Wireless Troubleshooting · Topic 2 of 3

Ack Timeout, Distance and CCQ

Learn how ack timeout and distance settings affect link throughput, how to read the CCQ value in the registration table, and what a low CCQ tells you.

5 min

In a nutshell

Every unicast frame must be acknowledged, or the sender repeats it.
A timeout that is too short causes needless retries on long links.
A timeout that is too long wastes airtime after real losses.

What it is

Ack timeout and distance control how long a sender waits for an acknowledgement, and CCQ measures how well the link uses its airtime.

Every unicast frame in Wi-Fi must be acknowledged. If the sender hears no acknowledgement in time, it sends the frame again.

On a short link the default behaviour is fine. On a long link, the delay of the signal itself becomes important, and a wrong timeout wastes capacity.

How it works

The timeout must cover the round trip, and CCQ shows the cost when it does not.

The acknowledgement timeout

After sending a frame, the sender starts a timer. If the acknowledgement arrives before the timer ends, all is well. If it does not, the sender assumes the frame was lost and sends it again.

Radio waves need time to travel. Over a long link, the acknowledgement arrives later, because it must make the round trip. The longer the link, the longer the timeout must be.

Too short and too long

Timeout	Effect
Too short	The acknowledgement arrives after the timer ends, so the sender repeats frames that were not lost. Throughput falls and retries rise
Too long	The sender waits too long after a real loss, so it wastes airtime. Throughput also falls
Correct	Lost frames are repeated quickly, and good frames are not repeated

A timeout that is too short is the more damaging mistake on a long link. The link can show a strong signal and still give poor speed.

Distance and ack timeout settings

RouterOS offers two related settings on the wireless interface for Atheros based cards:

/interface wireless set wlan1 distance=dynamic
/interface wireless set wlan1 ack-timeout=dynamic

distance tells the interface how far away the other side can be. It accepts a number of kilometres, indoors, or dynamic.
ack-timeout sets the timer directly, and it also accepts dynamic.

With dynamic, the router works out the value automatically. That is a good start. For a point to point link of known length, a fixed value set by hand can be more exact.

Note

Radio waves cover a kilometre in a few microseconds, and the acknowledgement must travel back, so each kilometre adds to the round trip time.

Seeing the result

The registration table shows an estimated distance for each peer:

/interface wireless registration-table print detail

Use that value to check the setting. If the estimate is much larger than the real distance, the link may have a timeout problem or suffer reflections.

What CCQ is

CCQ (Client Connection Quality) is a percentage. It compares the airtime that the link would need under ideal conditions with the airtime it really uses. A value near 100 percent means little time is wasted on retries and low rates.

The registration table shows tx-ccq and rx-ccq, one for each direction. The monitor command shows an overall value for the interface:

/interface wireless monitor wlan1

Reading CCQ

CCQ	Meaning
Near 100 percent	The airtime is used well
Falling	More retries or lower rates, so more airtime is wasted
Low in one direction only	A one sided problem, such as noise at one end

A low CCQ is a symptom, not a cause. Possible causes include interference, a weak signal, a bad ack timeout, hidden nodes and too many clients. Use the other tools in this chapter to find which.

A troubleshooting sequence

Check signal-strength and signal-to-noise first.
Check tx-ccq and rx-ccq for the weak direction.
On a long link, compare the estimated distance with the real distance.
Set distance or ack-timeout and watch whether the CCQ improves.
If CCQ stays low, look for interference and change the frequency.

Exam tip

After you change distance or ack-timeout, wait a few minutes and compare the CCQ before and after. One reading is not enough.

What to remember for the exam

Six facts cover this topic.

Every unicast frame must be acknowledged, or the sender repeats it.
A timeout that is too short causes needless retries on long links.
A timeout that is too long wastes airtime after real losses.
distance and ack-timeout can be set to dynamic or to a fixed value.
CCQ is a percentage, and a value near 100 percent is good.
tx-ccq and rx-ccq give the quality in each direction.

Common mistakes

Four errors come up often.

Leaving a long link on the wrong value. A strong signal with poor throughput can come from a timeout that is too short.

Setting the timeout far too long. A very large value wastes airtime after every lost frame.

Treating CCQ as a cause. CCQ is a symptom, so look for noise, signal and timing problems behind it.

Ignoring direction. tx-ccq and rx-ccq can differ, which tells you which side to fix.

Check yourself

Work through these three questions.

Why does a long link need a longer ack timeout?
The acknowledgement must travel back over the same distance, so it arrives later. A timeout that is too short makes the sender repeat frames that were not lost.
What does a CCQ near 100 percent mean?
It means the link uses its airtime well, with few retries and few low rates. A falling CCQ shows more airtime is being wasted.
Which values can distance take on an Atheros based interface?
It can be a number of kilometres, indoors, or dynamic. With dynamic the router works out the value automatically.

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