It is several years since I've looked in detail at that hardware, so be warned...
The earlier 6 port transmux is not as nice or flexible as the 12 port one in that you have to use the BNCs and if the DS3s you are using in the transmux originate in the same 15454, you must crossconnect them externally with short BNC jumpers between a regular DS3 port card and the transmux. If the DS3s are going elsewhere, the 6 port transmux is ok but less dense.
The 12 port model can use the BNCs, or not, and if not, ALL your mux ports are available in the 15454's crossconnect fabric and can be routed as desired.
Now for the REALLY big and important function of the transmux. 28 T1s can be packed in an STS sized bundle (call it an OC1 if that helps) in two very different ways. There is the older "M13" format, and the very handy VT1.5 format.
In something such as a PA-MC-2T3+ card, you were limited to having the T3 packed using the M13 format. And all the 15454 can do with an M13 format T3 is to carry it intact to somewhere and hand it back out on BNCs. If, however, the T1s were packed as Sonet VT1.5s, a 15454, with the correct crossconnect fabric could play DACS and crossconnect at the VT1.5 level. Each "T1" could be moved from its input bundle to any of the 28 slots in any other such bundle in that 15454.
This replaces a REAL 3:1 DACS. Or the really dreadful option of using M13 muxes wired to real copper DSX panels that get ports crossconnected with that special two pairs and a fifth tag light wire that get wire-wrapped at each end but which can be initially setup or altered at any time using plug in patch cords, until the new configuration is hard wired and the patch cords get pulled..
There are BNC ports that can carry the VT1.5 bundles, but they are NOT the normal 15454 DS3 port cards. Normally VT1.5 bundles will be on fiber and each as an STS in a OC-x bigger bundle.
The transmux card takes a DS3s with M13 format packed T1s on one side and gives you STSes with VT1.5 packed T1s on the other. Either take the same space on an OC-x bigger fiber link.
Once in the VT1.5 packing, a 15454 or series of them across the country can play DACS and crossconnect any VT1.5 T1 from one time slot in to any other in VT1.5 packed STSes letting you add or drop individual t1s at any 15454
If you MUST hand off an M13 formatted DS3 to an ILEC in their COLO (smart way to avoid individual T1 local loop charges is to hand the bigger bundle where the break even used to be about 9 T1 local loops cost about what the T3 handoff was and the other 19 t1s were basically FREE. Some ILECs might be able to take an OC-x handoff with some STSes designated as being packed in VT1.5 format, but many really just want M13 format. The transmux lets you start with a pile of PA-MC-2T3+ cards, convert their output into Sonet VT1.5 packing, and then shuffle T1s individually from bundle to bundle going across the state or country on 15454 hardware and then, if necessary, converting somewhere near the final destination back to M13 format with a transmux
I think some bigger cisco routers than I used actually had cards that were like the PA-MC-2T3+ cards but that either used OC-3 or OC-12 ports and could pack in the VT1.5 format directly. Sadly I never saw that in a card for a 7206, though I suspect PMC Sierra did make the appropriate chips and cisco had sampled them.
