Cal. Code Regs. tit. 21 § 1704.5
The transponder-to-reader (uplink) modulation scheme shall be amplitude modulation of an RF carrier backscatter created by varying the reflecting crossection of the antenna as seen by the incident carrier signal. The antenna crossection shall be varied between upper and lower limits with a 50 percent duty cycle and rise and fall times of less than 75 nanoseconds. The transponder baseband message signal shall modulate the subcarrier using FSK modulation with a center frequency of 900 kHz and frequency deviation of ± 300 kHz. The lower and upper subcarrier frequencies correspond to data bits '0' and '1' respectively. The message information is conveyed by the subcarrier modulation frequencies of the transponder backscattered signal and not by amplitude or phase.
The data bit rate for transponder-to-reader data messages shall be 300 kbps.
The field strength at which a transponder data message is transmitted using backscatter technology is dependent upon the incident field strength from the reader, the transponder receive and transmit antenna gains, and any RF gain internal to the transponder. The transponder and antenna gain taken together shall effect a change in the backscattering cross section of between 45 and 100 square centimeters.
The standard portion of a transponder data message shall consist of a header and transaction record type code. The subsequent length, data content, and error detection scheme shall then be established by the definition for that transaction record type.
There may be numerous transponder-to-reader data message formats. The format is determined by the transaction record type code sent by the transponder. The following is the reader-to-transponder message format presently specified for AVI electronic toll collection applications:
Transponder transaction type 1 (data message) allows for unencrypted transponder ID numbers to be transmitted. Type 1 (data messages) shall be structured using the following ordered data bit fields:
| Field Definition | No. Bits | Hexadecimal Value | ||
| Header Code | ||||
| Selsyn | 8 | AA | ||
| Flag | 4 | C | ||
| Transaction Record Type Code | 16 | 1 | ||
| Transponder ID Number | 32 | |||
| Error Detection Code | 16 | |||
| Total: 76 | ||||
The End-of-Message signal for transponder data messages shall consist of a minimum of 10 microseconds with no modulation.
Cal. Code Regs. Tit. 21, § 1704.5
2. Amendment of subsections (d)-(f) filed 5-1-98; operative 5-31-98 (Register 98, No. 18).
3. Change without regulatory effect amending subsection (e)(1) filed 7-10-98 pursuant to section 100, title 1, California Code of Regulations (Register 98, No. 28).
4. Renumbering of former section 1704.5 to section 1703.5 and renumbering of former section 1705.5 to section 1704.5, including amendment of subsection (f), filed 11-28-2017; operative 1-1-2018 (Register 2017, No. 48).
Note: Authority cited: Section 27565, Streets and Highways Code. Reference: Sections 27564 and 27565, Streets and Highways Code.
2. Amendment of subsections (d)-(f) filed 5-1-98; operative 5-31-98 (Register 98, No. 18).
3. Change without regulatory effect amending subsection (e)(1) filed 7-10-98 pursuant to section 100, title 1, California Code of Regulations (Register 98, No. 28).
4. Renumbering of former section 1704.5 to section 1703.5 and renumbering of former section 1705.5 to section 1704.5, including amendment of subsection (f), filed 11-28-2017; operative 1/1/2018 (Register 2017, No. 48).