DOWNHOLE COMMUNICATION
A downhole system (10) comprises a tool (16) suspended on a reelable support (18). The support (18) has an optical fibre and the tool communicates with a surface controller via the fibre. The tool (16) has a number of units (24, 16a, 16b, 16c), the unit (24) functioning as a router to transmit and receive signals to and from the other units (16a, 16b, 16c) and the surface controller via the optical fibre. Each of the units (24, 16a, 16b, 16c) is assigned a unique address which permits secure communication between each unit (24,16a, 16b, 16c) and the controller over the fibre.