PDF of this tutorialEPONs can be implemented using either a two-wavelength or a three-wavelength design. The two-wavelength design is suitable for delivering data, voice, and IP–switched digital video (SDV). A three-wavelength design is required to provide radio frequency (RF) video services (CATV) or dense wavelength division multiplexing (DWDM).
Figure 8 shows the optical layout for a two-wavelength EPON. In this architecture, the 1510 nm wavelength carries data, video, and voice downstream, while a 1310 nm wavelength is used to carry video-on-demand (VOD)/channel change requests, as well as data and voice, upstream. Using a 1.25 Gbps bidirectional PON, the optical loss with this architecture gives the PON a reach of 20 km over 32 splits.
Figure 8. Optical Design for Two-Wavelength EPON
Figure 9 shows the optical layout for a three-wavelength EPON. In this architecture, 1510 nm and 1310 nm wavelengths are used in the downstream and the upstream directions respectively, while the 1550 nm wavelength is reserved for downstream video. The video is encoded as Moving Pictures Experts Group–Layer 2 (MPEG2) and is carried over quadrature amplitude modulation (QAM) carriers. Using this setup, the PON has an effective range of 18 km over 32 splits.
Figure 9. Optical Design for Three-Wavelength EPON
The three-wavelength design can also be used to provide a DWDM overlay to an EPON. This solution uses a single fiber with 1510 nm downstream and 1310 nm upstream. The 1550 nm window (1530–1565 nm) is left unused, and the transceivers are designed to allow DWDM channels to ride atop the PON transparently. The PON can then be deployed with no DWDM components, while allowing future DWDM upgrades to provide wavelength services, analog video, increased bandwidth, etc. In this context, EPONs offer an economical setup cost, which scales effectively to meet future demand.
