The multiple roles of MPLS

It seems like many years since multiprotocol label switching first emerged as a key technology in the communications industry. The initial adopters of MPLS used the technology within existing IP core networks on router-based infrastructures as a mechanism to provide flow-based traffic engineering and later VPN capabilities. Today IP/MPLS core networks are ubiquitous with service providers' packet data infrastructures, delivering guaranteed end-to-end performance, efficient sharing of network resources and global VPN services.

Convergence of multiple data services onto one core network has long been a goal for carriers and service providers--core data networks are now migrating to a converged multiservice paradigm, integrating many disparate technologies such as ATM, frame relay, Ethernet and IP onto a common MPLS-based core infrastructure.

At the same time, we are seeing an explosion in data traffic. Many Industry experts predict that as much as 90% of the traffic in public networks will be data by 2007. Perhaps more important, analysts also estimate that revenue-per-bit for data services will drop much more rapidly than in the past, on the order of 30% to 35% per year.

Today one of the hottest growth data service segments is metro Ethernet. However, far from being competitive to IP, these emerging Ethernet services are very complementary, typically providing the physical interface for the delivery of IP services, such as RFC2547 VPNs.

Using a common network for IP and Ethernet services, a carrier could expect to realize significant cost savings in network equipment and in the simplification of network engineering and operations. Service Providers need to achieve the integration of Sonet/SDH features and packet data networks to enable new revenue generating services, and just as importantly to deliver significant opex and capex reductions required for profitability. Opex, in particular, drives for end-to-end managed services and a reduction in the number of network layers and technologies.

So, while MPLS has been used as a vehicle for this transition in the core network, it must now also be extended to access, metro and transport network infrastructures to provide a true end-to-end solution for data service delivery.

Metro and access networks: The new battleground

Today's metro and access networks are built from hierarchy of TDM centric Sonet/SDH rings, which were deployed during the 1980s and 90s to deliver circuit-based voice and leased line services. However Sonet/SDH networks are simply not optimized to carry new emerging bursty packet-based data efficiently and deal with the huge growth in IP and Ethernet services expected.

The requirements for both access and metro networks differ markedly from those in the core network in a number of key areas; for example, access and particularly metro networks are predominantly based upon ring-centric architectures. Also the access/metro network has the requirement to deliver all services over a single access link, in contrast to the single service platforms seen in the core network. Access and metro networks must provide a high degree of service aggregation in order to handle thousands of customer end-points.

While new data-centric technologies like generic framing procedure (GFP), virtual concatenation (VCAT) and link capacity adjustment scheme (LCAS) have been introduced on Sonet/SDH multi service platforms they go only some of the way toward addressing the issues of providing high-bandwidth efficiency and delivering new advanced packet data services. However, carriers cannot comprise on the key values they derive from of existing Sonet/SDH metro networks; for example, reliability/resilience and extended performance monitoring.

A carrier-grade, packet-optimized access and metro infrastructure must be able to guarantee performance and SLAs for a variety of data services; from Internet access to video distribution and voice over IP, and provide fair resource sharing under network congestion, as well as delivering sub-50 millisecond resilience for both ring and mesh topologies.

To achieve this end-to-end architecture for delivering next-generation IP and Ethernet services to end users, advanced functions providing efficient and resilient transport, aggregation and switching of many packet data technologies need to be embedded into access, metro and transport networks. MPLS promises to provide this capability, acting as the "glue" to converge disparate technologies within access and metro networks and also provide a common technology to integrate with core data network.

The appeal of MPLS as a single network layer to service providers is derived from the following benefits:

• Cost reduction: MPLS drives a major reduction in capex and opex costs by providing efficient multiservice network and operations environment, eliminating the need for multiple separate data networks--which is particularly important in access and metro segments. The use of MPLS as a service multiplexing scheme both in the access, metro and core of the network means end-to-end service control can be achieved, again driving down costs.

• Scalability: MPLS addresses the scalability issue of core network solutions using label stacking. Label stacking is a means of aggregating flows that share a common forwarding path. Inherent to MPLS, it greatly reduces the amount of state information that core MPLS devices must maintain. This not only allows networks to scale, but also vastly simplifies network maintenance and day-to-day operational tasks.

• Revenue generation and service innovation: service innovation: MPLS is viewed by many as the logical platform to underpin of a portfolio of next generation multimedia and mission-critical data services over flexible, Traffic-engineered network. Service providers are depending on these new services to drive future revenue growth, as revenues derived from voice continue to erode.

