If we are going to talk about ATM overhead when doing TCP/IP don't we need to talk about the overhead of partially filled ATM cells? Won't that cost you about 1/3 of your available bandwidth? -Jeff Ogden Merit
Jeff, would you or anyone give an example or two of when you get PARTIALLY filled ATM cells? I've interviewed dave sincoskie, steve tabaska, and Stephen von rump so far. they all rather like ATM. Don't believe I heard about this problem from them. You aren't talking about the switch not being able to tell whether a cell is mangled before it transmits by any chance are you????? ********************************************************************* Gordon Cook, Editor & Publisher Subscriptions: Individ-ascii $85 The COOK Report on Internet Individ. hard copy $150 431 Greenway Ave, Ewing, NJ 08618 USA Small Corp & Gov't $200 (609) 882-2572 Corporate $350 Internet: cook@cookreport.com Corporate Site Lic. $650 http://pobox.com/cook/ for new COOK Report Glossary of Internet terms ********************************************************************* On Tue, 19 Mar 1996, Jeff Ogden wrote:
If we are going to talk about ATM overhead when doing TCP/IP don't we need to talk about the overhead of partially filled ATM cells? Won't that cost you about 1/3 of your available bandwidth?
-Jeff Ogden Merit
Gordon Cook writes:
Jeff, would you or anyone give an example or two of when you get PARTIALLY filled ATM cells? I've interviewed dave sincoskie, steve tabaska, and Stephen von rump so far. they all rather like ATM. Don't believe I heard about this problem from them. You aren't talking about the switch not being able to tell whether a cell is mangled before it transmits by any chance are you?????
Since TCP/IP packets are of variable length, the last ATM cell for the packet will not be full. The remaining bytes in the cell are wasted bandwidth. Gary Wright --------------------------------- gwright@connix.com TCP/IP Illustrated Volume 2 ----- http://www.aw.com/cp/Vol2.html
If we are going to talk about ATM overhead when doing TCP/IP don't we need to talk about the overhead of partially filled ATM cells? Won't that cost you about 1/3 of your available bandwidth?
-Jeff Ogden Merit
Jeff, would you or anyone give an example or two of when you get PARTIALLY filled ATM cells? I've interviewed dave sincoskie, steve tabaska, and Stephen von rump so far. they all rather like ATM. Don't believe I heard about this problem from them. You aren't talking about the switch not being able to tell whether a cell is mangled before it transmits by any chance are you?????
Gordon, He's talking about the overhead due to carrying variable length IP packets in fixed length ATM cells. Consequently the last cell of an AAL5 frame will contain 0 - 39(?) bytes of padding, which is wasted bandwidth. Assuming random length distributions (which they're not), the average waste is about 20 bytes per packet. My rough estimate, based on an average packet size of 200 bytes (used to be correct, not sure anymore), is that the waste due to cell padding is about 10%. Due to the highly skewed packet size distribution the actual overhead might vary substantially. Note that this 10% is on top of the ~10% overhead due to the 5 byte ATM cell headers (5/53 ~= 10%), and various other overheads (some of which are also present in frame over Sonet schemes). There's beginning to be some expectation that there will be a transmission capacity crunch in the carrier's Sonet nets, and this ~25% ATM cell tax may be looked at carefully as packet over Sonet solutions emerge. -- Jim
participants (4)
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Gary R Wright -
Gordon Cook -
Jim Forster -
jogden＠merit.edu