Page 1026 - 5G Basics - Core Network Aspects

2 Transport aspects

Figure S.6 – Busy response handling

The nested state machines in the DPU and NT can conceivably get out of step in a number of unspecified
ways; nor is it specified how to escape from a loop of transmission failure and retry. As a recovery
mechanism from detectable state errors, it is recommended that the NT reply with command processing
error result codes to both the acknowledged download section and end software download commands,
and that the DPU send a final end software download command with a known bad CRC and image size (e.g.,
all 0), whereupon both the DPU and NT reset to the state in which no download is in progress, that is, state
S1/S1' of Figure S.3. Likewise, the DPU can abort the download operation at any time by sending an end
software download message with invalid CRC and image size.

S.6.3 Software image activate and commit
Figure S.7 shows the details of software image activate and commit. When the NT has downloaded and
validated a new image, that image is initially not-committed and not-activated. The DPU may then send the
activate image command. After the NT sends a positive activate image response, the NT loads and executes
the new software image, but without changing the committed state of either image. The DPU may then
send the commit image command, causing the NT to set the commit state true for the new image, and false
for the previous image. The time between the download, activate and commit phases is not specified.

If there is a problem with the newly activated image that causes the NT to fail (e.g., watchdog timeout), the
NT may do a soft restart on the (other) committed image. Activating prior to committing may thereby allow
for automatic failure recovery by the NT.

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