ATM designers select cells over packets because of the following facts:

• Cells are fixed length and memory management for them is easy. Handling variable length packets tends to memory fragmentation.

• Variable length data packets need hardware to accommodate the maximum possible packet, and thus to check the end of the packet. With cells bits may just be numbered as they reach.

• The length of time needed to send a variable length packet is variable and needs complicated interrupt scheme to check completion of transmission. QoS could not be guaranteed with variable length data packets as simply as it cannot be variable length cells.