The ATLAS Computing Model is under continuous development. Previous exercises focussed on the Tier-0/Tier-1 interactions, with an emphasis on the resource implications and only a high-level view of the data and workflow. The work presented here attempts to describe in some detail the data and control flow from the High Level Trigger farms all the way through to the physics user. The current focus is on the use of TAG databases for access and the use of streaming at various levels to optimise the access patterns. There has also been detailed consideration of the required bandwidth to tape and disk, which then informs storage technology decisions. The modelling draws from the experience of previous and running experiments, but has also been tested in Data Challenges and will be tested in the LCG Service Challenge 4.