Mike Dano of Light Reading reports that Verizon is considering using wireless backhaul to reach as many as 20% of small cell sites. Verizon says they will use wireless backhaul for locations where they want to provide 5G antennas but can’t get fiber easily or affordably. The article sites an example of using wireless backhaul to provide connectivity where it’s hard to get the rights-of-way to cross railroad tracks.

This prompts me today to write about the issues involved with wireless backhaul. Done well it can greatly expand the reach of a network. Done poorly it can degrade performance or cause other problems. This is not an anti-Verizon blog because they are one of the more disciplined carriers in the industry and are likely to deploy wireless backhaul the right way.

Dano says that Verizon has already addressed one issue that is of concern today to municipalities that are seeing small cell deployments. Cities are worried about small cell devices that are large and unsightly. There are already pictures on the web of small cells gone awry where a mass of different electronics are pole-mounted to create an unsightly mess. Verizon describes their solution as integrated, meaning that no additional external antennas are needed – implying that the backhaul is likely using the same frequencies being used to reach customers. The small cell industry would do well to take heed of Verizon’s approach. It looks like courts are siding with municipalities in terms of being able to dictate aesthetic considerations for small cells.

Another issue to consider is the size of the wireless backhaul link. For instance, if Verizon uses millimeter wave backhaul there is a limitation today of being able to deliver about 1-gigabit links for 2 miles or 2-gigabit links for about a mile. The amount of bandwidth and the distance between transmitters differ according to the frequency used – but none of the wireless backhaul delivery technologies deliver as much bandwidth as fiber. Verizon has been talking about supplying 10-gigabit links to cell sites using next-generation PON technology. Wireless backhaul is going to be far less robust than fiber. This is likely not an issue today where many cell sites are using less than 2 gigabits of bandwidth. However, as the amount of broadband used by cellular networks keeps doubling every few years it might not take long for many cell sites to outgrow a wireless backhaul link.

The primary issue with wireless backhaul is the bandwidth dilution from feeding multiple wireless sites from one fiber connection. Consider an example where one cell site is fiber-fed with a 10-gigabit fiber backhaul. If that site them makes 2-gigabit wireless connections to four other cell sites, each of the 5 sites is now upward limited to 2 gigabits of usage. The bandwidth of the four secondary sites is limited by the 2-gigabit link feeding each one. The core site loses whatever bandwidth is being used by the other sites.

That’s probably a poor example because today most cell sites use less than 2 gigabits of bandwidth. Verizon’s use of 10-gigabit fiber backhaul moves them ahead of the rest of the industry that has cell sites with 1- to 5-gigabit backhaul connections today. The weaknesses of wireless backhaul are a lot more apparent when the wireless network beings at a site that only has a 1- or 2-gigabit fiber connection.

I’m sure that over time that Verizon plans to build additional fiber to relieve network congestion. Their use of wireless backhaul is going to push off the need for fiber by a decade or more and is a sensible way to preserve capital today.

The issues with wireless backhaul are far more critical for carriers that don’t have Verizon’s deep pockets, fiber networks, or discipline. It’s not hard today to find wireless networks that have overdone wireless backhaul. I’ve talked to numerous rural customers who are buying fixed wireless links from WISPs who are delivering only a few Mbps of bandwidth. Some of these customers are getting low speeds because they live too far away from the transmitting tower. Sometimes speeds are low because a WISP oversold the local antenna and is carrying more customers than the technology comfortably can serve.

But many rural wireless systems have slow speeds because of overextended wireless backhaul. In many cases in rural America, there are no fiber connections available for fixed wireless transmitters, which are often installed on grain elevators, water towers, church steeples or tall poles. I’ve seen networks that are making multiple wireless hops from a single gigabit fiber connection.

I’ve also seen preliminary designs for wireless ‘mesh’ networks where pole-mounted transmitters will beam wireless broadband into homes. Every wireless hop in these networks cuts the bandwidth in half at both radio sites (as bandwidth is split and shared). If you feed a mesh wireless network with a gigabit of bandwidth, then by the fifth hop a transmitter only sees 62 Mbps of raw bandwidth (which is overstated because by not accounting for overheads). It’s not hard to do the math to see why some rural wireless customers only see a few Mbps of bandwidth.

I’m sure that Verizon understands that many of the cell sites they serve today wirelessly will eventually need fiber, and I’m sure they’ll eventually build the needed fiber. But I also expect that there will be networks built with inadequate wireless backhaul that will barely function at inception and that will degrade over time as customer demand grows.