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

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 marketplace isn't always a neat and tidy place. When companies were
trying to make business decisions how they should expend their resources,
ATM appeared to be a much larger marketplace. The POS marketplace was
viewed as just a few niche markets like ISPs, while ATM was viewed
in terms of huge telecommunications market. So many companies first put
their resources into making ATM interfaces.

ATM interfaces can be much more complicated, but if the marketplace is
larger you can afford to spend more money and still come out with a
less expensive product. So even though an ATM interface may be 10%-20%
less efficient, if the pricing was 30% better, ATM could still be a good
choice. On the other hand, if you are paying the same price for the
underlying physical facilities, you probally want to optimize for the
goodput of user data.