VPLS lets carriers create a service that looks and feels just like a wide-area LAN, but is more robust and reliable. In case of failure anywhere in the network, the carrier will route a user's traffic along backup paths automatically.In case you haven't noticed, the transition to MPLS-based services is in full swing.In a recent Nemertes benchmark, 42% of participants reported using MPLS, up from 24% in 2004, with another 9% planning to deploy MPLS-based services within the next 12 months. That's at the expense of legacy technologies, such as frame relay (which dropped from 78% deployment in 2004 to 46% today) and ATM.Why? MPLS-based services offer a host of benefits. For one thing, they're generally less expensive than most alternatives - carriers are pricing MPLS-based services at 10% to 25% lower than what existing data services cost. Another primary benefit is the ability to provide QoS-based support for converged voice and data services. Many companies are rolling out MPLS in preparation for, or in conjunction with, VoIP initiatives. This makes sense, given MPLS' built-in ability to support latency-sensitive, any-to-any applications such as voice. Moreover, many of the folks I work with are taking advantage of MPLS' capabilities to deliver interactive videoconferencing across the WAN - particularly to remote and branch offices. By converging their voice, video and data over a common MPLS backbone, companies can save 25% to 40% over existing telecom costs.But lower costs and QoS aren't the only benefits gained from moving to MPLS. As I've noted in previous columns, MPLS isn't so much a service as an architecture that can flexibly support multiple service offerings.One such offering is Virtual Private LAN Service (VPLS), which relies on MPLS underpinnings to extend an enterprise's LAN transparently across the WAN. Within the carrier cloud, VPLS relies on an MPLS architecture. However, users connect to this cloud, not by routers but with Ethernet switches. The carrier uses a protocol such as Border Gateway Protocol or Label Distribution Protocol to connect sites across the WAN using pseudo-wires.This lets carriers create a service that looks and feels just like a wide-area LAN, but is more robust and reliable. In case of failure anywhere in the network, the carrier will route a user's traffic along backup paths automatically.Carriers are aggressively deploying VPLS: In February, Verizon announced an expansion of VPLS in the United States, Europe and Asia-Pacific as an access option to its MPLS-based IP services, and earlier this year telcos Broadwing and Hutchison Global Communications announced a VPLS-based intercarrier Ethernet network.Although a standard VPLS network-to-network interface doesn't exist yet, some efforts are underway to develop one. An area of particular promise is hierarchical VPLS (H-VPLS), which enables greater VPLS scalability by limiting the number of participating routers. As a corollary benefit, H-VPLS provides a mechanism for carriers to interconnect their VPLS networks.If you're interested in getting up to speed on VPLS, H-VPLS and other MPLS initiatives, stop by MPLSCon in New York in a couple of weeks. You'll have a chance to hear yours truly discuss enterprise trends in MPLS rollouts. See www.mplscon.com for details.Johnson is president and senior founding partner at Nemertes Research, an independent technology research firm. She can be reached at email@example.com.