We have briefly introduced the automatic switched optical network (ASON) framework for the control plane of optical networks in Section 2.5. The ASON framework facilitates the set-up, modification, reconfiguration, and release of both switched and soft-permanent optical connections. Switched connections are controlled by clients as opposed to soft-permanent connections whose set-up and tear-down are initiated by the network management system. An ASON consists of one or more domains, where each domain may belong to a different network operator, administrator, or vendor platform. In the ASON framework, the points of interaction between different domains are called reference points. Figure 5.1 depicts the ASON reference points between various optical networks and client networks (e.g., IP, asynchronous transfer mode [ATM], or Synchronous Optical Network/synchronous digital hierarchy [SONET/SDH] networks), which are connected via lightpaths. Specifically, the reference point between a client network and an administrative domain of an optical network is called user–network interface (UNI). The reference point between the administrative domains of two different optical networks is called external network–network interface (E-NNI). The reference point between two domains (e.g., routing areas), within the same administrative domain of an optical network is called internal network–network interface (I-NNI).

Multiprotocol label switching

The ASON framework may be viewed as a reference architecture for the control plane of optical switching networks. It is important to note that the framework addresses the ASON requirements but does not specify any control plane protocol. In transparent optical networks, such as ASON, intermediate optical add-drop multiplexers (OADMs) and optical cross-connects (OXCs) may be optically bypassed and thereby prevented from accessing the corresponding wavelength channels.

Optical fiber provides huge amounts of bandwidth which can be tapped into by means of dense wavelength division multiplexing (DWDM), where each fiber may carry tens or even hundreds of wavelength channels, each operating at electronic peak rate (e.g., 40 Gb/s). Given this huge number of high-speedwavelength channels, one may think that network capacity will not be an issue in future optical networks and it seems reasonable to deploy dynamic optical circuit switching (OCS) to meet future service requirements in support of existing and emerging applications. Typically, these optical circuits may be lightpaths that are dynamically set up and torn down by using a generalized multiprotocol label switching (GMPLS) based control plane to realize reconfigurable optical transport networks, leading to multiprotocol lambda switching (MPλS), as discussed at length in Chapter 5. While OCS may be considered a viable solution that can be realized using mature optics and photonics technologies, economics will ultimately demand that network resources are used more efficiently by decreasing the switching granularity from optical wavelengths to optical packets, giving rise to optical packet switching (OPS) (O'Mahony et al., 2001). Especially given the fact that networks increasingly become IP data-centric, OPS naturally appears to be a promising candidate to support bursty data traffic more efficiently than OCS by capitalizing on the statistical multiplexing gain. Furthermore, the connectionless service offered by OPS helps reduce the network latency in that OPS avoids the two-way reservation overhead of OCS. Note that in Chapter 9 we have seen that the same holds for optical burst switching (OBS) as well.

We have seen in Chapter 15 that wavelength division multiplexing (WDM) upgraded Ethernet passive optical networks (EPONs) are expected to become mature in the near term. In this chapter, we consider WDM EPONs and, arguing that the key tasks of cost reduction and design of colorless ONUs will be addressed successfully in the near term, elaborate on the question “WDM EPON – what's next?” Our focus will be on evolutionary upgrades and further cost reductions of WDM EPONs and the alloptical integration of Ethernet-based WDM EPON and WDM upgraded RPR networks. The resultant Ethernet-based optical access-metro area network, called STARGATE, was recently proposed in Maier et al. (2007) and will be described at length in the following.

Research on the interconnection of multiple (E)PONs has begun only very recently. In Hsueh et al. (2005a), multiple PONs of arbitrary topology are connected to the same central office (CO) whose transmitters may be shared for downstream transmission among all attached PONs. In An et al. (2005), a common fiber collector ring network interconnects multiple PONs with the CO whose transmitters are used not only for downstream from CO to subscribers but also for upstream transmission from subscribers to CO by means of remote modulation. Note that in both proposed PON interconnection models, any traffic sent between end users residing in different PONs has to undergo OEO conversion at the common CO (i.e., PONs are not interconnected all-optically).

RPR can easily bridge to Ethernet networks such as EPON and may also span into metropolitan area networks (MANs) and wide area networks (WANs). This makes it possible to perform layer 2 switching from access networks far into backbone networks (Davik et al., 2004).

