Protocol used by many cable television (CATV) operators to provide internet access over their existing cable infrastructure.
The European version is called euroDOCSIS. The main difference is that PAL/SECAM channels use slightly more bandwidth (8Mhz in stead of 6Mhz for NTSC).
DOCSIS uses TDMA and optional DES encryption so that individual subscribers can be given higher or lower speeds to access the internet. TDMA is used within a channel; if more bandwidth is needed because more users have subscribed to cable internet, any (unused) TV channel can be used to host an additional downstream signal. Potential download speeds are limited only by the number of television channels left unused.
In contrast to the downstream channels, the upstream bandwidth can't be upgraded by adding channels since existing cable networks may not be equipped to handle new frequencies being used for the return signal. CATV networks were originally set up to be broadcast channels only, not interactive two-way channels.
The 'modem' used by the cable company to connect to all the users' modems in a neighborhood is called a 'head-end'. It is sometimes possible to split up sections of the cable network so one head-end is replaced by two, doubling the amount of bandwidth available.
It is a myth that DSL can provide 'unshared' bandwidth and cable internet cannot. QoS is built in to the DOCSIS standard, and different subscribers can get different amounts of bandwidth, either shared, exclusive or prioritized. However, DSL offers better upstream performance.