s; also performing channel aggregation enables up to 100Kbps. However, those are the theoretical possibilities. In general practice, the operational speed is closer to 28.8Kbps. HSCSD also provides for several levels of error correction, which can be deployed according to the quality of the radio link. This means that with HSCSD, assuming the best of conditions, a time slot that would normally carry 9.6Kbps (in GSM's original CSD mechanism) can carry 14.4Kbps. By using multiple time slots at the same time, it is possible to achieve an increase in maximum transfer rates up to 57.6Kbps. The time slots being used for such higher-speed data transfer must be fully reserved for a single user. Because the GSM network is configured such that normal voice calls take precedence over additional time slots for HSCSD users, at some point during a call, it might not be possible to satisfy an HSCSD user's full request. In order to maintain the user connection, the user will be charged at a higher rate, sometimes multiplied by the number of time slots allocated and based on the amount of time the user has an active connection. This means that HSCSD can be relatively expensive; therefore, GPRS (discussed in the following section) is becoming the more common choice. HSCSD is designed for applications that require continuous availability of band- width, such as large file transfers, remote access to corporate facilities where a user will be working on a remote server for some time, videoconferencing, and multime- dia transmission. HSCSD is offered to subscribers using either voice terminals that support the feature or a special PC card with a built-in GSM phone that turns a notebook computer or other portable device into a complete high-speed mobile office, with the ability to make hands-free voice calls and engage in data transfer. All operators that support HSCSD have implemented national roaming agreements. GPRS, an enhancement to the GSM system, supports data packets. It is the world's most ubiquitous wireless data service, ava