Why the specific metro Ethernet interest? Two main reasons. First, bandwidth: Ethernet services typically deliver some of the highest-available bandwidth in the WAN. Second, simplicity: Ethernet services are often plug-and-play.
Carrier Ethernet (relative to its popularity) is that for carriers, it often represents a radical departure from their existing architectures. Service providers continue to depend on traditional SONET/SDH-based access, metro and transport technologies, even as they watch demand increase for IP and Ethernet. That means they're managing separate transport hardware and provisioning systems to handle both their legacy networks and the new generation of packet-transport protocols that include Ethernet.
The current crop of Ethernet services is defined to run over multiprotocol label switching (MPLS). But from the carrier perspective, turning up new customers on MPLS-based services like Ethernet requires a complex set of steps involving multiple different operational support systems (OSS).
Carrier Ethernet standards designed to create single control plane
To streamline deployment and management of these new services, carriers are seeking a way to merge their Layer 1, 2 and 3 operations and management infrastructures so they can operate a single control plane for provisioning. Two emerging specifications seek to do exactly that:
- MPLS-transport protocol (MPLS-TP) is an approach that started life as T-MPLS (Transport-MPLS) within the within the International Telecommunications Union (ITU). Due to concerns about interoperability between the ITU's T-MPLS proposal and existing MPLS standards, the ITU turned over T-MPLS development to the Internet Engineering Task Force (IETF).
- Provider Backbone Bridge -- Traffic Engineering (PBB-TE ) is the main competitor to MPLS-TP. This approach is based on leveraging existing IEEE 802.1 standards to enable carriers to natively deploy Ethernet services using existing Ethernet technologies.
The benefits of MPLS-TP. MPLS-TP is essentially an MPLS extension based on the concept of extending MPLS resiliency and provisioning mechanisms to Ethernet via a new transport-focused profile. With MPLS-TP, carriers can mix and match circuit or packet-based services in the same network, using a single control plane and operational support system (OSS) for service provisioning.
Perhaps MPLS-TP's most important quality is that it applies circuit-switching-like functionality to MPLS, treating MPLS label switch paths as dedicated circuits. This approach enables operators to define bi-directional paths (same path forward and backward), eliminating the LSP (label switch path) merging capability of MPLS, whereby packets going to the same destination can be merged into a single LSP. By eliminating LSP, MPLS-TP enables providers to isolate customer traffic into separate end-to-end virtual circuits.
In addition, MPLS-TP eliminates the need for IP at the end of the LSP by extending the label all the way out to the end device in a path. This allows service providers to eliminate the need to configure IP services on edge devices, instead allowing MPLS-based provisioning of lower-layer services such as Ethernet connections at Layer 2.
Proponents of MPLS-TP also tout the following benefits for carriers and service providers that already have MPLS cores:
- MPLS-TP leverages existing MPLS standards, providing a smooth migration path, adding only extensions for Layer 2 forwarding, provisioning and management. That means a more seamless deployment for service providers already using MPLS.
- MPLS-TP can support a mix of traditional circuit-switched Layer 1 and 2 services (like SONET/SDH or WDM), as well as packet-based services like Ethernet, enabling service providers to protect existing customer revenues while modernizing their own transport infrastructures.
- Re-use of existing MPLS services means additional services can be provisioned using existing OSS with minimal modifications.
PBB-TE advantages. A competing proposal is PBB-TE, based originally on Nortel's proprietary Provider Backbone Transport (PBT) but now undergoing standardization within the IEEE's 802.1Qay working group. Unlike MPLS-TP, PBB-TE only supports Ethernet, meaning that other Layer 2 services must be tunneled within MPLS or converted via a gateway. Proponents of PBB-TE tout the following benefits:
- Simplified infrastructure based on using existing 802.1 protocols such as VLAN tunneling (also known as double-tagging or Q-in-Q) to enable a provider to tunnel customer VLAN within provider VLANs
- Elimination of Ethernet inefficiencies by replacing MAC learning and spanning tree with a new protocol (Provider Link State Bridging -- PLSB) a link-state protocol that uses the common IS-IS routing protocol to calculate optimal and redundant paths.
- Re-use of existing ITU and IETF standards for resiliency such as resilient-packet-ring (RPR) and IEEE 802.1ag carrier OAM standards
It's too soon yet to say which approach will ultimately win out, but the existence of both specs spells good news for users. Both standards offer carriers the opportunity to achieve lower operating costs while improving service delivery. And that means that Carrier Ethernet services users are growing to love will be more widely-available than ever in coming years.
About the author: Irwin Lazar is the vice president for Communications Research at Nemertes Research, where he develops and manages research projects, develops cost models, conducts strategic seminars and advises clients. His background is in network operations, network engineering, voice-data convergence and IP telephony. He is responsible for benchmarking the adoption and use of emerging technologies in the enterprise in areas including VOIP, unified communications, Web 2.0 initiatives, social networking, and collaboration.
This was first published in June 2009