Gfast, Now With More Fiber

While Gfast updates won't actually help your dietary needs, they do push the service offerings into alignment with those of a fiber-only connection. From a service provider view, this simplifies operational processes, since providers need only maintain a single set of service profiles for their customers to purchase.

Put another way, it wouldn't matter if a customer has a fiber only connection or a fiber connection extended by a Gfast connection: The user experience is the same.

That's because the latest profile update to Gfast extends the bandwidth the technology uses from 106 MHz to 212 MHz. For those keeping track, this is double the spectrum usage which, given the modulation Gfast uses, doubles the number of subcarriers (or tones) used to transport data from 2,048 to 4,096. This pushes data rates available to the user above the 1Gbit/s mark; in some cases, it even comes closer to 1.5 Gbit/s.

These increases in spectrum usage and data rates present some interesting challenges when testing Gfast transceivers. Let's look at those challenges and how they differ from testing 106MHz profiles.

More bandwidth, new challenges
One key point in how Gfast systems use the additional spectrum is that total transmit power was not increased from the original +4 dBm level. In other words, a Gfast signal operating over the full 212MHz of bandwidth basically operates at half the original power density at any given subcarrier. Gfast's initialization process ensures each subcarrier's operating signal to noise ratio meets the requirements the operator sets. Still, it's vital to consider this lower power density in how test noises are expected to impact the line.

In the Gfast certification program, these test noises are designed to trigger very specific behaviors within Gfast transceivers, often without triggering other behaviors. For example, if a test case is designed to test only the seamless rate adaption (SRA) feature, it must inject a noise onto the line that will cause transceivers to alter their bit loading and adjust the total net data rate. However, it must not cause the transceivers to trigger the fast-rate adaption (FRA) feature; after all, the certification test case has to correctly verify the devices to implement the SRA feature. In updates to the Gfast certification test plan, we examined each test scenario to ensure it was properly stressing and testing the Gfast transceivers and causing the required actions in those transceivers.

Another, more obvious, complexity is testing transceivers' transmit power and transmit power spectrum density (PSD). You might say, "That's not complex. Every communication system ever built has had this testing." Ah, but Gfast presents some unique challenges to this measurement.

First, the Gfast interface is typically an RJ11 or RJ45 interface (twisted pair). While this interface is convenient for end users, it is not specified to the tight tolerances required for the PSD measurement at 212MHz. Similarly, there are complexities in performing a PSD measurement over such a wide bandwidth. And, if those challenges weren't enough, Gfast uses a time division duplex (TDD) scheme, so transceivers are not continuously transmitting a signal that can be measured.

The TDD aspect further complicates measurement when the transceiver under test must be connected to another Gfast device to cause it to transmit the signal being measured. To address these complexities, the certification test plan provides detailed requirements on the PSD measurement system (jig), in terms of return loss and accuracy. The PSD measurement jig is essentially an impedance-matched differential attenuator, with high impedance measurement differential probes attached to each side of the attenuator.

One way to measure the TDD signal is using a digital capture system (A/D) and post-processing the signals captured from each side of the attenuator. This method allows the PSD measurement to be calculated only for times when the transmitter is active, and tosses away the measurement signal when the other transmitter is active (i.e. the signal that bled through the attenuator). This approach provides an accurate measurement of the transmitter PSD, while also remaining robust enough to account for differences between each Gfast implementation (i.e. the use of idle / quiet symbols -- but that is another blog for another time).

Overall, the addition of the 212MHz profile increased the system's available data rates, aligning Gfast with most fiber deployments in use today. The updated Gfast certification testing continues to provide a detailed analysis of transceivers and their implementations of the features defined in the ITU-T standards. Certified Gfast systems help operators ensure systems adhere to the standard's requirements and improve both the current and future interoperability of their deployments.

Related posts:

• Arvig Takes On Many Roles in Competitive Broadband Arena
• Openreach Deploys Gfast in 81 New Locations
• Operators Win Smart City Hearts With Mix of Broadband Techs

— Lincoln Lavoie, Senior Engineer for Broadband Technologies, UNH-IOL and serves on the Broadband Forum (BBF) Board of Directors, as well as several BBF groups.