Challenges and Evolution of Optical Transport Networks

We discuss the evolution of interface rates and transport capacities in optical networks in view of the observed network traffic growth. Based on consistent historic scaling trends, we anticipate increasingly parallel interfaces and spatially multiplexed optical transport systems. Network traffic growth Over the past 20 years, the build-out of the Internet, and with it of a plethora of bandwidthhungry data services, has fuelled an exponential transport bandwidth growth. As shown in Fig. 1 (red circles), the growth rate has been close to 60% (i.e., 10 log10 (1.6) = 2 dB) per year over the past 10 years1, with a current total of about 20 Tb/s for the US network. Similar growth rates are seen in other regions2. (The fact that exponential traffic growth rates differ somewhat among carriers and geographical regions is immaterial to our discussion.) Future data-centric cloud applications may accelerate the observed network traffic growth rates, based on the notion that cloud services let the network take the role of a distributed computer interface. Following Amdahl's rule of 3,4 thumb , a processor's interface bandwidth should be chosen proportional to its processing power. These two arguments tie the expected network traffic growth due to emerging cloud applications to the historically strictly exponential scaling of microprocessor performance at 2.7 dB per year, as illustrated by the evolution of supercomputers5 in Fig. 1 (green squares). In addition to cloud applications, multi-media real-time applications with large bandwidth demands (e.g., telepresence6) are expected to increasingly gain importance.