An upgrade for 2G CDMA 1xRTT networks that provided up to four times greater capacity.
There are no longer any CDMA networks operating the US.
Some devices used CDMA 1xRTT (and 1x Advanced) for voice and text messaging, while for data they used CDMA 1xEV-DO and/or LTE networks. Therefore the benefits of 1x Advanced applied mostly to voice service.
The four times greater capacity means that a network upgrading from 1xRTT to 1x Advanced can handle four times as many voice calls using the same radio spectrum. Or - more likely - it can handle the same number of voice calls in one-fourth the radio spectrum, freeing up radio frequencies (bandwidth) for additional 1xEV-DO or LTE service.
To achieve the full four-times capacity boost, all phones must support 1x Advanced as well. Carriers saw some immediate benefit when upgrading network towers to 1x Advanced, but only realized the full four-fold capacity boost over time, as consumers upgraded to newer phones that supported 1x Advanced.
A 1x Advanced network can optionally be configured by the carrier to provide greater coverage compared to 1xRTT, at the expense of the improved capacity benefit.