Optical WANs

Here the main interest is in the role optical networks (Fig.4) might play in future wide area BBNs, namely, backbone networks where it now seems clear that ATM and IP will play a major role.

Figure 4 - Target physical optical transport network architecture (WAN)

In the most ambitious vision, very large ATM/IP switches are directly interconnected with fibers; in this scenario, all switching/routing is done at the ATM/IP layer. However, long distance fiber links today contain bundles of tens of fibers, each of which can support transmission rates of up to 10 Gb/s. This could lead to WAN nodes being required to support hundreds of gigabits per second of traffic, which is currently impractical using only ATM/IP switches (Fig. 2). In addition, WANs tend to be sparsely connected, and therefore only a small fraction of the ATM/IP traffic may terminate at a node. In these situations, it is far more practical and economical to build a hierarchical ATM/IP over SDH network where ATM/IP switches connect to SDH cross-connects, which in turn are interconnected by fibers.

Figure 5 - Generic BBN alternatives

Fig. 6 - Layered multiplexing.  

Fig. 7 - Layered switching architecture

Figure 8- Layered traffic and statistical gain

With the introduction of long-haul WDM and OTDM technology, the bandwidth through a node may exceed several terabits per second. It seems likely that some form of optical switching services would be useful in switching these large bandwidths. The questions we address below are which optical services should be supported, and how this choice affects the overall BBN architecture.