I currently am using a WIC, which has an ethernet cable T1 connected to it, and we're considering getting a MUX to tie in an additional T1 for more bandwidth. I can see how the MUX would connect from the MUX to the provider, but what about from the MUX to my router? Do I need a different module than the WIC to do this, or do I need to just change the config on my single WIC to include 48 channels, or would I need to use a 2nd WIC to utilize the second T1? Or, are none of my ideas correct?
Yeah, I would be hesitant to use MPPP with a 1721 at one end...
The reason why you would use a physical layer mux is to get your link to appear as one single interface on your router.
The difference between doing that and using CEF is a subtle one. CEF will load-balance per-destination over each line. This means that the maximum rate acheived by a single flow can be no more than the size of a single link since a single flow will not be sent out over more than one link (since this could cause packet re-ordering issues). That means that if you do something like a FTP download, the max you will get is a single T1 rate. This limitation is not much of an issue if you have got lots of different flows flowing over the link.
The following link has a good discussion of nxE1/T1 links: