1. Introduction

The Internet is a massive network infrastructure on a global scale that interconnects LAN- and broadband-access users through WAN. It currently supports a wide range of social, business, and individual activities and is expected to provide even more support in the years to come. A major factor in this explosive and ongoing growth of the Internet is the rapid progress of LAN and WAN. These two types of networks are significantly different in several respects. They each have their applicable area, design ideology, and applicable technology and they differ in lifetime, reliability, and other system features. Yet at the same time, they have both been supported by technological progress over the last 20 years especially in optical communications, LSI technology, and computer technology, and they both have made significant gains on the path to ultra-high-speed and large-capacity systems. Figure 1 shows this trend toward higher speeds and capacities in terms of line speeds and network-interface speeds (per wavelength) 1 ^, 2

High-speed WAN technology in Japan has undergone a number of developments over the years. The transition from Plesiochronous Digital Hierarchy (PDH) to Synchronous Digital Hierarchy (SDH) began in 1989 (FTM-150M, FTM-600M), and a 2.4-Gbit/s system (FTM-2.4G) and 10-Gbit/s system (FA-10G) were introduced in 1991 and 1996, respectively. In more recent years, the application of Wavelength Division Multiplex (WDM) technology has taken off around the world, and with the aim of increasing the transmission speed per wavelength, standardization of a 40-Gbit/s system had been completed 3 ^, 4...