Dynamic Bandwidth Allocation for Ethernet Passive Optical Networks

Introduction

With the explosive growth of the Internet and ever-increasing users’ demand for various broadband applications, such as Internet telephony, high-definition television (HDTV), interactive games, and video on demand, subscriber access networks, which cover the “last mile” area, and serve numerous residential and small business users, have become a bandwidth bottleneck in providing broadband services to subscriber users (Zheng & Mouftah, 2004]. In recent years, subscriber access networks have been extensively upgraded with the deployment of innovative xDSL and CaTV technologies. However, these technologies have their own limitations and are insufficient to meet the ever-increasing bandwidth demand of subscriber users. To alleviate this bottleneck, fiber to the home/curb/building (FTTH/FTTC/FTTB) technologies have been long envisioned as a preferred solution, and passive optical networks (PONs) have been widely considered as a promising technology for implementing various FTTx solutions.

As one of the promising solutions, EPON has attracted a great interest from both industry and academia in recent years. EPON combines low-cost Ethernet equipment and low-cost passive optical components, and thus has a number of advantages over traditional access networks, such as larger bandwidth capacity, longer operating distance, lower equipment and maintenance cost, and easier update to higher bit rates (Kramer & Pesavento, 2002). An EPON system is a point-to-multipoint fiber optical network with no active elements in the transmission path from source to destination. It can use different multipoint topologies, such as bus, ring, and tree (Kramer & Pesavento, 2002), and different network architectures (Foh, 2004; Kramer & Pesavento, 2002; Shami, 2005; Sherif, 2004). The standard PON architecture is based on a tree topology and consists of an optical line terminal (OLT), a 1:N passive star coupler (or splitter/combiner), and multiple optical network units (ONUs), as shown in Figure 1. The OLT resides in a central office (CO) that connects the access network to a metropolitan area network (MAN) or a wide area network (WAN), and is connected to the passive star coupler through a singe optical fiber. The passive coupler is located a long distance away from the CO but close to the subscriber premises. Each ONU is located either at curbs or at subscriber premises, and is connected to the passive coupler through a dedicated short optical fiber. The distance between the OLT and each ONU typically ranges from 10 to 20 km. In an EPON system, all transmissions are performed between the OLT and the ONUs. In the downstream direction, an EPON is a point-to-multipoint network, in which the OLT broadcasts data to each ONU through the 1:N splitter, where N is typically between 4 and 64. Each ONU extracts the data destined for it based on its media access control (MAC) address. In the upstream direction, a PON is a multipoint-to-point network, in which multiple ONUs transmit data to the OLT through the 1:N passive combiner. The line data rate from an ONU to the OLT and the user access rate from a user to an ONU do not necessarily have to be equal and the line data rate is usually much higher than the user access rate. Since all ONUs share the same upstream transmission medium with limited bandwidth, an EPON system must employ a MAC mechanism to arbitrate the access to the shared medium in order to avoid data collisions in the upstream direction and at the same time to efficiently share the upstream transmission bandwidth among all ONUs. For this reason, bandwidth allocation is critical for ensuring the quality of network services in an EPON system and a large amount of research work has been conducted on this critical issue in recent years. The purpose of this chapter is to give an introduction of the fundamentals on bandwidth allocation in an EPON system and present an overview of the state-of-the-arts dynamic bandwidth allocation (DBA) algorithms proposed thus far for EPON systems.

Figure 1.

EPON architecture

The remainder of the chapter is organized as follows. In Section II, we introduce some fundamental concepts on bandwidth allocation in an EPON system. In Section III, we discuss the major challenges in designing a polling protocol for bandwidth allocation. In Section IV, we present an overview of the state-of-the arts DBA algorithms proposed for EPONs. In Section V, we conclude this chapter.