Nortel Networks boasted today about having been selected, along with Siemens, to supply carrier Ethernet gear for British Telecom’s 21^st Century Network initiative. In particular, Nortel claimed the win was a “major vote of confidence” from a top-tier carrier for Nortel’s chosen metro technology, Provider Backbone Transport.

“PBT played a significant role in convincing BT” to select Nortel, said Philippe Morin, president of Nortel’s Metro Ethernet Networks division.

The Siemens system Nortel selected also employs PBT.

Since last year, Nortel has been marketing PBT as a simpler, less expensive alternative to multiprotocol label switching (MPLS) in metro networks. Based on the IEEE's 802.1ah standard, PBT sets up point-to-point Ethernet service tunnels by affixing a 24-bit media access control (MAC) header atop each existing customer MAC header in packet-based traffic. It’s simpler and cheaper than MPLS in the metro, Nortel says, because carrier Ethernet switches need only keep track of their own MAC addresses, not the customers'.

Nortel unveiled PBT in its 8600 Metro Ethernet Routing Switch with plans to eventually add the technology to its Optical Multiservice Edge 6500. BT will deploy both products—the 8600 starting this quarter, and the 6500 in the second half of this year.

Nortel is currently marketing PBT solutions most intently for wireless backhaul and business services applications.

Rather than replace MPLS, PBT can complement it, Morin said, by handling metro networks while MPLS handles backbones. But where carriers don’t already have MPLS backbones, PBT could creep into the core, he said. “PBT can play a role in the core,” he said, adding that Nortel is talking to some carriers about doing exactly that.

As PBT is a point-to-point technology, some analysts have questioned how scalable it is. Unlike Layer 2 carrier networks based on virtual local area networks, which can only scale to 4094 tags, PBT can theoretically issue up to 16 million tags, Nortel said.

“In reality, you’re limited by your network processor,” said John Hawkins, Nortel’s senior marketing manager. “The network processor has to create messages that flow through the tunnels. Processing those messages is what tends to be the bottleneck. I know [the 8600] can do several thousand [service tunnels], but I don’t know what ‘several’ is.”

The precise upper limit may be academic, Hawkins suggested, because the 8600 will provide more than enough tunnels for carriers’ needs.

In practice, carriers will use PBT trunks for point-to-point links connecting high-bandwidth data centers and “plain old” Provider Backbone Bridging (a superset of PBT, also based on IEEE standards) for multicast traffic. And their metro switches will be segmented into two parts: PBT trunks and Ethernet virtual local access networks. “The two don’t interfere,” Hawkins said. “They live happily side by side.”