From GSM to LTE: An Introduction to Mobile Networks and Mobile Broadband

4.9 Interconnection with UMTS and GSM

When a mobile device is at the border of the coverage area of the LTE network, it should switch to another network layer such as UMTS and GSM to ensure connectivity. In the worst case, the mobile device looses the LTE network coverage and if it does not find a suitable LTE cell on the current channel it will search for LTE cells on other channels and also switch to other frequency bands and other RATs to regain contact with the network. These actions take a significant amount of time, typically between 10 and 30 seconds, during which the device is not reachable for services trying to contact it such as push e-mail or incoming voice calls. It is therefore better if the network supports the mobile device to find other suitable channels, bands or radio technologies. There are three basic procedures for these purposes that are described in the following section:

• cell reselection from LTE to UMTS or GSM;
• RRC connection release with redirect from LTE to UMTS or GSM;
• inter-RAT handover from LTE to UMTS or GSM.

Irrespective of whether the mobile has to find a GSM or UMTS network by itself or if it is supported by the network, the LTE network has to be connected with the GSM and UMTS networks so that the subscriber's context, that is, the assigned IP address, QoS settings, authentication and ciphering keys, etc. can be seamlessly exchanged between all core network components involved. One option connects the LTE core network and the GSM/UMTS core ...