3/19/98
John Zwiebel
uniethernet.txt

                    Using Ethernet as a Unidirectional Link
                    ---------------------------------------

Unidirectional satellite links are becoming more and more important.  It is
difficult to justify lease of a satellite link to test new protocols and 
procedures.  The following special ethernet cables can be used to set up
truly physical unidirectional links.

All of the cables below are based on 10baseT ethernet.  I have found that
the order of the RX pinouts does not matter.  (pins 3 and 6).  
	
				      RJ-45
	
				 +--------------+
				/              /|
			       /              / |
	      Cable           +--------------+  |---+
	----------------------|              |  |  /|
		   pair   --- |------    1   |  +-+ +
		   three  --- |------    2   |  | |/
		  ----------- |------    3   |  +-+
	 pair   /   pair  --- |------    4   |  |
	 two    \    one  --- |------    5   |  |
		  ----------- |------    6   |  |
		    pair  --- |------    7   |  +
		    four  --- |------    8   | /
	----------------------|              |/
			      +--------------+
	
				 EIA TIA 568A
	
	
		
One to One with a Single Cable
------------------------------

     Transmit Router:  Two ethernet interfaces are required

    Interface 1   Interface 2
     RJ-45 #1       RJ-45 #2
     1 2 3 6          1 2
     | | | |          | |
     | | | +----------+ |
     | | +--------------+
     | |
     | |
     3 6
    Interface 1
  
     Receive Router



One to Many Transmit Cable to an Ethernet Hub
---------------------------------------------

     Transmit Router:  Two ethernet interfaces are required

    Interface 1   Interface 2
     RJ-45 #1       RJ-45 #2
     1 2 3 6          1 2
     | | | |          | |
     | | | +----------+ |
     | | +--------------+
     | |
     | |
     1 2

      Hub



One to Many Receive Cable from an Ethernet Hub
----------------------------------------------

      Ethernet Hub: Two ports are required

      Port 1         Port 2

       3 6          1 2 3 6   
       | |          | | | |
       | +------------+ | |
       +------------+   | |
                        | |
                        | |
                        3 6
                     Interface
              
          Receive Router