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what the sequence of message exchanged between UE <-enode-> and the stack layer pdcp <->rlc<->mac for the data flow.

+3 votes
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I am interested to know the entire data flow from EnodeB to UE across all the RA stack layers

posted Sep 30, 2013 by Sony Mohanty

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Bit confused, are you looking for the Message Flow Diagram between ENODEB and UE for Random Access(RA). Please clarify...

2 Answers

+1 vote

eNodeb and UE both have same RA protocol stack.
Please find the Flow from eNodeB to UE for downlink data.
----------------------------------------------------------->
There are S1-U layer at eNodeB which receives coming GTP packets from SGW.
S1-U layer at eNodeB has one to one mapping between core network ERAB and logical channel at eNodeB.
There are 8 max bearers are supported for data plane. After receiving GTP packets, it removes GTP header and give plane IP packets to PDCP layer of eNodeB. PDCP layer first compress using ROHC header compression and then cypher the compressed packets and finally send to RLC layer.
At RLC layer segmentation and assembling is performed based on the ask of bytes from MAC.
At this stage If you remember, eNodeB MAC has already sent out DCI for the same that where data is coming across the bandwidth and how to decode. It means eNodeB MAC ask only such amount of data from RLC layer which can send to UE and already eNodeB MAC has informed to UE.
Based of ask from MAC layer, RLC does segmentation/assembling if required and send PDU to MAC.
MAC receives this SDU and add it's own header and send to PHY layer. As MAC has 8 HARQ process running and phy do its own job (like bits to signalling conversion ans so many things ).

Hope this description will help out to you.

answer Sep 30, 2013 by Neeraj Mishra
+1 vote

Assuming u are looking for Random Access -

Random access procedure consist of four steps explained below (Only contention based procedure is shown below)
http://2.bp.blogspot.com/-U0V9TztYKZI/UX5RdpbQGsI/AAAAAAAAErY/F2FP6F1SXmU/s400/Rach+steps.png

Step 1: Msg1
1 UE selects one of the 64 available RACH preambles
2 Now UE also needs to give its own identity to the network so that network can address it in next step. The identity which UE will use is called RA-RNTI (Random access radio network temporary identity). Basically its not some value sent by UE but interestingly RA RNTI is determined from the time slot number in which the preamble is sent
3 If UE does not receive any response from the network, it increases its power in fixed step and sends RACH preamble again

Step 2: Msg2
1 eNodeB sends "Random Access Response" to UE on DL-SCH (Downlink shared channel) addressed to RA-RNTI calculated from the timeslot in which preamable was sent, as explained in step 1 (about RA-RNTI calculation)
2 The message carries following information
* Temporary C-RNTI: Now eNB gives another identity to UE which is called temporary C-RNTI (cell radio network temporary identity) for further communication
* Timing Advance Value: eNodeB also informs UE to change its timing so it can compensate for the round trip delay caused by UE distance from the eNodeB
* Uplink Grant Resource: Network (eNodeB) will assign initial resource to UE so that it can use UL-SCH (Uplink shared channel)

Step 3: Msg3
1 Using UL-SCH, UE sends "RRC connection request message" to eNodeB
2 UE is identified by temporary C-RNTI (assigned in the previous step by eNodeB)
3 The message contains following
* UE identity (TMSI or Random Value )
* TMSI is used if UE has previously connected to the same network. With TMSI value, UE is identified in the core network
* Random value is used if UE is connecting for the very first time to network. Why we need random value or TMSI? Because there is possibility that Temp-CRNTI has been assigned to more than one UEs in previous step, due to multiple requests coming at same time (Collision scenario explained later)
* Connection establishment cause: The shows the reason why UE needs to connect to network

Step 4: Msg4
eNodeB responds with contention resolution message to UE whose message was successfully received in step 3. This message is address towards TMSI value or Random number (from previous steps) but contains the new C RNTI which will be used for the further communication

answer Sep 30, 2013 by Salil Agrawal
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