Wireless-N Configuration

From DD-WRT Wiki

Revision as of 06:33, 23 January 2010 by Phuzi0n (Talk | contribs)
Jump to: navigation, search

Contents

General Wireless N Information

First your router must be upgraded to a recent firmware build, check the appropriate forum for your hardware for recommended builds, and obviously MUST be an N capable unit. There are many different factors that can affect N performance: RF interference, router brands, congested wireless locations, etc. If you have B or G clients on your network, they will reduce the overall performance of the network.

All N capable routers are MIMO (Multiple Input/Multiple Output). Technical Specs Due to this fact, multiple antennas will operate together to provide better signal coverage and increased data rates.

To ensure the best operating conditions, open the Web-GUI and navigate to Status->Wireless. At the bottom there is a tab called "Site Survey." Click it to view all wireless traffic nearby and find a channel that is not being broadcasted (1, 6, or 11). Use this channel you've selected if having problems with connectivity.

Basic Configuration

20 MHz vs 40 MHz

The N spec is able to use 40MHz of bandwidth, but to maintain compatibility with legacy systems, it requires one main 20MHz channel plus a free adjacent channel at ±20MHz. The main channel is used for legacy (a/b/g) or other clients that aren't able to transmit at 40MHz. The spec also requires the whole WLAN to only use the main 20MHz channel if it detects anything using the additional 20MHz channel.

In 5GHz band, it shouldn't be that difficult to find two free channels, so go ahead and turn 40MHz on.

However, when using 2.4GHz, using 40MHz isn't nearly as easy since the channels were already overlapped in 11b and 11g mode.

Effectively to use this feature, you will block 7–9 of the 13 channels and, of course, ch12 and ch13 aren't available in North America. So yes, it's possible that your neighbors will hate you if you turn this option on. This is why there was a lots of pushback at the end of the final IEEE 802.11n ratification to entirely ban 40Mhz in 2.4GHz. That didn't happen as 40MHz still appropriate for some situations (ie: a warehouse) but the bottom line remains, enabling 40MHz in the 2.4Ghz band isn't a good idea for dense residential areas.

Though the end result is the same, to be even more confusing different manufacturers are using different methods to have the user pick their 20/40 channels.

  • Recent builds of DD-WRT has you choose the main 20 MHz channel followed by "upper" or "lower".
  • Some stock firmwares have you choose a Main and Aux (or in the case of my Buffalo router a horribly misnamed "control" channel)
  • It wouldn't surprise me if some had you also choose a main and then the center frequency of the 40Mhz channel.

Channels are specified by their center frequency but instead of displaying (center of main 20 MHz + center of additional 20MHz) the devs opted to display it as (center of main 20MHz + center of 40MHz).

Reference thread: [phuzi0n troubleshoots 40 MHz problem]

Illustrations of 2.4GHz Channels

These illustrations show ALL Wifi channels but channel 14 is only available in Japan for B spec, so you can pretty much forget about your dreams to fit two 40MHz channels in the 2.4GHz Spectrum. As you can see, there is barely enough room to fit three non-overlapping 20MHz channels, or one 40MHz channel + one 20MHz channel. The tops of the blocks reflect the actual 20 or 40 MHz required by the channel whereas the bottoms account for the extra 1MHz on each end that the channel is allowed to attenuate, hence the 22MHz and 42MHz maximum widths. It may be possible to use channel 1 Upper and 13 Lower to produce two 40MHz channels but the signals may bleed into each other with that 1MHz they're each allowed to before attenuating.

Illustration of non-overlapping 20MHz channels.
Illustration of non-overlapping 20MHz channels.
Illustration of Channel 6 Lower. Channels 6 and 10 are combined and Channel 8 is where the center of the combined channel is.
Illustration of Channel 6 Lower. Channels 6 and 10 are combined and Channel 8 is where the center of the combined channel is.
Illustration of Channel 6 Upper. Channels 6 and 2 are combined and Channel 4 is where the center of the combined channel is.
Illustration of Channel 6 Upper. Channels 6 and 2 are combined and Channel 4 is where the center of the combined channel is.

Table of 2.4GHz Channels

Green are good choices that have minimal overlap, Red are bad choices that overlap too much, and White are uncommonly used channels that should only be used in substitute of channel 11.

Main
Channel
20 MHz 40 MHz Lower 40 MHz Upper
blocksAux.centerblocksAux.centerblocks
1 1-3 5 3 1-7 Not Available
2 1-4 6 4 1-8 Not Available
3 1-5 7 5 1-9 Not Available
4 2-6 8 6 2-10 Not Available
5 3-7 9 7 3-11 1 3 1-7
6 4-8 10 8 4-12 2 4 1-8
7 5-9 11 9 5-13 3 5 1-9
8 6-10 12a 10a 6-13 4 6 2-10
9 7-11 13a 11a 7-13 5 7 3-11
10 8-12 Not Available 6 8 4-12
11 9-13 Not Available 7 9 5-13
12a 10-13 Not Available 8a 10a 6-13
13a 11-13 Not Available 9a 11a 7-13

a = Setting not available in North America.

Wireless Security

You MUST use WPA2 authentication with AES encryption only, or use no security at all if you wish to achieve N rates. Anything else is against the N spec and typically results in the client falling back to G rates.

Problems and Resolution

Actual Speeds

Even if you are connecting to your N router at 130 mbps or 270 mbps, actual throughput will be roughly half the link rate due to modulation and error correction. The best test is to play with transmit power and channels to figure out what works best for you. Typically lowering the transmit power to 20-40 mW is good for N spec routers, going above the default 70 mW will likely reduce your throughput in addition to potentially overheating the radio and damaging it.

In a crowded apartment complex with 20 routers, a 20 Mhz width channel was able to get about 40 mbps (5 MB/s) as an actual transfer rate.

Testing

When testing throughput do it ~1 meter from the AP. Doing it far away/behind walls/downstairs/etc will have reduced rates and is a different problem to solve. Being extremely close can also cause problems.

Device Drivers

As always, make sure your wireless cards have the latest drivers. Ensure that the advanced properties of this card are set to match what your router is transmitting (examples: different channel widths, N-only)


Device Specific Information

Broadcom

[phuzi0n] - I've done many tests with NEWD and NEWD2 wireless drivers on various builds with a WRT150N v1.1 and WRT300N v1.1 and found the 40MHz channel (2.4GHz spectrum) throughput to be extremely lackluster. Most often it gave worse throughput than 20MHz width even though I have no nearby interference on any channels.

Ralink

[phuzi0n] - I've had very strong 40MHz channel (2.4GHz spectrum) throughput with my WHR-G300N. It capped at ~90mbps throughput due to the 100base-t Ethernet ports.