A communication system, comprising: an optical fibre along which radiation can be transmitted; a light source for generating radiation to be transmitted along the optical fibre, wherein the light source is a swept-wavelength laser or the light source is configured to generate broadband radiation; a first node comprising: a first optical fibre grating formed within the optical fibre, the first optical fibre grating having a structure, and being configured to reflect radiation at a particular wavelength; and a first instrument coupled to the first optical fibre grating and configured to receive a data signal and controllably modify the structure of the first optical fibre grating in response to the received data signal, thereby changing the wavelength at which the first optical fibre grating reflects radiation, to encode a signal in the radiation at the particular wavelength in response to the received data signal; a second node comprising: a second optical fibre grating formed within the optical fibre, the second optical fibre grating having a structure, and being configured to reflect radiation at a particular wavelength; and a second instrument coupled to the second optical fibre grating and configured to receive a data signal and controllably modify the structure of the second optical fibre grating in response to the received data signal, thereby changing the wavelength at which the second optical fibre grating reflects radiation, to encode a signal in the radiation at the particular wavelength in response to the received data signal; and a receiver configured to receive first radiation reflected by the first optical fibre grating and second radiation reflected by the second optical fibre grating, wherein the light source and the receiver are located at a first end of the optical fibre, and wherein the receiver, or a computer or processor associated with the receiver, is programmed with threshold values of wavelengths or discrete bands of wavelengths so that, if a signal is received from the first or second node above a particular threshold value, or falling within a particular wavelength band, it can be interpreted accordingly. an optical fibre along which radiation can be transmitted; a light source for generating radiation to be transmitted along the optical fibre, wherein the light source is a swept-wavelength laser or the light source is configured to generate broadband radiation; a first node comprising: a first optical fibre grating formed within the optical fibre, the first optical fibre grating having a structure, and being configured to reflect radiation at a particular wavelength; and a first instrument coupled to the first optical fibre grating and configured to receive a data signal and controllably modify the structure of the first optical fibre grating in response to the received data signal, thereby changing the wavelength at which the first optical fibre grating reflects radiation, to encode a signal in the radiation at the particular wavelength in response to the received data signal; a first optical fibre grating formed within the optical fibre, the first optical fibre grating having a structure, and being configured to reflect radiation at a particular wavelength; and a first instrument coupled to the first optical fibre grating and configured to receive a data signal and controllably modify the structure of the first optical fibre grating in response to the received data signal, thereby changing the wavelength at which the first optical fibre grating reflects radiation, to encode a signal in the radiation at the particular wavelength in response to the received data signal; a second node comprising: a second optical fibre grating formed within the optical fibre, the second optical fibre grating having a structure, and being configured to reflect radiation at a particular wavelength; and a second instrument coupled to the second optical fibre grating and configured to receive a data signal and controllably modify the structure of the second optical fibre grating in response to the received data signal, thereby changing the wavelength at which the second optical fibre grating reflects radiation, to encode a signal in the radiation at the particular wavelength in response to the received data signal; and a second optical fibre grating formed within the optical fibre, the second optical fibre grating having a structure, and being configured to reflect radiation at a particular wavelength; and a second instrument coupled to the second optical fibre grating and configured to receive a data signal and controllably modify the structure of the second optical fibre grating in response to the received data signal, thereby changing the wavelength at which the second optical fibre grating reflects radiation, to encode a signal in the radiation at the particular wavelength in response to the received data signal; and a receiver configured to receive first radiation reflected by the first optical fibre grating and second radiation reflected by the second optical fibre grating, wherein the light source and the receiver are located at a first end of the optical fibre, and wherein the receiver, or a computer or processor associated with the receiver, is programmed with threshold values of wavelengths or discrete bands of wavelengths so that, if a signal is received from the first or second node above a particular threshold value, or falling within a particular wavelength band, it can be interpreted accordingly.