s transmission formats using RZI, 4-PPM or HHH modulation methods. The Ir pulse durations are nominally 25% of the bit or symbol duration. All transmissions start at the lowest data speed of 9.6 kbps in order to ensure interoperability, and higher data rates are negotiated, depending on the capabilities of the communicating devices, as part of the process of establishing the link. Data Link Layer IrDA's two mandatory protocols, the link access (IrLAP) and link management (IrLMP) protocols, are Data Link (Layer 2) protocols in terms of the OSI model (Figure 10-11). Clock 16 X bit rate Data bits (NRZ) Return to zero inverted (RZI) Bit period 7 off 6 off 16 clock cycles per bit period Figure 10-12: IR Pulse Shortening (SIR RZI Modulation) Chapter Ten IrLAP establishes the device to device connection, controlling the discovery and addressing of devices within range and establishing the best common data transmission rate. On discovery, devices randomly choose and exchange 32-bit IrLAP addresses. The devices in an IrLAP connection have a master-slave relationship with the master responsible for sending command frames, initiating connections and transfers, organising and controlling the flow of data, including dealing with data link errors. The slave device sends response frames, responding to the commands and requests of the master device. Once the IrLAP connection is started, media access is controlled by time division (TDMA) with master and slave taking alternate 500 millisecond time slots. The distinction between master and slave devices is relevant at the Data Link level. However, at the application level, once a connection is made between two devices, an application on a slave device can initiate an operation on the master device just as easily as the other way round. IrLMP, the link management protocol, multiplexes services and applications on the connection established by IrLAP. IrLMP also handles address conflict resolution in the case of a new device being discovered which requests the same IrLAP address. IrDA