X2 (Chipset)
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US Robotics's 56K modem protocol for upload under V.34+ at 33.6 kbit/s and download under PCM at 56 kbit/s. This competed with Lucent and Rockwell's joint K56flex protocol. Though it was found to consistently have a lower top end speed, it consistently was faster than K56flex, on the averages. The Lucent and Rockwell incompatibilities were also avoided as there was only one X2 standard. The K56flex and X2 protocols have been superseded by V.90.