Ethernet: the panacea for passive networks?

The debate over which protocol is best for framing traffic in passive optical networks (PON) may boil down to this: a promise of lower costs for more bandwidth versus a lack of standards.

By Kathleen Richards, Lightwave

Having made its way out of the enterprise into metro networks, Ethernet is already emerging as the framing technology in PON solutions at several optical access start-ups. Ethernet makes sense because of its advantages over ATM: lower costs; scalability beyond ATM-PONs (APON) current 622Mbit/s, and simplicity. Currently, in APON configurations that conform to the ITU-T standard, as many as 32 customers may share 622Mbit/s downstream and 155.52Mbit/s upstream - certainly not the high level of bandwidth that business customers expect from fibre.

The FSAN coalition is working on Gigabit PON (GPON), a proposal to increase the bandwidth specified in ITU-T G.983.1 from 622Mbit/s to 1.2Gbit/s. Some in the ATM camp also disagree with the general assumption that Ethernet framing offers cost and scalability advantages.

"In the optical domain, cost would not be determined by the protocol efficiency but by the desire to cover certain optical specifications that include power level, distance, split ratio, and the spacing between bursts that go upstream," says Boris Auerbuch, chief technology officer at APON vendor Terawave Communications (Hayward, CA). "That's the fundamental cost trade-off for selection."

But Herbert Martin, president and CEO of Salira Optical Network Systems (Santa Clara, CA) counters, "With PONs, the underlying communication protocol is TDM downstream; it is at the framing layer that you get the differentiation. We can handle many more users. Ethernet is a plug-and-play technology and it is much less expensive."

Salira's product will enable carriers to support native TDM traffic and use their existing operations support systems and test procedures. Salira's EPON offering also addresses common Ethernet sticking points such as a lack of quality-of-service (QoS) capabilities and security. ATM readily handles such concerns, which is why ATM is entrenched in many carriers' networks.

Multi-service, carrier class
As EPON offerings make their way into lab tests and network trails, high-profile APON vendors such as Quantum Bridge Communications (Andover, MA) and Terawave report trials and contracts with a range of customers. Quantum Bridge looked at an Ethernet approach in 1999, but chose instead to pursue APON because of the FSAN-led standards.

Despite its APON successes, Quantum Bridge is focused on customer solutions, not "protocol holy wars", says the company's Charlie Guyer. Thus, the company also offers an "Ethernet in, Gigabit Ethernet-out" approach, via an Ethernet interface card on the customer premises system and a Gigabit Ethernet WAN card in the central office.

Quantum Bridge has been elected to serve as the liaison between the FSAN coalition and the IEEE 802.3 Ethernet in the First Mile (EFM) subcommittee, which is working on developing a physical layer standard for Ethernet in access networks.

The IEEE 802.3 EFM subcommittee is working to define several aspects of EPON over a single strand of single-mode fibre. The EFM standard will also cover point-to-point Gigabit Ethernet and point-to-point copper specifications.

A few points of contention have already surfaced. "Carriers want range," says Salira's Diestel. "They are looking at a 20km range to maximise the split ratio to 32:1. Carriers are also interested in operations, maintenance, and provisioning, which is not part of the EFM standard."

Meanwhile, the FSAN coalition is debating the structure requirements for the GPON architecture. These discussions started in September 2001. According to Terawave's chief technology officer Boris Auerbuch, the protocol will be cell-based and allow variable-sized bursts upstream. Ultimately the FSAN coalition will make its recommendations to the ITU-T.

APON versus EPON
"When we started the company, there was no APON or EPON, just PON," says Quantum Bridge's Guyer. "Some are trying to divide it up, but right now, people have bandwidth needs; carriers want to offer new services. Before the downturn, people had funded the backbone, they had funded the metro, and those started to dry up a little bit. The next obvious place to invest is in the access networks."

Venture capitalists and other investors apparently agree, as multiple optical access vendors specialising in PONs received funding during the past two years, despite a nascent market. "Optical is maturing and reaching point-to-multipoint connectivity," says Terawave's Auerbuch. "That's really the most essential thing in the PON story: the ability to share multiple subscribers over a single fibre and to reduce the number of ports in the central office. Nobody is saying that the future will be PON in the proprietary format. The trend is to standardise the solution that will allow at least the physical elements to be the same from one equipment vendor to another."

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