At the beginning of this decade there was a lot of fanfare about the Optical Transport Network. OTN is intended to be the next evolutionary step in a carrier’s infrastructure. It’s designed to address the requirements of next-generation networks by efficiently transporting heterogeneous data-oriented traffic directly over light beams carried on the same optical backbone.
The International Telecommunication Union‘s OTN standards provide the ability to support multiple light beams within a fiber, combine multiple signals within a light beam, realize optimal utilization of transport capacity and deliver improved cost-effectiveness of the transport system, while enabling switching at service rates of 2.5G, 10G and 40Gbps.
The expectation has been that existing carrier transport networks must evolve to fulfill emerging requirements, such as fast and automatic end-to-end provisioning, optical re-routing and restoration, support of multiple clients and client types, and interworking of IP-based and optical networks.
SONET, the de-facto technology that OTN would, in effect, replace, was developed in the mid 1980s to handle voice and medium-speed leased line services. In some instances, the growing trend of data traffic could pose technical challenges to SONET networks, especially in relation to the bursty and asymmetrical nature of such traffic.
There are several types of carrier network infrastructures in place, including: transport systems (such as Async DS1/DS3 systems, SONET-based systems); Layer 2 packet systems (such as ATM overlaid on async or sync optical transport, Metro Ethernet); and, Layer 3 packet (such as private lines overlaid on async or sync optical transport, IP overlaid on ATM, IP overlaid on async or sync optical transport, and MPLS).
At face value it is desirable to migrate to a single infrastructure to reduce costs and pass thoe savings on to the user. This is the stated goal of OTN. It makes use of an optical channel layer: each light beam is wrapped in an envelope that consists of a header (for overhead bytes) and a trailer (for FEC functions.) The payload section enables existing network protocols to be mapped, thus making OTN protocol-agnostic.
The flexibility of OTN is built on the protocol and bit-rate independence of the information-carrying light beams in the fiber waveguide. This transparency enables the OTN to carry many different types of traffic over an optical channel regardless of the protocol (Gigabit Ethernet, 10 Gigabit Ethernet, SONET and so on) or the bit-rate. The Generic Framing Protocol provides a means for packing non-voice traffic into an OTN frame: the OTN allows transmission of different data packet types using the GFP mapping. This mapping reduces the layers between the fiber and the IP layer and thus makes much more efficient use of bandwidth. It follows that one of the advantages of OTN is its backward-compatible functionality, specifically, support for existing SONET, Ethernet, ATM, IP, and MPLS protocols without changing the format, bit rate or timing. OTN nodes can receive, generate and transmit management and control information throughout the network, making performance monitoring and other network management possible on a per-light beam basis.
With all of this going for it, one would expect to see aggressive deployment of OTN technology. The reality is that very little has happened since its development in the late 1990s; it has been mostly dormant, although some products have emerged. It is not clear when OTN-based technology will be deployed in the United States on a broad scale, but 2012 or beyond would not be unreasonable. So, don’t hold your breath waiting: plan your enterprise and institutional networks on traditional services for the time being.
