Everything You Need to Know about Point-to-Point Wireless Links

Point-to-point wireless links are being deployed increasingly often, both in busy urban areas and in more remote areas. Free-to-air installation is getting more and more popular.

The initial wave of adoption was driven by companies that needed to interconnect nearby offices, but could not obtain the approvals required to install their own cables. In time, though, the reasoning diversified. Point-to-point wireless links, such as FSO and microwave links, offer an interesting compromise between the convenience of a wired link and the speed, security and flexibility of a wired connection.

When should you consider using FSO or microwave links? Point-to-point wireless links can match dedicated wired links in terms of speed and reliability, but they are usually more expensive to operate and maintain. What makes this solution so compelling?

Most businesses that rely on point-to-point wireless links do so for reasons such as:

• They are unable to get the approvals required to install their own cabling infrastructure, or would rather avoid the waiting and the legal hassle involved.

• They operate in areas where leased lines are expensive enough that operating your own equipment is a compelling option in financial terms. The additional flexibility and independence that they get from controlling their infrastructure also offsets some of the maintenance cost.

• They operate over long distances (over 5-10 km) that are not feasibly covered with dedicated wired infrastructure and available leased-line options do not meet their requirements.

• They have specific security or performance (especially latency) requirements that cannot be satisfied by leased lines

If you find yourself in one of these situations, installing a point-to-point wireless link is one of the options that you may want to look into.

What are my options?

“Point-to-point wireless” is a term that describes the structure of a network, not a particular technology or connection method. Point-to-point wireless simply means that a transmitter can only talk to a single receiver at a given time (which is then connected to a network router and, through it, to the rest of the network). It does not specify how that is done – except, of course, that it’s done without cables.

There are several technologies that can be used to implement this network structure, such as free-space optical (FSO) links or short-distance microwave links. There are three fundamental properties that distinguish them from each other:

• The distance that they can cover and whether or not each antenna must be visible from its counterpart
• The type of the carrier wave (microwave/radio waves, or light)
• Whether or not you need to hold a license in order to operate the equipment

Line-of-sight (LoS) vs. non-line-of-sight (NLoS) transmission is the distinction that is most easily made. Some means of transmission work only as long as there is a direct, unobstructed “line of sight” between the antennas of the receiver and the transmitter.

This requirement puts a cap on:

• After a certain distance (which depends on the height of the two antennae), the Earth’s natural curvature causes the two antennae to become invisible to each other. An observer sitting on top of one antenna would not be able to see the other one, because it’s below the horizon.
• Deployment environment. Even over short enough distances, there are obstacles, such as tall hills or broad, tightly-packed groups of tall buildings that render the antennae invisible to each other.

Non-LoS methods do not require a direct, unobstructed line of sight between the two endpoints. However, most non-LoS methods do rely on LoS communication with intermediary nodes called repeaters — so you can think of non-LoS communication as being achieved, more or less, by “stringing together” several LoS sections.

Optical vs. microwave/RF links. Wireless data transmission over non-trivial distances can be carried out by two means, and both of them rely on sending data using electromagnetic waves.

One option is to send data encoded in pulses of light; the other is to send data over a stream of radio waves — typically, though not necessarily, in the microwave region of the spectrum.

Free-Space Optical (FSO) links are the most widely-deployed solution that relies on optic data transmission. FSO links are fast, line-of-sight links with several useful properties.

Microwave links can be either LoS or non-LoS, so they work over higher distances. Traditionally, the maximum distances have been in the 60-100 km region, but there is no hard limit.

Licensed vs. License-Free/Unlicensed operation. The electromagnetic spectrum is divided into several slices or *regions*, based on frequency. Some of these regions, such as the region between 694 and 790 MHz, can only be used by operators who hold a *license*, and only for specific purposes. Other regions can be used without a license, as long as the devices operate below a certain power threshold.

Private operators can be (and often are) granted a license, but the process is not very straightforward, and operating within a licensed band is neither easy nor cheap. When it comes to internal communication/data traffic, operating in a license-free region is far more hassle-free and is sufficient for most businesses.

Three technologies see significant use and we are going to discuss each one of them. These technologies are:

• Free-Space Optical (FSO) links
• Short-distance microwave bridges
• Long-distance microwave link

Free-Space Optical (FSO) Links

FSO is an optic, line-of-sight, license-free data transmission method which uses a tightly-focused beam of light to transmit data between two endpoints. More to the point, it transmits data by modulating a laser beam.

If you think this sounds an awful lot like fiber optics, you’re right — except that signals are transmitted through air, rather than through a dedicated transmission medium. The lack of a dedicated transmission medium (i.e. the cable, made from a material with special optical properties) that you can bend and route in order to guide the light means that there has to be a free, straight path between the two endpoints. But it also means that there is no fibre cable to bury or break.

FSO links use light beams at specific wavelengths in the infrared spectrum, between 750 nm and 1550 nm. These values are not chosen at random — they are chosen so as to be safe for the skin and eyes, and to guarantee good signal propagation properties.

Speed. With today’s technology, FSO links can reach speeds of 622 Mbps, 1.25 Gbps, 2.5 Gbps or even 10 Gbps. This is on-par with other point-to-point wireless technologies.

