The metropolitan optical network — given its need to support a diverse range of services using a variety of transport technologies — is an ideal candidate forgeneralized multiprotocol label switching (GMPLS)-based control and provisioning.

Specified by the International Engineering Task Force (IETF), GMPLS is a protocol suite that uses advanced network signaling and routing mechanisms to automate set up for end-to-end connections for all types of network traffic: time-division-multiplexed, packet- or cell-based, wavebands, or wavelengths.

A GMPLS-based metro network can offer many capabilities: network resource discovery and routing control; dynamic provisioning and traffic engineering for end-to-end connections; bandwidth-on-demand for just-in-time service creation; and new quality of service (QoS)-defined value-added services.

GMPLS has evolved from MPLS, the original IETF standard intended to enhance the forwarding performance and traffic engineering intelligence of packet-based networks. Basically, GMPLS extends these capabilities to other traffic types. Unlike MPLS, which is supported mainly by routers and data switches, GMPLS can be supported by a variety of optical platforms including Sonet add-drop multiplexers, optical cross-connects and dense wave division multipexing systems. This will allow an entire infrastructure, extending from the access network to the core network, to utilize a common control plane.

While MPLS requires a Label Switched Path (LSP) between two end-point devices for connection set up, GMPLS expands this concept beyond simple point-to-point connections. In a GMPLS-based network, it is possible to find and provision end-to-end paths that traverse different networks.

For instance, a packet/cell LSP can be nested in a TDM LSP for transport over a Sonet network. The TDM LSP can similarly be nested in a wavelength-level LSP for transport over a wavelength network. Finally, these "lambda" LSPs can be nested within a fiber LSP set up between two fiber-switching elements. This forwarding hierarchy of nested LSPs allows service providers to seamlessly send different types of traffic over varying network segments.

GMPLS also provides the ability to automate many of the network functions that are directly related to operational complexities. These functions include end-to-end provisioning of services, network resource discovery, bandwidth assignment and service creation. Traffic engineering parameters relating to Sonet protection support, available bandwidth, route diversity and quality of service are distributed throughout the network, allowing every node in the network to have full visibility and configuration status of every other node — ultimately making the optical network intelligent.

Therefore, as service providers introduce new network elements into their networks, add/remove facilities, or turn up new circuits, the control plane will automatically distribute and update the network with the new information. The contrast today is that many of these upgrades and updates are performed manually.

Self-sufficient networks

The complexity of current metro overlay architectures means the provisioning of connections often requires a substantial amount of coordination among operations staff located throughout the network. Capacity is assessed, optimal connection and restoration paths are determined, and the connection must be fully tested once it is established. On the other hand, GMPLS makes the network sufficiently self-discovered to dynamically advertise the availability or lack of resources throughout the network.

This is done through advanced routing protocols such as Open Shortest Path First (OSPF) and Intermediate-System-to-Intermediate-System (IS-IS), or signaling protocols such as the Resource Reservations Setup Protocol (RSVP) and MPLS' Constraint-Based Routed Label Distribution Protocol (CR-LDP). With this capability, multi-hop connections with optimal routes and backup paths, can be established in a single provisioning step.

Using a simple network management system-based provisioning mechanism, with point-and-click capabilities, the new-generation GMPLS metro optical network can first signal a request for a label-switched path (LSP). Upon confirmation, it dynamically sets up the requested path. Source: Polaris

While GMPLS has potential, this protocol has emerged very quickly and is definitely still in its infancy. One advantage of GMPLS is that it leverages much of the development work and field deployment experience of MPLS. However, the IETF working groups are expected to continue their development and tailor the protocol specifications to ensure that GMPLS is the comprehensive and open standard required by the industry.

Stumbling blocks include multi-vendor interoperability, integration with legacy operational support systems (OSS), and the need to redefine other operations. Overall, wide-scale deployment of GMPLS is likely to occur gradually, driven by phased deployment strategies.

One deployment scenario is where a metropolitan service provider pursues deployment in a closed network environment, such as a metro backbone, using a single vendor's proven solution. Having certified, deployed and field-proven the potential of GMPLS, the next phase of deployment could involve extending the control plane to the metro access network. This would involve interoperability between metro core optical transport switch platforms and access/edge devices.

And, with that deployment milestone complete, the metro network would become fully GMPLS-enabled and automated. As a final milestone, the service provider could focus on extending the control plane to the long-haul network; this could be through an interexchange carrier (IXC) partner.