DecodingUMTS/PS

Packet-Switched Services in UMTS

Radio Access Network

In addition to the obvious differences between UMTS and GPRS at the radio interface, packet-switched (PS) services are managed very differently in UMTS. For example, UMTS does not utilize the concept of Temporary Block Flows (TBFs), and most mobility management is managed via soft handover rather than cell change orders. As a result, there is no disruption in data flow as a UMTS handset moves between cells.

Virtually the entire cell capacity can be assigned to a single user in UMTS - via "multi-code", or the simultaneous use of multiple code channels. In GPRS, handsets are hard-limited to using only a single frequency channel at any one time, limiting the maximum cell capacity available to any one user at 8 time slots. However, since a GPRS handset needs time both to transmit the uplink and to measure neighboring cells, the practical peak number of time slots assigned to any one user is 4.

PS services were supported in UMTS from day one, and simultaneous use of voice (CS) and data (PS) services have always been possible. Support for Dual Transfer Mode (DTM) which permits simultaneous voice and data in GSM/GPRS is still comparatively rare.

Core Network

Core networking for UMTS PS services was designed to be backward compatible with GPRS. As a result, the PS core network architecture and signaling are virtually identical. UMTS - like GPRS - utilizes the GPRS Service Management (GSM) and GPRS Mobility Management (GSM) protocols in the core.