A method of converting AAL2 cells that includes an original user data set into AAL5 cells and another method of converting AAL5 cells that includes an original user data set into AAL2 cells are disclosed. In the method of converting AAL2 cells, the length of the data subset included in each AAL5 cell generated is stored in a memory. Therefore, it is not necessary to store all the payloads of generated cells. When a last CPS packet is extracted, a total length of the original user data can be found using a set of lengths stored in the memory. As a result, the present invention requires only a small memory space, and any unnecessary delay problem is eliminated.