How is trade in services conducted under the WTO in the absence of the Doha conclusion? The Doha round of negotiations for the WTO had a focus on services, but that round has not been concluded.

Does that mean that trade in services between trading partners without a preferential trade agreement in place, for example the EU and the USA is effectively prohibited? Or legislated for on a case-by case basis?

Or is trade in services effectively unaffected by the traditional tariff model of the WTO?
It's definitely not prohibited; the US-EU trade in services is worth some $200 billion (each way).