In the preceding chapters, we witnessed that optical fiber is widely used as the transmission medium of choice in wide, metropolitan, access, and local area (wired) networks. Passive optical networks (PONs) might be viewed as the final frontier of optical wired networks where they interface with a number of wireless access technologies. One interesting approach to integrate optical fiber networks and wireless networks are so-called radio-over-fiber (RoF) networks. In RoF networks, radiofrequencies (RFs) are carried over optical fiber links to support a variety of wireless applications. In this chapter, we describe some recently investigated RoF network architectures and their support of various wireless applications. After reviewing the use of optical fiber links for building distributed antenna systems in fiber-optic microcellular radio networks, we elaborate on the various types of RoF networks and their integration with fiber to the home (FTTH), WDM PON, and rail track networks.

Fiber-optic microcellular radio

Distributed antenna system

To increase frequency reuse and thereby support a growing number of mobile users in cellular radio networks, cells may be subdivided into smaller units referred to as microcells. The introduction of microcells not only copes with the increasing bandwidth demands of mobile users but also reduces the power consumption and size of handset devices. Instead of using a base station antenna with high-power radiation, a distributed antenna system connected to the base station via optical fibers was proposed in Chu and Gans (1991).

Future broadband optical access networks not only have to unleash the economic potential and societal benefit by opening up the first/last mile bandwidth bottleneck between bandwidth-hungry end users and high-speed backbone networks but they also must enable the support of a wide range of new and emerging services and applications (e.g., triple play, video on demand, video conferencing, peer-to-peer [P2P] audio/video file sharing, multichannel high-definition television [HDTV], multimedia/multiparty online gaming, telemedicine, telecommuting, and surveillance) to get back on the road to prosperity. Due to their longevity, low attenuation, and huge bandwidth, asynchronous transfer mode (ATM) or Ethernet-based passive optical networks (PONs) are already widely deployed in today's operational access networks (e.g., fiber-to-the-premises [FTTP] and fiber-to-the-home [FTTH] networks) (Abrams et al., 2005). Typically, these PONs are time division multiplexing (TDM) single-channel systems, where the fiber infrastructure carries a single upstream wavelength channel and a single downstream wavelength channel. To support the aforementioned emerging services and applications in a costeffective and future-proof manner and to unleash the full potential of FTTX networks, PONs need to evolve by addressing the following three tasks (Shinohara, 2005):

1. Cost Reduction: Cost is key in access networks due to the small number of costsharing subscribers compared to that of metro and wide area networks. Devices and components that can be mass produced and widely applied to different types of equipment and situations must be developed. Importantly, installation costs which largely contribute to the overall costs must be reduced. A promising example for cutting installation costs is NTT's envisioned do-it-yourself (DIY) installation which deploys a user-friendly hole-assisted fiber which exhibits negligible loss increase and sufficient reliability, even when it is bent at right angles, clinched, or knotted, and can be produced economically.

2. […]

Optical fiber provides an unprecedented bandwidth potential that is far in excess of any other known transmission medium. A single strand of fiber offers a total bandwidth of 25 000 GHz. To put this potential into perspective, it is worthwhile to note that the total bandwidth of radio on Earth is not more than 25 GHz (Green, 1996). Apart from its enormous bandwidth, optical fiber provides additional advantages such as low attenuation loss (Payne and Stern, 1986). Optical networks aim at exploiting the unique properties of fiber in an efficient and cost-effective manner.

Optical point-to-point links

The huge bandwidth potential of optical fiber has been long recognized. Optical fiber has been widely deployed to build high-speed optical networks using fiber links to interconnect geographically distributed network nodes. Optical networks have come a long way. In the early 1980s, optical fiber was primarily used to build and study point-to-point transmission systems (Hill, 1990).As shown in Fig. 1.1(a), an optical point-to-point link provides an optical single-hop connection between two nodes without any (electrical) intermediate node in between. Optical point-to-point links may be viewed as the beginning of optical networks. Optical point-to-point links may be used to interconnect two different sites for data transmission and reception. At the transmitting side, the electrical data is converted into an optical signal (EO conversion) and subsequently sent on the optical fiber. At the receiving side, the arriving optical signal is converted back into the electrical domain (OE conversion) for electronic processing and storage.

Current Ethernet passive optical networks (EPONs) are single-channel systems; that is, the fiber infrastructure carries a single downstream wavelength channel and a single upstream wavelength channel, which are typically separated by means of coarse wavelength division multiplexing (CWDM). In the upstream direction (from subscriber to network), the wavelength channel bandwidth is shared by the EPON nodes by means of time division multiplexing (TDM). In doing so, only one common type of single-channel transceiver is used network wide, resulting in simplified network operation and maintenance. At present, single-channel TDM EPONs appear to be an attractive solution to provide more bandwidth in a cost-effective manner.

