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مشاهدة النسخة كاملة : why ATM cell size is 53 bytes ??



A7med Baraka
04-15-2009, 08:37 PM
How come an ATM cell is 53 bytes anyway?



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.

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.