airMAX - Frequently Asked Questions (FAQs) About GPS Sync

TDD fixed framing options are available on all AC products when in AP PTMP AC mode. Support for airMAX M CPEs was added in airOS version 6.1.3. For scenarios where you use M devices as CPEs and AC devices as APs, the AC devices must be on (at least) airOS v8.4, and the M devices must be configured in Station WDS mode.

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.

Note: Both the AP and the CPEs need to be updated to v8.3/v6.1.3+ (5GHz) and v8.5/v6.1.4+ (2.4GHz) in order to support the fixed framing protocol. CPEs must be in Station PTMP mode.

• 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

Frame durations of 5ms, 8ms, 10ms are supported. It is also allowed to split these in 75 DL/25 UL, 67 DL/33 UL or 50 DL/50 UL ratios.

Note: 75:25 is available only for 8ms and 10ms frame duration

See tables below for comparison:

20 MHz Channel Width
	75:25	67:33	50:50
5ms	NA	90:24 (Mbps)	63:52 (Mbps)
8ms	105:25	95:36 (Mbps)	70:60 (Mbps)

40 MHz Channel Width
	75:25	67:33	50:50
5ms	NA	200:50 (Mbps)	140:111 (Mbps)
8ms	245:50	210:70 (Mbps)	150:120 (Mbps)

In a PTMP fixed framing system, some part of the UL allocation is used to allow the connected CPEs to request the AP for time allocation. This part of the UL cannot be used to send data. Additionally, there is some amount of turn around time reserved for the transition between DL and UL, and also between UL and DL. There are already some improvements to the UL throughput, and they will be provided in subsequent releases.

The expected latency is 2x to 3x the TDD framing duration.

• 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.

Note: Retries and scheduling may affect the instantaneously observed latency.

Since flexible framing dynamically allocates the time used by DL and UL, capacity is reported assuming that specific direction uses 95% of the time to send data. Additionally, the amount of time used changes dynamically and is elastic.

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.

The latency in fixed frame mode is similar to any other PTMP fixed frame system for the same framing duration. airMAX-AC flexible framing intelligently shifts the unused portion of time between the two directions. This allows much lower idle latency.

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.

There are two reasons:

• 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.

Note:  We have an experimental mode where an AC AP with only one client connected can get latency close to the TDD framing duration like AF5x. We have not enabled it at this time though.

The airMAX-AC scheduler always prioritizes VoIP and the link latency is below standard VoIP jitter buffer limits. In our testing, we find that fixed framing provided better VoIP MOS scores on a loaded network.

Yes, flexible is the same as the original AP PTMP airMAX-AC mode and has had no changes in the v8.3 line. The same distance limitations continue to apply for flexible mode PTMP usage as before.

In flexible framing, the number of allowed CPEs is 85. In fixed framing, the currently allowed number of CPEs is 60, due to memory constraints on our airMAX-AC coprocessor. This limit will be increased in the future to match the flexible framing limit.

In fixed framing, while there is no limit to the supported distance, we have internally limited it to 75km or 46 miles. This limit is regardless of the channel width. Based on community requirements this limit can be changed.

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.

In fixed framing, we have an additional step between when a CPE first registers with the AP to when it is allowed to complete its association process with the AP. We call that “admission control”. During this step, just like a DOCSIS cable modem, the CPE needs to complete its ranging measurements with the AP and synchronize timing, before being allowed to send data.

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.

Simply enable the GPS Sync option from the TDD Framing section, on all the APs you wish to synchronize, like so: 

The AP’s dashboard page will show the TDD framing and sync status:

Pay special attention to the following:

1. Synchronized APs must use the same TDD framing and ratios.
2. 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.

Yes, when using 5ms TDD framing on both the AC and AF5/AF5x links. Please make sure the same ratios are used, and you consider that only AP syncing is currently supported. Note:  For AF5s in FDD mode, sync is not relevant as it is transmitting and receiving at the same time.

If no GPS signal is available, or if the signal is lost, the APs shall continue to operate in TDD fixed frame mode. As soon as the GPS signal is recovered, they shall automatically resynchronize and continue operation.

TCP-based download traffic requires TCP ACKs to be sent using the UL. In a PTMP network, there will be multiple TCP downloads going on, each requiring TCK ACKs being sent using the UL. Having enough time on the UL to allow multiple CPEs to send TCP acknowledgments, reduces the latency experienced by TCP, thereby increases the actual DL throughput.

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.

Yes. Any airMAX-AC product when set to Access Point airMAX-AC mode allows selecting 5ms and 8ms TDD framing. The only difference is the products that have the GPS component are able to synchronize their cycles.

GPS Sync is built in, there is no cost to use this technology.

GPS Sync is all about time sync, allowing access points to transmit at the same time, resulting in better performance whilst using less channels.

Only your Access Points require the GPA antenna. This is because only the Access Points need to be synced together. Station devices can benefit from having GPS to enable the Map and Fresnel diagrams on the dashboard, but that can also be enabled by adding the location of the Station on the System Tab.