ISO-TP Protocol: Key Benefits, Features & Limitations

Introduction : ISO-TP provides a robust transport layer over CAN (and CAN-FD) enabling segmented transfer of longer payloads, flow control and addressing modes.

What is ISO-TP

This protocol emerges as a critical component, acting as a transport layer on top of the ubiquitous Controller Area Network (CAN) bus. It fulfills the need of robust and reliable method to transfer large amounts of data between ECUs (Electronic Control Units). Essentially, ISO-TP overcomes the inherent limitation of standard CAN frames, which can only carry a mere 8 bytes of data.

At its core, ISO-TP provides a standardized method for sending data packets that are larger than the payload capacity of a single CAN frame. It achieves this through a process called segmentation and reassembly. These frames are accompanied by specific metadata, known as Protocol Control Information (PCI), which allows the receiving ECU to correctly reassemble the original message. ISO-TP protocol utilizes several frame types to manage the communication process.

• Single Frame (SF): It is used for small messages which can be carried by single CAN frame (Up to 7 bytes of payload).
• First Frame (FF): It carries total length of the message and initial segment of the data.
• Consecutive Frame (CF): After first frame, rest of the frames are sent as consecutive frames with unique sequence numbers so that it will be easy to reassemble them in the correct order.
• Flow Control Frame (FC): This crucial frame is sent by the receiver to the sender. It manages the flow of data by indicating the receiver’s readiness to accept more Consecutive Frames.

Addressing modes

ISO-TP supports different addressing modes to identify the communicating ECUs.

1. Normal addressing : The CAN identifiers themselves are used to distinguish between different ECUs.
2. Extended addressing : In this mode, the first data byte of the CAN frame is used as an additional address extension. This is particularly useful in complex network architectures, such as when communicating through a gateway ECU.
3. Mixed addressing : This is a combination where the CAN ID and an address byte are used.

Advantages of ISO-TP Protocol

Following are some of the benefits of ISO-TP protocol.

1. It can transmit messages significantly larger than the 8-byte CAN frame limit, with the standard allowing for up to 4095 bytes and even larger payloads with newer revisions.
2. As an international standard (ISO 15765-2), it promotes interoperability between diagnostic tools and ECUs from different manufacturers.
3. It offers reliable communication as protocol incorporates flow control mechanisms that prevent the sender from overwhelming the receiver with data. Moreover it uses sequence numbers in consecutive frames which enables detection of lost frames.
4. ISO-TP can be used over both classical CAN and the more modern CAN FD (Flexible Data Rate), which offers a higher data throughput. This adaptability ensures its relevance as vehicle networks evolve.
5. The protocol is designed for one to one communication between a sender and a receiver, which is ideal for diagnostic sessions and ECU reprogramming.

Disadvantages of ISO-TP

Following are some of the limitations of ISO-TP.

1. Inclusion of Protocol Control Information (PCI) in each CAN frame introduces overhead which is noticeable for small messages.
2. There is no mechanism for retransmission of single lost or corrupted frame. If this occurs, entire transmission of multi-frame needs to be aborted or re-initiated.
3. Implementing a full ISO-TP stack can be more complex than simple CAN communication, requiring careful management of state machines for segmentation, reassembly, and flow control.
4. Protocol is mainly designed for unicast i.e. one to one communication. This makes it less suitable for applications requiring efficient broadcast of large data to multiple ECUs simultaneously.
5. The process of segmentation, transmission of multiple frames and flow control can introduce latency, which might be a concern for time critical applications.

Conclusion: A clear grasp of ISO-TP’s strengths and constraints empowers vehicle network architects and diagnostic tool developers to make informed decisions when implementing or analysing transport layer communications. This ensures efficient and reliable data transfer while avoiding potential bottlenecks.