Ethernet passive optical network (Part I)

4. Operational Principle

From OLT to ONUs, downstream traffic transmission is easy, which is based on a broadcast principle to send each ONU a copy of signal, and each ONU judges whether the packet belongs to itself or others simply based on the MAC address in the header of each frame. If the MAC header in the frame matches the MAC address of an ONU, then this ONU will access this frame; otherwise, it just simply drops the frame. Based on such a mechanism, there is no collision in downstream traffic transmission as there is only a single traffic source from OLT.  

From ONUs to OLT, there is an issue of frame collision if there is no good coordination between the frame transmissions of these ONUs. For example, if two ONUs send frames to OLT at the same time. These two frames will be destroyed by each other and OLT cannot tell which frame comes from which ONU. Thus, it is very important to design a protocol that can coordinate the upstream traffic transmission. This problem is often call upstream bandwidth allocation. There are two ways to allocate capacity to ONUs in the upstream direction. The simplest way is based on a fixed allocation strategy; that is, each ONU is assigned a fixed time slot to transmit its traffic to OLT. Each time when it transmits traffic, there would be no other ONUs sending traffic to OLT. Because data traffic from users are often bursty, sometimes there can be some traffic,  while sometimes there can be a long quiet period. Thus, if we allocate a fixed time slot for each ONU, when there is no or little traffic transmitted during the time slot, the capacity of this time slot will be wasted due to the fixed allocation. Just because of this, a more advanced technique is based on a dynamic allocation strategy. That is, it is the OLT that is responsible for the capacity allocations between ONUs. When an ONU has more traffic to transmit, it is assigned more capacity; otherwise, it is allocated less capacity. By doing this, EPON can significantly improve the upstream capacity utilization and other performance criteria such as average packet delay and throughput. To realize this, OLT specifically run a polling protocol which polls each ONU periodically asking how much bandwidth it needs for the next round of transmission. When the ONU sends data upstream to OLT, such a bandwidth request information will be piggybacked on the uploaded data. The OLT gets this information, and then it will schedule the time for the ONU to transmit data in the second round. Once scheduled, the OLT will send a GRANT message to the ONU to tell it the detailed information. Thus, overall this is an interleaving polling process. It has been verified to achieve an efficient performance in terms of bandwidth utilization and average packet delay.  

In addition, to support different quality of service of upstream traffic data, more advanced bandwidth allocation and scheduling have been devised too. One important scheduling approach is called intra-ONU scheduling, in which several queues are set, and each of the queues has different priorities. The frames with higher priorities are always transmitted earlier than those in lower priority queues. This can help assure QoS of higher priority services such as VoIP.  

5. Advantage and disadvantage 

Compared to the traditional APON technique, due to the extensive deployment of Ethernet technique, EPON is expected to achieve a lower cost to set up a system. In addition, it is more efficient in network capacity utilization due to its lower control overhead in each Ethernet frame than the overhead in each ATM cell. However, compared to APON, it may not be able to support QoS for application services as well as ATM, as APON has a more sophisticated QoS-supporting scheme inherited from the traditional ATM networks. 

6. Current Status of EPON

Currently, EPON system have been commercially available and some testbed and commercial deployments have been made widely in the world. It is expected that the access network market based on EPON will further enlarge due to the cheap cost of the system and high efficiency in capacity utilization compared to other PON techniques. It will also be expected to take over the access network market from traditional DSL techniques because of the fast increase of bandwidth-intensive services such as IPtV, VoIP, etc. PON is the final destination that we have to face for the future access network development and deployment. 

7. Future Perspective

Today's EPON can support up to 1GB/s upstream and downstream capacity. More advanced systems are being developed in two directions. One is to increase the transmission capacity in time domain. Some 10Gb/s EPON systems are being developed and tested by many vendors, and standardized process has been started recently. On the other direction, people are concentrating on the wavelength domain to develop WDM EPON. That is, rather than transmitting only a single wavelength, several wavelengths are transmitted in a fiber such that the total capacity of the whole EPON system can be upgraded for many times. Such WDM-based systems are called WDM-PONs. It is also possible to combine WDM technique with 10G-EPON so as to support even higher capacity.