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Engineering-Wise: Boosting the last mile access speed

Wed, 09/11/2013 - 7:50pm
Daniel Howard, SVP of engineering & CTO of the SCTE

The broadband industry is now defining many of the last mile physical layer (PHY) technologies that will take us into the next decade such as DOCSIS 3.1, EPoC, EPON, RFoG, and now Remote PHY. But which combination is best for a given cable operator to use for a given location and competitive landscape? That’s a question we’ll be striving to answer at SCTE Cable-Tec Expo this year.

Let’s start with DOCSIS 3.1. By ultimately promising 10 Gbps downstream and 1 Gbps upstream, it is arguably the most important of the new coax PHY technologies.

That’s why this year’s Expo agenda includes a pre-conference symposium on D3.1 hosted by SCTE and CableLabs.

We’ll cover key technical points like the new PHY (OFDM DS/OFDMA US, LDPC, new frequency plan), but also the convergence layers (especially downstream profiles), how latency is controlled and the new MAC features such as how legacy equipment will be shared/multiplexed and why bonding was used in 3.1. The latter is pretty easy to grok: A simple math check says you need six 192 MHz wide D3.1 channels totaling 1152 MHz RF at the recently demonstrated 9.23 bits/Hz to hit that 10 Gbps downstream speed target. With a target of 11 bits/Hz using 4096 QAM and strong LDPC protection, we could squeeze 10 Gbps into 5 bonded 192 MHz D3.1 channels and fit that into the 1 GHz plant that Cox and Suddenlink already have in their networks. Sweet.

But now we’re developing the EPON Protocol over Coax (EPoC) as another option. EPoC is about extending the high speed EPON MAC layer to the coax PHY layer, so that we’re doing Ethernet end-to-end, with all the associated cost sav ings, MAC simplicity and integrated network benefits. But the EPoC PHY, and in particular, latency still looms as a big question. A TDD scheme has been proposed, but that might add 1-2 ms of latency and create round trip times (RTTs) of over 3 ms that would make some Metro Ethernet Forum’s MEF 23 services impossible. But rest easy: EPoC developers are working through these issues so operators soon will have a way to build out EPON networks that are 100 percent IP and can be extended over existing or new coax.

The question of when to use D3.1 vs. EPoC is going to be very interesting for cable operators, so another session at Expo will provide tutorials and explore use cases as well as comparative pros and cons of each coax PHY. But if you’re convinced that moving to FTTX as soon as possible is the best long-term competitive strategy, you can get there costeffectively via RFoG. The SCTE RFoG standard now provides a natural way to transition from it to a full Ethernet PON architecture. A PHY session on FTTX at Expo will discuss how leveraging EPON rollouts to business can make the RFoG business case even better, especially when we leverage DWDM technology in the fiber.

But since DOCSIS 3.1 is going to extend the life of both the coax network as well as DOCSIS, how can we migrate the network gracefully to this new Ethernet/full IP/PON-type solution as an endgame scenario? The latest answer offered is Remote PHY, which means ramming 880 Gbps via DWDM to the fiber node and then peeling out whatever is needed from the node— EPON, GPON, RFoG, DOCSIS HFC, EPoC, etc. — to serve the neighborhood.

That means putting more smarts in the node such as the PHY or even the MAC/ PHY to create a remote CMTS, remote CCAP, or even a mini-CCAP in the node or MDU. Some argue against more complexity in the plant/edge, so you’re likely to hear a lively discussion in the Remote PHY session at Expo.

The answer to which PHY to use for last mile access ultimately depends on the specifics of the cable operators’ current access network technology, their business objectives and their strategic technology plans. The only way to find the right answer is to explore all of these options and compare them. More than ever at Expo, you can compare the strengths and weaknesses of each approach vs. the potential returns on the investment in terms of increased revenue, customer satisfaction, and operational efficiencies such as reducing the number of truck rolls and calls to customer care.

So if you’re a PHY guy or gal with an interest in exploring these new technologies while keeping business objectives in mind, you won’t want to miss that track. Want to prepare? Head to www.scte.org/phy where you’ll see definitions of all the acronyms in this article, full session descriptions of the PHY track, and links to short videos by the PHY track moderators and presenters, as well as those Expo sessions in the IP Networking, Quality Assurance, and Advanced Services tracks. For the digital natives who increasingly make up our workforce, the videos are the latest SCTE tools for achieving the SCTE’s mission of providing information and resources to the industry, and now in a more easily consumed manner given our busy and mobile schedules. Happy planning, and we look forward to seeing you at Expo.

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