Monthly Update - March 2023
This month we’re following up with more backbone information because some people asked “What does a backbone site actually consist of?”. Then news from AREDN at SCaLE in Pasadena this month where we won an award!
Anatomy of a Backbone Site
Every backbone site is unique in many ways. What can be seen from the location and what is obstructed? Is it high above the desired coverage area, or far away? Are there rules restricting what can be mounted? Is there emergency power? Is there any power? While there are many variables, there are some general building blocks any site will have. Here we’ll give an overall view of what is involved when building a new backbone site.
AREDN Radios
First, we need some AREDN radios. AREDN radios at backbone sites provide two major functions. First, they distribute the AREDN network to locally operated nodes in the community. Second, they provide point-to-point links to other sites in the network.
For local coverage we use some form of sector antenna. With the pending release of AREDN’s support for “AC” radios, we’ve started to deploy either the Ubiquiti Rocket 5 AC Lite or the Mikrotik mANTBox 19s, radios with good transmit power and excellent receiver sensitivity. The Mikrotik radio comes with a built-in 120 degree sector antenna which makes installs simple. The Ubiquiti requires a separate antenna, which increases the overall cost, but enables more flexible coverage options which some sites need.
For point-to-point connections there are lots of good alternatives. Sites often use variations of the Ubiquiti PowerBeam, and the new “AC” models provide improved throughput.
Backbone Radios
A backbone site would not be a backbone site without at least one backbone radio. What those are and how they work was discussed at length last month. To quickly summarize, a backbone radio is a non-AREDN radio used to run AREDN traffic between two backbone locations. It uses larger antennas and more expensive radios, which results in better throughput.
Ideally, any backbone site would have more than one backbone radio - the backbone comes in and goes onwards to the next site. For redundancy, each backbone site needs to have at least three backbone radios, so if one link fails the network remains operational.
Switching and Power-over-Ethernet
Once we have a site with multiple radios, we need some sort of ethernet switch to tie them together. Here there are many options. If we also want to use this switch to power the devices, using power-over-ethernet (PoE), then the choices become a little more limited. AREDN radios use passive 24vdc power, and most PoE switches don't support this. We generally use versions of the Ubiquiti EdgeSwitch, although these are increasingly difficult to find. Don’t be tempted by Ubiquiti’s UISP switches - they work, but they have no local UI and must be configured via their cloud. Mikrotik also offers various PoE switches which have a low-power PoE setting; for example, the CRS112-8P-4S-IN is an excellent and flexible option.
You can also opt for a non-PoE switch and just use the PoE injectors that come with the radios. But we prefer the control a PoE switch provides, enabling us to power cycle radios remotely if we have to.
Backup Power
When the main power goes down (and during the storms this year, this has happened a lot) we want to keep the site online. At some sites we’re fortunate enough to share previously established failsafe power systems; at others we need to provide our own.
A typical setup starts with a Ubiquiti EdgePower. This device manages the charging of backup batteries, as well as seamlessly switching between them and mains power. For batteries we use old-school lead-acid; cheap and well understood. Alternatively, we’ve started to deploy LiFePO4 batteries (lithium batteries with the bonus of not bursting into flames) which provide better power density per kilogram and have a life of 3x or more compared to lead-acid. But they cost more.
How many batteries? That depends on how long we want to keep a system running before the mains comes back, and how many radios we need to keep operational.
Finally, solar power can be an option at some sites, providing an off-grid way to charge batteries.
AREDN at SCaLE
SCaLE is the largest community-run open-source and free software conference in North America. It is held annually (pandemics permitting) in Pasadena, in the greater Los Angeles area. AREDN has been at the conference for many years as there’s always been a crossover between the open-source world and amateur radio. The high speed, network oriented nature of AREDN is a particular sweet spot.
This year the team went all out with demonstrations of AREDN connected cameras (both onsite and off), VoIP phones, and hands-on with the latest hardware. Thanks in particular to KE6BXT, W6BI, and KM6SLF who brought most of the booth demos. Their work paid off; AREDN won the Best Presentation Award. Thanks everyone and see you next year.
AREDN News
The next release of AREDN is nigh. With fingers crossed, version 3.23.4.0 will be released sometime mid-April. More on that next time.
73,
Tim - KN6PLV - tim@sfwem.net