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Ethernet Passive Optical Networks - INTRODUCTION, EPON ARCHITECTURE, MULTI-POINT CONTROL PROTOCOL

frames onu epons pesavento

Mário M. Freire
Universidade de Beira Interior, Portugal

Paulo P. Monteiro
SIEMENS S.A. and Universidade de Aveiro, Portugal

Henrique J. A. da Silva
Universidade de Coimbra, Portugal

Jose Ruela
Faculdade de Engenharia da Universidade do Porto (FEUP), Portugal

INTRODUCTION

Recently, Ethernet Passive Optical Networks (EPONs) have received a great deal 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; Pesavento, 2003; McGarry, Maier & Reisslein, 2004). An EPON combines the transport of IEEE 802.3 Ethernet frames over a low-cost and broadband point-to-multipoint passive optical fiber 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 and focuses on the following issues: EPON architecture; Multi-Point Control Protocol (MPCP); quality of service (QoS); and operations, administration, and maintenance (OAM) capability of EPONs.

EPON ARCHITECTURE

EPONs, which represent the convergence of low-cost and widely used Ethernet equipment and low-cost point-to-multipoint fiber 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 fiber 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 tradeoff between the number of ONUs and the distance limit of the EPON, because optical losses increase with both split count and fiber 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).

Although several topologies are possible (i.e., tree, ring, and bus) (Kramer, Mukherjee & Maislos, 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).

EPONs cannot be considered either a shared medium or a full-duplex point-to-point network, but a combination of both depending on the transmission direction (Pesavento, 2003). In the downstream direction, an EPON behaves as a shared medium (physical broadcast network), with Ethernet frames transmitted from the OLT to every ONU. In the upstream direction, due to the directional properties of passive couplers, which act as passive splitters for downstream, Ethernet frames from any ONU will only reach the OLT and not any other ONU. In the upstream direction, the logical behavior of an EPON is similar to a point-to-point network, but unlike in a true point-to-point network, collisions may occur among frames transmitted from different ONUs. Therefore, in the upstream direction, there is the requirement both to share the trunk fiber and to arbitrate ONU transmissions to avoid collisions by means of a Multi-Point Control Protocol (MPCP) in the Medium Access Control (MAC) layer. An overview of this protocol will be presented in the next section.

EPONs use point-to-point emulation to meet the compliance requirements of 802.1D bridging, which provides for ONU to ONU forwarding. For this function, a 2-byte Logical Link Identifier (LLID) is used in the preamble of Ethernet frames. 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. An ONU transmits frames using its own assigned LLID and receives and filters frames according to the LLID. An emulation sublayer below the Ethernet MAC demultiplexes a packet based on its LLID and strips the LLID prior to sending the frame to the MAC entity. Therefore, the LLID exists only within the EPON network. When transmitting, an LLID corresponding to the local MAC entity is added. Based on the LLID, an ONU will reject frames not intended for it. For example, a given ONU will reject broadcast frames that it generates, or frames intended for other ONUs on the same PON (Pesavento, 2003).

In the downstream direction, an EPON behaves as a physical broadcast network of IEEE 802.3 Ethernet Frames, as shown in Figure 3. An Ethernet frame transmitted from the OLT is broadcast to all ONUs, which is a consequence of the physical nature of a 1:N optical splitter. At the OLT, the LLID tag is added to the preamble of each frame and extracted and filtered by each ONU in the reconciliation sublayer. Each ONU receives all frames transmitted by the OLT but extracts only its own frames; that is, those matching its LLID. Frame extraction (filtering) is based only on the LLID since the MAC of each ONU is in promiscuous mode and accepts all frames. Due to the broadcast nature of EPONs in the downstream direction, an encryption mechanism often is considered for security reasons. In the upstream direction, a multiple access control protocol is required, because the EPON operates as a physical multipoint-to-point network. Although each ONU sends frames directly to the OLT, the ONUs share the upstream trunk fiber, and simultaneous frames from ONUs might collide if the network was not properly managed. In normal operation, no collisions occur in EPONs (Pesavento, 2003).

MULTI-POINT CONTROL PROTOCOL

In order to avoid collisions in the upstream direction, EPONs use the Multi-Point Control Protocol (MPCP). MPCP is a frame-oriented protocol based on 64-byte MAC control messages that coordinate the transmission of upstream frames in order to avoid collisions. Table 1 presents the main functions performed by MPCP (Pesavento, 2003). In order to enable MPCP functions, an extension of MAC Control sublayer is needed, which is called Multipoint MAC Control sublayer.

MPCP is based on a non-cyclical frame-based Time Division Multiple Access (TDMA) scheme.The OLT sends GATE messages to ONUs in the form of 64-byte MAC Control frames. The GATE messages contain a timestamp and granted timeslot assignments, which represent the periods in which a given ONU can transmit. The OLT allocates time slots to the ONUs. Depending on the scheduler algorithm, bandwidth allocation can be static or dynamic. It is not allowed frame fragmentation within the upstream time slot, which contains several IEEE 802.3 Ethernet frames.

For upstream operation, the ONU sends REPORT messages, which contain a timestamp for calculating round trip time (RTT) at the OLT, and a report on the status of the queues at the ONU, so that efficient dynamic bandwidth allocation (DBA) schemes can be used. The ONU is not synchronized, nor does it have knowledge of delay compensation. Moreover, for upstream transmission, the ONU transceiver receives a timely indication from MPCP to change between on and off states (Pesavento, 2003).

OPERATIONS, ADMINISTRATION, AND MAINTENANCE CAPABILITY OF EPONs

OAM capability provides a network operator with the ability to monitor the network and determine failure locations and fault conditions. OAM mechanisms defined for EPONs include remote failure indication, remote loopback, and link monitoring. Remote failure indication is used to indicate that the reception path of the local device is non-operational. Remote loopback provides support for frame-level loopback and a data link layer ping. Link monitoring provides event notification with the inclusion of diagnostic data and polling of variables in the IEEE 802.3 Management Information Base. A special type of Ethernet frames called OAM Protocol Data Units, which are slow protocol frames, are used to monitor, test, and troubleshoot links. The OAM protocol also is able to negotiate the set of OAM functions that are operable on a given link interconnecting Ethernet devices (Pesavento, 2003).

CONCLUSION

EPONs have been proposed as a cost-effective solution for next-generation high-speed access networks. An overview of major issues in EPONs has been presented. The architecture and principle of operation of EPONs were briefly described. The Multi-Point Control Protocol used to eliminate collisions in the upstream direction was briefly presented. Quality of service, a major issue for multimedia services in EPONs, was also addressed. The operations, administration, and maintenance capability of EPONs was also briefly discussed.

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