What is CSD?

CSD most likely refers to Circuit-Switched Data. It's a method of transmitting digital data over a circuit-switched network. In the context of mobile telecommunications, it was a technology used in older (2G) mobile networks (like GSM) to provide data services before the introduction of packet-switched data (like GPRS or EDGE). It works by establishing a dedicated circuit (connection) between the mobile device and the network for the duration of the data transmission. It was typically used for things like basic internet access, fax, and short message service (SMS).