This article answers some of the most common questions about GPS Sync, released for airMAX AC in airOS v8.3 and for airMAX M in v6.1.3. Find more information in the GPS Sync page.
Frequenty Asked Questions
What devices are supported?
Currently, 10 Mhz, 20 MHz, and 40 MHz channel widths are supported. While airMAX priority and ATPC are operational in fixed Frame mode, The TDMA Filter setting on the AP is not connected to the fixed frame implementation. The option to synchronize with the GPS clock is only available on products that have the GPS component e.g. Rocket PRISM 2AC, Rocket PRISM 5AC, Rocket PRISM 5AC GEN2, PRISMStation 5AC, and LiteAP GPS.
What is the minimum firmware required for GPS Sync?
- 8.3.0 added GPS Sync support for AC only sectors
- airMAX 5AC: 8.4.3 (for all)
- airMAX M5: 6.1.3 (for all XM/XW/TI)
- airMAX 2AC: 8.5.0 airMAX M2: 6.1.4
What frame durations and ratios are supported?
What is the difference between clean channel throughput data?
|20 MHz CHANNEL|
|5ms||NA||90:24 (Mbps)||63:52 (Mbps)|
|8ms||105:25||95:36 (Mbps)||70:60 (Mbps)|
|40 MHz CHANNEL|
|5ms||NA||200:50 (Mbps)||140:111 (Mbps)|
|8ms||245:50||210:70 (Mbps)||150:120 (Mbps)|
Why doesn't the UL throughput match the ratio exactly?
What is expected latency?
- For 5ms framing, an average latency between 10 to 15ms will be seen.
- For 8ms framing, an average latency between 16 to 24ms will be seen.
- For 10ms framing, and average latency between 20 to 20ms will be seen.
Why are the reported capacity numbers much lower than flexible framing?
|The actual capacity of the AP is the average of the DL and UL capacity in flexible mode.|
When using fixed framing, the time allotted to each direction is fixed, hence the capacity is proportional to the allowed time.
|The actual capacity of the AP is the sum of the DL and UL capacity in fixed frame mode.|
Additionally, in fixed frame, there is a brief "ramping up" period for the Rate Adaptation algorithm. The capacities upon initial connection start low and ramp up to the actual possible number.
Why is the latency higher?
A packet entering the CPE over the Ethernet, needs to wait for the CPE to get an opportunity to transmit. In flexible framing, the time a CPE needs to wait before asking is lower when the network is idle. In fixed framing, this time is proportional to the TDD FRAMING duration. When traffic is passing, the latency between flexible and fixed is similar. In other words, fixed framing may have a higher idle latency, but this does not increase much with traffic.
Why don't we see a latency equal to the TDD frame duration with airFiber X products?
- AFx being a PTP product, the slave always has a time allocation to use the UL. In a PTMP product, the CPE needs to compete with others to get an allocation before transmitting. This is the primary cause of the difference and is common to any PTMP fixed frame protocol like LTE/WiMAX/etc.
- The secondary cause of the difference is due to the amazing performance of the UBIQUITI INVICTUS silicon in airFiber.
If latency is higher, what about VoIP?
Is the new "flexible" duration option the same as the original AP PTMP airMAX-AC mode?
How many CPEs can be connected to one AP?
What is the maximum supported distance?
There is a natural reduction in expected throughput based on the distance, due to propagation delay. For example, the round trip propagation delay for a 75km link is 500 microseconds, and this translates into a 10% reduction in performance for a 5ms frame and a 6.25% reduction when using an 8ms frame.
Why does the association process take longer?
In the current release, the WEBUI does not show the clients when in this state, but internally they are associated a lot sooner than when the WEBUI shows them. A subsequent release will show the stations associated in this state on the WEBUI.
What are the recommended settings?
How do I synchronize my APs?
The AP’s dashboard page will show the TDD framing and sync status:
Pay special attention to the following:
- Synchronized APs must use the same TDD framing and ratios.
- Only AP syncing is currently supported. E.g. If AP1 is co-located with a device in station mode which in turn is connected to AP2, synchronizing AP1 and AP2 will make AP1 and the station interfere.
In subsequent releases, a time offsetting option will be added for such scenarios.
Can I synchronize with airFiber?
What happens when the GPS signal is not available?
Most of the data on my network is "download heavy", why can't I have higher ratios like 85/15 or 95/5 for example?
As an example:
|There are 50 clients connected to an AP. If 10 of these clients are simultaneously downloading and the UL allocation is extremely small like in 95/5, the UL time may only be enough to carry the TCP ACKs of one of the stations. The others will need to wait for their turn, which will arrive according to the TDD framing cycle. But during this time the TCP may stall its window and not send any further traffic until the ACKs are received. This causes underutilization of DL.|
Hence extremely low UL allocations may end up reducing the effective throughput of the DL on larger PTMP networks.