Doubt in SONET

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Hello,
Let me attach a picture here.

In this picture, why is attaching a router to a SONET Multiplexer depicted? Does that make sense? If yes, how? Wouldn't just a MODEM be sufficient?

Also, I read somewhere - "ATM or simply Asynchronous Transfer Mode uses SONET as a transport mechanism." If this is right, can anyone explain to me what does this sentence means??
Thanks.

@wes-bryan

Hey @Atharva-Bet anytime you need to combine multiple signals onto a single medium you will need a multiplexer and demultiplexer. You also need a multiplexer to allow protocols that are not compatible on a medium to convert the single from one form to another.