[Sephiroth]

The upstream is offered by the CMTS on criteria determined by the CMTS. From BradyVolpe:

Once a cable modem is powered on and connected to the RF cable of a CATV network, it will begin a “hunt” for a valid downstream DOCSIS channel. First the cable modem looks for and locks to any 64- or 256-QAM digital channel. Fortunately for the modem, the CMTS is sending out a “Sync” broadcast at least every 200 msec, which is used for system timing. In addition, the CMTS sends out an Upstream Channel Descriptor (UCD) every two seconds, which tells modems the upstream frequency to transmit on, symbol rate, modulation profile, and other parameters necessary to communicate on the network. Finally the CMTS is sending out Media Access Protocol (MAP) messages to allocate “talk time” to each cable modem. You see, since there are many cable modems and only one upstream frequency, the cable modems must “time-share” the upstream channel, which is called Time Division Multiple Access (TDMA). Now, when the cable modem successfully locks to a QAM channel, it looks for the Sync, UCD and MAP messages from the CMTS. If it finds these it knows it is on an active DOCSIS channel. If they are not present, the cable modem assumes the DOCSIS channel is offline or it is on a video QAM channel and continues its search.