Controller Area Network- Carrier Sense

Carrier Sense -

• Carrier here is nothing but a CAN Bus. Carrier sense is to check the bus before transmitting anything on the Bus.

• CAN Controller checks if the bus is Idle.

• It starts transmission only if the bus is idle.

• If the bus us not idle then it will wait for current transmission to end.

• Consider a Scenario when CAN Node wants to send a message on the CAN bus.

Refer below link to see a Video to visualize it.

Comments

What is UDS on CAN or Diagnostic on CAN ?

OSI Model and Diagnostics : OSI model has 7 layers and each layer has defined functionality. To understand this you can refer this excellent IEEE publication. It explains OSI Model for CAN , Ethernet, flexray and MOST. External reference : http://www.ieee802.org/1/files/public/docs2013/new-tsn-diarra-osi-layers-in-automotive-networks-0313-v01.pdf So let us understand, what is this UDS on CAN ? When a Diagnostic Message formatted as per ISO 14229 -1 i.e UDS Standard is sent over Controller Network we say Diagnostics Communication is happening as per UDS on CAN. So as per this : Below layers participate in Diagnostic Communication Over CAN : 1. Application Layer : ISO 14229-1:2013 ROAD VEHICLES — UNIFIED DIAGNOSTIC SERVICES (UDS) — PART 1: SPECIFICATION AND REQUIREMENTS ISO 14229-3:2012 ROAD VEHICLES — UNIFIED DIAGNOSTIC SERVICES (UDS) — PART 3: UNIFIED DIAGNOSTIC SERVICES ON CAN IMPLEMENTATION (UDSON...

Diagnostic Session Function 3 - Security and authorization of the user – Use of Seed and Key for authenticating -

Let us see how the function of Security Access is served by Diagnostic Session. Security is an important aspect of an UDS implementation. It is required to avoid unauthorized access to ECU. As one can control ECU functionality through usage of UDS services, it becomes vital to secure ECU from unintended or vulnerable access. Hence UDS has provisioned a special service called Security Access SID 0x27 to authorize a user.It works on the principle of Seed and key mechanism. You can get lot many websites explaining how security service works. The intention of this write up is to know how session is important for the implementation of security of an ECU. As mentioned in the blog (https://thevehiclediagnostics.blogspot.com/2020/01/what-is-diagnostic-session.html ) a diagnostic session of an ECU is a timed window. Session management helps in following manner for security 1 . Limit a secured access for timed window i.e session. 2 . End se...

Diagnostic Session : Function 5 - Session Oriented Access to Services

There are multiple users those need to use UDS protocol for various purposes like Garage person for fault detection, ECU manufacture for Testing activities and development purpose. Also OEM needs access to ECU memory for flashing purposes. UDS services can be categorized in different groups like few services will just read some Data from memory,a few will write or update the data to memory, some will erase/update ECU sw itself , some will control Inputs outs of ECU directly. So there are chances that any improper usage of UDS service may result in safety issues , security breaches or damage to ECU resources. Hence, it doesn't make sense to give access to all services to everyone. To achieve this UDS has designed some mechanisms like Sessions, Security Authentication, Preconditions. I hope you are aware about what a Diagnostic Session is. Please visit : https://thevehiclediagnostics.blogspot.com/2020/01/what-is-diagnostic-session.html if you ...

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