices (G).

N slowdown is most evident when both types of devices are simultaneously active for long periods of time (streaming video, transferring files). For short periods of activity like web browsing, email, etc. you won't see the effect.

Wrong, wrong, wrong - When in mix-mode operation, the AP will use 802.11g framing, but the actual PDU's for 802.11n clients will use the appropriate modulation schemes for the detected MCS. It's more overhead than Greeenfield, but it's not 802.11g

Another myth to bust - 802.11n does not fallback to 802.11g or 802.11b for lower data rates - it uses MCS ratings, but it's still 802.11n - the OFDM numerics remain the same, and there 802.11n is still better, due to the MAC layer improvements in the spec.

 

[thiggins]

Wrong, wrong, wrong - When in mix-mode operation, the AP will use 802.11g framing, but the actual PDU's for 802.11n clients will use the appropriate modulation schemes for the detected MCS. It's more overhead than Greeenfield, but it's not 802.11g

What specifically is wrong?

Another myth to bust - 802.11n does not fallback to 802.11g or 802.11b for lower data rates

I did not say that N falls back to slower rates.

 

[sfx2000]

What specifically is wrong?

I did not say that N falls back to slower rates.

http://forums.smallnetbuilder.com/showthread.php?t=2505

Yes you did...

"Under weak / fringe signal conditions, N routers drop back to using 802.11g, then b protocols. "

No they don't - they're still 802.11n, just lower MCS values...

 

[thiggins]

Well, during testing in Location F, I'm pretty sure I've seen link rates lower than 6.5. But I could be mistaken. I've changed the sticky.

Please let me know if there are other places where I have made this mistake and I'll change them. I did a quick check and didn't find any.

 

[SamS]

Very interesting discussion, gents. I continue to learn quite a bit, reading here.

 

[tipstir]

What about link speed shows 54mbps on 802.11g and 802.11n shows 65mbps in mix mode. I really don't have issue setting the Auto/20Mhz and 40MHz as option on mix mode. To me results going mix to full 802.11n @ 40Mhz is slightly greater but not by that much.

GF (Green Field) on Android is disabled on 802.11n @ 40MHz on what I created for Android on ROM Updates that feature is enabled. Like with Windows and Android 802.11g and 802.11n depending on what's in the system will use what the hardware can offer.

But again I never seen any fallback from 802.11n to 802.11g on a device that has 802.11n from PC to Tablets never the case.