The ultimate goal of the Internet and communication networks in general is to provide access to information when we need it, where we need it, and in whatever format we need it in. To achieve this goal, wireless and optical technologies play a key role. Wireless and optical access networks can be thought of as complementary. Optical fiber does not go everywhere, but where it does go, it provides a huge amount of available bandwidth. Wireless access networks, on the other hand, potentially go almost everywhere, but provide a highly bandwidth-constrained transmission channel susceptible to a variety of impairment. Clearly, as providers need to satisfy users with continuously increasing bandwidth demands, future broadband access networks must leverage on both technologies and converge them seamlessly, giving rise to fiber-wireless (FiWi) access networks.
RoF networks are attractive since they provide transparency against modulation techniques and are able to support various digital formats and wireless standards in a cost-effective manner, for example, wideband code-division multiple access (WCDMA), IEEE 802.11 wireless local area network (WLAN), personal handy phone system (PHS), and Global System for Mobile Communications (GSM). To realize future multiservice access networks, the seamless integration of RoF systems with existing and emerging optical access networks is important, such as FTTX and wavelength-division multiplexing (WDM) PON networks. RoF networks are also well suited to avoid frequent handovers of fast-moving users in cellular networks. An interesting approach to avoid handovers for train passengers is the use of an optical fiber WDM ring-based RoF network installed along the rail tracks in combination with the moving cell concept, as recently proposed . The concept of moving cells enables a cell pattern and a train to move along on the same radio frequency during the whole connection in a synchronous fashion without requiring handovers.
2.WIRELESS ACCESS NETWORKS
2.1 WIRELESS MESH NETWORKS
Recent advances in wireless communications technology have led to significant innovations that have enabled cost-effective and flexible wireless Internet access, and provided incentives for building efficient multihop wireless networks. A wireless ad hoc network precludes the use of a wired infrastructure and allows hosts to communicate either directly or indirectly over radio channels without requiring any prior deployment of network infrastructure. Wireless mesh networks (WMNs), on the other hand, are networks employing multihop communications to forward traffic en route to and from wired Internet entry point. In contrast to conventional WLANs and mobile ad hoc networks (MANETs), WMNs promise greater flexibility, increased reliability, and improved performance. WMNs can be categorized into infrastructure, client, and hybrid WMNs (Fig. 1). A router in an infrastructure WMN has no mobility and performs more functions than a normal wireless router. Among others, a router performs mesh functions (routing and configuration) and acts as a gateway. In a client WMN, clients perform mesh and gateway functions themselves. Efficient routing protocols provide paths through the wireless mesh and react to dynamic changes in the topology, so mesh nodes can communicate with each other even if they are not in direct wireless range. Intermediate nodes on the path forward packets to the final destination. Due to the similarities between WMNs and MANETs, WMNs can apply ad hoc routing protocols (e.g., ad hoc on demand distance vector [AODV] and dynamic source routing [DSR], among others).
2.2 ENABLING TECHNOLOGIES
New technologies and protocols in the physical (PHY) layer, medium access control (MAC) protocols, and routing protocols are required to optimize the performance of WMNs. In the PHY layer, smart antenna, multi-input multi-output (MIMO), ultra wideband (UWB), and multichannel interface systems are being explored to enhance network capacity and further enable wireless gigabit transmission. Recently, gigabit transmission resulting from a combination of MIMO and orthogonal frequency-division multiplexing (OFDM) has been demonstrated. MAC protocols based on distributed time-division multiple access (TDMA) and CDMA are expected to improve the bandwidth efficiency of carrier sense multiple access with collision avoidance (CSMA/CA) protocols.
Currently, IEEE 802.11 a/b/g (WiFi) technologies are widely exploited in commercial, products and academic research of WMNs due to their low cost, technological maturity, and high product penetration . However, since these protocols were originally designed for WLANs, they clearly are not optimized for WMNs. Proprietary wireless technologies and WiMAX have been proposed. Unlike WiFi, IEEE 802.16 allows for point-to-multipoint wireless connections with a transmission rate of 75 Mb/s and can be used for longer distances.
