WiMAX, LTE, UMB, etc. etc., buzz words in the emerging 4G wireless space. Different interests, standardization groups and politics but they all have one thing in common: All are based on IP and all will rely on Voice over IP (VoIP) in one form or another (e.g. IMS or SIP) to carry voice calls. With sheer bandwidth, IP header compression and optimized handover strategies between cells I can imagine it happening. But what happens when you run out of network coverage and only a GSM network is available to continue the call in?
A number of alternatives exist. The first one might be evolved EDGE which could deliver GPRS data rates high enough to sustain a VoIP call begun in a 4G network on the packet switched side of the network. However, I wouldn’t bet on this one happening everywhere. It’s more likely that the VoIP call must be continued in the circuit switched side of the GSM network. But how can that be done?
Voice Call Continuity (VCC) could come to the rescue. A first version is already standardized in 3GPP TS 23.206 and it can do this and many other interesting things. I’ve done a short intro on VCC before, take a look here. Yes, it’s standardized but it’s not a home run:
One of the problems with VCC is that the mobile needs to be connected to both the 4G network and the GSM network at the same time to perform a handover. This consumes more energy then only being connected to one network at a time. Furthermore, such a dual connection might be difficult to establish if the two networks use the same frequency band. If the 4G network is deployed in the 2.5 or 3.5 GHz band then this is not going to be a problem. In case classic 2G frequency bands (850, 900, 1800, 1900 MHz) are partly re-farmed and the GSM network to be handed over to is nearby then VCC will become a challenge. 3GPP Release 8 might yet get a work item to study the possibility of single radio VCC (SR-VCC) to deal with these issues and I am looking forward to see how handover speeds in the order of a few hundred milliseconds can be achieved.
All-IP wireless networks will be a great thing to have but solving the handover to legacy wireless networks to prevent calls from dropping is going to be a difficult thing.