Transport wars: MPLS-TP vs PBB-TE

10 Nov 2008
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It's never a dull moment in the ongoing quest to transform the world's TDM grid into sleek, efficient packet-based networks - particularly when it comes to transport. Sonet/SDH may be from the circuit-switched old school, but it has set the standard for reliability, manageability and scalability that carriers need to run their networks at 'five nines' level reliability - a benchmark that packet-based technologies have struggled to match.

Take Ethernet, which has long been championed as the most cost-efficient and simplest architecture for the packet-based future. But getting even carrier-grade Ethernet to perform its magic at the transport layer has been a struggle. Ethernet by nature is a connectionless technology - to beat Sonet/SDH, it needs to be connection-oriented.

'With Ethernet or any sort of packet-based technology, the advantages you get with it are statistical multiplexing, and you can look at multiple flows of information, which is acceptable for data traffic,' explains Tom Goodwin, marcoms VP for the optics division of Alcatel-Lucent's carrier business group. 'But when you get more delay- or jitter-intolerant apps like video and voice, you really need that connection-oriented parameters that you get with the TDM control plane parameters that can better deliver quality of service, but have flexibility to pack more packet traffic more efficiently into the network.'

As usual, telecoms vendors have rallied around two competing standards to make this happen.

On the one side has been the ITU-T, whose Study Group 15 took MPLS - the technology standard created by the IETF over a decade ago that's running on networks worldwide - as a starting point and has labored to develop Transport MPLS (T-MPLS), a stripped down version suitable for connection-oriented transport.

On the other side has been the IEEE, the industry group behind Ethernet's evolution from a technology suited for office LANs to an increasingly preferred technology for metro networks and backhaul links. Their technology, PBB-TE (Provider Backbone Bridge - Traffic Engineering), is a native Ethernet approach that comes with the usual Ethernet promises of greater simplicity and lower cost.

Both have their pros and cons, but one con that they have shared is lack of standardization. By that measure, PBB-TE (whose draft standard nomenclature is 802.1Qay) was at the biggest disadvantage, as work on T-MPLS was expected to be completed sooner. On the other hand, T-MPLS had one distinct problem of its own: it was completely incompatible with existing MPLS.

Now there's a new twist in the saga. In April this year, the ITU-T - realizing that T-MPLS with compatibility problems was going to be a hard sell - set up a joint working team with the IETF to come up with another approach. Result: T-MPLS was officially scrapped in June in favor of MPLS - Transport Profile (MPLS-TP), and some vendors are claiming that it's so good that PBB-TE now doesn't stand a chance.

The death of T-MPLS

To understand the case for MPLS-TP, it's worth looking a little more closely as to what was wrong with T-MPLS.

'Basically, T-MPLS was an attempt to enhance MPLS but it was done by a different body [the ITU-T],' says Pekka Viirola, head of technology for the IP transport business unit of Nokia Siemens Networks.

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