Wireless Troubleshooting · Topic 3 of 3

Frames, HW Frames and Data Rates

Learn how to compare frames and hw-frames in the registration table to find retries, and how supported and basic data rates control the rates a link uses.

5 min

In a nutshell

frames counts handled frames, and hw-frames counts hardware transmissions including retries.
A hw-frames count much larger than frames means many retries.
tx-rate and rx-rate show the current data rate in each direction.

What it is

Frame counters show how many transmissions a link really needs, and data rates show how fast each one is.

If a link needs many hardware transmissions for each frame, it is wasting airtime on retries. If it runs at low rates, the same traffic takes longer on air.

Both facts are visible in the registration table, so you can judge a link without extra tools.

How it works

You compare two counters for retries, and you shape the rate list with supported and basic rates.

Frames and hw-frames

The registration table has two counters for each peer:

frames counts the frames handled for the peer.
hw-frames counts the frames that the hardware actually transmitted, including repeats.

When a frame is lost, the hardware sends it again. The repeat counts in hw-frames but not in frames. If hw-frames is much larger than frames, many retries happen, and the link is under stress.

/interface wireless registration-table print stats

The two numbers keep growing, so look at how they grow together. The ratio of the two is more useful than either number on its own.

Note

Frame aggregation on 802.11n can put several frames into one hardware transmission, so the two counters can differ even on a healthy link. Compare readings on the same peer over time.

What a high retry level means

A high retry level is a sign, not a cause. Common causes include weak signal, interference, a wrong ack timeout, a hidden node and too long a distance for the settings. Use the signal values, noise and CCQ pages to find which one applies.

Retries cost airtime. A link with many retries has low throughput and poor CCQ, even if the signal looks acceptable.

Data rates

The data rate is the speed at which one frame goes over the air. The device picks a rate for each peer, and lowers it when frames are lost. A good link holds high rates, and a bad one falls back to low rates.

The registration table shows the current tx-rate and rx-rate. A client stuck at a low rate in one direction points to a problem in that direction.

Supported rates and basic rates

Two settings control the rate list on the interface:

Setting	Meaning
`supported-rates-a/g`	The 802.11a and g rates that the interface may use
`supported-rates-b`	The 802.11b rates that the interface may use
`basic-rates-a/g`	The 802.11a and g rates that every client must support
`basic-rates-b`	The 802.11b rates that every client must support

A basic rate is a rate that all clients must be able to use. A device that cannot use the basic rates cannot join the cell. The access point also sends its management frames at basic rates.

To edit the lists, switch the rate set to a configured list first:

/interface wireless set wlan1 rate-set=configured
/interface wireless set wlan1 supported-rates-a/g=24Mbps,36Mbps,48Mbps,54Mbps
/interface wireless set wlan1 basic-rates-a/g=24Mbps

This removes the slowest rates. Clients with a weak signal can no longer connect, but the cell is not slowed by slow frames. The 802.11n rates have their own settings, which the 802.11n chapter explains.

Choosing a rate policy

Goal	Rate policy
Maximum range	Keep the low rates in the lists
Maximum cell speed	Remove the low rates, and accept a smaller cell
Stop old 802.11b devices	Drop the 802.11b rates or choose a band without b

Removing low rates is a good way to push far away clients to roam, and to stop one weak client from taking a lot of airtime.

A troubleshooting routine

Compare frames and hw-frames for the problem client.
Look at tx-rate and rx-rate in both directions.
If rates are low, check signal and noise.
If retries are high with a good signal, look at ack timeout and interference.
Change the rate lists only when you want to limit the cell on purpose.

Exam tip

Do not raise basic-rates-a/g without checking your clients. A client that cannot do the basic rate can never connect.

What to remember for the exam

Six facts cover this topic.

frames counts handled frames, and hw-frames counts hardware transmissions including retries.
A hw-frames count much larger than frames means many retries.
tx-rate and rx-rate show the current data rate in each direction.
supported-rates-a/g and supported-rates-b limit the rates the interface may use.
A basic rate must be supported by every client, or it cannot join.
Editing the lists needs rate-set=configured.

Common mistakes

Four errors are typical.

Reading one counter alone. The comparison of the two counters shows the retries.

Forgetting aggregation. On 802.11n the counters can differ without any fault.

Setting basic rates too high. Clients with a weaker signal cannot connect at all.

Editing rate lists without the configured set. The default rate set ignores your lists.

Check yourself

Work through these three questions.

How do frames and hw-frames show retries?
hw-frames counts every hardware transmission including repeats, and frames does not. When hw-frames is much larger, many retries happen.
What is a basic rate?
It is a rate that every client must support. A device that cannot use it cannot join the cell.
Which setting lets you edit the supported and basic rate lists?
Set rate-set=configured first. Then the supported-rates and basic-rates lists take effect.

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