By Peter Lunk, Extreme Networks and Esmeralda Swartz, Soapstone Networks
CommsDesign Mar 14, 2008

Rate this article WORSE | BETTER 1 2 3 4 5

Multiprotocol label switching (MPLS) has found great success in the service provider core network to enable the reliable transport of a broad range of services and applications spanning E-Line business services, IP VPNs and IPTV transport. However, as MPLS begins to push out into the metro, its advantages begin to quickly erode given the high expense and complexity of MPLS hardware relative to standard Ethernet-based switches and the growing recognition that MPLS solutions cannot scale effectively, particularly in the metro.

Typical MPLS networks utilize a distributed control plane for performing path computation where each node in the network participates independently in the computation. However, the most fundamental reason why the control plane needs to be moved off equipment is that as we look forward to more complex services and network policies, fundamental technical concerns emerge that drive the shift to a separate service control plane with a centralized view of the entire network. Carriers are recognizing that they need to deploy a service network that is independent of the underlying transport. They would like the freedom to deploy a wide variety of vendors, but the use of a built-in control plane often gives rise to interoperability issues. An external control plane eliminates these issues and is an important consideration for carriers wanting to deploy equipment from multiple vendors and avoid being locked into the duopoly of MPLS hardware platforms.

The emerging Provider Backbone Bridging-Traffic Engineering (PBB-TE) standard brings new capabilities to the metro network to address the limitations of MPLS and enable more cost-effective Ethernet transport and aggregation while substantially simplifying network management. The need for service predictability makes PBB-TE's circuit-like behavior very appealing for services deployment. And, in order to make Ethernet viable in the carrier environment there was universal recognition very early by PBB-TE suppliers that management (OAM&P) was critical. This is in direct contrast with MPLS where management was added well after the initial technology was rolled out to customers. PBB-TE is not intended to replace MPLS but rather work in conjunction with a carrier's existing MPLS core infrastructure. Specifically, PBB-TE brings Ethernet cost economies and service flexibility to the Metro while enabling multi-vendor interoperability on the network control plane, as well across protocols and over the data plane. In addition to maximizing provisioning and management flexibility, PBB-TE improves quality of service through deterministic path selection and increases reliability with SONET/SDH speed failover to pre-provisioned backup paths. The use of an external control plane also enables administrators to take full advantage of XML technology to further simplify and automate network management.

PBB-TE Ready for Prime Time
Because of the deterministic network behavior, enhanced reliability, and cost savings it brings to the metro, PBB-TE has caught the interest of carriers worldwide. By decoupling the Ethernet switches and the control plane, PBB-TE also simplifies management of complex, heterogeneous networks and enables carriers to more economically deliver revenue-rich services to increase market share and profitability. (see Figure 1.)

Figure 1. External control plane providing a top level services view while managing a heterogeneous switch infrastructure.

The first applications for PBB-TE have been to replace SONET/SDH switches for the leased line services now standardized by the Metro Ethernet Forum (MEF) as E-Line services, that still drive a significant percentage of service provider revenues.

Additionally, as carriers reach the scalability limits of legacy Frame Relay and ATM networks, we have seen a shift toward MEF E-LAN services. As carriers to once again turn to Ethernet to provide the lowest cost per bit transport, PBB-TE is also posed to support multicast-heavy IPTV and other video services as well as mobile backbone connectivity over MEF E-Tree services. By working in conjunction with the control plane vendors, innovative switching vendors are reducing development times and quickly moving past the basic point to point capabilities of PBB-TE and adding support for these applications with MEF E-LAN and E-Tree services.

To address the understandable reluctance of carriers to deploy proprietary technologies, much work has been done within the IEEE, IETF, and MEF to enable deployment of PBB-TE technology upon a foundation of open standards. For example, consider an application where network services are provided wholesale to other carriers (see Figure 2).

Figure 2. PBB-TE provides the lowest cost transport infrastructure with Service Level Agreement (SLA) capabilities. Coupled with open standards such as the External Network to Network Interface (E-NNI), PBB-TE simplifies connecting to other service provider networks for native Ethernet service extensions and regulatory compliance.

Achieving an efficient handoff of services between carriers is extremely complex using MPLS, given its varying implementations. In contrast, the base PBB-TE standard provides the resiliency and determinism required for such applications. In additional, hand-offs between carriers are explicitly defined in PBB-TE through the External Network to Network Interface (E-NNI). Under development by the MEF, E-NNI provides a well-understood and standard means for detailing the negotiation of bandwidth, quality of service, and other transmission parameters to define how services will be supported and to what level this support extends. The External-NNI standard is just one example of how the industry is working to provide the key underlying technologies to enable and simplify a wide range of advanced applications and services through PBB-TE.