PDF of this tutorialThe key difference between EPONs and APONs is that in EPONs, data is transmitted in variable-length packets of up to 1,518 bytes according to the IEEE 802.3 protocol for Ethernet, whereas in APONs, data is transmitted in fixed-length 53-byte cells (with 48-byte payload and five-byte overhead), as specified by the ATM protocol. This format means it is difficult and inefficient for APONs to carry traffic formatted according to the IP. The IP calls for data to be segmented into variable-length packets of up to 65,535 bytes. For an APON to carry IP traffic, the packets must be broken into 48-byte segments with a 5-byte header attached to each one. This process is time consuming and complicated and adds additional cost to the OLT and ONUs. Moreover, 5 bytes of bandwidth are wasted for every 48-byte segment, creating an onerous overhead that is commonly referred to as the “ATM cell tax.” By contrast, Ethernet was tailor-made for carrying IP traffic and dramatically reduces the overhead relative to ATM.
Managing Upstream/Downstream Traffic in an EPON
In an EPON, 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 EPON are illustrated in Figures 4 and 5.
In Figure 4, 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 carries 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 4, ONU–1 receives packets 1, 2, and 3; however, it delivers only packet 1 to end user 1.
Figure 4. Downstream Traffic Flow in an EPON
Figure 5 shows how upstream traffic is managed utilizing TDM technology, 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 is 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.
Figure 5. Upstream Traffic Flow in an EPON
EPON Frame Formats
Figure 6 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, each of which carries 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 one-byte code that is transmitted every 2 ms to synchronize the ONUs with the OLT.
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 Gbps. The expanded view of one variable-length packet shows the header, the variable-length payload, and the error-detection field.
Figure 6. Downstream Frame Format in an EPON
Figure 7 depicts an example of upstream traffic that is TDMed onto a common optical fiber 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.
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, in Figure 7, 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 expanded 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.
Figure 7. Upstream Frame Format in an EPON
