airFiber - Frequently Asked Questions (FAQs)

The minimum distance between Master and Slave ends of the link will depend on the model:

• airFiber5: 10 meters
• airFiber 2X, 3X, 4X, and 5X: this varies depending on the antenna gain, but a general rule would be 25 meters
• airFiber24: 25 meters (using firmware 2.0 or higher)
• airFiber11: 25 meters 

Hybrid Division Duplexing (HDD), is a special, proprietary wireless mode which allows for half-duplex wireless operation, where same frequency is used on both TX/RX chains. This proprietary mode is similar to Time Division Duplexing (TDD) and allows for more resilient radio links, at longer distances. 

The Frequency Division Duplexing (FDD), is a wireless protocol that allows for full-duplex wireless operation. It uses separate frequencies for TX/RX chains on the same radio. Given the isolation, this wireless mode allows for the highest available throughput.

airFiber radios can operate in Full-Duplex (FDD) or Half-Duplex (HDD) mode. Full-Duplex offers the highest performance and lower latency (below 1ms) but generally speaking, requires perfect Line of Sight and perfect alignment. On the other hand, Half-Duplex mode is more tolerant to misalignment and in certain cases can work in nLoS (near Line of Sight) scenarios, especially at lower frequencies, like in the 2.4GHz band (ex. AF-2X).

Important: When the radios are running in Half-Duplex mode, it's recommended to always set the same Wireless Mode (Master or Slave) in all the units located at a specific location. 

Which airFiber device models support FDD & HDD modes of wireless operation?

Device Model	Full-Duplex (FDD)	Half-Duplex (HDD)
AF-2X	-	Yes
AF-3X	-	Yes
AF-4X	-	Yes
AF-5X	-	Yes
AF-5/AF-5U	Yes	Yes
AF24	Yes	Yes
AF24HD	Yes	Yes
AF-11	Yes	-
AF-5XHD	-	Yes
AF60	-	Yes
AF60-LR	-	Yes

For more information, along with other airFiber features in the different User Guides downloadable in our Downloads page, under the Documentation section. 

How do Half-Duplex & Full-Duplex modes work specifically with airFiber devices?

When you think of any airMAX and airFiber X-series products, you immediately think Half Duplex. With those products, you choose one frequency and width. Both the AP and CPE use that same frequency/width to send data in both directions. The AF5 and airMAX both do it by taking turns sending data, one side at a time. While there are largely technical and hardware differences in how airMAX and airFiber send data using a single channel, just remember that in both cases it's one side sending at a time. That's the AF5 in Half Duplex and AF-2X/AF-3X/AF-4X/AF-5X.

If you are used to picking a single open channel using airMAX, or replacing another Half Duplex link, using Half Duplex with AF5 or any AF-X model will seem very familiar and easy to understand. Just pick a channel that's open on both sides, and use it for both directions.

One very important thing to understand is that Half Duplex with AF5 is not half speed. It's not like Half Duplex ethernet where you use fewer wires and run at half the speed. With AF5, you get the same throughput per spectrum used, in both Half and Full Duplex. It's not whether you use half or full that determines throughput, it's the amount of spectrum you use. And in fact, under some conditions, Half Duplex will provide better total throughput than Full Duplex.

The other important Half Duplex difference between AF5 and airMAX is the AF5 send/receive time slots are fixed, while in airMAX they're variable. airMAX will adjust the time slots for the direction that needs it most at any given time, being able to assign nearly all the bandwidth to one direction. But in AF5, in order to keep the latency down, it doesn't have time to make those adjustments. The send / receive time slots or Duty Cycle is fixed so both sides can instantly transfer any data that comes along. In v2.1, the Duty Cycle is always 50% which means 50/50 send/receive. Since firmware version 3.2, you can choose among different duty cycle menu choices like 75%, which would mean 75% download and 25% upload; and 67%, which would mean 67% download and 33% upload. 

There are more advanced ways to use Half Duplex, but we'll skip those for now and move on to Full Duplex. With Full Duplex, you use separate channels for each direction. One channel is exclusively used for transmission, and another different channel is exclusively used for reception. Both sides can send at the exact same time, hence the name Full Duplex. Because each frequency is only used in one direction, the one-way flow is doubled compared to Half Duplex. But, in order to send in both directions, you now need two channels and more spectrum. It's important to remember that any speed increase in using Full Duplex, is only achieved by increasing the use of spectrum.

Using Full Duplex does have some interesting advantages. For starters, each end of the link has noise on a different frequency, so you can independently tune the best reception frequency for each side based on noise detected. You no longer have to find a common ground channel that's open on both sides.

The other advantage of Full Duplex is that the channel width for each direction can be different. You can provide a wider channel for faster downloads, and a narrower channel for uploads and save spectrum by doing that.

Note: Full Duplex can self-interfere if the power is too high, and the channels too close. Cranking the power to the max is counterproductive with Full Duplex. Between 43-45 dBm seems to be a limit above which higher constellations can't be maintained and performance will start to drop.

Now getting back to Half Duplex. You don't have to use just a single channel for Half Duplex. Nothing says you can't use one for each direction. But the AF5, will still only send in one direction at a time, so this can waste spectrum. But using two channels in half can actually be very useful at times. Let say you can only find a different 20 wide free channel on each side of the link. And they both overlap with 10 shared widths in the middle. You can't use Full Duplex because they will self-interfere. Or you could settle for the common 10 wide in the middle. But the best solution is to use Half Duplex with two slightly different channels. Because each side sends one at a time, you won't self-interfere in the middle 10. And the increased bandwidth allowed by this will double your throughput compared to settling for the shared 10 wide in the middle.

