Answer: ATM cells are standardized at 53 bytes because it seemed like a good idea at the time! As it turns out, during the standardization process a conflict arose within the CCITT as to the payload size within an ATM cell. The US wanted 64 byte payloads because it was felt optimal for US networks. The Europeans and Japanese wanted 32 payloads because it was optimal for them. In the end 48 bytes was chosen as a compromise. So 48 bytes payload plus 5 bytes header is 53 bytes total.

The two positions were not chosen for similar applications however. US proposed 64 bytes taking into consideration bandwidth utilization for data networks and efficient memory transfer (length of payload should be a power of 2 or at least a multiple of 4). 64 bytes fit both requirements.

Europe proposed 32 bytes taking voice applications into consideration. At cell sizes >= 152, there is a talker echo problem. Cell sizes between 32-152 result in listener echo. Cell sizes <= 32 overcome both problems, under ideal conditions.

For several years the *near* consensus was 64 octets. France wanted 32 because they figured with 4 ms. cell fill time, they could *just* scrape by from one end of the country to the other without echo cancellers, while in the US we need em 'anyway. So France held its breath, took a few smaller European countries with them, and demanded that 64 be lowered. Hence the "split the difference" 48 size. This was at a CCITT SG XVIII meeting ca. 1989.

CCITT chose 48 bytes as a compromise. As far as the header goes, 10% of payload was perceived as an upper bound on the acceptable overhead, so 5 bytes was chosen.

Answer: The EFCI bit is in the cell header. Check out the definition of the PTI field. In essence, the 2nd bit of the PTI is the EFCI bit when the 1st bit indicates that this is a user cell. PTI mappings:

         
     PTI                     Meaning
     
     000  User cell, no congestion encountered, user-to-user indication = 0
     001  User cell, no congestion encountered, user-to-user indication = 1
     010  User cell, congestion encountered, user-to-user indication = 0 
     011  User cell, congestion encountered, user-to-user indication = 1
     100  OAM segment associated cell
     101  OAM end-to-end associated cell
     110  Resource management cell
     111  Reserved for future use