<P> Circuit - switched connections do have the advantage of providing a constant, guaranteed quality of service, which is useful for real - time applications like video conferencing . </P> <P> The General Packet Radio Service (GPRS) is a packet - switched data transmission protocol, which was incorporated into the GSM standard in 1997 . It is backwards - compatible with systems that use pre-1997 versions of the standard . GPRS does this by sending packets to the local mobile phone mast (BTS) on channels not being used by circuit - switched voice calls or data connections . Multiple GPRS users can share a single unused channel because each of them uses it only for occasional short bursts . </P> <P> The advantage of packet - switched connections is that bandwidth is only used when there is actually data to transmit . This type of connection is thus generally billed by the kilobyte instead of by the second, and is usually a cheaper alternative for applications that only need to send and receive data sporadically, like instant messaging . </P> <P> GPRS is usually described as a 2.5 G technology; see the main article for more information . </P>

Phone and data transmission share the same line