The only thing that contradicts what newtype_alpha is saying is that we know Bajoran impulse ships don't have warp speed and that warp fields are synonmous with subspace mass reducing fields.
There's plenty of on-screen evidence to the contrary, but the best example is in the pilot to "Deep Space 9."
DS9 isn't capable of generating a "warp field." Yet it certainly is capable of generating a "subspace field."
See, "warp field" is a SUBSET of "subspace field." Every "warp field" as you call it is actually a distorted form of subspace field.. a "warped" subspace field.
But many systems which are not even intended to be self-motive (such as computer cores) have subspace field generation incorporated. And DS9 used the cardassian shield generator system (which is evidently subspace-field-based) to reduce the station's mass sufficiently to allow it to move to the wormhole in time to "save the day."
Ultimately if there is another form of mass reducing field that still makes impulse at least partially driven but field propulsion.
Just keep repeating this to yourself... "a subspace field isn't necessarily a 'warp field'."
We know that subspace fields do two things... increase the local value of C, and decrease the "projected mass shadow" of any object inside the field. This has been repeated established on-screen.
I don't think ANYONE is seriously suggesting that "impulse drive" as we see it used on Trek is a "simple rocket" propulsion system. Am I mistaken here?
No... for "impulse" to work, and yet to remain meaningful, it needs to be a COMPOSITE SYSTEM. Use a field to reduce the mass (and thus the inertial effects, allowing greater acceleration/decelleration for far less actual thrust) and, consequently, to also increase the local speed of light, so that your "rocket thrust" (that is, an IMPULSE based propulsion system) is able to do thing that a "regular rocket" would never be practical for.
This, also, is established in-canon as something that's really done, at least during the TNG-era.
But it's not necessary to say "it's one or the other." It's not. It doesn't HAVE to be. There's no compelling argument for it to be. And there are a LOT of compelling arguments for it to be the "hybrid FTL impulse, supplemented by a static subspace field" that I've been pushing for the past... what, three years now?
The thing is, if it's going to be called "impulse," it needs to be an "impulse" based propulsion system... that is... apply a certain impulse to the ship (which has a certain mass) and you get a certain resultant velocity.
Impulse, in "physics" terms, and translated to "Trek-understandable" terms, simply means "apply this much thrust for this much time."