QoS-Aware Radio-and-Fiber (R&F) Access-Metro Networks (Invited Paper) Martin Maier and Navid Ghazisaidi Optical Zeitgeist Laboratory, INRS, 800, Gaucheti` ere West, suite 6900, Montr´ eal, QC, H5A 1K6, Canada Phone: (514) 875-1266, Fax: (514) 875-0344, Email: {maier,navid}@emt.inrs.ca Abstract—Future bimodal fiber-wireless (FiWi) access-metro networks may deploy both radio-over-fiber (RoF) and radio-and- fiber (R&F) technologies. RoF networks are rapidly becoming mature, but they fall short of interworking with distributed wire- less MAC protocols such as DCF in widely deployed IEEE 802.11 WLANs. While R&F networks are able to avoid this limitation by means of protocol translation at the optical-wireless interface, recent testbed activities demonstrated that their multimedia QoS performance is far from acceptable, giving rise to various open R&F networking issues. In this paper, we report on our ongoing research activities on providing improved QoS support in R&F access-metro networks by means of (i) hierarchical scheduling and hybrid access control in integrated RPR/WiMAX metro networks, and (ii) hierarchical frame aggregation in integrated EPON/next-generation WLAN-based mesh access networks. I. I NTRODUCTION Future broadband access networks will be bimodal, capital- izing on the respective strengths of both optical and wireless technologies and smartly merging them in order to realize future-proof fiber-wireless (FiWi) networks that strengthen our information society while avoiding its digital divide. By com- bining the capacity of optical fiber networks with the ubiquity and mobility of wireless networks, FiWi networks form a powerful platform for the support and creation of emerging as well as future unforeseen applications and services, e.g., telepresence [1]. Radio-over-fiber (RoF) networks have been studied for many years as an approach to integrate optical fiber and wire- less networks. In RoF networks, radio frequencies (RFs) are carried over optical fiber links between the central office (CO) and multiple low-cost remote antenna units (RAUs) in support of a variety of wireless applications, e.g., microcellular radio systems [2]. It was experimentally demonstrated that RoF networks can have an optical fiber range of up to 50 km [3]. However, inserting an optical distribution system in wireless networks may have a detrimental impact on the performance of medium access control (MAC) protocols [4]. The additional propagation delay may exceed certain timeouts of wireless MAC protocols, resulting in a deteriorated network perfor- mance. More precisely, MAC protocols based on centralized polling and scheduling, e.g., IEEE 802.16 WiMAX, are less affected by increased propagation delays due to their ability to take longer walk times between the CO and wireless subscriber stations (SSs) into account by means of interleaved polling and scheduling of upstream transmissions originating from different SSs. However, in distributed MAC protocols, e.g., the widely deployed distributed coordination function (DCF) in IEEE 802.11a/b/g WLANs, the additional propagation delay between wireless stations (STAs) and access point (AP) poses severe challenges. Due to the acknowledgment (ACK) timeout optical fiber can be deployed in a standard 802.11b WLAN- based RoF network with a default ACK timeout value of 20 µs only up to a maximum length of 1948 meters to ensure the proper operation of DCF [5]. The aforementioned limitations of WLAN-based RoF net- works can be avoided in so-called radio-and-fiber (R&F) networks [6]. While RoF networks use optical fiber as an analog transmission medium between the CO and one or more RAUs with the CO being in charge of controlling access to both optical and wireless media, in R&F networks access to the optical and wireless media is controlled separately from each other by using in general two different MAC protocols in the optical and wireless media, with protocol translation taking place at their interface. As a consequence, wireless MAC frames do not have to travel along the optical fiber to be processed at the CO, but simply traverse their associated AP and remain in the WLAN, thus avoiding the negative impact of fiber propagation delay on the network performance. In general, FiWi networks may deploy both RoF and R&F technologies. While significant progress has been made at the PHY layer of FiWi and in particular RoF transmission systems, FiWi networking research on layer-2 related issues has begun only very recently, addressing key challenges such as integrated channel assignment and bandwidth allocation, integrated path selection, hierarchical optical burst assembly and wireless frame aggregation, as well as flow and congestion control [7]. Another important challenge is quality-of-service (QoS) which plays a key role in running various multimedia applications and services over FiWi networks. In this pa- per, we study an Ethernet-based R&F access-metro network, referred to as SuperMAN, which integrates next-generation WiFi and WiMAX networks with optical access and metro networks. Our focus is on layer-2 QoS provisioning which largely depends on the performance of routing and resource management algorithms, including bandwidth allocation and channel assignment algorithms, with absolute and relative QoS assurances.