Additionally, orthogonal frequency-division multiple access (OFDMA) and smart antenna technologies extend the scalability of WiMAX. These technologies are exploited to enhance the capacity, reliability, and mobility of WMNs.
Ultra-high-bandwidth standards such as IEEE 802.16m, which aims to provide 1 Gb/s and 100 Mb/s shared bandwidth, can be employed to further enhance the bandwidth and mobility of WMNs. Since packets are routed among mesh routers in the presence of interference, shadowing, and fading, a cross-layer design is required to optimize the routing in WMNs. For instance, DSR uses link quality source routing (LQSR) to select a routing path according to link quality metrics. LQSR includes three performance metrics: per-hop packet pair, per-hop round-trip time (RTT), and expected transmission count (ETX). ETX shows the best performance in networks ,with fixed nodes, while minimum hop count shows good performance in networks with mobile nodes.
2.3 FUTURE DEVELOPMENTS
Given the increased demand for mesh networks, a task group was formed in 2004 to define the Extended Service Set (ESS) mesh networking standard; its goal is the development of a flexible and extensible standard for WMNs based on IEEE 802.11. The IEEE 802.11s amendment can be split up into four major parts: multihop routing, MAC enhancements, security, and general topics. It also defines a new mesh data frame format that can be used for transmitting data within the WMN. Traffic in mesh networks is predominantly forwarded to and from wire line gateway nodes forming a logical tree structure. The 802.11s defines a default mandatory routing protocol (Hybrid Wireless Mesh Protocol [HWMP]) that uses hierarchical routing to exploit this tree-like logical structure and on demand routing protocols to address mobility; the on demand routing protocol is based on AODV, which uses a simple hop count routing metric. Alternatively, the standard allows vendors to operate using alternate protocols, one of which is described in the draft (Radio Aware Optimized Link State Routing [RA-OLSR]). RA-OLSR uses multipoint relays, a subset of nodes that flood a radio-aware link metric, thereby reducing control overhead on the routing protocol. Other interesting developments are concerned with the integration of different access technologies; for instance, the authors of ?presented an approach for integrating WiMAX and WiFi technologies, and discussed several issues pertaining to protocol adaptation and QoS support.
4.1 ENABLING TECHNOLOGIES
Currently, there are two technologies used to implement fiber-wireless (FiWi) networks:
? Free space optical (FSO), also known as? optical wireless (OW)
? Radio over fiber (RoF)
4.1.1 ?Free space optical (FSO)
? FSO is a type of direct line-of-sight (LOS) optical communications that provides point-to point connections by modulating visible or infrared (IR) beams . It offers high bandwidth and reliable communications over short distances. The transmission carrier is generated by deploying either a high-power light emitting diode (LED) or a laser diode, while the receiver may deploy a simple photo detector. Current FSO systems operate in full-duplex mode at a transmission rate ranging from 100 Mb/s to 2.5 Gb/s, depending largely on weather conditions. Given a clear LOS between source and destination and enough transmitter power, FSO communications can work over distances of several kilometres. At both source and destination, optical fiber may be used to build high-speed LANs, such as Gigabit Ethernet (GbE).
4.1.2 Radio over fiber (RoF)
?????? RoF, on the other hand, allows an analog optical link to transmit a modulated radio frequency (RF) signal. There are different techniques available to realize RoF networks. Typically, an RoF transmitter deploys a Mach- Zehnder intensity (MZI) modulator in conjunction with an oscillator that generates the required optical carrier frequency, followed by an Erbium doped fiber amplifier (EDFA) in order to increase the transmission range. RoF networks provide both P2P and point-to-multipoint connections. Recently, a full-duplex RoF system providing 2.5 Gb/s data transmission over 40 km with less than 2 dB power attenuation was successfully demonstrated using the millimeterwave band . There are many cost-efficient optical approaches to mixing and up converting millimetre wave signals.
Table 1 summarizes and compares the salient features of both enabling technologies of FiWi networks.
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