After acquiring the E-Plus mobile network in Germany, Telefonica O2 has started integrating the two physical networks. For their customers the first visible change is that Telefonica has enabled national roaming between the two networks. There are several ways to do this so I had a closer look which option Telefonica has taken in Germany.
As reported by the press, national roaming has been enabled for the two formerly separate GSM and UMTS networks but not yet for LTE. In Cologne, there's advertising in the streets now to make customers aware of it. And indeed the two UMTS networks are now open for national roaming while the LTE networks are still separate and registration attempts with a SIM card of E-Plus are rejected on a Telefonica LTE cell. At this point in time in Cologne, however, the GSM networks are not yet shared and my E-Plus SIM card is rejected in Telefonica's GSM network. From a policy point of view that's quite interesting and shows that mobile Internet access is now in the focus rather than 2G voice telephony.
To make devices aware that they can roam between the networks, there's the Equivalent PLMN (Public Land Mobile Network) ePLMN mechanism defined in 3GPP. The ePLMN indication can be put into the GSM, UMTS and LTE broadcast information which is what is done in Austria for example to indicate national roaming between ex. Orange, 3AT and T-Mobile. Telefonica in Germany, however, has chosen not to go down this route as I could observe no ePLMN information in the broadcast information of any network technology.
Instead, ePLMN information is sent to the device at the end of the registration process in the Location Area Update Accept and Routing Area Update Accept messages. I played around a bit and this works just as well. As designed, my mobile went from an E-Plus cell to an O2 cell despite manual network selection to E-Plus.
While being 3GPP standards compliant not all customers are happy with this in practice. As reported in this article, some Telefonica/O2 customers are not happy to find their mobile on the E-Plus side of the combined network which at times and in some locations seems to be more congested than the O2 network that is also present. But once in the E-Plus UMTS network the only way to automatically go back to the O2 UMTS network is to loose E-Plus UMTS and GSM network coverage. It would of course also be possible to define neighbor cell relationships between the two networks so the mobile could find cells of the 'other side' but I guess that's too much trouble for an interim solution.
Network sharing is also bad news for subscribers in dense population areas where only one of the two UMTS networks has been available so far. Due to the sharing the network now has to cope with twice the numbers of subscribers. According to the article above that has some rather unpleasant slow-down effects at times.
A quirk caused by sharing the two UMTS networks but not the LTE networks is that once a device is in the 'partner' UMTS network it is unable to go back to the LTE network of its home network operator until coverage of the shared UMTS/GSM network has been lost. As the LTE networks are not shared it would not even be possible to announce neighbor cell relations ships. I can see why that will make some customers with patchy LTE coverage quite unhappy.
So while national roaming is a good idea to extend coverage for customers and probably a requirement to shutdown the E-Plus network over time it will be interesting to observe how many customers are affected by the negative consequences of a move that the marketing departments try to give a positive spin.