The Passive Optical Networking industry has made some key steps forward since our last column on the subject two months ago. Most significantly, the ITU-T has approved the first two components of the Gigabit PON standard - G.984.1 and G.984.2 - and reached an "almost frozen" draft stage on a third, G.984.3.

This is an official "Really Big Deal" because the .1 and .2 standards have defined both the Gigabit Service Requirement (GSR) for service providers and the Gigabit Physical Media (GPM) standards. The physical media standards, among other things, define the optical characteristics of the network, and the speeds and feeds required in the upstream and downstream directions. The third piece of the standards puzzle, the protocol layer, hasn't been ratified yet but a nearly complete draft is in place and expected to be ratified at the next meeting in October.

What this really means is that vendors in the GPON space can begin to develop products in earnest (if they haven't already). One vendor, FlexLight Networks, has already announced general availability of a G.984.1- and .2- compliant system, and is releasing its 984.3 system ahead of the final spec with plans to make modifications per the final standard. Other Full Service Access Network (FSAN) member vendors can be expected to do so this year.

In the IEEE, work continues on standards development for EPON systems. One of the next big decisions for Ethernet PON (EPON)/Ethernet-in-the-First Mile (EFM) committee members concerns PON timing parameters. In particular, a decision is pending regarding whether the finalized EPON/EFM standard will support tight or loose timing.

Because PON Optical Network Terminals transmit data in synchronized bursts and not continuously, each transmission is proceeded by a guard time - the delay between transmissions by adjacent ONTs - as well as an initial overhead time, which allows the ONT to synchronize and adjust power levels. Tight and loose timing refers to the actual elapsed time for this guard time and initial overhead. Systems with tight timing have shorter overhead periods than loose timing systems.

The regional Bell operating companies, in their FSAN requirements, have been supporters of tight timing. We spoke about this with Brian Ford, manager of Exploratory Services in BellSouth's Science and Technology division: "BellSouth will support tight timing (which closely matches GPON specifications) because it gives us more confidence on delivering the services we need to deliver."