If you want to have true 1:1 protection with the traffic receiving unchanged bandwidth guarantees during the failure, you have to dedicate half of the bandwidth to backup paths. If you're willing to reduce the available end-to-end bandwidth during the failure period, you can oversubscribe the ring and thus gain additional bandwidth during the normal operation of the ring.
However, please note that PBB-TE uses external network management system to provision end-to-end Ethernet Switched Paths (ESP) across the PBB-TE infrastructure. While the PBB-TE standard does not prohibit oversubscription -- in fact, the only time you'll find Quality of Service or per-ESP bandwidth mentioned is in the notes, not in the main text -- the capability to do it lies solely with the network management system.
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