generally speaking POS interfaces are not equivalent to ATM interfaces at the same speed.
POS = packet over sonet is for point to point links and uses a much simpler L2 protocol you can see like a sync serial over SDH.
ATM interfaces are much more complex they use VPI/VCI and send ATM cells and can support PVCs and can be point to multipoint different PVCs lead to different remote routers.
Both use SDH at physical layer but they are quite different.
We should look if the new SPA can work as ATM interface
The general datasheet states the ATM SPA is in End of Sale
From there going to specific datasheet for the SPA 4 POS OC3 V2 sends me to the following link
That describes only POS mode of operation.
You would need to move to a different WAN technology to use POS interfaces.
Hope to help
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