Several different styles of OPGW are made. In one type, between 8 and 48 glass optical fibers are placed in a plastic tube. The tube is inserted into a stainless steel, aluminum, or aluminum-coated steel tube, with some slack length of fiber allowed to prevent strain on the glass fibers. The buffer tubes are filled with grease to protect the fiber unit from water and to protect the steel tube from corrosion; the interstices of the cable are filled with grease. The tube is stranded into the cable with aluminum, alumi.