North American Network Operators Group

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Re: Q:Why router with ATM interface comes out earlier than pure SONET interface?

  • From: Tony Li
  • Date: Mon Aug 03 18:17:53 1998

|  >One other fallacy here: ATM and POS do not provide the same effective
|  >bandwidth when used as an IP transport.  Due to the high encapsulation
|  >overhead and the sizing overhead of fitting packets into cells, ATM has
|  >proven to be about 20% less efficient at carrying packets than SONET.
|  >While this is probably not an issue in a campus or within a building, the
|  >implications for long lines is enormous.
|  >
|  	Okay, so given all the great features that ATM is supposed to have
|  and the only thing that really sucks about it is the overhead due to the 53
|  byte cell size, the obvious question is why can't there be an ATM standard
|  with, say, 197 ( 4 times the current 48 byte payload) or even 389 ( 8
|  times 48 ) byte cells?
|  	Is there something magic about 53 or is the IP over ATM application
|  still so 'obscure' that there is no interest?

Increasing the cell size lowers the efficiency further.

53 is an ATM architectural constant.  Change it, and it's no longer ATM.
Change it, and you're no longer interoperable.