SONET differs from this approach by using a byte-interleaved, synchronous, multiplexing technique with several variations on payload types.
SONET payload can carry multiplexed lower rate streams (DS1, E1, DS3, etc.)
as well as any octet-based format such as TCP/IP, ATM, frame relay, Ethernet, etc.
If a complete DS3 signal is to be delivered to a SONET drop, then the entire DS3 is transmuted intact into an STS container and no subsequent cross-connect capability exists with the SONET payload at a VT1.5 level.
White Rock Networks Description of a product with Transmux Support: "For applications where conversion between DS3 and VT1.5 mapped STS-1s is required, transmux allows DS3 embedded within an STS-1 circuit to be adapted to a VT1.5 mapped STS-1 without physically terminating the circuit as a DS3 or EC-1."