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.
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1 comment:
This was a great idea inside 3GPP, although it won't help much without proper radio engineering (at least in the initial stage of LTE rollout).
In LTE, the TA list will minimize the number of needed TAU for a moving MS, because in the beginning the LTE coverage will be spotted we can easily assume that a lot of UTRAN<->E-UTRAN Inter-RAT transitions will take of for that particular moving MS, in this case the number of RAU+TAU will be ~ the same with or without the ISR.
Taking into account that the real networks will start as overlay, ie TAI and RAI will have different borders... this will be a mess, I think in the real world it won't help so much for a moving MS BUT it will help a lot on the borders of cell coverage as you stated.
I'm curious to see a live implementation and real world figures...
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