Cisco SR2016T-NA Reference Manual page 1052

interface cable-modem
cable-modem upstream
preamble qpsk
cable-modem voip best-effort
Cisco Broadband Cable Command Reference Guide
6-50
Enables the QPSK modulation scheme in the upstream direction
from the cable interface to the CMTS.
Allows voice traffic to be transmitted on the upstream using a
best-effort QoS.
Chapter 6 Cable CPE Commands
OL-1581-07