by Stephen Hardy
Business services represent the biggest current driver for MSO fiber to the premises architectures. PON, and particularly EPON, best fits the requirement now; in the future, an MSO-specific variant of PON may emerge.
It's not only increasingly difficult to tell cable companies and their telco competitors apart by their broadband service offerings, but even the infrastructures by which these services reach customers have begun to converge. Service providers from both camps continue to push fiber deeper into their networks. Still, telcos in general appear to be more aggressive when it comes to fiber to the premises (FTTP). That difference in enthusiasm is likely to continue for the near term, says a consultant active in the cable space. When it does change, PON will likely prove the architecture of choice—but even then, a cable PON will likely not be identical to telco PONs.
Speaking at Lightwave's Optical Access '08 e–conference in early December, consultant Victor Blake pointed out that one can find a deployment of every optical access technology available, from Active Ethernet to APON, BPON, EPON, and GPON, within the ranks of North American cable operators. However, traditional DOCSIS–enabled hybrid fiber/coax (HFC) remains the most ubiquitous approach to service provision. When it comes to triple–play service provision, particularly to the home, that's likely to remain the case for most applications for some time to come, Blake suggested.
Besides the cable operators' desire to wring every megabit out of their existing infrastructures, Blake pointed out that HFC networks are more robust and efficient than the twisted–pair plant that telco FTTH networks are meant to replace. “I think what we've seen when we look at telcos is that a lot of their justification of the expense of fiber to the premise is in a reduction in their opex costs as they move from a highly active network to a passive network,” he said. “This is because of a lot of technical difficulty operating twisted pair in the network. This is really not the case in the MSO market today.” For the most part, HFC networks can compete with FTTH on the basis of reliability and operational expense, and can probably exceed FTTN approaches in terms of performance, Blake said.
However, that doesn't mean there isn't a requirement for fiber in the MSOs' last mile. High–end business services that call for pipes too big for the HFC to accommodate or a burgeoning wholesale Ethernet business would create a need for an FTTP approach for commercial customers, Blake suggested. On the residential side, many new housing developments (back when there were many new housing developments) have adopted FTTH as a part of their amenities. In such instances, “fiber to the home...is really being driven, not by bandwidth concerns, but actually by marketing concerns,” Blake said.
The question for these applications becomes which FTTP technology to deploy. Emerging RF over glass (RFoG) technology, now the subject of standardization efforts by the SCTE, has attracted attention from cable operators as a simple and direct way to extend DOCSIS–based provisioning methods to FTTH while using existing customer premises equipment (CPE) and headend interface gear. However, Blake saw limitations in such an approach.
“It does fulfill the requirement to get fiber to the home, so it does actually involve all the costs of fiber construction,” he explained. “It really doesn't offer any additional benefits beyond the existing hybrid fiber/coax plant; it just gets you that check mark that says, ‘Yes, we have fiber to the home.'”
If cable operators want the most cost–effective means of deploying an optical access network, they should look elsewhere, Blake asserted. “If a cable operator does decide to build into a new housing development that requires FTTH, PON is really the only reasonable choice. Active Ethernet technologies that require a fiber run from the MSO network directly to each customer clearly consume more fiber mileage than passive optical networks.”
The majority of MSOs have opted for EPON when following this path for several reasons, Blake said. Since it is the world's most popular PON technology in terms of total subscribers, EPON benefits from the cost reductions inherent in comparatively high–volume production. The fact that it's based on Ethernet, and therefore matches the services that will be delivered to commercial customers, also appeals to cable operators. Its quality of service mechanisms also look a lot like those of DOCSIS networks—and for good reason. “The architects of the EPON specification are actually folks that had a lot of experience in cable, and they emulated what DOCSIS did. So it turns out that the way the service is provisioned and operated is similar, which makes it an easy technology for MSOs to adopt,” Blake explained.
Finally, unlike GPON, EPON is service–layer agnostic, which makes it easier to implement DOCSIS–based service provision. Add to that a clear roadmap toward 10 Gbps and, perhaps, WDM–based service delivery, and the case for EPON is complete.
However, there are no standards for DOCSIS–based EPONs, which leaves cable operators to rely on a combination of MSO–friendly EPON offerings and their own wits in most cases. Only one vendor—Salira Systems (www.salira.com)—currently offers a true DOCSIS–enabled EPON in Blake's opinion; Cisco's (www.cisco.com) DOCSIS PON is more of a spin on RFoG, he said. However, several vendors offer EPONs with some sort of RFoG–like capability. Blake listed Alloptic (www.alloptic.com), Pacific Broadband Networks (www.pbn.com.au), and Enablence Technologies (www.enablence.com) among this camp.
Despite the appeal of mixing DOCSIS and EPON, Blake said he doesn't see this combination as the ultimate endgame for FTTP in cable operators' networks. Particularly as services become more IP–based, Blake said he can foresee cable operators leveraging the MoCA interfaces on current and future set–top boxes to provide two–way video capability. This approach would combine MoCA with the Digital Set–top Gateway (DSG) protocol, a combination that Blake referred to as DSGoMoCAoDPON. However, networks based solely on this approach would take “many, many years to achieve,” in his opinion.
Stephen Hardy is editorial director and associate publisher of Lightwave.
LINKS TO MORE INFORMATION
Optical Access '08: “The MSO Market Opportunity,” Victor Blake
Lightwave: “MSOs Want Their FTTH and DOCSIS, Too”
On the Lightwave Channel: “Intro to DOCSIS PON”