Hi all I am trying to read RX/TX data from RJ45 cable, but without success. So my question is how can I read with arduino the actual data from this cable that connect us to internet ?
You need an Ethernet shield. The data from an RJ45 is not serial data that can be read directly by the arduino.
Ok can I send it somehow without using ethernet shield ?
Ethernet is a complex layered protocol.
You need to undersand the protocol stack which is explained by the OSI model: http://en.wikipedia.org/wiki/OSI_model
The physical layer is a high speed differential pair. Overview: http://en.wikipedia.org/wiki/Ethernet_physical_layer
The 8-bit microcontrollers used on Arduino boards simply were not designed to act directly as ethernet devices. You need a PHY chip like those on the Ethernet Shield.
mitko290: Ok can I send it somehow without using ethernet shield ?
To summarise the advice given by James No.