serializer deserializer mostly used in high speed serial links to convert the parallel digital data to serial before sending it then send it serially to another SERDES that restores it back (example SATA HDD)
we send data with some media,such as fiber,coaxial-cable,this time it is very high speed.but in the board it is slow speed signal.
so we need serial the slow signal to high speed to send data;then
we deserial high speed data to slow speed data to handle them.
and we always use PLL with CDR in the deserial process,and use multiply
PLL in the serial process.