How much does rainfall impact airFiber links?

Radio signals undergo significant levels of attenuation from humidity and precipitation: around 8GHz and higher. Therefore, it's always recommended to consider humidity and rainfall as factors in link planning.

Due to their acute wavelengths, signals in the 24GHz band encounter additional atmospheric attenuation beyond that which is expected from free space path loss due to water absorption, including rain fade. A general guideline is an additional loss of 1-3 dB per kilometer. The amount of additional atmospheric attenuation will vary, depending on your specific installation environment (the world has a variety of rainfall zones).

For example, links exposed to 25 mm/hr of rainfall could undergo additional 4dB of attenuation per km. The table below illustrates the estimated additional attenuation at various rainfall rates and distances, based on information from the FCC bulletin.

Estimated rainfall attenuation rates at 24GHz. 

Distance	1.25 mm/hr	5 mm/hr	25 mm/hr
1 km	0.2 dB	0.7 dB	4 dB
2 km	0.4 dB	1.4 dB	8 dB
3 km	0.6 dB	2.1 dB	12 dB
4 km	0.8 dB	2.8 dB	16 dB
5 km	1 dB	3.5 dB	20 dB

Can the AF5 connect to the AF5U?

What Output (TX) Power and Receive (RX) Sensitivity levels are required for certain modulation rates?

In order to negotiate at a given Modulation & Coding Scheme (MCS) rate, radios require minimum signal thresholds. Radio sensitivity information is available on our product datasheets (found on our Downloads page).

Additionally, radio output power also determines modulation rate, since higher output powers cause distortion in symbol mapping at transmitter/receiver radios. Compared to competitor radios, airFiber radios operate cleanly to maximize throughput efficiency at every output power level.

The following two tables shown below help explain these two concepts:

1. The first table shows modulation rates according to maximum TX Power.
2. The second table shows modulation rates according to minimum radio sensitivity, based on given channel widths.

Note: Remember that EIRP (and consequently, maximum output power) depends on country code and frequency.

Suggested Max TX Power:

Modulation	Output Power
8x	19-20 dBm
6x	21-22 dBm
4x	23-24 dBm
1-2x	26 dBm
1/4x	26 dBm

From the table above we can conclude that to achieve up to 8x modulation it's highly recommended to use output power up to 20dBm. If higher levels are used, for example 21dBm output power, it's possible that you'd only get up to 6x.

Suggested RX Sensitivity Levels:

Radio sensitivity depends on channel width and modulation, for example if you want to get 8x modulation using 30MHz channel you need a received signal strength of -62dBm or better (like -60dBm, -55dBm, etc...).

User Tip: It's a good practice to keep a certain fade margin to prepare for weather changes or other situations that could temporarily affect your link. Recommended fade margins depend on your area and link distance. In most cases, it's recommended to keep at least 5dBm of fade margin.

BER (Bit Error Rate) is a very popular measurement of the percentage of packets with errors on a digital link. A BER value of 10 - 6 = (1/1000000), means that of every 1.000.000 received packets 1 has errors and must be retransmitted, which is considered a very good link. On the other hand, a link with a BER of 10 - 3 = (1/1000), is considered a very bad or damaged link (according to the IEC 608720-3 and IEC 60801-4).

Thanks to airFiber's revolutionary system it has an effective BER = 0, which means that of every 1.000.000.000 received packets 0 has errors. How is this possible? airFiber uses hitless adaptive modulation and an ARQ scheme over the radio link to ensure that no bit errors are delivered over the link. More information about BER available here: https://en.wikipedia.org/wiki/Bit_error_rate

Is auto modulation a hitless protocol?

Yes, airFiber Adaptive Modulation is hitless. airFiber uses a selective acknowledgment-based protocol to ensure that no packets are lost as a result of link adjustments such as changes in modulation. The only thing you should see during a transition is a short spike in latency while the rates change. Make sure Flow Control is enabled to avoid any potential Rx buffer overflow when the link is heavily loaded.

If the link needs to modulate down it will be capable of carrying less traffic and thus may lose packets as a result of overflow (i.e. the user rate is 700 Mbits and the link negotiated to 500 Mbits).

Which antennas are compatible with AF-4X?

The airFiber 4X (AF-4X) works with any 4.9GHz-capable MIMO antenna. The AF-4X is a high-performance radio designed to work in the licensed 4.9GHz band, also known as the public safety band. A license is required to use these radios in the United States. US units operate from 4,940MHz to 4,990MHz, and international units (only for export) can operate from 4.7-4.99GHz.

Important: You must have a license to use any 4GHz frequency. You will be breaking the law if you fail to comply.

The AF-4X works with any 4.9GHz-capable MIMO antenna. It is compatible with the Ubiquiti airFiber Dish 30dBi and 34dBi models, as well as the RocketDish 30dBi and 34dBi antennas. Unfortunately, since those antennas have been designed for the 5GHz band, the performance in the 4.9GHz range is lower. Therefore, it's important to consider this and use the following values in your calculations and in the data you enter into the airFiber 4X Wireless tab.

Shown below are a list of Ubiquiti antennas that can be used with the AF-4X and their corresponding gain.

Antenna	Gain at 4.9GHz
AF-5G30-S45	26dBi
AF-5G34-S45	30dBi
RD-5G30	26dBi
RD-5G34	30dBi
MonsterDish	34dBi

Note: The AF-5G23-S45 antenna is not recommended for the 4.9GHz, since its antenna gain at 4.9GHz is around 17-18dBi.