By MEGHAN FULLER
Since early 2000, various standards bodies and industry forums have been working to define an open interface for interoperability between the electrical and optical layers, principally in a quest to enable direct interaction between the Internet Protocol (IP) and optical layers. While the number of groups working on this issue might raise fears of competing standards, a consensus on how to enable such communication appears to be emerging.
In the traditional IP network infrastructure model, intelligence resides solely in the electrical layer; the optical layer is a transport medium only, a collection of high-capacity but low-intelligence pipes that connect electrical-layer devices like IP routers and ATM switches. Multiprotocol Label Switching (MPLS), itself a new standard, is the control mechanism within the packet layer.
MPLS works on top of the IP layer, assigning a set of overheads or labels to a group of packets that specify an explicit route for those packets to take. However, packet-based routers at intermediate points around the network often cannot handle huge amounts of traffic. In an internetworking model, the router would only aggregate the traffic and then signal to the optical switch to set up a channel for that period of time. But that interoperability cannot happen without some kind of standard protocol or open interface.
The Optical Domain Service Interconnect (ODSI), Internet Engineering Task Force (IETF), Optical Internetworking Forum (OIF), and International Telecommunications Union (ITU) all are working on defining a specification for seamless communication between the layers. They are not necessarily working on exclusionary models for interoperability, however; despite the various acronyms and different models they employ, there is a certain synergy among the standards bodies' activities.
"The good thing is that all the standards that are emerging have a common theme: To accomplish a more dynamic and more easily provisioned optical layer, you need to have intelligence there," maintains Scott Larson, director of strategic marketing programs at Sycamore Networks (Chelmsford, MA). "And IP-based protocols are going to enable intelligence at the optical layer."Established in January 2000, the ODSI was the first to begin working on defining a standard open interface to enable electrical-layer devices to request high-speed optical channels on demand. In this model, interaction between the layers occurs across a public user-to-network interface (UNI) that is based on a new signaling protocol that runs over the transmission control protocol (TCP). The IP routers and optical switches operate via separate control planes.
The IETF, in contrast, is currently working on a model known as general MPLS (GMPLS), in which the electrical-layer and optical-layer devices run the same protocol on a peer-to-peer basis, sharing all the network-level information. It is essentially an adaptation of the IP-router MPLS to make it applicable in the optical layer. One GMPLS signaling scheme operates across the optical layer-in time-division multiplexing (TDM) systems, wavelength switching systems, and spatial systems-and electrical layer, in mainly packet-switched systems.
The router and optical switch are on the same footing in terms of information exchange, even though each performs different functions. From the protocol perspective, they exchange all the network information among themselves. This model is typically favored by the router community, as it enables routers to route and reroute traffic automatically across the optical network.
But what if a carrier does not want to share network-level information with a customer's router? The OIF is working on a compatible model, a hybrid of the ODSI and IETF versions, in which the network would not have to reveal this information to a router that is not within its domain of trust. Like the ODSI model, the OIF model is also based on a UNI configuration. However, it operates via the resource reservation protocol with traffic extensions (RSVP-TE), which is the basic building block of GMPLS.
Under this model, the electrical layer and optical layer act independently of one another. The router signals the optical switch, indicating that it needs to make a connection from point A to point B. The optical switch fulfills the router's request without sharing the specific channels on which the traffic will flow. The router then acts as a client to the optical network.
Spearheaded by Cisco Systems and Ciena Corp., the OIF's model also includes two additional features designed to simplify network management: a neighbor discovery mechanism, which enables equipment on both ends of the fiber link to identify each other, and a service delivery mechanism, which enables clients to determine the services available from the optical network.
"The GMPLS is a peer-to-peer model in which all the network elements share information; it can run within one network operator's trust boundary," ex plains Sid Chaudhuri, OIF vice president and director of new architecture at Tellium Inc. (Oceanport, NJ). "On the other hand, UNI can run between two network boundaries."
Meanwhile, the ITU has begun work on the development of another new recommendation, Automatic Switched Optical Network (ASON) architecture. Known as the G.ASON, the ITU recommendation can be categorized as belonging to the UNI or overlay model.
There has been some confusion in the industry about the exact relationship between the standards bodies, particularly in the wake of the OIF's rejection of the ODSI's UNI signaling protocol last August. The OIF instead approved protocols proposed by Cisco and Juniper Networks Inc., RSVP, and Constraint-routed Label Distribution Protocol (CR-LDP).
The differing makeup of the ODSI and OIF may explain why the efforts of the two organizations may not appear to be in harmony. Whereas the OIF benefited from the strength of having established companies such as Cisco and Ciena as founding members, ODSI had difficulty attracting the membership of some of the bigger names in the router industry, which many believe was a setback it never quite overcame in having its work fully adopted by the OIF and other standards bodies. Having successfully completed interoperability testing last December, the ODSI voted to suspend further activity at its January meeting. How many companies will implement the ODSI UNI specifications into their products remains to be seen.
The ODSI has offered its specifications to the other standards bodies to help them in their work. While the OIF's choice to go in a different direction for its UNI signaling protocol can be seen as a dark sign, ODSI proponents remain upbeat about their work and its influence on other standards efforts. "Most of the same companies who were in ODSI are now working through the OIF," asserts Rick Thompson, director of product marketing for Sycamore's core switch business unit. The OIF plans to demonstrate interoperability among at least a dozen vendors at SuperComm 2001 in Atlanta next month. It hopes to approve its specification shortly thereafter. Still in the draft stage, the IETF's GMPLS may take slightly longer to mature into a more detailed specification.
While the model for an open interface remains in debate, the one thing that is not debatable is the need for such an interface. "I spend a lot of time with our customers and carriers," says Larson, "and everyone seems to agree that fundamentally, next-generation broadband networks are going to encompass the electrical layer, the IP layer, and the optical layer. To enable innovative types of broadband services, these layers need to work together. That's what GMPLS and the implementation of IP protocols in the optical layer will provide."