In figure $1$ below, the PCs L$1,$ L$2,$ and L$3$ are sending packets to the PCs S$1,$ S$2,$ and S$3,$respectively, and at the same time, the network system is smart enough not to clash these packets. Explain the following:

D$.$ In what form a digital message can be sent through a wired link.

E$.$ How the single link between Switch $0$ and Switch$1$ is shared for sending the data based on:

$1.$ Synchronous Time Division Multiplexing $($STDM$).$

$2.$ Frequency Division Multiplexing $($FDM$).$

$3.$ What is the problem of STDM$?$ And how Statistical Multiplexing can overcome the problem?