3 km gigabit link on laser modems

3 km gigabit link on laser modems

TL; DR This article describes our experience in building a wireless connection at a distance of 3 kilometers using Lantastica TZR laser modems without using radio. Conclusion: it really works, the real channel width is ~ 950mbit/s in duplex with delays of & lt; 1ms.

Typically, classical radio bridges like ubiquiti and mikrotik are used for such tasks, and few people imagine that radio is not the only available method. It turns out that laser modems exist in civilian performance, are sold openly and do not even require a license.

In practice, we will go through all the steps to set up a laser link: from installing equipment on the roof to setting up and testing bandwidth and delays.

Main features of the laser modem:

  • Does not use radio frequencies - data is transmitted using a laser beam in the invisible range of 780 ~ 850 nm. Does not cause radio interference, does not require permission to use radio frequencies
  • Low delays - compared to WiFi equipment, laser modems have delays comparable to cable connections.
  • More difficult to intercept data - radio bridges are easy to listen to without breaking the signal. It is almost impossible to intercept the traffic of the laser bridge without breaking the connection.
  • Stealth - the laser link is difficult to detect due to a narrow beam. A radio link is easy to detect, even at a considerable distance from the antennas.


Schematic representation of the principle of laser modem operation

How it works in practice: Each modem has a transmitter on a laser diode and a receiver on a photodiode. Modems "shine" into each other with a laser with a wavelength of 780 or 850 nanometers. The laser modem is picky about aiming accuracy, so the built-in motors automatically adjust the position of the transmitter and lens for the most accurate hit. Laser brightness and beam divergence are also automatically adjusted, depending on weather conditions.

At the output, the device is connected to a regular Ethernet network through a twisted pair or optics.

Weather Limits

Since data transmission requires direct visibility, precipitation in the form of snow, rain, fog makes it difficult to transfer data. In case of deterioration of the visibility of the device, the brightness is increased and the angular size of the light beam is reduced. The same device can reduce the data transfer rate, to ensure the best noise immunity.

The manufacturer claims that the device works successfully up to 99% of the time in a year in the weather conditions of central Russia. When weather conditions deteriorate, the modem switches to a reduced speed mode of 200Mbit/s. To improve link reliability, it is proposed to use channel duplication using a radio bridge, which can automatically be activated upon a command from a laser modem, in case of complete loss of communication on the laser link.

Table of availability of laser link during the year at the request of the modem manufacturer:
Range Maximum Speed ​​ Reduced Speed ​​ Backup Channel
1km 99% 0.9% 0.1%
3km 98% 1.5% 0.5%
5km 95% 3% 2%


--------------------------------- -------------------------------------------------- -------------------------------------------------- -------------------------------------------------- ---------------------------
Modems look like a surveillance camera in a metal stainless steel case. Mounted on a tripod with swivel bracket. ------------------------------------------ -------------------------------------------------- -------------------------------------------------- -------------------------------------------------- -------------------------------------------------- -------------------------------------------------- -------------------------------------------------- -------------------------------------------------- -----------------
  • Full Duplex speed: at a distance of 3km - 1 Gbit/s (98% of the year) + 100Mbit/s (1.5%); at 5km - 1 Gbit/s (95%) + 100 Mbit/s (3%).
    Often, wireless equipment manufacturers indicate a total speed in both directions, for example, at TX 50Mbit/s and RX 50Mbit/s, a speed of 100Mbit/s is indicated. Therefore, it is important to pay attention to the duplex parameter.
  • SFP port, two Ethernet ports (one with PoE out)
  • Built-in heating for working in the winter
  • Maximum consumption of 15 watts in the warm season; 65 watts in cold weather
  • Operating temperatures: -50 ° C to + 50 ° C, IP65 protection.

Lantastica TZR modem back panel


We needed to connect two buildings across the river at a distance of about 3 km. The case takes place near St. Petersburg. Pulling the optical line across the Neva is a real task, but the cost of such a project is tens, if not hundreds of times more expensive than any of the options for a wireless bridge.

First of all, we carry out reconnaissance of the area, we are convinced that there is a direct visibility between the roofs of houses, and we consider the approximate distance between the points with the help of the map.

