Why ATM cell is size 53 bytes?

Answer 1

The American standard for DS0 is 64 Kbs, the European standard is 32 Kbs. The Americans pushed for an ATM payload of 64 Bytes while the Europeans pushed for 32 Bytes. To come to a compromise, the committee added 64 + 32 = 96. They then divided this sum by 2 for a result of 48. A 5 Byte header was added for a total of 53 Bytes.

Answer 2

When the CCITT was standardizing ATM, parties from the United States wanted a 64-byte payload because this was felt to be a good compromise between larger payloads optimized for data transmission and shorter payloads optimized for real-time applications like voice; parties from Europe wanted 32-byte payloads because the small size (and therefore short transmission times) simplify voice applications with respect to echo cancellation. Most of the European parties eventually came around to the arguments made by the Americans, but France and a few others held out for a shorter cell length. With 32 bytes, France would have been able to implement an ATM-based voice network with calls from one end of France to the other requiring no echo cancellation. 48 bytes (plus 5 header bytes = 53) was chosen as a compromise between the two sides. 5-byte headers were chosen because it was thought that 10% of the payload was the maximum price to pay for routing information.

From: http://en.wikipedia.org/wiki/Asynchronous_transfer_mode