SERDES
as far as i know there r many modes of operation , like:
1- the Tx and the Rx are separate each working on a system.
2- there are testing modes called loop back where some parts of the SERDES are operated and others are not.
i think there was some modes where the whole TX and Rx are connected to each other and the i/p data are feeded to the Rx and compared to the o/p from the Tx.
other modes where the some parts of the system are operative and other are not , these depend on the system design and are for testing purposes.
u may refer to some of the data sheets for SERDES from broadcom , TI >>> i think sonet oc-48 was good