If we were to run a contest for the most deceptively simple question in the networking world, “who do you interconnect two networks?” would be one of the most likely winners. It’s a very common problem: companies with multiple sites, as well as companies with a single site but multiple buildings, or located in remote areas, frequently have to solve it.
You’d think the answer is obvious – you run a cable between them – but that’s not always an option. In fact, today we’re going to talk about the other option: wireless bridges.
A wireless bridge is a solution (specifically, a set of devices) that allows you to connect two networks, or more accurately two network segments, over a wireless channel – sort of like a bridge, if you will, hence the name. Wireless bridges can be used to interconnect networks such as those in buildings in the same campus, in shipping and manufacturing sites in neighbouring districts or nearby towns, offices across the street from one another and so on. You may also find a wireless bridge referred to as a WiFi bridge.
Wireless bridges use radio (RF) signals in the microwave (MW) or optical/laser (free space optics – FSO) links to interconnect two access points. They use antennae – usually highly directional antennae – to send and receive wireless signals on one side, and connect to regular IP networks on the other side. This way, two IP networks can be connected through a wireless link.
Since they transmit and receive signals through air, using antennae or optical receptors, wireless bridges are sometimes referred to as free-to-air (FTA) bridges. And, since communication over a wireless bridge is always done between two transceivers (as opposed to one emitter that broadcasts a signal to any receiver that is willing to “listen”), these wireless links are point-to-point links.
Why go through all that trouble?
The most common answer is that you need a physical link, which rules out a VPN connection, and a wired link isn’t possible, either. In some areas of London, for example, high-performance leased lines can be prohibitively expensive. In other locations, leased lines may simply be unavailable.
Deploying and maintaining your own long-distance wired infrastructure is also expensive, and sometimes outright impossible for technical reasons. It may also require various approvals from public authorities. In short: sometimes, a wireless bridge is the only feasible option.
In other cases, a wireless bridge may simply turn out to be the better option, for technical, performance or security reasons, price or legal constraints, business goals, or maintenance capacity constraints.
Some of the most common reason for deploying wireless bridges include:
There are two sets of parameters that define a wireless bridge’s performance: its network performance – bandwidth, latency and so on – and its range.
In terms of distance, wireless bridges can cover distances ranging from a few dozen meters – indeed, comparable to those of a Wi-Fi access point, as some of them really use the same technology – to a few dozen kilometers, typically up to 50-60 km.
In terms of performance: practical caps on bandwidth are in the 1 – 1.25 Gbps range, with latency figures that vary between those of a regular leased line or better (for FSO links) to those of a good 4G network (for long-distance microwave links). As a rule of thumb, you can expect both figures, especially latency, to be better than what you could get over a VPN connection, but the exact numbers depend on the bridging technology and equipment.
That being said, we should point out that wireless bridges are not inherently better or worse than other interconnection technologies. Just like wired bridges over leased lines, or VPN connections over public networks, they excel in some regards but not in others.
Note: confused by bandwidth and latency? Here’s a quick explanation:
Imagine you develop the following data exchange method: you copy data on 64 GB USB sticks, fill up a pizza box with everyone’s sticks, and drive from one office to another. Let’s say the first office is in Canary Wharf and the other one is in Chiswick, and assume you can fit about 100 sticks in a pizza box.
Now, it takes you about 45 minutes to get there, but you are ferrying 6.4 TB of data, which is a lot: you are effectively transferring data at a bandwidth of 2.5 GB/s. However, if all you want to send is a 4 MB image, it would still take about 45 minutes – nearly 100 times more than over a broadband connection. That’s latency: sending a single bit takes 45 minutes. That’s a high-bandwidth channel – you can pack a lot of other bits (up to 6.5 TB!) in a single go – but it also has a very high latency.
This is a surprisingly confusing topic. If you try to look up wireless bridges online, you’ll get dozens of seemingly unrelated results. That’s because “wireless bridge” is more of a technical concept than a specific technology. There are many ways to build one.
The most straightforward way to break them down, which is what we will use here, too, is by the connection technology. There are two large classes:
Which technology is used, and in what way, has two important implications.
Licensed vs. license-exempt use. First of all, operating in some regions of the radio spectrum requires a license. That’s exactly what it sounds like – a document, issued by Ofcom, which allows you to use a specific type of equipment in a particular region, for a specific purpose.
This isn’t just bureaucratic pedantry – when operated incorrectly, radio equipment can cause dangerous interference, sometimes in completely non-obvious ways. Some of you may remember a particularly hilarious incident from 2020, but there is a real danger of disrupting important communication services.
However, some regions of the spectrum, which are “far” enough (in terms of signal frequency and technical properties of the RF equipment) can be operated at low enough power without a license. This limits their range, typically to 1-2 km, but that’s still long enough to be useful in a lot of cases, and you don’t need a license.
Line-of-sight (LoS) vs non-Line-of-Sight (NLoS). Imagine that, instead of a fancy, high-speed laser link, you’re trying to send data in Morse code, using a laser keychain. The only way this works, of course, is if the other person can see your laser pointer.
Now, replacing the cheap laser keychain with a high-speed laser installation will dramatically improve the data rate, but the restriction that the light has to make it to the other optical receptor remains in place. Since light only moves in a straight line, that means there has to be a direct (i.e. straight), uninterrupted line-of-sight between the two endpoints of the transmission.
This is called Line-of-Sight (LoS) communication. If both endpoints are at ground level, the maximum distance is about 4-5 km. This maximum distance can be increased by raising the antennae, but not indefinitely, as radio and optical signals attenuate with distance.
