How the LTE Core Network talks to UMTS and GSM

An important functionality that has to be in place when LTE networks are launched from day one is the ability for mobiles to roam from LTE to other types of radio access networks. In most parts of the world except for the US and Canada, that is UMTS and GSM. While doing some research on this topic as to how that works from a network point of view, all books I have come across so far point to the new S3, S4 and S12 interfaces between the 2G and 3G network nodes (the SGSN and RNC) and the LTE core network nodes (or the Evolved Packet Core (EPC) to be precise), i.e. the Mobility Management Entity (MME) and the Serving Gateway (S-GW).

One might be happy with this answer from a theoretical point of view but in practice this approach might be a bit problematic. As the functionality has to be there from day one, using the new interfaces means that the software of the 2G/3G SGSNs and RNCs need to be modified. Now one thing you don't want to do when introducing a new system is to fiddle with the system that is already in place as you've already go enough work at hand. So I was wondering if there was an alternative to introducing new interface, even if only for Inter-RAT (Inter Radio Access Technology) cell reselection triggered by measurements on the mobile side.

It turned out that there is. After some digging, annex D in 3GPP TS 23.401 provided the answer (sometimes I wonder what is more important, the specification text or the annexes…). Here, a network setup is described where the 2G and 3G SGSN is connected to the LTE world via the standard Gn interface (Gp in the roaming case) to the MME and the PDN-Gateway. To the SGSN, the MME looks like an SGSN and the PDN-Gatweay looks like the GGSN. No modifications are required on the 2G/3G side. On the LTE side, this means that both the MME and the PDN-Gateway have to implement the Gn / Gp interface. But that's something that has to be done on the new network nodes which means its not a problem from an real-live network introduction point of view. With the Gn / Gp interface support in place, the introduction of LTE and roaming between different radio access networks could be introduced as follows:

Cell Reselection Only at First

To make things simple, LTE networks are likely to be launched with only cell reselection mechanisms to 2G and 3G networks instead of full network controlled handover. That means that the mobile is responsible to monitor signal strengths of other radio networks when connected to LTE and autonomously decide to switch to GSM or UMTS when leaving the coverage area of the LTE network. When using the GSM or UMTS network the mobile also searches for neighboring LTE cells and switches back to the faster network once the opportunity presents itself (e.g. while no data is transmitted).

Handovers Follow Later

The advantage of cell reselection between different types of access networks is that they are simple and no additional functionality is required in the network. The downside is that when a network change is necessary while a data transfer is ongoing the mobile will either not attempt the change at all or the change results in an temporary interruption of the ongoing data transfer. The answer to the downside is to perform a network controlled handover between the different radio systems. This makes the change between access networks a lot smoother but requires changes in both the new and the old radio networks. On the GSM/UMTS side, the software of the base stations and radio network controllers have to be upgraded to instruct the mobile to also search for LTE cells while the mobile is active and to take the results into account in their existing handover mechanisms. As far as I can tell, no modifications are required in the SGSN, as transparent containers are used to transfer non-compatible radio network parameters between the different networks.

Packet Handovers Today

At this point I think it is interesting to note that packet handovers are already specified today for GPRS/EDGE to UMTS and vice versa. However, I haven't come across a network yet that has implemented this functionality. Maybe it is the speed difference between the two radio access networks that makes the effort undesirable. Between UMTS and LTE, however, such packet handovers might finally make sense as in many scenarios, the speed difference might not be that great.

The GGSN Oddity

One last thought: In annex D, the 2G/3G GGSN functionality is always taken over by the PDN-GW. That means that an LTE capable mobile should never use a 2G/3G only GGSN when first activating a PDP context in GPRS/EDGE or UMTS. If this was done I don't see how it would be possible to reselect to the LTE network later. This is due to the fact that the GGSN is the anchor point and can't change during the lifetime of the connection. If an "old" GGSN would be the anchor point, then the MME and S-GW would have to talk to the "old" GGSN after a cell reselection or handover from GPRS/EDGE or UMTS to LTE instead of a real PDN-GW. That's a bit odd and I don't see this described in the standards.

There are several ways how that could be achieved. Using a special APN for example that triggers the use of a combined GGSN/PDN-GW when the connection is established could be a possibility or the analysis of the IMEI (the equipment ID). While the first idea wouldn't require new software in the SGSN, the second one probably would and then there is always the chance that you miss some IMEI blocks in the list on the SGSN, especially for roamers, so it's probably not such a good idea after all. Another option would be to replace the GGSNs in the network or upgrade their software so they become combined GGSNs/PDN-GWs. However, there some risk involved in that so some network operators might be reluctant to do that at the beginning.

If you know more about this or have some other comments or questions in general, please leave a comment below.