Whatever is vOLT-HA?

Many network engineers find the entire world of telecom to be confusing—especially as papers are peppered with a lot of acronyms. If any part of the networking world is more obsessed with acronyms than any other, the telecom world, where the traditional phone line, subscriber access, and network engineering collide, reigns as the “king of the hill.”

Recently, while looking at some documentation for the CORD project, which stands for Central Office Rearchitected as a Data Center, I ran across an acronym I had not seen before—vOLT-HA. An acronym with a dash in the middle—impressive! But what is, exactly? To get there, we must begin in the beginning, with a PON.

There are two kinds of optical networks in the world, Active Optical Networks (AONs), and Passive Optical Networks (PONs). The primary difference between the two is whether the optical gear used to build the network amplifies (or even electronically rebuilds, or repeats) the optical signal as it passes through. In AONs, optical signals are amplified, while ins PONs, optical signals are not amplified. This means that in a PON, the optical equipment can be said to be passive, in that it does not modify the optical signal in any way. Why is this important? Because passive equipment is less complex, and does not require as much power to operate, so a PON is much less expensive to build and maintain than an AON. Hence a PON is often more economically realistic when serving a large number of customers, such as in providing service to residential or small office customers.

A PON uses optical splitters to divide out the signal among the various connected customers. Like any other shared bandwidth medium, every customer receives all the data on the downstream side, switching only traffic destined for the local network onto the copper (usually Ethernet) network beyond the optical termination point (called an OLT, or Optical Line Terminal). In a PON, the upstream signal is divided up into timeslots, so the system uses Time Division Multiplexing (TDM) to provide (a much slower) path from the end device into the provider’s network. As signals from each edn device reach the splitters in the network, the path is reversed, and the splitter ends up becoming a power combiner, which means the signal can “gain power” on the way up towards the central office (CO). These kinds of systems are typically sold as Fiber to the Home, which is abbreviated FTTH (of course!).

Is your head dizzy yet? I hope not, because we are just getting started with the acronyms. 🙂

The Optical Line Terminal, or OLT, must reside in some piece of physical hardware, called an Optical Network Unit (ONU). The OLT, like a server, or an Ethernet port on a router or switch, can be virtualized, so multiple logical OLTs reside on a single physical hardware interface. Just like a VRF or VLAN, this allows a single physical interface to be used for multiple logical connections. In the case, the resulting logical interface is called a vOLT, or a virtual Optical Line Terminal.

Now we are finally getting to the answer to the original question. vOLT must somehow relate to virtualizing the OLT, but how? The answer lies in the idea of disaggregation in passive optical networks (remember, this is a PON). One of the key components of disaggregation is being able to run any software—especially open source software—on any hardware—so-called “white box” hardware in particular. To get to this point, you must have some sort of “open Application Programming Interface,” or API, to connect the software to the hardware. You might think the HA in vOLT-HA stands for “high availability, but then you’d be wrong. 🙂 It actually stands for Hardware Abstraction.

So vOLT-HA, sometimes spelled VOLTHA, is actually a hardware abstraction layer that allows the disaggregation of vOLTs in an ONU in a PON.

Got it?