Connections to the building network equipment is made via fiber optic cables. These connections make up the physical network backbone. Each building has at least 12 strands of multi-mode fiber. Two strands are required to effect a connection. Most connections are brought back to Green Hall (with the exception of the Townhouses, which are connected to Green via a CB3500 and a Gigabit link). With the exception of the Gigabit fiber interfaces, the fibers are driven by visible (red) or infra-red LEDs. The Gigabit equipment uses Class 1 lasers. According to specification, a half-duplex 100Mbps connection can be up to 400 meters long, while a full-duplex connection can be 2Km. Compare this to a length limit of 100 meters for copper.