G.fast is the hot new ultra-broadband standard from the Broadband Forum and the ITU. How does it compare with the previous standard, VDSL? Check out the technical details here.

1. VDSL vs. G.FAST
Technology comparison
Parameter VDSL G.FAST
Frequency Range Up to 30MHz (30a) 2-106MHz, 2-212MHz (future)
Max Rate Up to 250Mbps (30a) and
150Mbps (17a) for short loops,
real-world is 30-80Mbps
1Gbps over short loops,
exceeding 500 Mbps over real
100m lines
Modulation OFDM OFDM
Number of carriers 4K 2K (for 106MHz profile)
Multiplexing scheme FDD TDD, synchronized among the
different copper pairs
Symbol time ~250 μsec (17MHz profile) ~20 μsec
Vectoring Via G.993.5 Supported, with adaptations
needed for higher frequency
Tx power Varies by profile, 14.5dBm 4dBm
FEC RS + Trellis code RS + Trellis code
Confidential | Sckipio Dec 12, 2014 1

2. VDSL vs. G.FAST
System comparison
Parameter VDSL G.FAST
Downstream/Upstream ratio Fixed Configurable: 90:10 to 30:70
Customer self-install Questionable. i.e. rates become
marginally low
Yes
xDSL Spectral compatibility Issues with 17M/30M mix
Issues with CAB/DP Mix
Yes, fresh start with new
frequency band
Retrain time Very long (30-90 second range) Very short – few seconds
Rate adaptation Very slow, 128 carriers at a
time, SOS not robust
Very fast, quick and robust
adaptation – within few msec
Low power mechanisms Under definition, expected
savings up to 50%, very slow
wake up time
Specified from day one,
discontinuous operation, scales
with traffic rate
Confidential | Sckipio Dec 12, 2014 2