EPON - Ethernet Passive Optical Network
*** also called EFM (Ethernet in the First Mile)
*** click Here and Here for two excellent EPON primers
EPON is a very exciting technology, because it finally allows Ethernet networks to be directly connected via fiber, and it supports point-to-multipoint connectivity. The EPON standard supports EFM technologies such as all of the FTTx standards . . . FTTC (Fiber To The Curb), FTTP (FTT Premises) FTTB (FTT Building)
There are several primary components of a last-mile PON - the OLT, the fiber and splitters, and the ONT:
OLT (Optical Line Terminal) - located at the CO, the
OLT interfaces with the metropolitan network. It must be high power because
it sends optical signals out, which are immediately broken into several
streams. The main functionality of the OLT is to adapt the incoming
traffic (Voice and Data) from the metropolitan rings into the PON transport
ONT (Optical Network Termination) and ONU (Optical Network Unit) - ONT and ONU are basically the same device - however, the ONT is located at the customer premise, and the ONU isd located outside the home. These devices are the interface between the customer equipment and the PON. They talk to the OLT via the PON.
Upstream/Downstream EPON Traffic
In an Ethernet PON the process of transmitting data downstream from the OLT to multiple ONUs is fundamentally different from transmitting data upstream from multiple ONUs to the OLT. The different techniques used to accomplish downstream and upstream transmission in an Ethernet PON are illustrated in Figures 5 and 6.
In Figure 5 data is broadcast downstream from the
OLT to multiple ONUs in variable-length packets of up to 1,518 bytes
according to the IEEE 802.3 protocol. Each packet carriers a header
that uniquely identifies it as data intended for ONU-1, ONU-2 or
ONU-3. In addition some packets may be intended for all of the ONUs
(broadcast packets) or a particular group of ONUs (multicast
packets). At the splitter the traffic is divided into three separate
signals, each carrying all of the ONU-specific packets. When the
data reaches the ONU it accepts the packets that are intended for it
and discards the packets that are intended for other ONUs. For
example, in Figure 5 ONU-1 receives packets 1, 2, and 3, however it
only delivers packet 1 to end user 1.
Figure 5. Downstream Traffic Flow in an Ethernet PON
Figure 6. Upstream Traffic Flow in an Ethernet PON
ONUs transmit data upstream to the OLT in ONU-specific time slots using time division multiplexing to avoid transmission collisions.
Each variable-length packet is addressed to a
specific ONU as indicated by the numbers, 1 through N. The packets
are formatted according to the IEEE 802.3 standard and are
transmitted downstream at 1 Gb/s. The expanded view of one
variable-length packet shows the header, the variable-length
payload, and the error detection field.
Figure 7. Downstream Frame Format in an Ethernet PON
The ONU-specific time-slots are transmission
intervals within each upstream frame that are dedicated to the
transmission of variable-length packets from specific ONUs. Each ONU
has a dedicated time-slot within each upstream frame. For example,
referring to Figure 8, each upstream frame is divided into N time
slots, with each time slot corresponding to its respective ONU, 1
Figure 8. Upstream Frame Format in an Ethernet PON
Figure 9. Optical Design for Two-Wavelength Ethernet PON
Figure 10 shows the optical layout for a three-wavelength Ethernet PON. 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 MPEG2 and is carried over Quadrature Amplitude Modulated (QAM) carriers. Using this setup, the PON has an effective range of 18 km over 32 splits.
Figure 10. Optical Design for Three-Wavelength Ethernet PON
The three-wavelength design can also be used to provide a DWDM overlay to an Ethernet PON. This solution uses a single fiber with 1510 nm downstream and 1310 nm upstream. The 1550 nm window (1530 nm 1565 nm) is left unused and the transceivers are designed to allow DWDM channels to ride on top of the PON transparently. The PON can then be deployed with no DWDM components, while allowing for future DWDM upgrades to provide wavelength services, analog video, increased bandwidth, etc. In this context Ethernet PONs offer an economical set-up cost, which scales effectively to meet future demand.