Some of Netopia's products based on their generic router platform
consisting of the UMB3 motherboard and WAN modules
were not routers!
In the golden days of Copper Mountain Netopia
had made a D7100 SDSL CSU/DSU
and a corresponding product for
IDSL. (I'm unsure of the exact model number of the latter due to being lost
in Netopia's model artificiality, and I don't know if it was CM-specific or
generic.)
Although at the time I had very much longed for a true DSU first for IDSL, then for CM SDSL, I had a difficult time accepting Netopia's solution as being what I wanted. In actuality it is quite a stretch to call the product in question a CSU/DSU. Even though the manual calls it such on the front page, reading further reveals that the device can actually function in two modes: either a DSU for Frame Relay (all documentation keeps saying FR explicitly, even though a DSU is supposed to be protocol- and encapsulation-transparent) or an Ethernet bridge (a filtering bridge to use Netopia's exact words).
If that wasn't enough, additional turn-offs are:
There is no user configuration to select between the different-as-night-and-day DSU and Ethernet bridge modes, instead the box tries to read the user's mind telepathically it seems.
In both modes the unit still has Ethernet, wants an IP address and is
thus obviously IP-aware. In the DSU mode the IP-aware Ethernet port is
supposedly only for O&M and isolated from the data path, but how is that
isolation
worth anything if there is no way to guarantee that the
unit will stay in the DSU mode and won't switch to the bridge mode based
on its psychic sense? (As for the bridge mode, isn't it rather
self-contradictory for a bridge to have an IP address of its own?
I suppose it can be viewed as a bridge and a node connected together
without giving the user an option to separate them.)
Now that I know that Netopia had no separate CSU/DSU
hardware and
that a D7100 is identical to an R7100 router in terms of hardware,
things fall into place a little better.
However, some choices made by Netopia still defy reason:
If the hardware is identical anyway, why didn't they support all three modes of operation (router, bridge, DSU) in one product? They could have made the bit stream from the WAN module explicitly switchable between the router's processor and the auxiliary port, and if it was switched to the processor, the firmware could then implement routing or bridging per further configuration options.
If there was some compelling reason to make two separate products, it would have surely made more sense to combine the router and DSU modes in one and put the bridge mode in the other. Given that routed and bridged DSL circuits are different in terms of how an ISP configures them, it would have been quite natural to have one product (router/DSU) handle routed circuits and have the other handle bridged ones.