Some technologies allow communication between endpoints that do not have a direct, uninterrupted line-of-sight between them. These links are, of course, called non-Line-of-Sight (NLoS) links.
RF bridges use radio signals to transmit and receive data. Technically, all high-speed RF bridges currently in use operate in the microwave region of the spectrum, but due to largely historical reasons, the term “microwave bridge” is applied more commonly to those that operate in the 30-80 GHz range of the radio spectrum.
Virtually all RF bridges used for long-distance, high-throughput communication are line-of-sight bridges. However, they are typically quite resilient to atmospheric conditions and some types of obstacles. RF links will not penetrate buildings, but will tolerate some rain, fog, or tree foliage. However, RF relays make it possible to extend the range of RF bridges beyond the line-of-sight requirement.
In terms of licensing, RF bridges cover both licensed and license-exempt regions of the spectrum.
In the UK, the most common license-exempt regions are 2.4 GHz, 5 GHz (bands A and B) and, more recently, 60 GHz. The first two bands are commonly used for low-distance (50-150 meters) Wi-Fi bridges, whereas the license-exempt 60 GHz range can be used over longer distances, up to 1-1.5 km. This is enough to bridge networks in adjacent buildings, for example.
Unlicensed links are quick to install and cheap to operate, and can be a very good alternative to leased lines. They are particularly useful in low-density suburban areas, or in remote regions, especially those with difficult weather, such as coastlines. In busy urban areas the unlicensed spectrum regions are likely to be very crowded, so the effectiveness of these links is limited.
The most common licensed regions used in the UK are bands in the 6-38 GHz region, and the 70 GHz and 80 GHz bands. A lower-frequency, 3 GHz band is also used quite commonly, but obtaining a license for this band can be more difficult, as several large operators are already using it. The 70 and 80 GHz bands are used for high-speed (commonly 1-1.25 Gbps, but higher speeds are possible) links, generally over very long distances, up 60-100 km, sometimes more.
Wireless bridges operating in licensed regions are obviously hard to beat in terms of price and efficiency when it comes to long-range, NLoS operation. However, they can be an efficient option for short-range links in busy urban areas, where leased lines are expensive or simply not available, and the unlicensed regions of the spectrum are too crowded.
Free-space optics (FSO) laser links are wireless bridges that use optical signals to transmit and receive data. At first sight, they look like they’re just the fancy version of RF bridges (optical signals are electromagnetic waves, too, just at a higher frequency than radio signals).
So what’s the point?
The useful property that laser links have is that they are immune to electrical and radio interference. There are sources of interference, such as lightning, high-power electrical lines, or other licensed operators, that can interfere with radio signals even in the licensed region of the spectrum. In fact, medium- and short-range RF bridges near busy industrial floors or electrical stations can be subject to interference from equipment that belongs to the same company that operates the bridge itself.
FSO laser links aren’t subject to this type of interference, and they are license-exempt. However, they require a clear line of sight: physical obstacles, even “soft” obstacles, like heavy fog or smoke, can interfere with a FSO wireless bridge’s operation.
This limits its operating distance to about 3 km for high-speed (1 Gbps) links, and 4-5 km for 100 Mbps links. That is still higher than what you can typically get with license-exempt RF bridges, though, especially in busier areas of London.
RF wireless bridges are typically installed outdoors, on top of buildings and/or on metallic poles or masts. They have a directional antenna (which looks more or less like a satellite dish) and require electrical power to operate. For short-range, low-power links, that can be supplied through PoE, but a dedicated power source is otherwise required. Each bridge endpoint is connected to its respective network via a wired connection.
FSO laser wireless bridges are quite similar, but they use optical transmitters and receivers instead of antennae. They are also mounted on top of buildings on masts, and most of them look a bit like tripod-mounted cameras.
Determining if, where, and how the devices can be installed is the more difficult part of installing LoS bridges. The installation process itself is fairly straightforward and usually takes a few hours, depending on distance and environment. For long-distance, NLoS communication, the process is slightly more elaborate – it requires a basic initial design effort and licensing, and installation is of course complicated by the fact that it has to be carried out on endpoints that can be many kilometers apart.
Both RF and FSO links require regular and careful maintenance. This is particularly important for equipment that operates in the licensed region of the spectrum, where the requirements for compliance are very strict.
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There is a silver lining to this seemingly endless diversity of technologies: there are lots of options to choose from. That’s a good thing: it gives you more flexibility in terms of performance, operational constraints, and budget.
The exact choice depends on your specific requirements, on deployment environment and regulatory restrictions, but typical choices are roughly as follows:
In short, no. High-quality equipment that’s correctly installed and receives proper, regular maintenance, wireless bridges are safe to operate: the RF and optical signals are safe, and the equipment is no more of a fire hazard than any other electrical equipment that operates outdoors.
RF and optical equipment for telecom use is designed to operate in areas of the EM spectrum that interferes with the human body as little as possible, at signal strength levels that cause as few biological effects as possible.
Furthermore, installation guidelines are specifically designed to minimize exposure to RF and optical signals. That’s facilitated by the fact that the antennae that RF wireless bridges use are highly directional – most of the RF waves are concentrated in a small, narrow region between the two antennae called the Fresnel region, and which looks sort of like a long, narrow barrel with the antennae at its ends.
As for optical links, they typically operate outside the visible spectrum, at low signal strengths, and are specifically installed so as to avoid exposure. Consequently, as long as they are correctly installed, wireless bridges do not pose a health hazard.
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