modified on 2 April 2012 at 21:24 ••• 75,804 views

AirOS and AirMax - FAQ

From Ubiquiti Wiki

Jump to: navigation, search

Contents

AirOS

AirMax

What do AirMax Capacity (AMC) and AirMax Quality (AMQ) represent?

airMAX Quality

Available if airMAX is enabled. airMax Quality (AMQ) is based on the number of retries and the quality of the physical link. If this value is low, you may have interference and need to change frequencies. If AMQ is above 80% and you do not notice any other issues, then you do not need to make any changes.

airMAX Capacity

Available if airMAX is enabled. airMax Capacity (AMC) is based on airtime efficiency. For example, if you have one client with a low data rate or you are using a 1x1 device (such as Bullet or airGrid) alongside other clients that are 2x2, then it will use up more airtime (slots) for the same amount of data, reducing time (or capacity) for other clients. The lower the AMC, the less efficient the AP is. If you only have one client, this may not matter, but when you have many clients (for example, more than 30), then AMC becomes very important, and you want it to be as high as possible.

If you are looking at the client, AMC shows the theoretical capacity of that client, based on current TX/RX rates and quality. AMC is a percentage based on what the maximum performance would be if the link were perfect. Clients with poor airtime efficiency can negatively affect other clients by taking up more airtime while transmitting at lower speeds. For example, client A is at MCS 12 (78 Mbps) because of low signal. The client could theoretically do MCS 15 (130 Mbps), so AMC is based on the ratio of current rate/maximum rate (78 Mbps divided by 130 Mbps), which is 60%. In a similar fashion, a 1x1 device will always have a maximum AMC of 50%, because it provides half the performance of a 2x2 device.

If you are looking at the AP, then AMQ and AMC are averages of all clients’ values. If you want to discover what is lowering your values on heavily populated APs, single out the weak clients. You can either use airControl™ (recommended), or you can go to each client individually. Try to upgrade to a higher-gain antenna (to allow a better data rate), or

Max Distances per Channel Width (When should Long Range Mode be used?)

AirOS supports a maximum ACK Timeout value (or distance) for each channel width. If you exceed this value, it may be required to use the "Long Range PTP Mode" (or "PTP NO ACK Mode" in older versions of AirOS). The following lists the maximum distances supported for each channel bandwidth.

  • 40MHz: 16.5 miles (26.5 km)
  • 20MHz: 35.6 miles (57.3 km)
  • 10MHz: 72.3 miles (116 km)
  • 5Hz: 144.7 miles (232.9 km)

How do you configure QoS

QoS is enabled automatically with airMax devices, and there is no configuration necessary to enable QoS. In order for the devices to prioritize traffic properly, the traffic will need to have the DSCP/ToS values correctly configured prior to the traffic entering the airMax equipment. For more information on the correct DSCP/ToS values, see:

QoS_DSCP/TOS_Mappings

AirSync

Product Questions

Do I need special CPE devices to use with airSync?

All M-Series devices can operate as CPE devices in an airSync network Note: Version v5.4.5 or above must be used on the CPE devices in order to utilize airSync

Do all APs in an airSync network need to be GPS models?

Yes, all devices participating as an AP in an airSync network must be GPS models. Each AP needs to get very specific timing information via GPS. Syncing over Ethernet is not reliable enough to transfer very accurate timing information.

Protocol Questions

What's the benefit of AirSync

AirSync reduces problems when collocating multiple radios nearby each other. When a radio is receiving, it works best with the highest signal to noise ratio (SNR). If a nearby radio is transmitting at the same moment the first is receiving, the SNR of the first will be much lower, since the transmissions of the nearby radio are received at such a high level.

When AirSync is enabled, all APs will transmit at the same time, and receive at the same time, reducing collocation interference.

How do the APs know to sync at the same time?

The airSync APs are configured with certain transmit and receive slots (amounts of time). All APs will obtain very accurate timing from GPS satellites, and based on this timing and the slots configured above, will know when they should transmit and when they should receive.

How do the APs the sync / timing information

In airSync you configure one AP as Master and the rest as Slaves. In the slave radio, you enter the IP Address of the Master. When the slave radios boot, they will contact the master radio, and obtain the sync slot timing information.

Is L2 communication between APs required

No, as long as the devices can communicate on L3 (UDP), airSync will work (in other words, if you can ping the Master AP from the Slave AP).

Design Questions

Can remote towers be synchronized?

Yes, assuming there is L3 connectivity, and the latency between towers is 30ms or less. (This is only the latency from the slave AP to the master AP) Note: It is important that the stations on one tower cannot "hear" the APs of a different tower, and the APs of a tower cannot "hear" the stations of other towers.

Can I use the same frequency on all APs

In most cases, no.

Why can't I use the same frequency on all APs

In a typical deployment, you may have 3 or 4 APs on one tower.

  • With airSync disabled, all APs will transmit randomly, interfering with each other.
  • With airSync enabled, all APs will transmit at the same time, no longer interfering with each other.

As far as this is concerned, all APs can use the same frequency.

However!

  • If a CPE on one AP can also see the signal from another AP, and there is no frequency separation (both APs are on the same frequency), the SNR for the CPE will be VERY low!
  • If an AP is picking up transmissions of clients on different APs, and they're on the same frequency, that AP will have very low SNR for its own clients!

Because of this, we recommend that APs that are side by side have a frequency separation of 10-20MHz between the edges of the APs channel bandwidth. For instance:

  • AP-1 is using 5825MHz and 20MHz bandwidth (5815 - 5835MHz)
  • AP-2 is using 5785MHz and 20MHz bandwidth (5775 - 5795MHz)

There is 20MHz of separation between AP-1 and AP-2 (5815 - 5795 = 20)

What is the recommended frequency plan on a tower
Four APs (ABAB)

With four APs on a tower, one CPE may see the AP it is associated with, along with either of the CPEs directly alongside its own AP. In this case, it is recommended to use an ABAB deployment, where A and B have at least 20MHz separation (see above).

airSync_Diagram_4APs.png

Three APs (ABC)

With three APs on a tower, one CPE may see the AP it is associated with, along with either of the two remaining CPEs. In this case, it is recommended to use an ABC deployment, where A, B, and C have at least 20MHz separation (see above).

airSync_Diagram_3APs.png

Two APs (AA)

With only two APs in a back to back deployment, the chance of a CPE device seeing both APs is minimal. In this case, it's fine to reuse frequencies on the APs, in an A-A design (A denotes the frequency of the AP)

airSync_Diagram_2APs.png