Tim Hubbard, head of solution strategy for the BT NGN, which is known as the 21st Century Network (21CN), said 15 equipment vendors were invited to tender last year to provide the Ethernet backhaul between the MPLS core, with its 106 nodes, and the 5,500 multi-service access nodes (MSANs), to which end customers will then connect.

These vendors proposed three different technological approaches, with some of them offering more than one approach in their bids, he explained. There was PBT, Ethernet VLAN or Q-in-Q VLAN stacking, and Ethernet-over-Pseudowire with an MPLS control plane, he said.

The first two technologies are at Layer 2, which was considered a better way of doing things because we wanted to keep things simple, said Hubbard, a requirement dictated by the fact that, as the former PTT, BT is legally obliged to offer equal access to the core-to-MSAN link to all its competitors. MPLS, as a Layer-3 technology, would have meant an added level of complexity, not to mention cost.

With the choice coming down to the two L2 offerings, it was the scalability of PBT, or Provider Backbone Bridging Traffic Engineering (PBB-TE) as it is now being called, that won the day.

Traditional VLAN stacking has a maximum limit of 4,095 VLANs to a single link, whereas PBT uses the 802.1ah standard also known as PBB to enable MAC-in-MAC stacking, whereby a customer’s entire Ethernet frame can be encapsulated and have a carrier header, which raises the total to 16 million VLANs per link. Or as Hubbard put it, With VLAN stacking, the VLAN is only for separation, whereas with PBT we can encapsulate the entire end customer frame.

Hubbard also explained that the requirement for end-to-end separation of customer and carrier frames derived from the fact that it enables the end customer to provide their own protection, in the form of restoration and recovery, if they so desire. He compared it with the way IP traffic is encapsulated for transport across an MPLS network.

The first set of services that PBT will need to support in the core-to-MSAN link will be TDM voice, broadband and Ethernet services (EPL, EVPL and E-LAN), which will be the first to go over the 21CN. Then we’ll need to support connection-oriented services such as leased line, partial private and private circuits, he went on, adding that this means defined paths, controlled traffic and QoS, all of which favored PBT because it enables is to offer point-to-point services.