Access networks connect business and residential subscribers to the central offices (COs) of service providers, which in turn are connected to metropolitan area networks (MANs) or wide area networks (WANs). Access networks are commonly referred to as the last mile or first mile, where the latter term emphasizes their importance to subscribers. In today's access networks, telephone companies deploy digital subscriber line (xDSL) technologies and cable companies deploy cable modems. Typically, these access networks are hybrid fiber coax (HFC) systems with an optical fiber–based feeder network between CO and remote node and an electrical distribution network between remote node and subscribers. These access technologies are unable to provide enough bandwidth to current high-speed Gigabit Ethernet local area networks (LANs) and evolving services and applications (e.g., distributed gaming or video on demand). Future first-mile solutions not only have to provide more bandwidth but also have to meet the cost-sensitivity constraints of access networks arising from the small number of costsharing subscribers.

In so-called FTTX access networks the copper-based distribution part of access networks is replaced with optical fiber (e.g., fiber to the curb [FTTC] or fiber to the home [FTTH]). In doing so, the capacity of access networks is sufficiently increased to provide broadband services to subscribers. Due to the cost sensitivity of access networks, these all-optical FTTX systems are typically unpowered and consist of passive optical components (e.g., splitters and couplers). Accordingly, they are called passive optical networks (PONs). PONs had attracted a great deal of attention well before the Internet spurred bandwidth growth.

GMPLS

LION

The European IST project Layers Interworking in Optical Networks (LION) is a multilayer, multivendor, and multidomain managed IP/MPLS over automatic switched optical network (ASON) with a GMPLS-based control plane (Cavazzoni et al., 2003). The ASON framework facilitates the set-up, modification, reconfiguration, and release of both switched and soft-permanent optical connections (lightpaths). Switched connections are controlled by clients as opposed to soft-permanent connections whose set-up and teardown are initiated by the network management system (NMS). An ASON consists of one or more domains, each belonging to a different network operator, administrator, or vendor platform. 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 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). The LION testbed comprises three domains consisting of optical adddrop multiplexers (OADMs) and optical cross-connects (OXCs) from different vendors. For video-over-IP (VoIP) and computer-aided design (CAD) applications, the set-up and tear-down of soft-permanent connections through different domains using GMPLS signaling and interworking NMSs was experimentally validated. Furthermore, multilayer resilience tests were successfully carried out demonstrating MPLS fast reroute combined with optical restoration using a holdoff timer at the IP/MPLS layer.

In our introductory discussion of all-optical networks (AONs) in Section 1.5.1 we have seen that the concept of lightpath plays a key role in wavelength-routing optical networks. A lightpath is an optical point-to-point path of light that interconnects a pair of source and destination nodes, where intermediate nodes along the lightpath route the signal all-optically without undergoing OEO conversion. As each lightpath requires one wavelength on every traversed link and the number of both wavelengths and links in AONs is limited for cost and efficiency reasons, it is impossible to interconnect every pair of nodes by a dedicated lightpath. Nodes that cannot be directly connected via a lightpath may use multiple different lightpaths to exchange data. In the resultant multihop optical network, each intermediate node terminating a lightpath performs OEO conversion. As a consequence, such opaque multihop optical networks are unable to provide transparency. Also, note that the transmission capacity between node pairs connected via a lightpath is equal to the bandwidth of an entire wavelength channel. This transmission capacity is dedicated and cannot be shared by other nodes, leading to wasted bandwidth under bursty nonregular traffic. To improve the bandwidth utilization of lightpaths, electronic traffic grooming becomes necessary at each source node.

To avoid the loss of transparency and the need for electronic traffic grooming of lightpath-based optical networks, a novel solution for the design of transparent mesh wavelength division multiplexing (WDM) wide area networks was proposed in Chlamtac et al. (1999b).

Ethernet networks have come a long way and are widely deployed nowadays. In fact, 95% of today's local area networks (LANs) use Ethernet. Ethernet's transmission rate was originally set at 10 megabits per second (10 Mbps) in 1980 and evolved to higher speed versions ever since. A 100-Mbps version, also known as Fast Ethernet, was approved as IEEE standard 802.3u in 1995. In order to save time and standards development resources, physical signaling methods previously developed and standardized for Fiber Distributed Data Interface (FDDI) networks were reused in the IEEE standard 802.3u (Thompson, 1997). Fast Ethernet was immediately accepted by customers and its success prompted the development of an Ethernet standard for operation at 1000 Mbps (1 Gbps), leading to Gigabit Ethernet (GbE). The standard for Gigabit Ethernet, IEEE standard 802.3z, was formally approved in 1998. At present, 10-Gigabit Ethernet (10GbE) is the fastest of the Ethernet standards. The standardization of 10GbE began in March of 1999 and led to the 10GbE standard IEEE 802.3ae, which was formally approved in 2002.

In this chapter, we highlight the salient features of both 1 and 10 Gbps Ethernet. While 10GbE is the fastest existing Ethernet standard at the time of writing, it is worthwhile to mention that 10GbE does not represent the end of the development of ever-increasing higher-speed Ethernet networks. The standardization of 100-Gigabit Ethernet (100GbE) is currently under development by the IEEE 802.3 Higher Speed Study Group (HSSG). The HSSG was formed in 2006 and aims at providing a standard for 100GbE by the end of 2009.

The aforementioned wavelength division multiplexing (WDM) ring networks appear to be natural candidates to extend existing optical single-channel ring networks (e.g., RPR) to multichannel systems by means of WDM. In WDM rings, optical single-channel rings are multichannel upgraded by exploiting the already existing fiber infrastructure without requiring any additional fiber links and modifications of the ring topology. Clearly, deploying WDM on the existing ring infrastructure saves on fiber requirements. At the downside, however, WDM rings require all ring nodes to be WDM upgraded at the same time (e.g., each ring node is equipped with a transceiver array or wavelength (de)multiplexer). Furthermore, WDM rings are able to survive only a single link or node failure due to their underlying ring topology, similar to their single-channel counterparts.

An alternative approach to multichannel upgrade optical single-channel rings relies on topological modifications of the basic ring architecture. Many ways exist to modify and enhance the topology of ring networks, resulting in so-called augmented rings (Aiello et al., 2001). In this chapter, we describe a novel multichannel upgrade of optical single-channel ring networks where the ring network is left untouched and only a subset of ring nodes needs to be WDM upgraded and interconnected by a single–hop star WDM subnetwork in a pay-as-you-grow fashion (Maier and Reisslein, 2006). The resultant hybrid ring-star network, called RINGOSTAR, requires additional fiber links to build the star subnetwork, as opposed to WDM rings. Unlike WDM rings, however, RINGOSTAR does not require all ring nodes to be WDM upgraded at the same time.

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.