com/go/BTTM Broadband Telecommunications Technologies and Management Historical Note Discrete data telecommunications require technologies sim- ilar to analogue signals as data need to be transmitted over a physical medium using electrical currents or electromagnetic waves. Modulation of discrete data onto a carrier signal dates back to electrical telegraphy and on-off keying: a telegraph key (pictured)¹ connects and disconnects the flow of electricity and through this process information is communicated [1]. The word 'key' defines to this day the digital modulation techniques used to transmit discrete signals. Modulation of discrete data onto an electro- magnetic wave carrier was shown by Guglielmo Marconi (pictured) in 1895. His experiments led to the development of the wireless telegraph, and he is famously credited as the inventor of wireless telecommunications. The technique Marconi used was on-off keying through which discrete data modulated a carrier signal. These techniques can be considered a form of ana- logue amplitude modulation where a specific amplitude is used to represent a period of on, and a zero amplitude to represent the period of off. In essence the carrier signal's amplitude is varied to transmit information. Amplitude modulation of discrete signals is known as amplitude shift keying (ASK). In addition to amplitude modulation, and in a similar fashion as analogue signal transmis- sion, discrete information can also be embedded in the phase or the frequency of the carrier electromagnetic wave. Frequency modulation for transmission of discrete data was reported in 1947 in devices developed by Cable and Wireless [2]. The system used a frequency shift keying (FSK) technique for telegraph transmission using two distinct frequencies to commu- nicate periods of mark (on) and space (off). Frequency modulation for data transmission was then reported by Doelz and colleagues at Collins Radio in 1956 following the work of Gilbert on information theory at the Bell Laboratories [3, 4]. Phase modulation fo