Infrared (IR) Communication

CSE 477 Winter 1999Introduction2/44 Light Spectrum Implementation costs rise significantly around 1-10 GHz. (But one important exception is IR at around 500 THz ; very inexpensive.) Signals above 100 GHz cannot penetrate walls Most signals below 300 GHz are regulated by the FCC Radio (RF) Microwaves Infrared (IR) Visible Ultraviolet X-RaysGamma Rays FCC $ Freq. (Hz)

CSE 477 Winter 1999Introduction3/44 How do you Transmit with IR No conductor No conductor –Signal spreads from origin –Signal strength decreases with distance Can’t use amplitude Can’t use amplitude –Too much noise from other IR sources (lights, people, etc.) Can’t use sign (no +/-) Can’t use sign (no +/-) Solution: Modulation Solution: Modulation –Send pulses of light to represent binary information

CSE 477 Winter 1999Introduction4/44 Modulation Schemes SIR - Serial Infrared SIR - Serial Infrared –2400bps to 115,200bps –SIR modem is simple and low-cost SDLC-based MIR SDLC-based MIR –576Kbps and 1.152Mbps –guarantees a minimal occurrence rate of light pulses FIR - Fast Infrared FIR - Fast Infrared –4Mbps –power usage constant (always the same)

CSE 477 Winter 1999Introduction5/44 SIR - Serial Infrared Modulation 1.6us (or 3/16 bit interval) pulse of light for each ‘0’ in a standard asynchronous data stream 1.6us (or 3/16 bit interval) pulse of light for each ‘0’ in a standard asynchronous data stream –Inverse of serial RS-232 where signal is held high, then pulled down for zeros Stream consists of a start bit, N data bits, and 1 stop bit (serial packet) Stream consists of a start bit, N data bits, and 1 stop bit (serial packet)

CSE 477 Winter 1999Introduction6/44 SDLC-based MIR A data stream ‘0’ is coded for by the presence of an optical pulse 1/4 of the bit interval in duration (217ns for 1.152Mbps) A data stream ‘0’ is coded for by the presence of an optical pulse 1/4 of the bit interval in duration (217ns for 1.152Mbps) The SDLC protocol, with its zero-insertion bit- stuffing approach, guarantees a minimal occurrence rate of zeroes (and therefore light pulses) in the data stream The SDLC protocol, with its zero-insertion bit- stuffing approach, guarantees a minimal occurrence rate of zeroes (and therefore light pulses) in the data stream This also ensures synchronization between a transmitter and receiver can be maintained throughout a packet This also ensures synchronization between a transmitter and receiver can be maintained throughout a packet

CSE 477 Winter 1999Introduction7/44 FIR - Fast Infrared More Ethernet-like in its framing More Ethernet-like in its framing –a packet is made up of a preamble, start of frame delimiter (SFD) and a data payload. Utilizes 1:4 Pulse Position Modulation (PPM) Utilizes 1:4 Pulse Position Modulation (PPM) –each pair of bits in the data stream are represented by a pulse of light emitted in one of 4 available slot positions which comprise a 4PPM symbol PPM always requires a pulse for each slot, so power usage is independent of the data PPM always requires a pulse for each slot, so power usage is independent of the data

CSE 477 Winter 1999Introduction8/44 IR Communication Protocols To use IR hardware easily, we build up layers of protocol that get us farther away from the implementation each step. To use IR hardware easily, we build up layers of protocol that get us farther away from the implementation each step. Common Protocols: Common Protocols: –Serial IR –IrDA

CSE 477 Winter 1999Introduction9/44 Differences between Waves and Wire Distance/$$$ Distance/$$$ –Wave transmissions are much more expensive than wire Speed Speed –Wire is a very simple and highly controllable medium which allows for much higher transfer rates Limitations Limitations –IR: Line of Sight - IR can not travel through opaque objects, so transceivers must be able to see each other. For many commercial products they must be within 15º of each other.Line of Sight - IR can not travel through opaque objects, so transceivers must be able to see each other. For many commercial products they must be within 15º of each other. Signal Power - even though we can not see IR, making the signal too strong can easily blind usSignal Power - even though we can not see IR, making the signal too strong can easily blind us –RF: FCC RegulationFCC Regulation Interference with objects and other RF wavesInterference with objects and other RF waves