After talking and understanding in a precedent article the variables of ERTMS/ETCS language that this language is based on it in addition to packets, telegrams, and messages. Now, it's possible to talk about PACKETS as it's essential to understand variables to evoke it.


Definition of Packets

Packets are " Packets are multiple variables grouped into a single unit, with a defined internal structure." as defined in §7.3.3.1, chapter 7 of the System Requirements Specifications (SRS) 026.

So a good knowledge of variables, their structure, and rules is mandatory to understand packets.


Packets Structures

There are two structures of packets whether the information is transmitted from train to track or the opposite. Both of them start with a packet header (NID_PACKET) followed by:

  • For Track to Train transmission:
    • A unique packet number
    • Length of the packet in bits
    • The orientation information
    • Optionally: 
      • The distance scale and 
      • Information section containing a defined set of variables

Track to Train Packet Structure
Track to Train Packet Structure

  • For Track to Train transmission:
    • A unique packet number
    • Length of the packet in bits
    • Optionally: 
      • The distance scale 
      • Information section containing a defined set of variables

Train to Track Packet Structure
Train to Track Packet Structure



Lists of Packets

There are 55 packets for Track to Train transmission and 8 packs for Train to Track transmission. These two lists are defined in chapter 7 of the SRS 026. Each list figure in the picture below:


Track to Train Packets
Track to Train Packets



Train To Track Packets
Train To Track Packets


How to find and read a packet?

Each packet in the two pictures above is associated with a page number. The latter is where you can find the related packet in the SRS 026 - Chapter.

Example: 
For packet number 5 (Linking) in the list of Track to Train, the page number associated is 15.
On page 15 of SRS 026 Chapter 7, packet number 5 (Linking) appear:


Find packet
Find packet




To find out more about any variable on it, all variables are defined in §7.5 Definitions of Variables (as of page 44) in the same above-mentioned document:


Example:

Find variable
Find variable






Source:
System Requirements Specification - Chapter 7 ERTMS/ETCS language