Optical transmission takes place on the IrDA layer. SIR specifies the data interface with its parameters, coding, etc. The prerequisite for IR transmissions is a line-of-sight connection between the transmitting and receiving devices. The wavelength range is between 850 nm and 900 nm, with a range of 3 m and a maximum radiation angle of 30 degrees. The transmission speed of Serial Infrared (SIR) is between 9.6 kbit/s and 115.2 kbit/s.