Hello @zakyarlya ,
a Cisco router serial interface can be synchronus or asynchronus.
In both cases you cannot put a cloud on the path and you cannot connect it to a DSL modem.
In packet tracer or GNS3 you can connect a sync serial to another sync serial on another router.
In real world the router with the DCE side cable needs to be configured with the clock rate interface command to provide the clock to the other side (this is the meaning of sync serial).
In emulation I think it is enough to configure the clock rate command on one side.
Hope to help
Giuseppe