Distance. The distance that FSO links can cover is limited by atmospheric conditions and power. Typically, FSO equipment can operate reliably, year-round, over distances of up to 1.5 – 2 km, but they are usually deployed over shorter distances, on the order of 400-600 meters.

Reliability. Historically, the reliability of FSO transmission used to be frequently brought into question.  However, modern FSO technology can offer carrier-grade reliability over distances typical for urban deployment. Where absolutely necessary, FSO links can be deployed along with a redundant, RF-based LoS solution (i.e. a microwave uplink).

Equipment. FSO links use remarkably compact, self-contained equipment. You will need to install a FSO laser link device at each communication endpoint. This device incorporates both the transmitter and the receiver, and looks somewhat like a bulky CCTV camera.

The only major installation hurdle is that it needs to be tightly secured in place, on a stable surface, so that its beam can be aligned to the receiver at the other end. This is why FSO emitters typically cannot be installed on top of poles and are rarely used for mobile or portable units.

Short-Distance Microwave Bridges

Microwave bridges (sometimes called RF links or just microwave links) are line-of-sight, RF, point-to-point links that transmit data over radio waves in the microwave spectrum. Depending on which region of the spectrum they operate in, the use of microwave bridges may be licensed or license-free, but most private business users prefer license-free operation if possible.

Speed. Most companies operate microwave bridges at speeds of up to 1.25 Gbps, but higher speeds (2.5 Gbps, 3 Gbps, up to 10 Gbps) are also available. Bandwidth-wise, the performance of microwave links is similar to that of FSO links, but FSO links can offer better latency.

Distance. Short-distance microwave links can be used to connect endpoints that are placed several kilometers apart. The maximum distance depends on antenna size, shape and transmission power but, as a rule of thumb, as long as LoS propagation is possible, microwave links can cover any urban or suburban deployment scenario.

Reliability. LoS microwave links are highly reliable. Like any form of wireless links, they are susceptible to various forms of interference, e.g. from unauthorised or incorrectly configured RF equipment.

They are also affected by atmospheric conditions such as rain, snow and fog, but not as much as FSO links. Nonetheless, they can offer carrier-grade reliability; in fact, mobile carriers routinely rely on this technology.

Equipment. What kind of equipment can you expect for this sort of link? A radio modem, a radio unit and a directional antenna will have to be deployed at each communication endpoint.

The modem and the radio are electrically powered, and will need a reliable mains connection. The antenna is typically 30 cm – 1 meter in diameter (but can be wider) and will need to be securely mounted on a rooftop or a pole, along with the radio unit.

Long-Distance Microwave Links

Long-distance microwave links are non-LoS, RF, point-to-point links that carry data over RF waves in the microwave spectrum over long, non-LoS distances. This is achieved by using relays or repeaters — radio devices which receive and relay microwave links to other relays or endpoint devices — stringed together to cover the necessary distance.

There are both licensed and unlicensed spectrum regions available for long-distance microwave links. Most business users prefer the unlicensed regions, but operation in a licensed region can offer additional guarantees regarding the absence of radio interference.

Speed. Historically, long-distance microwave links have been developed for large-capacity links, so they are in fact capable of high speeds, reaching up to 10 Gbps.

Distance. In theory, there is no limit to the distance that you can cover, but typical upper bounds are at 60-100 kilometers. Few businesses *need* dedicated links over longer distances, and high speeds over longer distances become too expensive with this approach.

Reliability. Long-distance microwave links are highly reliable — in fact, they are often used by mobile carriers. However, operation in a license-free area of the spectrum comes with no guarantees regarding interference with other equipment. As the distance increases, so does the chance of interference from other devices.

Equipment. In addition to the equipment required for a LoS microwave link, you will also need to install microwave relays at several points between the two communication endpoints. The relays, consisting of a radio unit and (at least) one antenna, are usually pole-mounted.

What Is the Best Option?

None of these options are universal. When deciding which one to pick, you should consider your requirements in terms of distance, performance, uptime requirements, as well as the budget you have for purchasing and maintaining equipment.

That being said, here are a couple of broad guidelines that can help.

FSO links are ideal for deployment on short distances, in areas with a lot of RF interference. They are ideal for license-free operation, over distance of a few hundred meters, in busy urban areas, where the unlicensed regions of the RF spectrum are busy or occasionally disrupted. Installation tends to be more sensitive, but maintenance tends to be easier.

LoS microwave links are ideal for deployment over short and medium distances, especially in areas where the unlicensed regions of the RF spectrum is relatively unoccupied (e.g. low-density suburban areas), or in areas such as coastlines, where weather conditions make FSO links unreliable.

Long-distance microwave links are ideal for medium- and long-distance communication, but the risk associated with RF interference in unlicensed regions of the spectrum grows with distance. Maintenance and equipment costs also increase with distance, and depending on how and where pole-mounted equipment is installed, you may still need various approvals from local authorities.

Conclusions

The performance, reliability and flexibility of a wired infrastructure is hard to beat. However, when this type of infrastructure is impossible to deploy, point-to-point wireless links, such as FSO or microwave links, can be a very compelling option.

Point-to-point wireless links are competitive in terms of performance, reliability, security and flexibility, but they are more expensive to install and operate. They are significant investments — especially when they cover long distances — that see a significant ROI only through long-term commitment.

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