[srslte-users] Size of SIB messages issue

[Taillefer Simon]

Hello srsLTE team,

Basically, we are encountering an issue on the RRC protocol within the srsLTE software. In our case, this protocol is in charge of sending periodically the posSIBs from the ENB to the UE in an SI message (the same message used to send the SIBs). We receive the posSIBs data from the MME in an LPPa message encapsulated in a S1-AP message. This LPPa message can have a size from 700 to 6000 bytes but the problem is that the maximum size of a SIB message contained in the SI message within srsLTE is very low (277 bytes) compared to the data available to send (going up to 6000 bytes).
It is possible for us to pack and encode a posSIB / SIB messages bigger than 277 bytes but there are errors when the ENB try to send them to the UE (see error_error.png).

Moreover, we did not find anything about the size of the SI message or size of RRC protocol messages in the 3GPP documentations. There is a mention about the maximum size of the SIB content which is limited to 277 bytes ("Content" part in the Figure rrc_message.png). However, this size limit does not impact the messages because the posSIBs are already segmented in parts of maximum 217 bytes, so even if the posSIBs content is limited to 277 bytes, there will be no errors.
But on the srsLTE software, it is the full SIB message contained in the SI message that has a maximum size of 277 bytes (red part in the Figure rrc_message.png).
Here is the problem because we cannot put all the posSIBs types and content in the SI message.

Would you have any recommendation as to how to remove it? Or from where comes this limitation ?

Kind regards,

Simon Taillefer




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