So, what is backhaul?
According to Wikipedia, the telecommunications definition is "the intermediate links between the core network, or backbone network and the small subnetworks at the "edge" of the entire hierarchical network." But exactly what does this mean, and what technologies can be deployed to make backhaul work reliably?
The core network usually refers to that portion of the network where core equipment is located. This can be a central office, data center, Mobile Telephone Switching Office (MTSO), or video head-end. The edge usually refers to our access network, which may also include equipment like a Digital Subscriber Loop Access Module (DSLAM), Remote Digital Terminal (RDT), cell tower, or remote video node. Then, there’s backhaul. In the past, terms that have been used for backhaul include middle mile and transport.
Many technologies have been used in backhaul networks over the years. TDM technologies such as DS-1 and DS-3 worked on copper or coaxial facilities. Once fiber was introduced, those TDM technologies were multiplexed together to make larger bandwidths available, predominately using Synchronous Optical Network (SONET). In addition to copper, coax, and fiber, wireless technologies including microwave and Free Space Optics were also deployed.
With all these facility types and technologies available (many still in use today), backhaul can be very confusing. So to make this article clearer (and much shorter), we will focus on the dominate facilities in use today: fiber and microwave. We will also talk about IP, since that technology has largely eclipsed TDM in backhaul networks.
Everyone will agree that fiber is the best solution for backhaul when we consider capacity, reliability, and security. That’s for a good reason: it is pretty close to being future-proof. Early in my career, I marveled at being able to transport 45 Mbps over a pair of fibers. Yet today we have 100 Gbps channels in service, with 40 or more on a pair of fibers using Dense Wave Division Multiplexing (DWDM) technology.
Not surprisingly, systems capable of 1 Tbps or more are being tested in the lab, with no end in sight. Add to that fiber’s ability to provide five nines reliability and the security of a digital signal and you have the perfect backhaul facility. Today, every central office is connected with fiber and most types of remote access equipment are slowly being migrated off TDM to an IP based fiber backhaul system. The same applies to cellular networks, where fiber to the tower is the de facto standard.
Even the cable providers are getting into the fiber backhaul act, with Hybrid Fiber Coax (HFC) networks that use fiber to transport between the head end/Cable Modem Termination System (CMTS) and the remote nodes. Therefore, the answer to all the backhaul issues of the past have been largely answered by fiber and IP based electronics.
Recently, another version of a fiber backhaul network has gained traction: dark fiber. Dark fiber is plain old fiber cable with no optics or electronics.
There are 2 tactics used today to harvest that dark fiber.
Tactic #1: The customer leases dark fiber and supplies all the end-to-end transport electronics themselves. This can dramatically reduce the cost compared to paying for lit services in which the service providers maintain the electronics.
Customers typically choose to either lease the fibers in an existing cable for a specified period of time or they may even purchase those fibers by using an Indefeasible Right of Use (IRU). Both offer advantages and disadvantages, depending on the customer’s needs and the desired longevity of the contract (most IRUs tend to be at least 20 years).
The upside to leasing dark fiber is the ability for the customer to upgrade their connection without incurring additional fees from the fiber provider. This is very important for any customer that has bandwidth needs that continue to grow every year. Instead of renegotiating a contract for additional bandwidth every time, the customer merely makes a software change, adds a card, or, at worst, replaces a transport system.
But the downside of leasing dark fiber is knowing what to do. The engineering, purchasing, installation, and maintenance of these optical transport systems fall solely on the customer, so it’s not for the faint of heart. Getting it right the first time is paramount to reducing the future costs, so easily upgradable systems are a must.
Tactic #2: The customer uses lit fiber services.
Most cell towers, as an example use services on lit fiber. The costs are lower initially and all the system maintenance falls to the service provider, not the customer. The customer merely takes the connection and plugs it into their equipment. And with the newer optical equipment that is available, these connections can be anywhere between 10 Mbps and 100 Mbps (or more if they are aggregated).
Today, almost all smaller customers and most larger customers use lit fiber services, mainly because of the ease of operation. However, due to their ever-increasing bandwidth needs, cell service providers, hospitals, and larger schools are starting to turn more to dark fiber as a means to future-proof their bandwidth needs.
In fact, many cell providers are now starting to utilize dark fiber between their primary towers and new micro cell installations, even if the macro site is still served by a lit service. This means that a lot of fiber will be placed around residential areas, malls and shopping areas, schools, and other places where there is a lot of cellular traffic. These micro sites will aggregate to a larger macro site, where their traffic will be added together. The issue that is being solved isn’t a backhaul bandwidth restriction. Rather this is a way to defer radio upgrades or preserve spectrum from the macro site by pushing cellular traffic to the edge. As the backhaul to the macro site increases beyond the lit capabilities, it too will be replaced by dark fiber.
The Future
As much as the future will be largely served by fiber, there is still one factor to be considered with a fiber deployment: cost. Some areas are too remote to economically serve with fiber backhaul, or the build costs are just too expensive to justify a fiber connection for the low traffic volume.
In those cases, many companies are turning to old school technology with a new twist for their backhaul needs: licensed microwave with IP transport. While a microwave transport system will never be as reliable as a fiber-based system, they are still much more reliable than they were decades ago. Equipment has evolved and the changeover to digital transmission has resulted in microwave systems that can exceed 1 Gbps in capacity for long distances and with great reliability. In fact, even some unlicensed microwave systems have capacity over 1 Gbps, if the route is clean and short enough. Just remember that clean is a relative term, as unlicensed spectrum is exactly that, with no one having more rights than another. So what looks good today may degrade to the point of unusability tomorrow.
It’s likely we will see more new microwave systems being installed in the near future. One major cellular provider has even publically stated that it currently pays too much for lit fiber services and will begin transitioning some of its backhauls from fiber to licensed spectrum microwave in 2016. So maybe it really is true that what once was old is new again. Although I don’t believe that microwave transport will ever become the dominate technology for backhaul, it is certainly another option under the proper circumstances.
One thing that 40 years in this industry has taught me is the only constant is change. I have watched the network evolve from 8 party lines over open wire, to DS-1 and TDM, to fiber, first TDM and now IP. But unlike many of its predecessors, I believe the tremendous capacity, increased security, and top reliability will keep fiber the top dog of the backhauls for years to come. That is until something better comes along.