scope:

This Recommendation describes the modifications to [ITU-T G.9701] that are needed to support distribution point unit - fast backhaul (DPU-FB). DPU-FB uses G.fast technology for both the backhaul connections (back lines) and the connections to the G.fast NTs (front lines) in an environment where crosstalk cancelling between the wire-pairs of the back lines and front lines is required in addition to the FEXT cancellation provided by G.fast.

All transceivers connected to the back lines and front lines shall be fully compliant with [ITU-T G.9701].

This Recommendation specifies modifications that apply to the transceivers connected to both ends of the back lines as well as the required synchronizations between the transceivers of the DPU-FB.

More specifically, this Recommendation includes the following:

• Requirements on the synchronization between the ITU-T G.9701 framing of the back lines and front lines.

• Allocation of sync symbol positions for the derivation of the crosstalk channel between the wire pairs of the back lines and the front lines.

• Modifications to the initialization of the transceivers of the back line to support joining of back lines into active front lines.

• Requirements on the use of transceivers between the DPU and DPU-FB to support G.fastback backhaul in the presence of underlying services such as POTS, ADSL2 and VDSL2 on the same twisted pair.

G.fastback lines are likely to be between 100 m and 200 m long and use up to 12 wire-pairs in a bonded group.

The number of customers served by a DPU is expected to be up to 20.

Data rate and reach from a DPU to a customer is expected to be that of ITU-T G.9701 transceivers using either profile 106a, 106b or 212a.

The DPU-FB may be reverse powered (see Annex A of [ITU-T G.997.2]).