(a) Agency Code: This 16-bit code field identifies the agency that has authority to conduct the transaction.

(b) Byte Order: Numeric fields shall be transmitted by the most significant bit first. If a numeric field is represented as multiple bytes, the most significant bit of the most significant byte is transmitted first. This document represents the most significant and first transmitted to the left on a line and to the top of a multi line tabulation.

(c) Error Detection Code: The error detection code utilized in the defined records is the CRC-CCITT, with a generator polynomial of X¹⁶+X¹²+X⁵+1. This results in a 16-bit BCC transmitted with each data message. The data field protected by the CRC excludes any preceding header in every case.

(d) Filler Bits: Filler bits are used to adjust the data message length to a desired length and shall be set to zero.

(e) Header Code: The header is the first field in each data message for either reader or transponder transmissions and consists of an 8-bit and a 4-bit word for a total of 12 bits. The header provides a signal that may be used by a receiver to self-synchronize (selsyn) with the data being transmitted, thus the notation selsyn. The selsyn signal has binary and hexadecimal values: 10101010 and AA, respectively.

The header code also provides for a unique, 4 bit flag that is recognized by a receiver decoder as the end of the header with the data message to follow. The flag signal has binary and hexadecimal values: 1100 and C respectively.

(f) Reader ID Number: This 32-bit field is used to uniquely identify the reader conducting the transaction.

(g) Transaction Record Type Code: This 16-bit code uniquely identified a specific type of valid transaction between a reader and a transponder. This code uniquely defines the transponder message fields and functions permissible with the transaction type specified by the polling message as described in Section 1703.5(e)(1). Hexadecimal numbers 1 through 7FFF are set aside for transponder message structures and 8000 through FFFF are dedicated for reader-to-transponder message structures.

(h) Transaction Status Code: Used to provide status information to the transponder.

(i) Transponder ID Number: This 32-bit code uniquely identifies which transponder is responding to a polling request or is being acknowledged.