In a network node, a method and apparatus assemble fixed cell length cells resulting from the segmenting of variable length frames into cells. A finite state machine fetches frame data moves frame data and control block data to the cell and outputs cell data. A counter points to the frame. A multiplexer selects the correct data to form the cell. The finite state machine builds cells corresponding to a frame using a cell pattern applied to these cells using two constraints. The first constraint is a 64 unit cell size with a 6 unit cell header and 10 unit frame header; the second constraint is that the insert and overlay field have to be even. The method replaces a field by a definite value when a frame is modified during the segmenting processing, when a new field is inserted in the frame or when frames are packed into one cell.