Shimano's simplified, modular wiring system for its Ultegra Di2 electronic shifting system is an improvement over the Dura-Ace Di2's wiring setup. Mounted to the cable guide beneath the bottom bracket shell (for externally wired systems), the junction box neatly and securely connects the wires running from the front derailleur, rear derailleur, shift levers and battery, powering and integrating the entire system.