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

  • From: George Janosik
  • Date: Mon Aug 03 19:32:58 1998

At 02:47 AM 8/3/98 , Tony Li wrote:
>> I find in an article, and it says: in 1995,1996 most nationalwide ISP
>> face the demand of higher bandwidth than T3, and at that time the only
>> avalible high bandwidth interface in router in ATM port(say 155M), then
>> this makes many of them choose ATM as their backbone tech. Later, Giga
>> router with POS(Packet over SONET/SDH) 155M(or higher) interface comes
>> out, then the situation is changed.
>> Then my question is: why 155M ATM interface is easier to make than 155M
>> POS interface?  Although the net capacity of 155M ATM is less than 155m
>> POS, their capacity for IP packet is really close. I think its nearly the
>> same challenge to pump up a 155M ATM pipe as a 155M POS pipe with IP
>> packet for a router.
>The ATM interface is actually harder to make than a POS interface because
>you have to include the segmentation and reassembly (SAR) functionality in
>the hardware.  Also, the software gets a lot more complicated.
>Why then did ATM appear first?  There was a great deal of publicity given
>to it.  The vision was that it was going to be the 'media' of the future
>and the hype ran all out of proportion with reality.  It took quite a while
>for the hype to give way, and for engineering and economic realities to
>truly settle in.  And as can be seen from recent announcements, that
>process is not yet complete.
>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.

Lest everyone forget, ATM was developed as part of the ISDN broadband
standard for integrating voice video and data (In that order).  at the time
it was the only proposed solution for highspeed integrated voice, video and
data.  Remember Voice over IP was not even discussed and neither was video.
 It was the most efficient  (maybe not the best) solution at the time for
statistically muxing voice, video, and data instead of channelized TDM
which made the overhead a non-issue considering the savings of unused TDM
bandwidth.  The telco equipment companies took for ever (what else is new)
to develop product while the data companies (i.e. FORE, Lightstream, etc.)
reacted quicker for its use in the data enterprise arena, but before ATM
could solidify itself in the WAN Cisco came out with POS and the game was
over.  Just my 1/2 cent.
Best Regards,


"The first man gets the oyster, the second man gets the shell"
                                                  Andrew Carnegie