A system and method for management of bandwidth in a fiber optic, ethernet-based, TDMA communications system. A request/grant process is used to control the use of upstream bandwidth. A sense of time must therefore be shared by a headend and remote end-user devices. The invention provides for a gigabit media-independent interface in a media access controller to detect start-of-frame delimiters in incoming data. This allows for synchronization of a headend and end-user devices. The invention also allows for phase locking a transmit bit rate, at a headend, to the headend's clock. Transmitted data can the be used downstream to derive a local clock. Synchronization can also be maintained by the use of synchronization bytes in MPEG frames and/or variable length frames. Efficient bandwidth usage can also be facilitated by the use of maximum data units in allocating bandwidth in unsolicited grants, and by allowing flexible fragmentation and/or prioritization of internet protocol (IP) packets.