D-Link KX-TDA100 - Manual (Page 58)

2.2 Installation of the Hybrid IP-PBX
58 Installation Manual
Note
For every 4 cables, use 1 ferrite core; a ferrite core holds a maximum of 4 cables.
When connecting an extension card
Pass the cable through the ferrite core, then close the case of the ferrite core. Attach the ferrite core 3 cm
away from the connector.
If you need to open the ferrite core, use a flathead screwdriver to unlatch the case of the ferrite core.
3 cm