MPLS in the access and metro network

MPLS classifies packets at the edge of the network depending on predefined criteria. Flows of packets with common forwarding criteria are identified by attaching a label to that flow of packets--to create a label switched path (LSP)--and then switching packets through the network dependant only on the label. This LSP then is manipulated across the network by using the label as identification regardless of the underlying transport technology. For example, MPLS packets can be explicitly forwarded, queued according to quality-of-service (QOS) requirements or separated into VPNs.

The beauty of MPLS lays both its simplicity and its independence from the underlying transmission technology. The use of MPLS has several advantages:

• Interoperability between network layers, architectures and vendors: MPLS provides a common protocol for access, metro and core networks, allowing services to be transported end-to-end with common QOS and operations, administration & maintenance (OAM) capabilities. In addition, MPLS is supported by almost all equipment vendors, based upon mature standards.

• Transport layer independence: MPLS can ride over virtually any transport technology including Sonet/SDH, Ethernet or RPR.

• Service identification: Packet classification is accomplished at the edge (ingress) of the network, allowing a great deal of flexibility in identify specific services.

• Service differentiation: An MPLS LSP is analogous to a virtual circuit (VC) in the ATM world. As with a VC an MPLS LSP can be serviced depending on predefined criteria for example QoS, bandwidth requirements, priority etc.

• End-to-end signalling: MPLS control plane ensures simple end-to-end provisioning by use of standards based signalling protocols (e.g., RSVP-TE).

The IETF is enhancing MPLS to deliver OAM capabilities, end-to-end quality of service, and most critical to MPLS deployment in the access and metro networks, is extending the reach of L2 services over an MPLS backbone.

The IETF has standardised Pseudowire Emulation (PWE3), or "Pseudowires," based upon its draft Martini, which provides a framework for multiplex and transport Layer 2 technologies such as frame relay, ATM or Ethernet and encapsulate (or tunnel) them over an IP/MPLS network.

Combining MPLS Pseudowires with enhanced techniques such as GFP and VCAT can provide an excellent evolution mechanism for Sonet/SDH networks, converging the packet and circuit networks together. This approach has the advantage to deliver statistical multiplexing of packet traffic (high-bandwidth efficiency), with traffic engineering for service guarantees, but at the same time retains all the benefits of Sonet/SDH technology, such as resilience and performance monitoring.

However, this transition from TDM-based Sonet/SDH rings to MPLS-centric packet networks in the optical metro and access domains requires that MPLS be extended to include features, which are critical within the metro network. Considering the ring-centric architecture found within access and metro networks the ability to provide packet-based ring protection and dual-node interconnect is critical, as well as extending OAM tools to MPLS and the services it carries.

Enabling advanced services

Customer-specific Pseudowires can be defined, which carry the traffic of a single customer, providing a logically segregated elastic pipe that delivers QOS guarantees (for example, controlled delay, jitter, packet loss etc.), as well as enforceable and verifiable SLAs for each customer flow. Using MPLS provides the ability to engineer several classes of service, as well as guaranteed, regulated (overbooked) and best effort per customer, which can then be applied to network applications with differing transmission requirements.

As well as variable and sustainable QOS definitions, the use of MPLS provides support for the provision multiple "virtual" topology options allowing the network to be designed as per customer requirements. Customers' services can be point-to-point (transparent leased-line replacement) services, point-to-multipoint or multipoint-to-multipoint connectivity transparent to the customers' network topology, protocol and addressing structure.

An example of such services are L2 Ethernet VPNs, based upon the IETFs VPLS concept, which provides multipoint "transparent LAN" service between multiple enterprise locations through Ethernet MAC switching.

Although Ethernet services are often thought of as with the "metro," they must be capable of being extended from one metro area to another to achieve the vision of global Layer 2 connectivity. The use of MPLS in both access/metro and the core allows this connectivity to be achieved by signalling an end-to-end traffic engineered MPLS connection with common QoS and OAM mechanisms. 

Conclusion

MPLS once confined to Core network routers is migrating into Access, Metro and Transport networks based upon a similar set of drivers: the need to converge multiple data technologies onto a single flexible, efficient transport mechanism for the explosive growth in packet data services.

Colin Evans is director of marketing and business development at Native Networks and a member of the board of the Metro Ethernet Forum (MEF).

Visit Native Networks online at www.nativenetworks.com.