 an elegant process to resolve collisions which will occur when several targets respond at the same time, particularly when targets are responding in passive mode (Figure 10-14). Collision detection at the bit level is made possible by the use of Manchester coding, since a collision is detected when a full bit period occurs without a transition being sensed. This can only occur when a 1-bit transmitted by one target collides with a 0-bit transmitted by another target. Bits received before the collision can be recovered and the targets are requested to re-send data starting with the unrecovered bit. A random delay used by responding targets ensures that this process does not get stuck in a repeating loop. The data link between devices is transaction based, with initiation and termination occurring around a single data transfer. Initiator and Target negotiate a communication speed, starting with the lowest (106 kbps), in a parameter selection step during transport protocol initiation. Data bits (Responder 1) Random retransmit delay Data bits (Responder 2) Random retransmit delay Combined response Manchester coded Detected bits (Initiator) First two bits Initiator transmits Next two bits Resend successfully Collision resend request successfully Collision request detected detected detected detected Figure 10-14: NFC Collision Detection with Multiple Responding Targets Wireless PAN Standards NFC in Practice Four basic NFC usage models are currently envisaged, as shown in Table 10-20. Apart from these usage models in which the NFC connection is used to transfer end-user data, NFC can also be used to securely initiate another connection between two NFC enabled devices. For example, NFC Table 10-20: NFC Usage Models Usage model Description Example Touch and go The user brings the device You see a poster advertising an storing a ticket or access event such as a concert you code close to the reader for want to attend. Bring your applications such as event or PDA or mobile phone near the transport ticketing and access poste