EPON - Ethernet Passive Optical Network

IEEE 802.3ah

*** 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:


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.

Downstream Traffic Flow in an Ethernet PON
Figure 5. Downstream Traffic Flow in an Ethernet PON

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 6 shows how upstream traffic is managed utilizing time division multiplexing, in which transmission time slots are dedicated to the ONUs. The time slots are synchronized so that upstream packets from the ONUs do not interfere with each other once the data are coupled onto the common fiber. For example, ONU-1 transmits packet 1 in the first time slot, ONU-2 transmits packet 2 in a second non-overlapping time slot, and ONU-3 transmits packet 3 in a third non-overlapping time slot.

Upstream 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.

EPON Frame Formats

Figure 7 depicts an example of downstream traffic that is transmitted from the OLT to the ONUs in variable-length packets. The downstream traffic is segmented into fixed interval frames, which each carry multiple variable-length packets. Clocking information, in the form of a synchronization marker, is included at the beginning of each frame. The synchronization marker is a 1-byte code that is transmitted every 2 ms in order to synchronize the ONUs with the OLT.

Downstream Frame Format in an Ethernet PON
Figure 7. Downstream Frame Format in an Ethernet PON

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 8 depicts an example of upstream traffic that is time division multiplexed onto a common optical fiber in order to avoid collisions between the upstream traffic from each ONU. The upstream traffic is segmented into frames and each frame is further segmented into ONU-specific time-slots. The upstream frames are formed by a continuous transmission interval of 2 ms. A frame header identifies the start of each upstream frame.

Upstream Frame Format in an Ethernet PON
Figure 8. Upstream 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 through N.

The TDM controller for each ONU, in conjunction with timing information from the OLT, controls the upstream transmission timing of the variable-length packets within the dedicated time-slots. Figure 8 shows an expended view of the ONU-specific time slot (dedicated to ONU-4) that includes two variable-length packets and some time-slot overhead. The time-slot overhead includes a guard band, timing indicators and signal power indicators. When there is no traffic to transmit from the ONU a time-slot may be filled with an idle signal.

EPON Optical System Design Options

Ethernet PONs can be implemented using either a two-wavelength or three-wavelength design. The two-wavelength design is suitable for delivering data, voice, and IP switched digital video (IP-SDV). A three-wavelength design is required to provide RF video services (CATV) or dense wave division multiplexing (DWDM).

Figure 9 shows the optical layout for a two-wavelength Ethernet PON. 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/channel change requests upstream as well as data and voice. Using a 1.25 Gb/s bi-directional PON the optical loss with this architecture gives the PON a reach of 20 km over 32 splits.


Optical Design for Two-Wavelength 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.

Optical Design for Three-Wavelength Ethernet PON
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.