Once a system definition is completed the transmitted packet can be defined. A packet is what is actually sent over the air. It consists of the data that is to be transferred combined with other information to protect the data and inform the receiver of other necessary or useful information. A packet has three main sections: the start code, data payload and error check. The start code is a way of setting up, synchronizing the receiver, and identifying the start of the packet. This distinguishes the packet from random noise and packets from other systems. The data payload is the actual data to be transferred. This is the sensor reading, the file, or whatever information is required by the application. Depending on the application other information may be included in the packet. Some common fields are payload length, hop counter for network systems, and node addresses. For the sake of this discussion, all of this type of information is lumped into the data payload. Last is an error check, a method of making sure that the received bits are correct and have not been corrupted.