Ethernet Multiplexers

Hello All,,


    I need to implement Ethernet Multiplexer in FPGA.
    I need to draft the requirement specifications also.
    Can any body guide me on what Ethernet Multiplexers are actually
used for ....?


     Any core area of application for Ethernet Multiplexers (FPGA
Based).
     Please suggest me some good documents, data sheets, URLs, books
what ever you can. 


     Thanks in Advance 
     Kedar