Aligning the Initial-Maintenance Intervals of Cable-Modem Upstream Channels

In a cable-modem network, a cable modem (CM) talks to a cable- modem termination system (CMTS) over a traditional cable-television network. The cable-industry--backed standard that specifies how a CM and CMTS talk to each other is called DOCSIS. In DOCSIS the CMTS schedules the upstream (CM-to-CMTS) channels, each of which is time-division multiplexed into a sequence of minislots. 

To schedule an upstream channel, the CMTS logically divides the channel into a contiguous sequence of intervals, each interval comprising a contiguous sequence of minislots. The CMTS assigns to each interval an interval type, which specifies how the CMs may use that interval. Initial-maintenance intervals (IMIs) are used by CMs when they first come on line. 

In some CMTS deployments, certain sets of upstream channels must be IMI aligned. A set of aligned channels has the property that for each IMI X mapped in any of those channels, all other channels in the set have an IMI that starts and ends at precisely the same time as X. This paper describes a novel algorithm for aligning the IMIs in a set of DOCSIS upstream channels. 

This algorithm, which has been implemented and deployed in a real CMTS, is both efficient and flexible. In particular, it enables any set of upstream channels to be aligned, even ones with different minislot sizes.