A message structure is provided for assigning transmission profiles to individual modems for modem transmissions in an upstream direction, on a modem by modem basis. The profiles define a set of transmission parameters, e.g., forward error correction information, symbol rate, frequency, modulation type, etc. that the modem should use to optimize their transmission in the upstream direction in order to maximize throughput. The invention is applicable to both wireless and land-based communications systems, particularly those adhering to the Data-Over-Cable Service Interface Specifications ("DOCSIS")—Radio Frequency Interface Specification standard, or other standards which are analogous to the DOCSIS standard. The message structure can consist of two parts—a first part that defines a plurality of different types of transmission profiles, and a second part that assigns a particular profile to a modem. In a DOCSIS embodiment, the first part is contained in a UCD packet and the second part is contained in a MAP packet.