Tuesday, July 20, 2010


Quick thought regarding the NAS layers between UE and MME. Since UE and MME directly cannot talk to each there could be a disconnect between the both. Consider this. When UE goes into idle mode, it is not known to the network until the network receives a TAU. In the mean time imagine that network initiated a bearer deletion procedure. If eNB doesn’t find the UE then bearer is implicitly detached on MME and eNB, but the bearer information is still present with the UE. At this point there could be a disconnect between UE NAS layer and MME NAS layer.

Assume another scenario where handover is in progress. Because of some reason the target eNB has not accepted a particular bearer. The same is conveyed to network in S1AP message and to UE in RRC messages. But NAS layers in MME and UE still have the bearer information. There could be several cases like this where there can be different NAS states in UE and MME. The question here is will RRC and NAS layers in UE be in sync. That is if a Data radio bearer for a particular bearer is not established, will the NAS layer be intimated the same by RRC and UE will delete the bearer from its NAS layer?



Ravi said...

Always a pleasure to read your blog.

To your first paragraph:

Why do you say that MME does not know? Shouldn't the eNB do either a "UE Context Release" or delete all the ERABs to indicate that the UE is in Idle mode?

If you are saying that UE never indicated to the network that it is going into the Idle mode, again, is it a real usecase? If UE wants to hold on to the RABs, yet go into the idle mode, what is the purpose? With RABs being on, the packets are going to arrive only on one specific eNB and it cannot transmit them. Paging does not apply because from network point of view, the RABs are still there.

May be I am missing it completely, can you please clarify?

Santosh said...

Thanks Ravi.

When I said Idle mode, I had this in mind. Assume that a UE has default and dedicated bearer activated. UE goes into the idle mode, network releases the bearers on radio and EPC side. Note that bearers on S5 are still intact, which means PGW is still ready to accept a packet for UE. Now because of some reason PGw wanted to delete the dedicated bearer. I assume there wont be any paging done here. In this case the network has deleted the dedicated bearer but UE still has the information. Now when UE moves into the connected mode, the network will just establish a default bearer as dedicated bearer is already gone, but UE is still lingering with both bearers. There is NAS state mismatch here. The solution could be page UE if there are any bearer deletion activites, but I havent found anything like this in specs.

Handover is another case where a NAS mismatch very vulnerable. The spec simply says eNB shall take further action. I am really interested to know how UE manufacturers are dealing this or is it that I am not sleeping properly thinking crazy.

Cheers, Santosh

K said...

The "bible" 23.401, ch. says:
"When the UE is in ECM IDLE state, the UE and the network may be unsynchronized, i.e. the UE and the network may have different sets of established EPS bearers. When the UE and the MME enter the ECM CONNECTED state, the set of EPS Bearers is synchronized between the UE and network."

The exact way of synchronization is somewhat described in TS 24.301, ch.

As far as I understand, MME and UE will delete bearers for which no radio bearers are set up. So both of them will radio bearers as a guidance.

Santosh said...

Excellent. This is what I was looking for. Thanks!

Rauf said...

Hi Santosh,

What I have understood when RRC connection is released due to S1AP UE Context Release request, default, dedicated bearers are released in radio and S1 interface but remain active in S5 interface. Then is it appropriate to say that "PDN connection between the UE and the server hosting application contents on the IP network is typically remain active." (excerpt from book LTE Signaling: Troubleshooting and Optimization)?

Santosh said...

Hi Rauf

Yes, you can say that. Note that untill detach happens, IP address of UE is not supposed to change.

Regards, Santosh

praveen ghosh said...

Hii all,

wat wil happen to ue context in eNB if ue is inactive for long? and whats the max time eNb can hold the context?