PJDLR v3.0

Invented by Giovanni Blu Mitolo
Originally published: 10/04/2010
Latest revision: 07/07/2019
Related implementation: /src/strategies/OverSampling/
Compliant versions: PJON v12.0 and following
Released into the public domain

10/04/2010 v0.1 - First experimental release
18/01/2017 v1.0 - Packet preamble
31/03/2017 v1.1 - Response info
31/10/2018 v2.0 - Frame initializer, mode 1
07/07/2019 v3.0 - Response made safe, response initializer

PJDLR (Padded Jittering Data Link over Radio) is an asynchronous serial data link for low-data-rate applications that supports both master-slave and multi-master communication and it is optimized to obtain long range and high reliability using ASK, FSK or OOK radio transceivers. PJDLR can be easily implemented on limited microcontrollers with low clock accuracy and can operate directly using one or two input-output pins.

Communication modes

The proposed communication mode is the result of years of testing and optimization for ASK/FSK radio transceivers and have been selected to be easily supported also by low quality hardware.

MODE Bit timing Sync bit timing Pad-data ratio Speed
1 512 328 0.64 202B/s - 1620Bd

Binary timing durations are expressed in microseconds.

Medium access control

PJDLR specifies a variation of the carrier-sense, non-persistent random multiple access method (non-persistent CSMA). Devices can detect an ongoing transmission for this reason collisions can only occur in multi-master mode when 2 or more devices start to transmit at the same time. When a collision occurs it can be detected by the receiver because of synchronization loss or by the transmitter if an active collision avoidance procedure is implemented.

Byte transmission

Byte transmission is composed by 10 bits, the first two are called synchronization pad and are used to obtain sampling synchronization. The synchronization pad is composed by a high padding bit shorter than data bits and a low data bit. The following 8 data bits contain information in LSB-first (least significant bit first) order.

The reception technique is based on 3 steps:

  1. Find a high bit which duration is equal to or acceptably shorter than a high padding bit
  2. Synchronize to its falling edge
  3. Ensure it is followed by a low data bit

If so reception starts, if not, interference, synchronization loss or simply absence of communication is detected.

 _____ ___________________________
| Pad | Byte                      |
|_    |___       ___     _____    |
| |   |   |     |   |   |     |   |
|1| 0 | 1 | 0 0 | 1 | 0 | 1 1 | 0 |

Frame transmission

Before a frame transmission, the communication medium's state is analysed, if high communication is detected and collision is avoided, if low for a duration that is longer than the response time-out plus a small random time, frame transmission starts with a frame initializer composed by 3 consequent synchronization pads followed by data bytes. The presence of the synchronization pad between each byte ensures that also a frame composed of a series of bytes with decimal value 0 can be transmitted safely without risk of collision.

 ___________ __________ _______________ ______________
|Pad|Pad|Pad| Byte     |Pad| Byte      |Pad| Byte     |
|_  |_  |_  |     __   |_  |      _   _|_  |      _   |
| | | | | | |    |  |  | | |     | | | | | |     | |  |

When a frame is received a low performance microcontroller with an inaccurate clock can correctly synchronize with transmitter during the frame initializer and consequently each byte is received. The frame initializer is detected if 3 synchronization pads occurred and if their duration is coherent with its expected duration. Frame initialization is 100% reliable, false positives can only occur because of externally induced interference.

Synchronous response

A frame transmission can be optionally followed by a synchronous response sent by its recipient. Between frame transmission and a synchronous response there is a variable time which duration is influenced by latency.

Transmission end                                   Response
 ______  ______  ______                             _____
|      ||      ||      |                           |  6  |
|______||______||______|                           |_____|

In order to avoid other devices to detect the medium free for use and disrupt an ongoing exchange, the sender cyclically transmits a short high bit (1/2 high padding bit duration) and consequently attempts to receive a response. The receiver can transmit its response as soon as possible, and, in order to avoid false positives in case of collision, must transmit its response prepended with an additional synchronization pulse. If the response is not transmitted or not received the transmitter continues to keep busy the medium up to the maximum acceptable time between transmission and response.

Transmission end                                    Response
 ______  ______  ______   _   _   _   _   _   _ ____ _____
| BYTE || BYTE || BYTE | | | | | | | | | | | | |SYNC| ACK |
|------||------||------| | | | | | | | | | | | |----|-----|
|      ||      ||      | | | | | | | | | | | | |    |  6  |
|______||______||______|_| |_| |_| |_| |_| |_| |____|_____|

The maximum time dedicated to potential response reception for a given application can be determined practically transmitting the longest supported frame with the farthest physical distance between the two devices. The highest interval between packet transmission and response measured plus a small margin is the correct timeout that should exclude response losses. Consider that the longer this timeout is, the more bandwidth is wasted if the transmission is not successful.

This document is automatically generated from the github repository. If you have noticed an error or an inconsistency, please report it opening an issue here

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