Infrared, or IR, is the the portion of the electromagnetic spectrum between 0.750 and 1000 micrometers. The term infrared literally means "below red", reference to the spectrum's location just below the 0.700 micrometers that the human eye perceives as red. This super-wide spectrum is important because of thermal imaging, investigation, and communications applications that depend upon it. Common thermal imaging applications include meteorology, astronomy, and military imaging in which the temperature of distant objects can be determined. Historical, criminal, and medical investigations often use infrared cameras to determine the composition of materials, aiding in the detection of forgeries and diseases. Wireless communication is often done over infrared in applications where cables are impractical, such as television remote controls and older PDA's and cellphones.
Short range wireless communication is usually accomplished via standards set forth by the Infrared Data Association, or IrDA. The IrDA standards are broken down into protocol layers similar to TCP/IP. The lowest layer, IrPHY (IR physical), specifies hardware and physical properties of infrared communications equipment. IrPHY requires compliant devices to transmit a beam capable of detection at a minimum 1 meter distance and 15 degrees offset. As the bright transmitters required by the protocol would blind IR receivers in most practical applications, IrPHY makes no provision for full duplex communication. The second protocol layer, IrLAP (IR Link Access Protocol), defines a primary device and a secondary device in one-on-one infrared communications. This layer allows simulated full duplex communication by having the devices synchronize alternating transmission and receiving times. The topmost layer of IrDA specifications is IrLMP, or IR Link Management Protocol. This layer defines device types and services, as well as transmission channels for the actual data transmission. The IrLMP layer is where different devices are identified and perform their main functions.
In addition to the three base protocol layers defined by the IrDA, there are several optional layers that provide enhanced functionality and speed for certain common applications. IrCOMM is the standard for two way communications between smart devices such as computers and PDA's. IrCOMM simulates a virtual serial port over infrared, thus facilitating operations such as printing and data synchronization. Tiny Transport Protocol, or TinyTP, allows splitting of data similar to IP packets, thus allowing fast transmission of large amounts of data over several of the available IR channels. This provides a base layer to other high-level protocols such as IrOBEX and IrLAN. IrOBEX is used for transmitting data only, such as files or software. IrLAN provides conduits for transmitting control packets, thus allowing compatible devices to participate in a LAN or PAN network.
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