Once connected, the data is sent or received and the user is billed for the time spent online. CSD requires the phone to make a special connection to the network before it can transfer data (like making a voice call) which can take up to 30 seconds.
GSM phones use a technology called CSD (Circuit Switched Data) to transfer data. Whilst GSM (2G) networks are excellent for voice calls, they are limited when it comes to sending and receiving data.
