Optical networking has attracted a great deal of attention during the last few months. Cisco acquired several companies in the business, including Cerent, Monterrey and Pirelli's optical transmission division. And there have been others, including Nortel's purchase of Qtera. In a similar vein, companies like Sycamore have enjoyed phenomenal success in the stock market.
Many of those optical-networking groups have the same goal: to remove the Sonet layer of the network, allowing the service layer and physical layers to link directly. For this new two-tiered network to be truly viable, photonic networking systems must provide additional functionality, including ultralong-reach transmission, facilities restoration, bandwidth provisioning and performance monitoring.
A large number of vendors focus on the metropolitan area network, the rings of fiber that surround large metropolitan areas. But the first real deployment of optical networking technology is likely to be in the transport or backbone network of large carriers such as MCI and Qwest. In addition, the backbone core of the network is the portion that is most in need of the benefits this technology offers.
Consider the standard that is most widely used today: synchronous optical network, or Sonet. Formulated 15 years ago on the back of a napkin, Sonet has been the standard for wide area networking as long as many of those reading this article have been in the industry.
Ideal for circuit-switched voice applications, Sonet began to feel burdened by growing data traffic demands. In 1995-96, dense wavelength division multiplexing (DWDM) systems began to proliferate as a quick fix to the intense explosion of data traffic that started to clog service providers' networks (and does to this day). Sonet's inability to handle the huge influx of data traffic and the associated unpredictable traffic patterns signaled the end of this standard's long reign in the backbone transport network. While long-haul architectures based solely on Sonet will be far too expensive and complex to meet the needs of tomorrow's backbones, some of the functionality Sonet encompasses will live on-only through a different, more efficient means.
Sonet provides four primary functions:
- Intermediate multiplexing (manipulating lower bit rates to higher rates).
- High-capacity bandwidth provisioning.
- Performance monitoring.
- Network survivability.
These critical disciplines must be incorporated into any future data-centric network architecture. In the near-term, advanced photonic networking systems combined with high-performance multigigabit or terabit routers will be able to perform these functions. So, while some Sonet functionality is still needed, a separate Sonet layer is not. Linking the service layer to a photonic transport backbone will set the stage for the fully optical Internet.
The primary reason for the paradigm shift in network design is the nature of the traffic crossing today's long-haul backbones. Specifically, Internet Protocol (IP) applications are the fastest growing segment of a service provider's network traffic. This growth is expected to continue well into the next century. According to a 1999 report by industry analyst firm Ryan Hankin Kent, "the growth of IP creates the prospect of a public network with 30 times the traffic of today's public network in just four years." More importantly, the bandwidth required to service this traffic is expected to grow at an even faster pace.
The exponentially growing popularity of the Internet, in turn, significantly affects the origins and destinations of long-haul network traffic. Internet users trigger information flow between nodes thousands of miles apart with a simple click of a mouse-distance is not a restriction or even a consideration for most of them. This introduces a great deal of unpredictability into the network, which makes forecasting for growth all the more difficult and the matter of scalability a critical concern.
Unfortunately, Sonet was never designed to deal with these factors. As a standard, it was based on the idea that network traffic had predictable, stable patterns and growth was easy to forecast. By comparison, the optical Internet will bypass Sonet altogether, providing for a backbone without capacity restrictions or distance limitations.
The optical Internet is built on a purely photonic backbone, with the service or IP layer of the network directly linked with the transport or photonic layer. That way, data traffic can be packaged and transported much more efficiently, yielding significant benefits in terms of cost, scalability, restoration times, bandwidth provisioning and overall footprint savings. Today's view of a multi-tier network-service layer, Sonet layer and physical layer-is woefully outdated and will not be able to scale to meet the exploding volumes of future data traffic. The concept of sending IP traffic through a Sonet box and then onto a DWDM system is far too cumbersome and, even worse, expensive. In a multitier infrastructure, carriers are often forced to include an "aggregation layer" in order to justify the costs of initiating one wavelength worth of traffic.
The idea of eliminating the Sonet layer within the transport backbone has been germinating for some time. The benefits of this strategy are clear. However, many of the solutions proposed to eliminate the Sonet layer simply replace it with another box. These interim solutions significantly reduce the cost benefits associated with a direct service-to-transport strategy and add unnecessary complexity into the backbone.
A truly optical Internet will link the service layer to the transport or photonic layer without any intervening device or layer or both whatsoever. In this model, the transport of data packets is done over a purely photonic backbone. A purely photonic transport system, complete with bandwidth management and facilities-restoration capabilities, greatly simplifies long-haul networking over the backbone. Opto-electronic regeneration is no longer required, thus eliminating the need for the most expensive part of the long-haul backbone: electrical regenerators and add/drop multiplexers. This helps reduce the overall costs of the transport network by 40 percent or greater, depending on the configuration. Fewer boxes make for a cheaper, simpler backbone.
Routers will be the first integral piece of the two-tiered network puzzle. Multigigabit and terabit routers developed by such companies as Avici, Cisco, Juniper and others sit at the core of a provider's backbone network and have been designed to take direct advantage of an all-optical networking environment. Because high-performance routers feature extremely high-bit-rate optical interfaces such as OC-48c and OC-192c, the high-bit-rate multiplexing traditionally performed by a Sonet terminal is no longer necessary. Further diluting the need for a Sonet device, terabit routers can also handle equipment protection. By directly linking the service layer to the transport layer, these devices will act as the integrating platform for multiservice network traffic including leased-line services, voice, video and data. In other words, A new breed of photonic networking equipment is the second part of the puzzle that will comprise the optical Internet, providing significant improvements in scalability, restoration and bit-rate transparency.
From an architectural design standpoint, direct IP-over-photonics implementation does not require nonblocking cross-connect matrices that typically encounter port-count scalability restrictions. And, the baseline for next-generation photonic networks is 10 Gbits/second, with 40 Gbits/s on the horizon.
The ability to offer tremendous amounts of bandwidth highlights another benefit of the routers-plus-photonics model: facilities restoration. Total failure detection and restoration times can be reduced to a few milliseconds.
Nevertheless, Sonet will continue to perform critical functions in the access portion of the network, where distance and scalability are less immediate concerns. As carriers off-load their data traffic to a more efficient platform, they will have more room to carry voice and to lease line services on their embedded Sonet base. That migration strategy actually extends the life of the carrier's Sonet investment.