Traction elevator machine rooms are located immediately above the elevator shaft served by that equipment. In the lower drive type, machine rooms are usually located next to the elevator bank or at the bottom of the shaft runway. In older installations, especially in high-rise buildings where space is limited, the machine room is often divided into two levels to house the traction equipment, motor-generator (MG) sets, and relay controllers. Typically, upgrading to a solid state controller and variable frequency drive (instead of the MG set) can free up this space for other uses.