Tuesday, June 29, 2010

Idle Mode Signaling Reduction

When ever a UE moves to a new tracking area or routing area it triggers a tracking area update (LTE) or Routing area update (3G) procedure to inform the network about its current location. Note that these procedures are triggered while UE is in Idle mode or Connected Mode. Read below considering UE in Idle mode.

With the LTE systems coming in, both the network antennas (3G and LTE) might remain close to each other. This means a UE can be in a tracking area of a MME at one point and it can immediately move to routing area of SGSN at another. If both coverage areas are pretty close and UE is lingering at the border of both networks, it could result in ping-pong effect. That is a UE may get into a loop where it sends RAU to 3G system de-registering itself from LTE and vice versa by continuously doing cell reselection.

Solution to this is proposed in Annex J of 3GPP TS 23.401, which is ISR.

The solution assumes that SGSN is S4 SGSN. Also it is a requirement that both UE and network should understand ISR. ISR support is mandatory for E-UTRAN UEs that support GERAN and/or UTRAN and optional for the network.

Assume that UE is registered to the network through EUTRAN. Now UE re-selects a GERAN/UTRAN network and initiates a Routing Area Update. SGSN will send context request to MME asking for the UE context. Here MME may reply with context response along with ISR activate indication. Network shall switch on the ISR once it realizes the UE context is being pulled from a different RAT. Once the context response is received, RAU accept along with ISR active indication is sent to UE. Once UE receives the RAU accept along with ISR active, it shall store the GUTI from EUTRAN and also the newly received P-TMSI from GERAN/UTRAN. MME will also keep the UE context. SGW is informed about ISR activation by SGSN in Modify bearer request (ISR flag set). At this point UE is registered in both EUTRAN and GERAN/UTRAN, has both the network identifiers (P-TMSI and GUTI) and UE may not initiate a TAU or RAU (until the UE has identified a new TA/RA that is not in its list).

When data for UE arrives from network, SGW will send downlink data notification to both SGSN and MME. Both MME and SGSN will initiate paging. UE may respond to which ever paging message it has received and initiate a normal service request procedure later.

Thus the ping pong effect of TAU and RAU is minimized at the expense of paging the both networks.

Monday, June 14, 2010

Spec Ambiguity

Could somebody kindly tell me if a SGW could be relocated by a TAU request in connected mode? I think not? Interested in a debate? 3GPP TS 23.401 Chapter:- 5.3.3 Cheers, Santosh

LTE Tidbits IV

Continuation to LTE Tidbits I, II, III

There have been several questions on how RRC and EMM/ECM states are in sync. To begin with, RRC and EMM/ECM states are two seperate layers, each layer provides service to other but doesnt have authority to modify each other. eNB, acts as a relay between UE and MME, i.e it will receives RRC messages from UE and puts them in S1AP messages towards MME. This means eNB will simple pass the NAS pdu's from UE to MME without modifying them. SO at UE, there are two layers to maintain, RRC state and NAS state. We cannot have a state where there no radio bearers for dedicated bearer, but the NAS layer has the EBI for dedicated bearer.

S1AP Initial UE message holds an extereme importance. It is sent only when UE first establishes an RRC connection with eNB. This message is what makes UE move from ECM idle to connected mode.


1. In a normal TAU procedure, when MME decides to change the SGW, how does the new SGW come to know about the IP and TEID of eNB for data downlink flow and how does the eNB come to know about SGW IP and TEID?

[S] When a SGW has changed during TAU, MME will send a create session request towards the new SGW. This CSReq is for establishing the control plane.Now in CSRes the new SGW will respond with its own user plane IP and TEID that is used by eNB for uplink. This info is propagated to eNB in Initial context setup message. Now eNB will create radio bearers and once the radio bearers are created it will send downlink user plane IP and TEID in intial context setup response message to MME. This info is send to SGW in modify bearer request.

2. In Initial Attach procedure, how does the eNB comes to know about the SGW IP and TEID?
[S] Same philosophy as above applies. The trick here is radio bearers will be created only once the network has setup the EPS bearers. Once EPS bearer is created using CSReq/Res, only then then eNB creates the radio bearers and this info is sent to SGW by MME in Modify bearer req/resp

3. If OI=0 in both CSR during initial attach and S1 based HO, why does the SGW not send CSR towards the PGW in S1 based HO?

[S]CSReq is used to create EPS bearers. In S1 HO case the EPS bearers are already created, so CSReq will not be sent to PGW. BUt since the SGW has changed, PGW should be informed of new SGW, so a modify bearer request is sent to PGW.