In RLC AM Mode, How RRC Re-establishment procedure happend?

Question

in RLC AM Mode, to resumption of SRB-1 , how ongoing sequence number can be maintain in RLC ?

If we do RLC-reset and MAC-reset operations, How can this sequence number information maintained in RLC and how it informs to upper layers?

Regards,
Suchakravarthi

Answer 1

Hi,

All context with respect to the UE will be deleted and re-established in ENB side. So no state variable and SN will be there in old context. Resuming means it will start an SRB 1 connection again by doing re -establishment instead of normal RACH procedure.(Here resuming means reduced RRC connection establishment time by doing re-establishment , not starting from where it left.)

Comments

HI jaganathan, the explanation you given i too can agree with you in case of UM MOde.

But in case of AM mode, to serve without loss of data there should be saving of sequence number before trigger the Re-establishment .. yes or no?

If no , why not and can we serve promised GBR services ? Please give me clarity?

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Hi,

First re establishment wont happen in UM mode..
In AM mode its of no use to maintain the old bearer context, because first reason is always UE wont do re-establishment in serving cell, it can do to any any other cell.How ever if You drop those packets, application level retransmission will be there(TCP level) so no need to worry abount that. Dropping wont reduce GBR rate, because scheduler will take care in the next SF onwards by scheduling more data.

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Hi jaganathan, i will be thnakful to you if you can provide me some specification reference regarding same.

Q1.  Is it not needed to save sequence number for re-establishment even it is AM Mode?

Q2. Why re-establishment wont happen in UM Mode?

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I am just guessing the reason can be default bearer. One UE will have at least one default bearer and the default bearer is always in AM mode. That might be the reason of saying that re-establishment is triggered only when UE has AM mode. Please correct me because I am not 100% sure on my thought process.

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Hi,

In UM mode RLC cant detect the link failure, But RRC can initmate re-establishment irrespective of RLC modes.If it is AM mode transmitting RLC can detect the link failure(MAX RLC re-tX happened)  and can notify the upper layer.

Re-establishment is not related to default or dedicated bearer.In all bearers re-establishment can occur.

Yes, Once re-establishment occurs RLC transmitting entity will discard all packets and all state variables. (refer 36.322 - 5.4)