Chains, TX Power and Link Debugging for 802.11n

What it is

Chains are the parallel radio paths of an 802.11n card, and each one has its own antenna connection.

The number of chains sets how many streams the card can use. More chains give a higher speed, but they also need good antennas and cables on every path.

When an n link is slower than expected, the cause is very often one weak or dead chain. Debugging means checking each chain separately.

How it works

You choose which chains are active, you account for power across chains, and you compare per chain signals.

Chain settings

Each wireless interface lets you pick the chains for receive and transmit:

/interface wireless set wlan1 ht-rxchains=0,1 ht-txchains=0,1

• ht-rxchains lists the chains used for receiving.
• ht-txchains lists the chains used for transmitting.

The chains are numbered from zero.

A two chain card has chain 0 and chain 1. Using both gives the two stream rates. Using one limits the card to the one stream rates, which are MCS 0 to 7.

You might use fewer chains on purpose. For example, a link with one good antenna and one poor path can be more stable on a single chain. Or you might use one chain when only one antenna is fitted.

Antennas and chains

Each chain needs its own antenna feed. A dual polarised antenna provides two feeds in one housing. Two chains with one single polarised antenna cannot work well, because both streams take the same path and cannot be separated.

TX power for n cards

An 802.11n card sends from several chains at once, so the power is a sum. Adding a second chain adds power. The wireless interface setting is a figure in dBm, and the total on air depends on the card and the number of chains.

Check what your card reports for its power before you work out EIRP. The rules about the legal limit apply to the total radiated power, so the number of chains counts as well as the antenna gain.

With frequency-mode=regulatory-domain, RouterOS applies the limit for you. With frequency-mode=manual-txpower you must do the sum with the real card figures.

Debugging an n link

An organised check works from the top down:

• Read the rate in the registration table. Note the number of streams and the width.
• If the stream count is lower than expected, look at the chains.
• Compare the signal on each chain.
• Check the channel width and noise.
• Test the real throughput.

Per chain signal

The registration table gives a signal value for each chain:

/interface wireless registration-table print detail

Look at signal-strength-ch0 and signal-strength-ch1. With two good antennas of the same kind, the two values should be close.

Testing throughput

A rate in the table is not a speed test. Use a test between two RouterOS devices:

/tool bandwidth-test address=192.168.88.2 direction=both

The test loads the link and shows what it really carries. Compare the result with the rate. A large gap points to retries, noise, a CPU limit or a settings problem such as aggregation.

Putting it together

If the speed is low, ask in order: Is the rate low, or is only the throughput low? If the rate is low, check chains, width and signal. If the rate is high but throughput is low, check retries, noise and the CPU of the router.