There are many causes of sewer gas smell. Improper venting, no trap, or too long of a trap arm. If your drain has no vent, everytime you run water down it, it creates a positive pressure ahead of the water and a negative pressure (vacuum) behind the water flow. If there is no vent to break the syphon created by the water flow, then the trap will continually suck itself dry. When the water runs horizontally across the floor and reaches the vertical stack, the gravity is working at 1/4 inch per foot, but when it falls down the stack, gravity is in full force. The pressures created by the fall try to draw air behind it, without a vent it will draw it down the trap and suck all the water out of the trap. A trap is designed with a 2 - 4 inch seal to store water and keep the sewer gasses from coming up in the room, with out a trap, the sewer gasses (which are lighter than air) rise up into the room. In addition when the wind blows on the roof some of the wind is diverted down the vent stack sitting above the roof and it pressurizes the plumbing system, some of that pressure is relieved in a fixture with no trap.
Finally if the trap is too far from the vent, the whirlpool created when water runs down the drain will also suck the trap dry. A vent breaks this syphonic action and allows some of the water in the trap arm to flow back into the trap refilling it, so it has a trap seal to prevent sewer gasses from coming into the room.