VIRTUAL TRIBUTARIES

SONET framing incorporates lower rate signals (less that DS3)  into subdivisions of the STS payload. The VT (Virtual Tributary) functions as a separate signal with its own overhead bits.  They can be locked or floating.  Floating signals are asynchronous bits and requires minimal timing while locked signals require a common clock frequency and a fixed phase based on the DS1 frame within the SPE for digital cross connecting of the signal channel in the DCS.