" AT&T’s current 700 MHz LTE network, which is expected to provide coverage to nearly 270 million consumers in 400 markets by the end of 2013, currently supports only Band 17 devices. Band 12 support on AT&T’s network will require that AT&T deploy a new network technology that will permit simultaneous support of both Band 12 and Band 17 devices. Absent this dual support, millions of pre-existing Band 17 devices currently in use by AT&T’s customers would be stranded. To achieve this, AT&T must develop, implement and deploy throughout its network Multi-Frequency Band Indicator or MFBI capabilities. MFBI will permit AT&T’s network to operate simultaneously as both a Band 12 and Band 17 network and to support devices in both bands. This feature was recently standardized, and as with any new significant feature, deployment will require lab regression testing of the new major software release that will contain it, lab testing of the feature and its functionality, and field testing that includes extensive testing with existing Band 17 legacy devices as well as new prototype Band 12 devices. Additionally, testing of both types of devices with various carrier aggregation capabilities operating simultaneously with the MFBI feature will need to be completed. "
You can read more here if interested
http://assets.fiercemarkets.net/public/mdano/amis/att-interop-fcc.pdf