Given the steadily increasing number of users and bandwidth-hungry applications, current single-channel TDM EPONs are likely to be upgraded in order to satisfy the growing traffic demands in the future. Clearly, one approach is to increase the line rate of TDM EPONs. Note, however, that such an approach implies that all EPON nodes need to be upgraded by replacing the installed transceivers with higher-speed transceivers, resulting in a rather costly upgrade. Alternatively, single-channel TDM EPONs may be upgraded by deploying multiplewavelength channels in the installed fiber infrastructure in the upstream and/or downstream directions, resulting in wavelength division multiplexing (WDM) EPONs. As opposed to the higher-speed TDM approach, WDM EPONs provide a cautious upgrade path in that wavelength channels can be added one at a time, each possibly operating at a different line rate. More importantly, only EPON nodes with higher traffic demands may be WDM upgraded by deploying multiple fixed-tuned and/or tunable transceivers while EPON nodes with lower traffic demands remain unaffected.

A variety of optical networking technologies and architectures have been developed and examined over the past decades. Up to date, however, only a few of them led to commercial adoption and revenue generation. According to Ramaswami (2006), Erbium doped fiber amplifiers (EDFAs), reconfigurable optical add-drop multiplexers (ROADMs), wavelength cross-connects (WXCs), and tunable lasers are good examples of devices successfully deployed in today's optical networks. In contrast, other technologies and techniques such as wavelength conversion, optical code division multiple access (OCDMA), optical packet switching (OPS), and optical burst switching (OBS) face significant challenges toward widespread deployment.

Crucial to the commercial success of any proposed networking technology and architecture is not only its performance evaluation by means of analysis or simulation but also a thorough feasibility study of its practical aspects. Toward this end, proof-of-concept demonstrators, testbeds, and field trials play a key role.

In this part, we provide an up-to-date survey of testbed activities on the latest switching techniques proposed for next-generation optical networks. A number of different optical switching techniques have been studied over the last few years. In our survey, we outline current testbed activities of the following major optical switching techniques: generalized multiprotocol label switching (GMPLS), waveband switching (WBS), photonic slot routing (PSR), optical flowswitching (OFS), optical burst switching (OBS), and optical packet switching (OPS), which were explained at length in previous chapters. We note that our survey is targeted to networks rather than stand-alone components and devices. Furthermore, we note that regional overviews of optical networking testbeds in Europe and China were recently reported in Fabianek (2006) and Lin and Wu (2006), respectively.

We have seen in Chapter 5 that generalized multiprotocol label switching (GMPLS) networks are able to support various switching granularities, covering fiber, waveband, wavelength, and subwavelength switching. To realize GMPLS networks, the underlying network nodes need to support multiple switching granularities rather than only one. Hence, ordinary optical cross-connects (OXCs) that perform only wavelength switching, such as the one shown in Fig. 1.5, must be upgraded in order to support multiple switching granularities, leading to so-called multigranularity optical cross-connects (MG-OXCs). Compared to ordinary OXCs, MG-OXCs hold great promise to reduce the complexity and costs of OXCs significantly by switching fibers and wavebands as an entity without demultiplexing the arriving WDM comb signal into its individual wavelengths, giving rise to waveband switching (WBS).

Recently, WBS has been receiving considerable attention for its practical importance in reducing the size and complexity of photonic and optical cross-connects. Due to the rapid development and worldwide deployment of dense wavelength division multiplexing (DWDM) technologies, current fibers are able to carry hundreds of wavelengths. Using ordinary wavelength-switching cross-connects would require a large number of ports. WBS comes into play here with the promise to reduce the port count, control complexity, and reduce the cost of photonic and optical cross-connects. The rationale behind WBS is to group several wavelengths together as a waveband and switch the waveband optically using a single input and a single output port instead of multiple input/output ports, one for each of the individual wavelengths of the waveband. As a result, the size of ordinary cross-connects that traditionally switch at the wavelength granularity can be reduced, including the associated control complexity and cost (Cao et al., 2003b).

Optical fiber is commonly recognized as an excellent transmission medium owing to its advantageous properties, such as low attenuation, huge bandwidth, and immunity against electromagnetic interference. Because of their unique properties, optical fibers have been widely deployed to realize high-speed links that may carry either a single wavelength channel or multiple wavelength channels by means of wavelength division multiplexing (WDM). The advent of Erbium doped fiber amplifiers was key to the commercial adoption of WDM links in today's network infrastructure. WDM links offer unprecedented amounts of capacity in a cost-effective manner and are clearly one of the major success stories of optical fiber communications.

Since their initial deployment as high-capacity links, optical WDM fiber links turned out to offer additional benefits apart from high-speed transmission. Most notably, the simple yet very effective concept of optical bypassing enabled network designers to let in-transit traffic remain in the optical domain without undergoing optical-electrical-optical conversion at intermediate network nodes. As a result, intermediate nodes can be optically bypassed and costly optical-electrical-optical conversions can be avoided, which typically represent one of the largest expenditures in optical fiber networks in terms of power consumption, footprint, port count, and processing overhead. More important, optical bypassing gave rise to so-called all-optical networks in which optical signals stay in the optical domain all the way from source node to destination node.

All-optical networks were quickly embraced by both academia and industry, and the research and development of novel architectures, techniques, mechanisms, algorithms, and protocols in the arena of all-optical network design took off immediately worldwide.

