--------                                              --------
             |        |                                            |        |
             |        |                                            |        |
             |  X11   |                                            |  X21   |
             |        |                                            |        |
             |        |                                            |        |
              --------                                              --------
                  .                                                    .
                  .                                                    .
                  .                                                    .
                  .                                                    .
                  .                                                    .
   --------    --------       --------               --------       --------      --------   
  |        |  |        |     |        |             |        |     |        |    |        |  
  |        |  |        |.....|        |             |        |.....|        |    |        |  
  |  Y11   |..|  Sw1   |.....|   PE1  |=============|  PE2   |.....|  TDM   |....|  Y21   |  
  |        |  |        |     |        |             |        |     |  (MUX) |    |        |  
  |        |  |        |     |        |             |        |     |        |    |        |  
   --------    --------       --------               --------       --------      --------   
                  .                                                    .
                  .                                                    .
                  .                                                    .
                  .                                                    .
                  .                                                    .
              --------                                              --------
             |        |                                            |        |
             |        |                                            |        |
             |   Z11  |                                            |  Z21   |
             |        |                                            |        |
             |        |                                            |        |
              --------                                              --------


PE1 and PE2 are M Series Routers
Sw1 is a distribution switch that terminates in a 
gig ethernet interface on PE1.