Tone signalling with TFO communication links

Apparatus for transporting tone signalling information through a GCME link in a GSM network supporting TFO and speech compression modes for the passage of signals through the link, respectively using TFO frames and TRAU-like frames, the apparatus comprising:

• a detector (6a), arranged in a data stream to the local entrance to the GCME link, operable to detect a tone signal to be transported,
• a coder which is operable to code the detected tone signal into bits of a TFO or TRAU-like frame for sending through the GCME link, including setting bits to mark the frame as a containing tone signalling information and setting bits to identify the detected tone signal, whereby the tone signalling information is transported in-band through the GCME link as a marked TFO or TRAU-like frame,
• a detector (7a), at the remote exit from the GCME link, operable to detect the marked frame and to regenerate the tone signal, in dependence upon the bits of the marked frame.