Ethernet PON (EPON)

EPON networks (sometimes referred to as Gigabit Ethernet PON or GEPON), are based upon IEEE
standards (802.3ah). Unlike the ITU standards of BPON and GPON, EPON is not an ATM or TDM-centric
PON standard. Not surprisingly EPON is very much focused on maintaining the overall architecture of
the 802.3 Ethernet standard which makes EPON an attractive model for service providers who do not have
legacy ATM, SONET, or archaic billing systems. EPON was the natural progression for carriers who began
to recognize that Ethernet had won the technology race and that further deployment of the ubiquitous
architecture in their core networks was likely to decrease cost and complexity. While much of this is
true, the IEEE did not go very far in articulating the 802.3ah standards to accomodate the various
technical complexities of a PON network. Consequently, the many EPON manufacturers in existence today
are producing a widely varied product set with an equally varied feature set. Any specifics for timing,
security (e.g., encryption), or details around feature implementation are absent from the standards. For
all the loose standards development around EPON, the technology has certainly not suffered any lack of
deployments. Asia/Pacific markets have deployed more EPON ONTs than GPON and BPON deployments
combined and there is no sign of any slow uptake as China continues to build out its infrastructure.
Not surprisingly, Japan and South Korea have extensive deployments of EPON despite a robust wireless
network infrastructure. Today there are over 40 million EPON subscribers and the numbers are growing
rapidly.

Wave Division Multiplexing PON (WDM-PON)

WDM PON networks (Wave Division Multiplexing PON), is an emerging network technology. In fact, you can
count the number of WDM PON manufacturers on one hand and still have some fingers left over. But
of all the technologies utilizing a passive fiber architecture, none of them have quite so much
technical and future-proof advantages as WDM PON. As its name suggests, WDM PON utilizes wave
division multiplexing optics. This is no more different than other PON technologies except that WDM PON
dedicates a wavelength to each ONT by incorporating up to 32 wavelengths. Each one of these
wavelengths is used to transmit and receive the information associated with its respective ONT. Other
PON technologies utilizes 2 to 3 wavelengths and rely on a point to multi-point protocol to facilitate
communication between ONTs. WDM PON optics are either tuned to their respective "color" of light or utilize
a dynamic channel or wavelength assignment as the ONTs are ranged to the OLT. Instead of a traditional
PLC or FBT splitters (see our tutorial on passive optical splitters) WDM PONs rely on an Arrayed Waveguide
Grating (AWG) to accomplish the separation of the signals from the single feeder fiber to the multiple
distribution fibers. The result of all this is that you end up with all the benefits of a PON architecture
without the shared bandwidth requirement. You also realize a more secure architecture when an ONT is
dedicated to a specific wavelength and is "blind" to other wavelengths on the PON.

The one advantage not present with WDM PON technology is cost optimization. WDM PON is so new
and the number of deployments so few that the optics are extremely costly relative to GPON or EPON
optics. Also the lack of any formal standards body supporting WDM PON is likely to slow the development
of competing products. There are also technical considerations in outside plant deployments which may
render WDM PON problematic. One of these considerations is the fact that extreme temperature variations
cause optics in fiber networks to "drift" from their respective wavelength. Without proper separation or
guard band technologies, this approach may lead to significant service disruptions and operational
expense due to the large number of wavelengths in relative proximity to each other. So while the
industry watches this technology with eager anticipation, there is much more work to do before the
cost and standardization of WDM PON renders it a mainstream consideration.