I asked your question ("What's the catch?") to the Bell Labs experts and this is what they said: "Distance is one part of the answer; we're also doing some undisclosed optimizations to get the most out of the 2-pair/30m case."
It seems the spectral efficiency without bonding (1 pair) is 5.7 bps/Hz, while with bonding (2 pairs) it is 10 bps/Hz. What's the catch? Distance (70m vs. 30m)? I would still not expect such a big difference.
G.fast is an acronym for Fast Access to Subscriber Terminals and its main purpose is just that--to get higher speeds from digital subscriber lines (DSLs). It is modeled on Very-high-bit-rate digital subscriber line 2 (VDSL2) which was designed to get high-speed data over the copper telephone wires already in older buildings (VDSL2 was defined as standard ITU-T G.993.2 in 2005.) The only thing exotic about G.fast is its promise of 200-to-500 megabit per second speeds for DSL.
NASA's Orion Flight Software Production Systems Manager Darrel G. Raines joins Planet Analog Editor Steve Taranovich and Embedded.com Editor Max Maxfield to talk about embedded flight software used in Orion Spacecraft, part of NASA's Mars mission. Live radio show and live chat. Get your questions ready.
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