By Chris Bastian
Senior Vice President, Engineering/CTO
Over the history of data over cable, which became the DOCSIS® specifications, network operators have offered asymmetric speeds, with the downstream speeds (from the network to the customer) typically ranging from three to 30 times faster than the upstream speeds. This was fine for services of the day, as people used applications, such as streaming and downloading videos, which required much higher downstream than upstream capability.
Full Duplex (FDX) DOCSIS technology dramatically changes the downstream-to-upstream ratio, with the capability of offering 5 Gbps simultaneously in both directions. This translates into an increase in spectral efficiency of almost 100 percent. As future applications are developed that require higher upstream traffic profiles, such as IoT applications and virtual reality, the ability to offer symmetric speeds will be advantageous.
How is Full Duplex DOCSIS technology possible over the conventional hybrid fiber-coax (HFC) network?
First, let’s look at how frequencies on the cable network plant are allocated today. Frequency Division Duplex (FDD) is employed, where the lower frequencies (typically 5 to 42 MHz in the U.S.) are allocated to the upstream and the higher frequencies (typically 54 to 850 MHz or higher) are allocated to the downstream.
With full duplex, the upstream and downstream frequencies overlap. When such overlap occurs, there is interference between the transmitters and receivers. This occurs at both the network side (CMTS/Distributed Access Architecture (DAA) node) and the customer side (cable modem). The key enabler to mitigating this interference and supporting full duplex is echo cancellation.
On the DAA node side, the downstream signal is much stronger than the received upstream signal, and echo cancellation is needed at the node’s receiver to be able to properly “hear” the weaker upstream signal.
On the cable modem side, there are two types of interference: CM self-interference, which is also mitigated with echo cancellation, and interference among CMs in the same service group. For this latter inter-CM case, the interference is mitigated via smart scheduling.
An “N+0” network topology is assumed for supporting full duplex. N+0 means that there are no active elements (amplifiers) between the remote PHY node and the cable modem. Theoretically, full duplex could be supported across a network that is not N+0; however, that would require upgrading amplifiers to support full duplex, with their comparable echo cancellation techniques. In light of many network operators’ fiber deep efforts, the target topology for FDX is, therefore, N+0.
CableLabs® released the Full Duplex DOCSIS technology requirements as an annex to the DOCSIS 3.1 specifications in October 2017, and several interoperability events are being planned for this year. Full Duplex DOCSIS technology will be backward compatible with existing DOCSIS deployments.
The Society will educate attendees of SCTE•ISBE Cable-Tec Expo® 2018 this fall on the future of DOCSIS/FDX through Cable-Tec Expo’s Wireline Access Network technical workshops track.
As we get closer to Full Duplex DOCSIS deployment, vendor partners will be teaming with network operators to conduct field trials. The day is coming soon where full duplex will be in a network near you.