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

Tony Li tli at juniper.net
Mon Aug 3 06:47:12 UTC 1998

> 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.


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