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Handover from 3G to 4G while UE in connected state?

+2 votes

I know its possible to handover to 3G from 4G in connected state.

But do networks implemented the backwards, I mean 3G to 4G while UE in connected state?? Reselection happens for me but not Handover ?? When UE goes to Cell_PCH or IDLE state it moves back to 4G, not when I am streaming or downloading.

posted Jul 21, 2013 by Nora Jones

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1 Answer

0 votes

support for 3G to LTE handover (i.e. direct transition from 3G RRC connected state to LTE RRC connected state) is optional in both the network and the UE (modem/chipset) side. Handover is not the only mechanism to move the UE from one RAT (in this case 3G) to another (in this case, LTE). In many discussions, the term handover is used "loosely" as a general expression for moving from one cell or system to another, whereas in the 3GPP specifications lingo the terms are more explicit.

Coming back to the point even without support for 3G->LTE handover, there are mechanisms for the UE to move (on its own or commanded by the network) to move from 3G to LTE even when the UE has a RRC connection in 3G. For example:

  • 3G to LTE redirection: the 3G network can send a RRC connection release message to the UE , redirecting the UE to LTE. This is mandatory functionality for the UE, so the network can always use this if it chooses to do so.

  • 3G to LTE cell reselection: in 3G CELL_PCH and URA_PCH states, the UE can support autonomous cell reselection to LTE. In addition, the UE can also support cell reselection from 3G CELL_FACH state, if the network supports this feature.

In 3G idle mode, of course the UE shall support cell reselection to LTE as per the inter-RAT cell reselection algorithms (see 3GPP TS 25.304).

answer Jul 21, 2013 by Kevin Peterson
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+4 votes

We have found an interesting scenario we would like you to share with the SGSN and HSS experts .

Below Description of scenario
When the user moves from X SGSN to MME (3G to LTE network), X SGSN is never informed of this RAT TYPE change, and it keeps a false entry for this user in the SGSN despite it's not attached to the 3G network anymore. However, the HLR knows the UE is not attached anymore to the 3G network, because the SGSN number and address cannot be seen anymore from the HLR GUI after the RAT TYPE change is completed. We think this might be one of the main reasons why we see more users attached to the 3G network in X SGSN than those we see in the HLR.
We think this is happening because the HSS-FE removes the SGSN number and address itself from the HSS when it receives the ULR from MME. However, it cannot send a CANCEL LOCATION to X SGSN because S6D interface is not available .

In order to make myself clear, you can find more comments below about the scenario.
1) The UE is attached to the 3G network, that is, it’s attached to SGSN
2) It moves from SGSN to our MME (from 3G to LTE). The MME sends the LOCATION UPDATE via S6A interface towards the HSS.
3) However, the CANCEL LOCATION is never received by our SGSN because the S6D interface between the HSS-FE and the SGSN is not implemented in our network. Therefore, the SGSN does not know that this user moved from 3G to LTE, and keeps the record in the SGSN. On the other hand, the SGSN number and address are removed from the HLR, and we think this is done by the HSS-FE itself.?
4) The SGSN keeps the record while the HLR don't, which creates a difference in the number of subscriber attached to the 3G network between the SGSN and the HLR. The SGSN sees more subscribers attached to the 3G than those seen by the HLR.

Could you please confirm that you have now a clear picture of what the issue is?

Could you please provide your recommendations in order to fix this issue?
Is't S6d interface is mandatory for this scenario?

If you have any question, please, let me know.

+4 votes

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