await suitable channel conditions in order to be able to inject at least one packet into a datastream on the main channel the datastream being transmitted from the first host 102 to the second host 104 when the first host 102 does not need to communicate with the second host 104 the first host 102 instead of simply remaining inactive during an idle period sends data signifying the idle period in the datastream to the second host 104 in accordance with the ieee 802 3 standard as described in ep a1 1 524 807 the in line sub channel apparatus 110 exploits idle periods on the main channel to support the sub channel turning to fig3 the application logic 114 monitors the internal buffer 207 in order to determine 300 whether the internal buffer 207 has packets containing measurement data to be sent if the internal buffer 207 is empty the application logic 114 continues monitoring the status of the internal buffer 207 referring to fig4 in accordance with a by pass mode an incoming frame of data 400 is received 402 by the in line sub channel apparatus 110 the incoming frame of data 400 is passed to the first multiplexer 204 without further interference and hence delay caused by the application logic 114 whereupon an output frame 403 constituting an unmodified version of the incoming frame of data 400 is sent by the in line sub channel apparatus 110 for receipt 404 by the second host 104 referring back to fig3 in the event that the.