1.8 NEXT-GENERATION OPTICAL INTERNET NETWORKS
WDM-based optical networks are becoming the right choice for the next-generation Internet networks to transport high-speed IP traffic. In the first phase, lightpath-based circuit switching WDM networks are deployed as a means of carrying IP traffic. SONET and ATM networks have been widely deployed in the transport networks. SONET systems have several attractive features such as high-speed transmission and network survivability. ATM networks have several attractive features such as flexible bandwidth allocation and QoS support. Therefore, ATM and/or SONET layers can be used in between the IP layer and the WDM optical layer for transporting IP packets. A major drawback of this multilayer approach is that it incurs increased control and management overhead.
WDM technology is evolving from circuit switching technology to burst switching and packet switching technologies. The granularity of the basic switching entity is large in circuit switching networks, medium in burst switching networks, and small in packet switching networks. While circuit switching technology is mature, the other technologies are not. In a circuit switching network, a wavelength channel on a link is used by a circuit (lightpath) for a long time, until it is torn down. In a burst (packet) switching network, a wavelength channel on a link is reserved only for the duration of the burst (packet). The bandwidth utilization in burst and packet switching networks is higher when compared to that in circuit switching networks. This is because, the former networks use statistical multiplexing while the latter does not.
In a burst switching network, the basic switching entity is a burst. A number of IP packets which are destined for the same egress router are assembled into a burst at the ingress router. The major challenges in burst switching networks include the design of cost-effective and fast switches, burst switching protocols, and wavelength channel scheduling. In a packet switching network, the basic switching entity is a packet. The major challenges in packet switching networks include the design of cost-effective and fast switches, packet synchronization, and contention resolution. Since optical processing is technologically and economically infeasible, the packet/burst header is processed electronically while the payload is switched optically. Since optical random access memory (RAM) is not available, a packet or burst cannot be buffered optically for a long time. A possible way is to use fiber delay lines (FDLs) to buffer (delay) a packet or burst for a short time. As we will see in Chapter 9, a multiprotocol label switching (MPLS)4 framework has several advantages, such as traffic engineering, explicit path routing, fast packet forwarding, and network survivability. Due to the above advantages, future Internet networks employing circuit/burst/packet switching are likely to use the MPLS approach.