25 series - Radio interface access stratum and the radio access network
This diagram shows the three main components of UMTS. The core network handles functions that are independent of the chosen radio access technology (such as routing and billing), while the radio access network handles radio communications to and from the mobile, which is known as the UE. It also shows some of the components within the radio access network. The Node B transmits and receives signals to and from the UE, while the radio network controller manages a group of Node B's. Also shown is an optional component known as the SAS, which is used in UE location measurement.
The 25 series protocols define functions and messages that directly involve the radio access network. These include the internal operation of the radio access network, and the interfaces from the radio access network to the mobile and the core network. They are distinct from the non-access stratum protocols in the 24 series, which define messages between the UE and the core network that pass transparently through the UTRAN. Note that the 25 series includes communications between the radio access network and the core network; the 28 series is hardly used at all.
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