The IEEE standard 802.17 Resilient Packet Ring (RPR) aims at combining SONET/SDH's carrier-class functionalities of high availability, reliability, and profitable TDM service (voice) support with Ethernet's high bandwidth utilization, low equipment cost, and simplicity (Davik et al., 2004; Yuan et al., 2004; Spadaro et al., 2004). RPR is a ring-based architecture consisting of two counter directional optical fiber rings with up to 255 nodes. Similar to SONET/SDH, RPR is able to provide fast recovery from a single link or node failure within 50 ms, and carry legacy TDM traffic with a high level of quality of service (QoS). Similar to Ethernet, RPR provides advantages of low equipment cost and simplicity and exhibits an improved bandwidth utilization due to statistical multiplexing. The bandwidth utilization is further increased by means of spatial reuse. In RPR, packets are removed from the ring by the corresponding destination node (destination stripping). The destination stripping enables nodes in different ring segments to transmit simultaneously, resulting in spatial reuse of bandwidth and an increased bandwidth utilization. Furthermore, RPR provides fairness, as opposed to today's Ethernet, and allows the full ring bandwidth to be utilized under normal (failure-free) operation conditions, as opposed to today's SONET/SDH rings where 50% of the available bandwidth is reserved for protection. Current RPR networks are single-channel systems (i.e., each fiber ring carries a single wavelength channel) and are expected to be primarily deployed in metro edge and metro core areas.

In the following sections, we explain RPR in greater detail, paying particular attention to its architecture, access control, fairness control, and protection.

Optical burst switching (OBS) is one of the recently proposed optical switching techniques which probably received the greatest deal of attention (Chen et al., 2004). OBS may be viewed as a switching technique that combines the merits of optical circuit switching (OCS) and optical packet switching (OPS) while avoiding their respective shortcomings. The switching granularity at the burst rather than wavelength level allows for statistical multiplexing in OBS, which is not possible in OCS, while requiring a lower control overhead than OPS. More precisely, in OCS, the entire bandwidth of each lightpath is dedicated to one pair of source and destination nodes and unused bandwidth cannot be reclaimed by other nodes ready to send data. Thus, OCS does not allow for statistical multiplexing. On the other hand, in OCS networks no OEO conversion is needed at intermediate nodes. As a result, OCS networks provide all-optical circuits that are transparent in terms of bit rate, modulation scheme, and protocol. OCS is well suited for large data transmissions whose long connection holding time on the order of a few minutes, hours, days, weeks, or even months justify the involved twoway reservation overhead for setting up or releasing a lightpath, which may take a few hundred milliseconds. Since many applications require only subwavelength bandwidth and/or involve bursts that last only a few seconds or less, the coarse wavelength switching granularity of OCS becomes increasingly inefficient and impractical. Unlike OCS, OPS is able to provide a significant statistical multiplexing gain due to the fact that bandwidth is not dedicated to a single connection but may be shared by multiple data flows.

So far, we have focused exclusively on malicious behavior, the traditional area of interest of security. But it is very important to keep in mind that security has been dominated for many years by military considerations, meaning that the “attacker” was indeed an attacker, equipped also with lethal weapons. Consequently, the goal was to defeat it at any cost, and it was pointless to try to quantify its motivation.

Commercial applications are very different from warfare, as we pointed out in Chapter 3. It is our profound belief that focusing exclusively on (malicious) attacks, as frequently done in current practice, captures only one aspect of the problem. Part III will explain how to model the motivation of a given party to depart from its nominal behavior.

The first two chapters of this part take the point of view of a selfish wireless station, considering first the MAC layer and then a fundamental mechanism of the network layer, namely packet forwarding. In both cases, we explain how to model the behavior of the stations by means of game theory. The reader unfamiliar with this discipline is strongly encouraged to first read Appendix B.

In the third chapter, we take the point of view of wireless operators having to cope with each other's presence in the same spectrum. Again, we explain how this problem can be modeled by means of game theory.

Many wireless networking mechanisms, notably routing, require that wireless nodes be aware of their neighborhood. This means that the nodes must know which other nodes they can communicate with directly. The procedure used to acquire this knowledge is called neighbor discovery.

In wired networks, neighbor discovery is a simple issue, because neighbor relationships do not change often, and hence routers can be pre-configured with the list of their wired neighbors. In contrast, in mobile wireless networks, the neighbor relationships change dynamically, which makes neighbor discovery an important mechanism. It is particularly important in the upcoming wireless networks that we described in Chapter 2.

Neighbor discovery can be achieved through simple protocols, where a node that wants to determine who its neighbors are broadcasts a neighbor discovery request, and every node that receives this request responds with a neighbor discovery reply. Receiving a reply means that the requesting node and the responding node can hear each other's transmission. In other words, they can communicate with each other directly, and hence they should consider each other as neighbors. The neighbor discovery protocol is sometimes called “hello protocol,” and the request and the reply are called “hello messages.”

An adversary can try to thwart the successful execution of the neighbor discovery protocol, for instance, by jamming the communication between two nodes. In this way, the adversary achieves that two nodes, which otherwise could communicate directly, cannot establish a neighbor relationship.