The cable for the BRI can contain as many as eight conductors. However, four of these are optional. They are organised in pairs like this.

Notice how there is one pair of conductors which carry data away from the devices toward the NT1 and another pair which carry data from the NT1 toward the devices.

The red lines show the optional pairs. These are concerned with providing power to devices connected to the BRI. This is a convenience for an ISDN telephone, that, for example, has an LCD and LEDs. This therefore needs an external power supply which can be supplied through its ISDN interface rather than through a separate mains power adapter.