Wireless-N Configuration

From DD-WRT Wiki

Revision as of 00:09, 27 November 2009 by Phuzi0n (Talk | contribs)
Jump to: navigation, search

Contents

General Wireless N Information

First your router must be upgraded to the newest firmware (there has been talk that some older versions provide more reliable N speeds than the newest firmware, try each to figure out which fits your needs the best), 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. Also, if you have B and G clients on your network, all routers will automatically downclock to those speeds during times of shared traffic. If those devices log off, the speed should resume to N levels.

All N capable routers are MIMO (Multiple Input/Multiple Output). Technical Specs Due to this fact, multiple antennas will operate on different frequencies in order to provide the best signal quality regardless of the location.

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 (1-11). 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.

Regardless throw out the previously reliable 1-6-11 rule of thumb as there are no non-overlapping channel sets. Just pick the best one and stick with it.

Main
Channel
20 MHz 40 MHz Upper 40 MHz Lower
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.

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]

Wireless Security

You cannot use TKIP and still get N speeds. You MUST use AES security or none at all.


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.

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.

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 Setups

The settings previously listed here were general settings that were taken from forum threads that reported success. I (phuzi0n) have removed them because they were not actually specific settings needed by the devices. If you discover any settings that are truly specific to particular devices then you can list them here.