BellSouth, SBC Communications and Verizon are expected to issue a joint request for proposals of Gigabit passive optical networking, or GPON, equipment in the next few months — possibly as early as November, according to some sources. As their efforts help yield a sharper definition for GPON, they also clarify differences among the Bells' preferences for the technology.

All three Bells are expected to test GPON throughout 2006 and deploy it in 2007. But SBC may have a more pressing stake in the details of GPON than Verizon because future increases in bandwidth demands or competitors' speeds (or even problems with fiber-to-the-node technology) would naturally squeeze SBC's VDSL2 network long before it would pressure Verizon to upgrade its BPON to GPON.

Because BPON's 622 Mb/s downstream bandwidth speeds are “roughly equivalent” to those of the VDSL2 SBC is deploying in its FTTN rollout, “That's a motivator for us to move as quickly as we can to [GPON],” said Ralph Ballart, president of SBC Labs. Whether SBC deploys GPON on a wider scale than it does BPON (now applied generally to greenfields) will depend on a few things, Ballart said. One is the pace of high-definition TV adoption, which could strain the bandwidth capacities of SBC's VDSL rollout. Another is whether competitors, such as cable companies with ever-growing broadband speeds, force SBC to change course. And finally, there's the matter of how happy SBC ends up with VDSL.

“Now that the standards are done, [VDSL] looks pretty good, but we're not out at 2 to 3 million homes,” Ballart said. “When you have that kind of experience, you learn a lot. That will feed the decision of where to put fiber-to-the-prem versus fiber-to-the-node.”

On some issues regarding GPON, the Bells seemed to have reached consensus, vendors familiar with the RFP said. For example, all three seem interested in a link loss budget of 28 db over 20 km and speeds of 2.4 Gb/s downstream and 1.2 Gb/s upstream (a jump from the 1.2 Gb/s down, 622 Mb/s up of GPON's first generation, sold today by Optical Solutions).

But variations in strategy and technology have split the Bells on other GPON aspects. For example, vendors said Verizon wants its GPON gear, like its current PON gear, to support traditional RF video. SBC, meanwhile, is more committed to IPTV. Another point of departure, vendors said, is Verizon's preference for multimedia over coax technology (MOCA) in the optical network terminal (ONT) on the side of the house. Though SBC expects to use both coax and phone lines to connect devices in the home, it has less interest in a MOCA-enabled ONT. Vendors will likely create different ONTs to address these needs, all conforming to the same GPON standard, but some questions also remain unanswered within the standard.

One of the hazy spots still lingering in the International Telecommunications Union's G.984 GPON standard (guided by the FSAN group), is its treatment of legacy time-division multiplexing (TDM) traffic, an important consideration for offering T-1 service. Bells are leaning toward the standard's GPON encapsulation method (GEM), which maps all traffic for transport across a GPON network. But carriers could also use circuit emulation, a packet technology separate from GPON. SBC, for example, plans to transport all its GPON traffic as Ethernet-over-GEM, (which features low packet overhead) using circuit emulation for TDM traffic.

The Bells may agree on GEM “as a direction” in which to move, Optical Solutions CEO Mike Dagenais said, but most of the equipment operating in their networks today is ATM-based, like BPON, he said. “I think there will be some transitioning. They'll slow down BPON and migrate to GPON.”