The ultimate goal of the Internet and communications networks in general is to provide access to information when we need it, where we need it, and in whatever format we need it (Mukherjee, 2000). To achieve this goal wireless and optical technologies play a key role in future communications networks. Wireless and optical networks can be thought of as quite complementary. Optical fiber does not go everywhere, but where it does go, it provides a huge amount of available bandwidth. Wireless networks, on the other hand, potentially go almost everywhere and are thus able to support mobility and reachability, but they provide a highly bandwidth-constrained transmission channel, susceptible to a variety of impairments (Ramaswami, 2002). As opposed to the wireless channel, optical fiber exhibits a number of advantageous transmission properties such as low attenuation, large bandwidth, and immunity from electromagnetic interference. Future communications networks will be bimodal, capitalizing on the respective strengths of wireless and optical networks.

Historical review

Optical networks have been long recognized to have many beneficial properties. Among others, optical fiber is well suited to satisfy the growing demand for bandwidth, transparency, reliability, and simplified operation and management (Green, 1996). In this part, we have first reviewed the historical evolution of optical networks from point-to-point links to reconfigurable all-optical WDM networks of arbitrary topology. In our review, we introduced the basic concepts and techniques of optical networking, highlighted key optical network elements (e.g., reconfigurable OADM and OXC), elaborated on the rationale behind the design of all-optical networks, and outlined their similarities to SONET/SDH networks. Furthermore, we identified and explained the most important features of optical networks, namely, transparency, reconfigurability, survivability, scalability, and modularity.

In the previous section, we have seen that photonic slot routing (PSR) can be transformed into individual wavelength switching (IWS) and used to realize synchronous optical packet switching (OPS) networks with the restriction that packets need to be of fixed size. Unlike electronic IP packet switching networks, these OPS networks require network-wide synchronization and are able to transport only fixed-size packets. In contrast, IP networks do not require network-wide synchronization and support variable-size IP packets. In addition, contention resolution can be done more easily and more efficiently in electronic networks than in optical networks by using electronic random access memory (RAM). Packets contending for the same router output port can be stored in electronic RAM and sent sequentially through the same port without collision. In optical networks, RAM is not feasible with current technology. Instead, bulky switched delay lines (SDLs) and/or inefficient deflection routing need to be deployed in order to resolve contention in OPS networks. Clearly, electronic packet-switched networks are able to resolve contention more efficiently by using electronic RAM. Given the steadily growing line rates and amount of traffic, however, electronic routers may become the bottleneck in high-speed communications networks that use electronic routers for storing and routing and optical fiber links for transmitting packets of variable size. This bottleneck is commonly referred to as the electro-optical bottleneck.

One of the main bottlenecks in today's Internet is (electronic) routing at the IP layer. Several methods have been proposed to alleviate the routing bottleneck by switching long-duration flows at lower layers (e.g., GMPLS; see Chapter 5). In doing so, routers are offloaded and the electro-optical bottleneck is alleviated.

In this part, we explore a wide range of different optical metropolitan area network (MAN) architectures and protocols. MANs are found at the metro level of the network hierarchy between wide area networks (WANs) and access networks. Typically, MANs have a ring topology and are deployed in interconnected ring architectures that are composed of metro core and metro edge rings, as depicted in Fig. III.1. Each metro core ring interconnects several metro edge rings with the long-haul backbone networks. Apart from inter-metro-edge-ring traffic, metro core rings also carry traffic from and to the long-haul backbone networks. Metro edge rings in turn carry traffic between metro core rings and access networks, for example, hybrid fiber coax (HFC), fiber-to-the-home (FTTH), fiber-to-the-building (FTTB) networks, and passive optical networks (PONs). Ring networks offer simplicity in terms of operation, administration, and maintenance (OAM). Moreover, ring networks provide fast protection switching in the event of a single link or node failure.

Optical metro ring networks can be either single-channel or multichannel wavelength division multiplexing (WDM) systems. Optical ring networks were initially singlechannel systems, where each fiber link carries a single wavelength channel (e.g., IEEE 802.5 Token Ring and ANSI Fiber Distributed Data Interface (FDDI)). Optical singlechannel ring networks belong to the first generation of opaque optical networks where OEO conversion takes place at each node. Opaque ring networks have come a long way. Among others, the so-called Cambridge ring is a unidirectional ring network whose channel access is based on the empty-slot principle (Hopper andWilliamson, 1983). The Cambridge ring deploys source stripping, where the source node takes the transmitted packet from the ring.

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.