SONET and T-carrier are both synchronous TDM.

But T-carrier does not use atomic clock. Why does SONET?

Is it because SONET does not use buffer? But how does it prevent bit loss if there is no buffering?

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    SONET/SDH end-stations, like routers or PSTN switches, can be clocked from the line/network. I believe you only need an accurate clock if you are a node able to add/drop/mux SONET channels among different circuits. – Jeff Wheeler Sep 12 at 14:40

SONET, or rather the synchronous digital hierarchy it's running on, requires very tight clock coupling across the whole structure. In order to stay within the sub-millisecond tolerances, there's no other way than with atomic clocks.

Sharing a central clock wouldn't work due to the inherent limitation of the speed of light, which covers just 300 km in one ms (or only 200 km over fiber) - minus processing overhead, the hierarchy would need to be very limited in geographic size.

T-carrier uses a much simpler master-slave clocking, so there's no requirements for atomic clocks.

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