A system for transmission of data from a source device to a destination device over a transmission link, the system comprising a transmit side and a receive side, wherein the transmit side comprises: a transmitter for transmitting user data over the transmission link, the transmitter being configured to transmit the user data in a format in which the user data is adapted to suit the properties of the transmission link; and a breakout device for receiving at an input thereof a data stream in a tunnelled format containing the user data to be transmitted, the breakout device being configured to extract the user data from the input tunnelled data stream and to output the extracted user data to the transmitter for transmission in the adapted format, and wherein the receive side comprises: a receiver for receiving the user data transmitted by the transmitter; and a capture device for receiving at an input thereof the user data received by the receiver, the capture device being configured to re-format the received user data into an output data stream in the tunnelled format, such that the output data stream resembles the input data stream input to the breakout device, characterised in that: the breakout device is further configured to modify the extracted user data to cause the extracted user data to be routed to the capture device; the capture device is configured to modify the user data received at its input to cause the received user data to be routed to its original destination; and/ or in that the breakout device is configured to generate a unique data stream identifier for a stream of user data received by the breakout device, and to replace information in the user data identifying a destination port with the unique data stream identifier for the stream of user data. a transmitter for transmitting user data over the transmission link, the transmitter being configured to transmit the user data in a format in which the user data is adapted to suit the properties of the transmission link; and a breakout device for receiving at an input thereof a data stream in a tunnelled format containing the user data to be transmitted, the breakout device being configured to extract the user data from the input tunnelled data stream and to output the extracted user data to the transmitter for transmission in the adapted format, and wherein the receive side comprises: a receiver for receiving the user data transmitted by the transmitter; and a capture device for receiving at an input thereof the user data received by the receiver, the capture device being configured to re-format the received user data into an output data stream in the tunnelled format, such that the output data stream resembles the input data stream input to the breakout device, characterised in that: the breakout device is further configured to modify the extracted user data to cause the extracted user data to be routed to the capture device; the capture device is configured to modify the user data received at its input to cause the received user data to be routed to its original destination; and/ or in that the breakout device is configured to generate a unique data stream identifier for a stream of user data received by the breakout device, and to replace information in the user data identifying a destination port with the unique data stream identifier for the stream of user data. a receiver for receiving the user data transmitted by the transmitter; and a capture device for receiving at an input thereof the user data received by the receiver, the capture device being configured to re-format the received user data into an output data stream in the tunnelled format, such that the output data stream resembles the input data stream input to the breakout device, characterised in that: the breakout device is further configured to modify the extracted user data to cause the extracted user data to be routed to the capture device; the capture device is configured to modify the user data received at its input to cause the received user data to be routed to its original destination; and/ or in that the breakout device is configured to generate a unique data stream identifier for a stream of user data received by the breakout device, and to replace information in the user data identifying a destination port with the unique data stream identifier for the stream of user data. the breakout device is further configured to modify the extracted user data to cause the extracted user data to be routed to the capture device; the capture device is configured to modify the user data received at its input to cause the received user data to be routed to its original destination; and/ or in that the breakout device is configured to generate a unique data stream identifier for a stream of user data received by the breakout device, and to replace information in the user data identifying a destination port with the unique data stream identifier for the stream of user data.