Evolution of Optical Networks

In multi-mode fibers, different wavelengths can be propagated in a certain distance.

Different optical networks implementing different technologies and architectures are demonstrated in the evolution of optical networks. Since optical fibers had been manufactured in 1970s, a variety of optical networks have come into existence to replace traditional networks using copper cables. Optical networks can be classified according to different criteria. According to network topology, they can be ring, mesh, or star. Ring topology is superior to mesh topology in many ways: (1) the number of rings increases linearly with the number of nodes, (2) fault tolerance, (3) load sharing, and (4) reduced load at the router and no need for buffering. According the number of hops in optical networks, they are classified into single-hop networks and multi-hop networks. Nevertheless, according to evolution of optical networks, there are three generations in the trend of optical network development. The first generation started when fibers were chosen to replace copper media. The second generation emerged by providing network functionality by electronics. The third generation demonstrated in 1999 was intelligent optical networks to provide the capacity of routing and signaling for optical paths.

Optical transmission was first introduced in the first generation. Data signals must be

converted between optical and electronic equipment and the protocols used in copper-based network are still deployed. Due to the burden of OEO conversions, only a small fraction of bandwidth, less than 0.1 percent, is utilized. FDDI (Fiber Distributed Data Interface) and Gigabit Ethernet are two major products in this generation to provide 100~200 Mb/s and 1~10Gb/s bandwidths.

Breakthroughs in technologies of WDM and EDFA, the second generation networks exploit the bandwidth of optical fibers by traditional electric network equipment, such as switches, amplifiers, and so on. The broadcast-and-select network (BASN) [28] is a representative product which consists of a passive star coupler (PSC) and connected nodes to form a star-like network. Each node equipped with one or more fixed-tuned or tunable optical transmitters and one or more fixed-tuned or tunable optical receivers. Each transmitter in each source node will be tuned to use a different wavelength such that all the signals are transmitted simultaneously to PSC. All transmitted signals will be combined in the PSC, and then broadcasted to all nodes. All destination nodes will tune their receivers to the corrected wavelength such that the signals propagated in the wavelength will be received by the receiver. The requirement of fast tunability is required in transmitters and receivers. Because each transmission in BASN needs to be broadcasted to all other nodes, not only most of the transmitted power is wasted on receivers but also the number of transmitted messages is limited by the number of wavelengths in the network. Therefore, although BASN is suitable for local or metropolitan area networks, it is not suitable for wide area networks.

The third generation is wavelength-based routing networks that are presented as a scalable alternative by the help of optical WADM and optical crossconnecter (OXC) [84]. To avoid wastage of transmit power, channeling a signal from the transmitter of a source node to the receiver of destination node along a restricted route is needed instead of letting it spread out over the entire network as in BSAN. Therefore, at each intermediate node on the route,

light coming in at one incoming port in a given wavelength is routed out of one and only one outbound port by a wavelength router. Not only the need for traffic is groomed but also the capacity gain provided by wavelength conversion is justified. In order to transmit signals more efficiently, the problem of the virtual topology design for offline traffic environment and the problem of finding a route and assigning a wavelength are imperative problems. By the different technologies adopted to build wavelength-based routing networks, there can be linear lightwave networks (LLN) and wavelength routing networks (WRN).

In LLN discussed in [3], nodes are classified into two types end nodes and routing nodes.

Routing nodes provide the function to multiplex and to demultiplex optical signals in wavebands but not in wavelengths, where wavebands partitioned from lower attenuation band (for examples, 1550 nm band) consist of a number or eight wavebands transmitted in fibers.

Each waveband can be partitioned into a number of wavelengths. End nodes provide the function to multiplex and to demultiplex optical signals in wavelengths but not wavebands.

The objective of the architecture is to provide purely optical connections on demand, supporting a high degree of flexibility, including user-chosen modulation formats (digital or analog) and user-chosen bitrates (or bandwidths).

To build a more flexible multipoint optical network such that the signal can be routed based on wavelength level, the wavelength routing networks (WRN) [70] is developed by deploying optical wavelength crossconnect (OXC) in networks. Three problems in BASN, lacking of wavelength reuse, power splitting loss, and scalability to WAN (wide area network) can be resolved. Using point-to-point optical fiber links to connect input ports and output ports in OXCs, a WRN with an arbitrary topology can be established and data can be rerouted to other optical switches based on wavelengths. Data will be sent from one node to another according to the wavelength-level connections that exist between every two consecutive switches. A WRN which carries data without any intermediate OEO conversion is called an

all-optical transparent WRN [8][70]. The type of optical networks deploying the WDM technology is called WDM networks, including BASN, LLN, and WRN.