What kind of data is typically transferred over CDPD?

The correct answer is the transfer of data via unused bandwidth of mobile frequencies. CDPD, or Cellular Digital Packet Data, is a technology that was designed to utilize spare capacity in cellular voice channels to transmit digital data packets. This approach allows for efficient use of the frequency spectrum, given that voice channels often have unused capacity that can be leveraged for data transmission without interfering with voice service.

Focusing on this, CDPD enables devices to send and receive data while still allowing voice calls to occur on the same cellular network. By using this unused bandwidth, CDPD provides a means for mobile devices to access data services like email and basic internet browsing, effectively combining voice and data over existing cellular infrastructure.

The other options do not accurately represent the nature of CDPD. For instance, while voice calls and other forms of data may be involved in various technologies, CDPD specifically does not primarily cater to voice calls or high-speed internet content in the way suggested. Video streaming is also not a primary use of CDPD due to bandwidth limitations typical of that time period when CDPD was primarily in use. Thus, the focus on utilizing unused mobile frequency bandwidth distinctly aligns with the operational function of CDPD.