I got a question along these lines from a friend working in SP environment:

Customer wants to upgrade a 7200 with PA-A3-OC3SMI to ASR1001. Can they use ASR1001-2XOC3POS interfaces or are those different from “normal ATM interfaces”?

Both interfaces (PA-A3-OC3SMI for the 7200 and 2XOC3POS for the ASR1001) use SONET framing on layer 1, so you can connect them to the same SONET (layer-1) gear.

POS (Packet-over-SONET) interface behaves like a point-to-point serial interface above the bit transmission layer – you can run HDLC or PPP on it (maybe even Frame Relay or X.25; I never tried). You can use a POS interface to connect two devices (for example two routers) with an OC-x/STM-y link.

ATM interface is a totally different beast – it slices and dices packets into ATM cells, implements virtual circuits and allows you to connect a router to numerous remote devices across an ATM network.

If you used ATM interfaces for point-to-point inter-router connectivity across SONET infrastructure (sometimes you could get an ATM bundle that would be cheaper than buying a POS interface) you can replace them with POS interfaces. If a router's ATM interface connects to an ATM network (directly or across SONET infrastructure), you cannot.

And then there are Channelized SONET/SDH interfaces. You could treat SONET/SDH bit stream as a sequence of bits (like the POS interface does) or as a structure of layer-1 frames multiplexing bits from numerous lower-speed point-to-point connections (for example, four OC-3 streams or 12 DS-1 streams fit into an OC-12 link).

A channelized interface can demultiplex those bit streams. It contains numerous serial interfaces that you can configure independently, resulting in a high-density WAN concentration option (it’s also pretty complex to configure – just check the configuration guide for the Channelized OC-12 module in a Cisco 10000 router).