Abstract
Recently, Ethernet Passive Optical Networks (EPONs) have received a great amount of interest as a promising cost-effective solution for next-generation high-speed access networks. This is confirmed by the formation of several fora and working groups that contribute to their development, namely the EPON Forum (http://www. ieeecommunities.org/epon), the Ethernet in the First Mile Alliance (http://www.efmalliance.org), and the IEEE 802.3ah working group (http://www.ieee802. org/3/efm), which is responsible for the standardization process. EPONs are a simple, inexpensive, and scalable solution for high-speed residential access capable of delivering voice, high-speed data, and multimedia services to end users (Kramer, Mukherjee, & Maislos, 2003; Kramer & Pesavento, 2002; Lorenz, Rodrigues, & Freire, 2004; McGarry, Maier, & Reisslein, 2004; Pesavento, 2003). An EPON combines the transport of IEEE 802.3 Ethernet frames over a low-cost and broadband point-to-multipoint passive optical fibre infrastructure connecting the optical line terminal (OLT) located at the central office to optical network units (ONUs) usually located at the subscriber premises. In the downstream direction, the EPON behaves as a broadcast and select shared medium, with Ethernet frames transmitted by the OLT reaching every ONU. In the upstream direction, Ethernet frames transmitted by each ONU will only reach the OLT, but an arbitration mechanism is required to avoid collisions. This article provides an overview of EPONs focused several issues: EPON architecture, multipoint control protocol (MPCP), quality of service (QoS), and operations, administration, and maintenance (OAM) capability of EPONs.
TopEpon Architecture
EPONs, which represent the convergence of low-cost and widely used Ethernet equipment and low-cost point-to-multipoint fibre infrastructure, seem to be the best candidate for the next-generation access network (Kramer & Pesavento, 2002; Pesavento, 2003). In order to create a cost-effective shared fibre infrastructure, EPONs use passive optical splitters in the outside plant instead of active electronics and, therefore, besides the end terminating equipment, no intermediate component in the network requires electrical power. Due to its passive nature, optical power budget is an important issue in EPON design because it determines how many ONUs can be supported, as well as the maximum distance between the OLT and ONUs. In fact, there is a trade-off between the number of ONUs and the distance limit of the EPON because optical losses increase with both split count and fibre length. EPONs can be deployed to reach distances up to around 20 km with a 1:16 split ratio, which sufficiently covers the local access network (Pesavento, 2003). Figure 1 shows a possible deployment scenario for EPONs (Kramer, Banerjee, Singhal, Mukherjee, Dixit, & Ye, 2004).
Figure 1. Schematic representation of a possible deployment scenario for EPONs
Although several topologies are possible, such as tree, ring, and bus (Kramer et al., 2003; Kramer, Mukherjee, & Pesavento, 2001; Pesavento, 2003), the most common EPON topology is a 1:N tree or a 1:N tree-and-branch network, which cascades 1:N splitters, as shown in Figure 2. The preference for this topology is due to its flexibility in adapting to a growing subscriber base and increasing bandwidth demands (Pesavento, 2003).
Figure 2. Schematic representation of a tree-and-branch topology for EPONs
Key Terms in this Chapter
Ethernet Frame: Consists of a standardized set of bits, organized into several fields, used to carry data over an Ethernet system. Those fields include the preamble, a start frame delimiter, address fields, a length field, a variable size data field that carries from 46 to 1,500 bytes of data, and an error checking field.
DBA: Dynamic Bandwidth Allocation algorithms can be used with the MPCP arbitration mechanism to determine the collision-free upstream transmission schedule of ONUs and generate GATE messages accordingly.
MPCP: Multi-Point Control Protocol is a medium access control protocol used in EPONs to avoid collisions in the upstream direction.
LLID: Logical Link Identifier is a 2-byte tag in the preamble of an Ethernet frame. This 2-byte tag uses 1 bit as a mode indicator (point-to-point or broadcast mode) and the remaining 15 bits as the ONU ID.
PON: Passive Optical Network is a network based on optical fibre, in which all active components and devices between the central office and the customer premises are eliminated.
ONU: Optical Network Unit is usually located at the subscriber premises or in a telecom closet and is responsible for the transmission of Ethernet frames to OLT.
OLT: Optical Line Terminal is located at the central office and is responsible for the transmission of Ethernet frames to ONUs.