The distance between two buildings where modems will be installed

We see that the desired point is in direct line of sight and is not blocked by trees. It is important to consider that trees can grow in a couple of years.

Installation location for the second modem

First, mount the bracket on a tripod using anchors. It must withstand wind loads and not vibrate.

For primary guidance to the remote modem, an optical sight is used, at some point the installers become like snipers.

Primary arm positioning with a laser sight

Now you can install a modem. It is important to remember that laser radiation is dangerous for the eyes and observe safety precautions. You can not look into the glass of a working modem.

To access the contacts, the back cover of the device is removed.

Power connection

The final view of the modem after installing and removing the protective film

Installation of the second point and the rest of the photo under the spoiler:

More photos
View from the second roof of the first modem

Type of installed modem

Power connection

Install SFP Module

View of the installed modem with the lid closed

Side view


Optionally, a camera can be installed in the modem, shooting in the infrared range, it serves to visually assess the working conditions, check the contamination of glass, etc. The video stream from the camera can be obtained in the usual RTSP format and integrated into the video surveillance system.

This is how the image from the camera inside the modem installed at our second point looks like. The beam from the opposite modem is seen in the image as a bright iridescent dot.

Video from the camera inside the modem. You can see the light spot of the opposite modem

Telnet Interface

Setup of devices happens through console telnet. Consider step-by-step initial setup steps. To connect to the modem console, you can use the built-in telnet client in Windows, or putty.

The manufacturer uses special terminology for console commands: interactive menus are called scripts , actions within the script are called commands .

The modems are equipped with motors for automatic guidance, which from the factory are in parking mode. To start the setup, you need to remove the motors from the parking: either using the button on the modem, or a command in the console. To do this, run the script trk and execute the command .go/z
in it
  telnet ip_ modem # connect to modem

 Do APP & gt; trk # enter the track script
 trk & gt; .go/z # remove the motors from the parking lot and set them to zero position

After that, run the automatic configuration in turn on each modem. It takes about ten minutes. During this time, with the help of motors, the modem guides as accurately as possible to the opposite.

  & gt; setup
 setup & gt; .dist 3000 # set the approximate distance in meters
 setup & gt;  .modem # run the autotune script and wait
 setup & gt;  .use # apply settings

Successful completion of the autotune script

Now that the modems are configured, we can check the correctness of the settings and the signal quality. To do this, we enter the monitoring script, which displays the current signal level, current traffic, failure statistics and the exact link distance, which we turned out to be 3059 meters.

  & gt; m # go to the monitoring script

The monitoring script displays signal strength, current traffic and failure statistics
We are interested in the following metrics:

Divergence is the beam divergence in mrad (milliradian), in our case it is 1.96 and 1.74 mrad.
Gain - the current brightness level of the laser.
CINR - (Carrier to Interference + Noise Ratio) signal to noise ratio.
FSO Speed ​​ - the speed at which the optical link works, in our case 1000 megabits.

This is what a customized link looks like: a video stream and a monitoring script running on two modems simultaneously.

Wiedo: image from cameras on modems and connection statistics


Setup is complete, you can proceed to the actual tests. To begin, measure the bandwidth using the jperf utility, it is the equivalent of iperf3 for Windows. The really accessible width of the channel turned out to be in the region of 950 megabits symmetrically in both directions, that is, in duplex mode. This is a very good result.

(Clickable) Passband measurements.
Usually, radio bridges built on the WiFi protocol or proprietary analogs like nv2 from Mikrotik add a delay of around 2ms. With laser modems, under ideal conditions, the delay turned out to be indistinguishable from the cable connection & lt; 1ms. Below is a screenshot of the usual ping utility running before the remote host behind the laser link.

Delay like a wired connection


In general, the impression is positive, the stated characteristics correspond to real ones. Particularly pleased with the complete lack of delay. Setting is still difficult, without the help of the manufacturer, I would not understand. I would like to have a web interface, instead of the console. The price of one modem from 170 thousand rubles, depending on the configuration. Similar to the characteristics of the solution like AirFiber are in the region of 110 thousand. rubles.

Thanks to the company NPK Katarsis for providing the equipment and the company's engineers for help in setting up.

Source text: 3 km gigabit link on laser modems