Sunday, December 13, 2009

LTE: Inter RAT Handover (E-UTRAN to UTRAN)

Below is the case where UE moves from LTE network to 3G network.


Source network is LTE and target network is 3G. Target SGSN communicates with MME over S3 interface. GTPv2 is protocol used here. This means existing 3G network SGSN's need a software upgrade. (GTPv1 --> GTPv2). We also assume that a LTE SGW is serving the 3G network. This means that SGSN again uses S4 interface to communicate with SGW.(GGSN is no longer required?) Below is the call flow and is self explanatory. I am considering that Indirect Data Forwarding tunnel is used. Details are in 3GPP TS 23.401 Chapter


The messages are almost similar to what S1 handover uses. Only change is communication between RNC and SGSN which is RANAP based. Does this mean that no software upgrade is required in RNC?

I am interested in two things here;-

NSAPI: NSAPI is the identifier used to identify a control plane tunnel/PDP context. EBI is used in LTE for the same purpose. So when the UE moves from LTE to 3G, EBI is mapped to NSAPI. However this mapping is not complex as both NSAPI and EBI are 4 bit values. Also they both start with integer value of 5.

QoS:- QoS values are quite different in LTE when compared to 3G. EPS QoS to pre release 8 QoS mapping is explained in 3GPP TS 23.401 Annex 5.   


Greg said...


I just wondering how S-MME will find IP address of T-SGSN. Is there any DNS query required ?

What about UTRAN to E-UTRAN handover DNS query ?
How then the records looks like ? Are they similar to LAC.RAC.mnc.mcc.gprs notation ?


Santosh said...

Hi Greg

I think source MME uses an IE called Target RNC identifier to identify the target SGSN.

I am quite not sure of the exact mapping and DNS query, but take a look at 3GPP TS 23.003, it has lot of details regarding EUTRAN to UTRAN mapping and vice versa.

Cheers, Santosh

Rajesh Badveti said...

Hi santosh,
As per the last paragraph,
"QoS:- QoS values are quite different in LTE when compared to 3G. EPS QoS to pre release 8 QoS mapping is explained in 3GPP TS 23.401 Annex 5/E", the mapping needs to be done. The values of H and M are need to be hardcoded ? Could you brief me how to map those values. ?
Rajesh Badveti

Santosh said...

Hi Rajesh

QoS mapping is a bit complex. I will take it up in seperate post. May be soon.


Anonymous said...

Hi Santosh..

This is Garima this side.
We recently had a training on LTE and there was a question that was not answered. Pls if u could help resolve.
When a handover takes place from LTE to WCDMA (Data call), then after the call is released in WCDMA, the UE will lose its IP, as in WCDMA, IP is given to a user only in a data call whereas in LTE UE has the IP since the time it attached to n/w till it gets detached. Now the question is that a UE which has released a call in WCDMA and therefore does not have an IP, how will the LTE n/w find / search / recongnize it ??
Thanks in advance..

Roger said...

Hi santosh,

Thanks for your nice post.

I have a question is, will the IP address be changed during such inter-RAT handover, both at E-UTRAN to UTRAN or at UTRAN to E-UTRAN?

I didn't see the IP address change procedure in the spec TS 23.401 chapter and

If the IP address doen not change during handover, could you give me a brief answer about this?

Best Regards,

Santosh Kumar Dornal said...

Hi Roger

IP address will not change during the Inter Rat handover. IP address change is as good as Detach in one RAT and Attach in another. Never in handovers IP address is supposed to change.

Regards, Santosh

Roger said...

Hi Santosh,

Thanks for your reply.

After reading your reply, I have another question.

If the UE won't get a new IP address during handover, either intra-PDN or inter-RAT, so will UE gets an all-new IP address when e.g. roaming or access from other non-3GPP technologies such as WiMax, WLAN.. ?

I'm interest in this kind of questions...thx!!

Best regards,

EDDY 4G said...


after the HO is successfull from LTE to 3G I do receive a PDP context modification from the SGSN, is this normal?


Santosh Dornal said...

Yes, it is normal. This could be because of qos conversion from LTE to 3G. Even if you move from 3G to LTE, there can be bearer modification.