How are wheeled excavators steered?

Hi does anyone knows how is a wheeled excavator steered? Is it a mechanical steering like other wheel loaders. Seems impossible because the excavator cab can do 180 about the undercarriage. I thought it must be hydraulically operated or skid steered, but my CAD teacher asked it to mechanically couple the wheels with the steering wheel. Haven't been able to figure out as to how to do it yet. Any suggestions?