WDM-PON faces upstream battle

As Verizon begins deploying Gigabit passive optical networks this quarter, industry talk is increasing about a potential next step for WDM-PONs. But standards bodies are still mulling how to remedy some of the challenges of pushing tens of Gigabits per second to and from the home. One of those remedies may require a radical shift in PON architecture.

A thorny problem in particular for WDM-PON networks is how to manage upstream channels as fast as 10 Gb/s, which would require a lot of power for the optical network terminal (ONT) at the home. One way to deal with the issue might be to replace that 10 Gb/s signal with two upstream wavelengths, each having burst-mode bandwidth of either 2.5 Gb/s or 5 Gb/s. The power required would be more manageable for the ONT. And burst-mode receivers could free up more bandwidth in the pipe.

Sending one stream down and two streams up would be a reversal of typical fiber-to-the-premises architecture. (Verizon's network, for example, uses two downstream channels — one for broadcast video and one for everything else — and another going back up.) This approach would also be a big departure in that it would require carriers to deploy two different types of ONTs: a 2.5 Gb/s model and a 5 Gb/s model.

Both models would presumably feed into the same optical line terminals (OLTs) in the central office (CO), so the OLT would need to keep track of which ONTs are which. It would also require new filters at the CO and a new media access control protocol. Both upstream and downstream signals would be clocked and controlled via a single downstream link — another big leap for PON.

Still, the advantages of this approach are already gaining favor with some participants in the standards process.