LTE Tidbits III

Gn/GP handovers:

LTE will not be fully functional from day 1. We will need the legacy systems to support major number of customers. LTE specs describe the inter RAT handovers, but they do insist to upgrade the existing SGSNs and GGSNs. Now no operator wants to touch their existing fully functional system untill the new system is working without any problems. This means for some point of time both legacy and LTE systems should work together. This also means that operators would still want their legacy systems to talk to the LTE and vice versa.

To provision this 3GPP Ts 23.401 Annex D provides the call flows. According to the Annex existing SGSNs need not be touched.LTE functional elements such as MME and PGW will also support legacy protocols (GTPv1) so that both networks can communicate. To a SGSN, MME will just behave as another SGSN.

But these handovers are hard handovers which means there could be a connection loss. Also GGSNs from the existing networks should be replaced with LTE PGW, as the whole conecpt is based on LTE PGW as the comman anchor between the two networks. This shouldnt be much of a problem as PGWs are written keeping this mind and I personally believe that GGSN replacement will not be that much of a problem. The dissected call flows shall be posted soon.

ISR/TAU/RAU:-

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) procedures to inform the network about its current location. Note that these procedures can be 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 a routing area of SGSN at another. If both coverage areas are pretty close then it could result in ping-pong effect. That is a UE may get into a loop where it sends TAU to LTE network and next moment it sends RAU to 3G systems de-registering itself from LTE and vice versa.

Solution to this is proposed in Annex J of 3GPP TS 23.401. ISR - Idle-mode singalling Reduction.

The soultion suggests that UE be registered with both networks while it is in Idle mode. That is UE is registered to a tracking area of a MME and also to a routing area of a SGSN. This needs UE to explicitly support ISR and also both the networks should be capable enough to support the ISR. The common achor point between the networks is considered to be SGW, this also means that SGSNs and GGSNs should be upgraded to support Rel 8. This way ping pong effect is avoided. But this comes at an expense. As the UE is registered to both networks both the SGSN and MME will have to initiate a paging process to make UE move to connected state. However UE will initiate serivice request to network that it is currently camped on, i.e either LTE or 3G.

More to follow.