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AT&T, one of the world's largest telecommunications carriers, is getting a facelift.

Over the past 18 months, the company has been hard at work designing and implementing the AT&T Global Network (GN). When it's completed, the GN will consolidate AT&T's own network with that of the IBM Global Network it purchased in 1999, and Frame Relay infrastructure it acquired in early 2002 from British Telecom.

The project isn't just about consolidation, however. At the new network's core is MPLS (Multi Protocol Label Switching), a packet tagging technology that's gaining popularity amongst network operators looking for a way to improve their ability to offer differentiated service levels that enable true multi-service networks.

An IETF standard, MPLS provides a mechanism for tagging packets with forwarding information that can be used to assign the packets to a specific virtual circuit or give them a particular priority that allows for the enforcement of quality of service (QoS) guarantees. It's been built into high-end routers from Cisco Systems, Juniper Networks, Alcatel, Foundry Networks and other carrier-focused vendors, and is slowly working its way down the food chain to smaller devices.

MPLS tags are added by label edge routers (LERs) when data packets enter the network; tagged packets can then be moved quickly throughout the network fabric by intermediate label switch routers (LSRs) that are spared the burden of examining packets closely at every hop. When the packet leaves the network, another LER strips the extra information and points the packet to its ultimate destination. It's like pinning a name tag on a child before putting him on an airplane to visit relatives.

"We're taking existing networks and migrating them onto the network," says John Mulligan, AT&T Asia-Pacific's Director of Enablement. "The current technology that meets our needs is MPLS. It gives us the ability to do real service and quality control of the type of traffic, and we've been able to build a uniform specification around the world. Many of our new services will be designed to go onto this platform."

The AT&T GN currently spans some 140 nodes worldwide, including more than 20 in Asia and Australia. Using MPLS, traffic over the network can be allocated to one of four service classes that allow the carrier to offer services such as AT&T's new eVPN (Enhanced Virtual Private Network), which offers a VPN with guaranteed performance characteristics that makes it eminently suitable for securely connecting branch offices.

Building the multi-service network

Around the world, many carriers are finally making the switch to MPLS, which was introduced several years ago but has struggled to find relevance in a climate where carriers were more concerned with simply shifting their networks to IP.

These days, however, IP-satiated carriers are recognising that existing QoS protocols designed for IP - including RSVP (Resource Reservation Protocol) and DiffServ (Differentiated Services), in particular - are little more than a bandaid approach because they are still subject to general network congestion and the variability of IP's best-effort routing. MPLS-tagged packets, on the other hand, can be sent along defined paths that can be dynamically changed by networks to route data around heavy congestion points.

Another significant benefit of MPLS is the fact that unlike IP, it's content-agnostic. MPLS is as comfortable at OSI Layer 2 (data link) as at Layer 3 (network), whereas IP-based QoS schemes only operate at Layer 3. This means MPLS is protocol-agnostic, and can be used equally well to carry IP, Frame Relay, ATM (asynchronous transfer mode), NetBIOS, IPX, SNA and any other type of data packet.

For carriers, this means MPLS can be overlaid directly onto a fibre core, shuttling all of the carrier's data services across the same network. And that means cost savings.

"Carriers can run more efficiently using MPLS," says Bjarne Munch, Senior Research Analyst with META Group. "They have the ability to cut costs while providing a more reliable VPN in a virtual flat form where they can cater for a large amount of access. Because MPLS management is done in software, it's cheaper and more flexible to manage the VPN compared with a Frame Relay VPN or ATM (Permanent Virtual Circuit)."

Does this mean the death of ATM, the long-suffering protocol that died a quick death in the enterprise but has found a small home in carrier networks?

"ATM and Frame Relay are certainly not dead, but a lot of carriers are looking to carry them over their MPLS infrastructure," says Roger Geerts, Systems Engineering Manager with Juniper Networks Australia-New Zealand. "They'll use ATM and Frame Relay as more of access technology. But because the bandwidth capabilities of ATM are limited compared to MPLS, it makes sense to have high bandwidth through an MPLS (core) and use ATM links to terminate connections."

