Overhead I/O cabling is contained within the frames. Extension “chimneys” that were featured  with previous Z systems are no longer used. A view of the top rear of the frame and the openings for top exit cables and power is shown in  Figure 2-4. When FC 7917 is installed, the plated adjustable shields are removed and the top  exit enclosure is installed. A newly designed vertical cable management guide (“spine”) can assist with proper cable  management for fiber, copper, and coupling cables. Depending on the configuration, a spine  is present from manufacturing with cable organizer clips installed.  The cable retention clips can be relocated for best usage. All external cabling to the system  (from top or bottom) can use the spines to minimize interference with the PDUs that are  mounted on the sides of the rack.  Figure 2-4 Top exit without and with FC7917 40  IBM z15 (8561) Technical Guide