JANUS Baseline Bit Allocation Table

Version 0.2 (20230323)

JANUS Baseline Bit Allocation Table (64 bit total)


The field names are detailed in the following table:

JANUS Bits	1..4	5	6	7	8	9..16	17..22	23..56	57..64
Descriptor	Version Number	Mobility Flag	Schedule Flag	Tx/Rx Flag	Forwarding Capability	Class User ID	Application Type	Application Data Block	CRC8
Bits per Field	4	1	1	1	1	8	6	34	8

The row JANUS Bits refers to transmission order. JANUS follows the Most Significant Bit First (MSBF) transmission ordering scheme (the MSB is the first to be transmitted). Binary numbers are written left to right, with the Least Significant Bit (LSB) being the right-most bit on the table above.
E.g.: the "Version Number" field, where the JANUS version decimal 3 is coded using the 4 bits as 0b0011, will be transmitted as the bit sequence 0 0 1 1.

1st bit: 0
2nd bit: 0
3rd bit: 1
4th bit: 1

The same rationale applies to all fields.

If the Schedule Flag is set to 1, the JANUS standard subdivides the Application Data Block (ADB) 34 bits as follows:

JANUS Bits	23	24..30	31..56
ADB Field Bit #	33	32..26	25..0
Descriptor	Reservation Time/ Repeat Interval Flag	Reservation Time/ Repeat Interval	User Defined
Bits per Field	1	7	26

The Reservation Time/ Repeat Interval field contents are selected via the Reservation Time/ Repeat Interval Flag field according to:

Reservation Time/ Repeat Interval Flag	Reservation Time/ Repeat Interval field contents
0	Reservation Time (see note "Reservation and cargo bits" below)
1	Repeat Interval

The time values are specified in separate look-up tables in the ANEP-87.

__Note:_

ADB Field Bit #:

The bit numbering ADB Field Bit # is included to aid developers since it is a more common bit numbering scheme in Computer Science.

Reservation and cargo bits:

According to ANEP-87, the reservation time can be used to request silence on the acoustic channel after the transmission of a JANUS baseline packet without cargo. However, when cargo is used:
- the schedule flag bit (bit 6 of the JANUS baseline packet) must be set to 1;
- the Reservation Time/ Repeat Interval Flag bit (bit 23 of the JANUS baseline packet) must be set to 0, indicating reservation;
- the reservation bits (seven bits: 24-30 of the JANUS baseline packet) contain the reservation time, which must be long enough to at least cover the transmission time of the cargo bits.

For a given duration of the reservation time [T_res] in seconds, a maximum number of cargo bits [n] can be transferred depending on the value of the Frequency Slot width ([FSw], which is 160 for ANEP-87 Edition A Version 1) and of the transmitted Chirp duration (or dyadic multiplier [delta] – as defined in 2.1.7.3 of ANEP87). The maximum number of cargo bits to be transmitted can be computed as:


When computing the cargo size in bytes, the value [n] must be divided by 8, taking the largest integer value less than or equal to (n/8).

For example, if T_res = 1.15 and delta = 1:
- the maximum number of cargo bits that can be transmitted is 84
- the maximum number of cargo bytes that can be transmitted is 10

Revision History

Date	Version	Description	Authors
2023/03	0.2	Added information for reservation time and cargo bits	NATO STO CMRE