3G FACH capacity

With the rising number of push eMail devices in 3G networks and mobile applications such as instant messengers and voice over IP clients the number of small IP packets to keep the connections of such applications alive through network address translation routers is rising. For the network this means of lot of radio layer signaling and waste of bandwidth. For the mobile device, keep alive messaging means significantly increased battery consumption.

3G UMTS networks are thus putting devices that only send little data on the Forward Access Channel (FACH) which requires much less radio channel signaling overhead than if the device is instructed to remain or use the High Speed Downlink/Uplink channels for such kind of traffic. As more always-on devices are used in the networks, this will quickly become an issue since the total capacity of the FACH of a cell is limited to 32 kbit/s today. With the bandwidth so small I think most operators will be very thankful for the enhanced-FACH extension which reserves some capacity on the high speed downlink channels for FACH operation. Despite using the high speed channels, no additional radio layer signaling will be used so overhead and battery consumption remains limited at the expense of spectral efficiency. While networks and mobile devices do not support this feature today I expect that this is definitely a feature that will be implemented in the future.

For more on radio interface optimization for future devices and services have a look at my previous entries on continuous packet connectivity (here, here and here), some more background on enhanced FACH (here) and some thoughts on upcoming capacity issues due to keep alive messaging (here).

It seems there is now also an initiative in Release 8 of the 3GPP standards to improve the uplink behavior of the system while a device in in Cell_FACH state. More about that once I have taken a look at the details.