Manual:Wireless FAQ

Settings

By changing some wireless settings the wireless link works unstable

Sometimes when you change some wireless setting for tuning the links you got so far that the link isn't establishing any more or works unstable and you don't remember what settings you had in the beggining. In this case you can use the reset-configiration command in the wireless menu - it will reset the all the wireless settings for the specific wireless interface and you will be able to configure the interface from the start. Note that executing this command also disables the interface, so please be careful not to exacute this command if you are configuring router remotly using that wireless link that you want to reset the configuration.

What are wireless retransmits and where to check them?

Wireless retransmission is when the card sends out a frame and you don't receive back the acknowledgment (ACK), you send out the frame once more till you get back the acknowledgment. Wireless retransmits can increase the latency and also the throughput of the wireless link. To check if the wireless connection has wireless retransmissions you need to compare two fields in the wireless registration table: frames and hw-frames. If the hw-frames value is bigger than frames value then it means that the wireless link is making retransmissions. If the difference is not so big, it can be ignored, but if the hw-frames count it two, three or four times or even bigger than the frames count then you need to troubleshoot this wireless connection.

What TX-power values can I use?

The tx-power default setting is the maximum tx-power that the card can use and is taken from the cards eeprom. If you want to use larger tx-power values, you are able to set them, but do it at your own risk, as it will probably damage your card eventually! Usually, one should use this parameter only to reduce the tx-power.

In general tx-power controlling properties should be left at the default settings. Changing the default setting may help with some cards in some situations, but without testing, the most common result is degradation of range and throughput. Some of the problems that may occur are:

• overheating of the power amplifier chip and the card which will cause lower efficiency and more data errors;
• overdriving the amplifier which will cause more data errors;
• excessive power usage for the card and this may overload the 3.3V power supply of the board that the card is located on resulting in voltage drop and reboot or excessive temperatures for the board.

What TX-power-mode is the best?

TX-power-mode tells the wireless card which tx-power values should be used. By default this setting is set to default.

• default means that the card will use the tx-power values from the cards eeprom and will ignore the setting what is specified by the user in the tx-power field.
• card-rates means that for different data rates the tx-power is calculated according the cards transmit power algorithm from the cards eeprom and as an argument it takes tx-power value specified by the user.
• all-rates-fixed means that that the card will use one tx-power value for all data rates which is specified by the user in tx-power field.

Note that it is not recomended to use 'all-rates-fixed' mode as the wireless card tx-power for the higher data rates is lower and by forcing to use the fixed tx-power rates also for the higher data rates might results the same problems like in the previous question about tx-power setting. For most of the cases if you want to change the tx-power setings it is recommended to use the tx-power-mode=card-rates and it is recommended to lower and not to raise tx-power.

What is CCQ and how are the values determined?

Client Connection Quality (CCQ) is a value in percent that shows how effective the bandwidth is used regarding the theoretically maximum available bandwidth. CCQ is weighted average of values Tmin/Treal, that get calculated for every transmitted frame, where Tmin is time it would take to transmit given frame at highest rate with no retries and Treal is time it took to transmit frame in real life (taking into account necessary retries it took to transmit frame and transmit rate).

What is hw-retries setting?

Number of times sending frame is retried without considering it a transmission failure. Data rate is decreased upon failure and frame is sent again. Three sequential failures on lowest supported rate suspend transmission to this destination for the duration of on-fail-retry-time. After that, frame is sent again. The frame is being retransmitted until transmission success, or until client is disconnected after disconnect-timeout. Frame can be discarded during this time if frame-lifetime is exceeded.

What is disconnect-timeout setting?

This interval is measured from third sending failure on the lowest data rate. At this point 3 * (hw-retries + 1) frame transmits on the lowest data rate had failed. During disconnect-timeout packet transmission will be retried with on-fail-retry-time interval. If no frame can be transmitted successfully during disconnect-timeout, connection is closed, and this event is logged as "extensive data loss". Successful frame transmission resets this timer.

What is adaptive-noise-immunity setting?

Adaptive Noise Immuinty (ANI) adjusts various receiver parameters dynamically to minimize interference and noise effect on the signal quality [1] This setting is added in the wireless driver for Atheros AR5212 and newer chipset cards

Setups

Will an amplifier improve the speed on my link?

It depends on your signal quality and noise. Remember that you can probably get a better link with low output power setting, and a good antenna. Amplifier increases the noise and will only cause problems with the link.

How to fine-tune the wireless link with hw-retries?

You should understand that for 802.11 devices there is really limited amount of information (or "feedback" from the environment) that devices can use to tune their behaviour:

• signal strength, which could be used to figure out best transmit rate knowing receiver sensitivity. Sill this is not reliable taking into account that sesitivity for different receivers varies (e.g. changes over time), path conditions are not simmetric (and device can only measure signal strength it receives), etc.
• by receiving/not receiving acknowledgement for frame sent.

Taking into account that using signal strength is not reliable, 802.11 device is essentially left with only one "feedback" to tune its operation - success/failure of transmission. When transmission fails (ACK not received in time), there is no way how sender can figure out why it failed - either because of noise, multipath, direct interference (and wether that disturbed actual data frame or the ACK itself) - frame just did not make it and in general it does not matter "why". All that matters is packet error rate.

Therefore RouterOS implements algorithm to try to use medium most efficently in any environment by using only this limited information, giving users the ability to control how algorithm works and describing the algorithm. And there are only a few usage guidelines, not a set of values you should use in particular situation.

In general - the larger hw-retries, the better "feedback" device gets about medium ability to deliver frame at particular rate (e.g. if sending frame with rate 54mbps fails 16 times, it is telling you more than if it fails with 2 retries) and the better it can figure out optimal transmit rate, at the expense of latency it can introduce in network - because during all those failing retries, other devices in this channel can not send. So bigger hw-retries can be suggested for ptp backbone links - where it is known that link must be always on. Less hw-retries make rate selection adapt faster at the expense of some accuracy (going below 2 is not reasonable in most cases), this can be suggested for ptmp links, where it is normal for links to connect/disconnect and keeping latency down is important.

on-fail-retry-time and disconnect-timeout controls how hard device will try to consider remote party "connected". Larger disconnect-timeout will make device not "disconnect" other party, even if there are lots of loss at the smallest possible transmission rate. This again is most useful for "weak" links that are known that they "must" be established (e.g. backbone links). In ptmp networks large disconnect-timeout will again increase latency in network during the time e.g. AP will attempt to send data to some client that has just been disabled (AP will try to do this for whole disconnect-timeout).

frame-lifetime allows to tune for how long AP is attempting to use frame for transmitting before considering that it is not worth delivering it (for example, if sending frame fails at lowest possible rate, on-fail-retry-time timer is enabled, if during this timer frame-lifetime expires, particular frame is dropped and next transmission attempt will happen with next frame. Disabled frame-lifetime means that wireless will ensure in order delivery of "all" data frames, no matter how long it takes, "or" will drop the connection if everything fails). This allows to optimize for different types of traffic e.g. for real-time traffic - if primary use of wireless network is e.g. voip, then it can be reasonable to limit frame-lifetime, because voip tolerates small loss better than high latency.