DOCSIS is the Data Over Cable Service Interface Specification and dictates how the modem interfaces with the underlying coaxial cable physical link.
DOCIS 3.0 is the latest standard in broad use, and provides for 16 channels of downstream bandwidth and 4 channels of upstream bandwidth. That’s how your ISP provides greater upload or download speed, they segment the bandwidth on the coaxial cable into discrete channels. They allocate more channels for download, because that’s what the majority of home users do – they download – websites, streaming music and movies, etc. Upload – not so much. The allocation of discrete channels is called channel bonding – multiple channels bonded together to provide more bandwidth.
In the DOCSIS standard terminology, a 16×4 modem means it can accommodate 16 channels of download and 4 channels of upload bandwidth. Ultimately, the amount of channels allocated for upload and download is entirely up to your ISP.
In the future, DOCSIS 3.1 will be required for gigabit cable speeds, but we don’t recommend choosing one of those models just yet.
Wikipedia – DOCSIS