QoS at last

Given the strong and continuing growth of performance-sensitive services like Voice over IP and VPNs, the ability to charge a premium for a high-quality service is particularly appealing for carriers. AAPT subsidiary Connect has found this capability invaluable, recently upgrading its routers to combine its previous SDH, ATM, voice and IP networks.

"Our idea was to build a multi-service IP core and use that with aggregation, so we could fit all the ATM, voice and IP over one backbone network," explains John Greenhough, Director of Networks with Connect. "We have classes of service set up now, but MPLS makes it easier to use and manage. You can set up clear differentiated switched paths through the network, each having different classes of service. Lots of time you can get away without offering QoS at all, but this allows us to do SLAs."

In Connect's case, those SLAs revolve around three MPLS-enforced grades of service: Real Time, Premium Data and Business Grade levels of service. Although some customers may assume they need the top level of service for all their traffic, Greenhough says, being realistic can keep charges down. A typical bundle, for example, might allocate 30 per cent of bandwidth as Real Time (for Voice over IP), 60 per cent as Premium Data (for general network traffic and backups) and 10 per cent as Business Grade (for best-effort services like web surfing and emails).

Connect eventually plans to decommission its separate ATM and Frame Relay networks in favour of the consolidated MPLS-based network. But not every carrier is so keen to jump ship from ATM. Carrier RequestDSL, for one, has been using MPLS on its routers since 2001 in order to logically segregate the wholesale bandwidth available to some 160 channel partners for resale.

Despite the strong performance of MPLS so far, Chief Architect Greg Tilton says ATM still has its uses - particularly where customers require a PVC (permanent virtual circuit) with a committed information rate and performance is not so variable.

"Rather than dealing with QoS at a packet level, if a particular service requires a committed rate we'll deliver it using ATM's QoS," Tilton explains. "We can provide different ATM clusters of service on different ATM PVCs, then set ATM classes of service at an appropriate level. We then use MPLS in our core to partition networks at Layer 3, which allows us to have multiple customers access our core network."

Redefining the network

For now, MPLS projects are still underway in most carriers that have been implementing them. As at AT&T, the need for MPLS-capable equipment may well mean that MPLS isn't integrated into most carrier networks until they undertake a broader consolidation or upgrade project. That means it will be years before MPLS can be taken for granted by customers demanding enforceable service levels.

Regional carrier Asia NetCom isn't wasting any time, however. It recently built a completely new MPLS-based network that offers five classes of differentiated service.

"Without MPLS you would struggle to compare with Frame Relay and ATM in terms of privacy, security, and service across the network," says Asia NetCom Director of Product Marketing George Harb. "We hear the word 'convergence' in every second breath, and anyone that is currently looking at Frame or ATM is looking for (carriers) to at least be able to talk to how they'll migrate to an MPLS solution. Moving towards a mesh design, you see savings of anywhere between 30 and 60 per cent over Frame Relay since you're not having to purchase multiple PVCs."

While use of MPLS is currently focused on partitioning core bandwidth into multiple channels with definable service levels, expanding its scope will ultimately pave the way for a marked architectural change. MPLS will enable the use of VPLS (virtual private LAN service), effectively allowing carriers to provide a Layer 2 extension to corporate networks that eliminates the translation currently necessary for Frame Relay, ISDN and other WAN services.

Although Telstra currently offers 1 Gbps WAN connections over its IP Connect service, VPLS services take the paradigm one step further because IP is irrelevant. VPLS builds on MPLS' tagging capabilities to allow bridging between multiple MPLS-based VPNs. Since the bridging occurs at Layer 2, the VPLS network appears to a company's edge router as just another Ethernet LAN, even though that LAN may extend for thousands of kilometres around the world - at whatever speed the network core is able to support.

VPLS will be particularly useful as an improvement over current metropolitan area networks (MANs), as it will allow the seamless interconnection of multiple branch offices located across a metropolitan area - while eliminating the speed differential between LAN and WAN. Furthermore, the fact that IP is written out of the equation allows VPLS services to support any type of data transfer.

This, then, will be MPLS' legacy: a complete redevelopment of the rules of QoS and the interface between LAN and WAN. Although it's still a distant concern for most commercial customers, carriers moving to adopt MPLS now will be well positioned to deliver such services when customers start demanding them in the not-too-distant future.

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