CSD and HSCSD
CSD, short for Circuit Switched Data, is the most basic mode of transferring data over a circuit-switched connection like GSM. The connection is established by dialing the number of an ISP, in the same manner that a dial-up connection is started on a land-based telephone line using a PC modem. With CSD you do not need an extra data plan to send data, as you do for GPRS, which costs more. You can use your existing voice minutes. There are two major disadvantages to using CSD, however. First, it takes a long time to connect since it involves dialing a number and waiting for the server to answer the call. Second, it’s slow – data transfer speed is only about 9.6 Kbps. In GSM-based smartphones, this mode is referred to as ‘Dial’ or simply as GSM data. Earlier smartphones such as the Nokia 9290 rely entirely on this mode of data communication.
HSCSD is the high speed version of CSD. HSCSD is another 2.5G standard that supplies a comparable speed to that of GPRS (although on the lower side in many cases), but with a significant difference – the bandwidth is reserved to the smartphone throughout the connection. This is because HSCSD, like CSD and GSM, is a circuit-switched technology. This makes HSCSD better suited for applications that require a constant bit rate, although the practical bandwidth is rather low for good real-time multimedia transfers – which benefit the most from constant bit rates. HSCSD is not widely used due to the high costs of implementation. The Nokia 6600 and the Motorola A920 are examples of smartphones that support HSCSD.