A Node B is the UMTS equivalent of a GSM Base Transceiver Station (BTS). This is the physical equipment that constitutes a cell site, including the antennas on the tower and the associated radio equipment cabinets on the ground. The primary function of a Node B is to handle the physical radio transmission and reception with the User Equipment over the Uu interface.

Compared to its GSM predecessor, a Node B is often described as being less "intelligent." Much of the high-level decision-making and radio resource control logic is offloaded to the RNC. The Node B's key responsibilities are primarily at the physical layer of communication:

• Air Interface Transmission and Reception: Managing the complexities of transmitting and receiving signals using the WCDMA protocol.
• Modulation and Spreading: Applying the correct spreading codes to outgoing data and demodulating incoming signals.
• Power Control Execution: A Node B does not decide the power levels, but it executes the rapid power control commands sent to it by the RNC, telling individual UEs to increase or decrease their transmit power up to 1500 times per second.
• Soft Handover Support: A Node B provides the physical radio links for a UE to be connected to multiple cells simultaneously during a soft handover, though the process itself is managed by the RNC.

The RNC is the master control unit of the radio access network. It is a powerful network element that manages all the radio resources for a group of Node Bs connected to it, often overseeing a large geographical area. The RNC acts as the bridge between the Core Network and the cell sites, making it the central point of intelligence in the UTRAN. It is connected to the Core Network's MSC (for voice) and SGSN (for data) via the $Iu$ interface, and to its Node Bs via the $Iub$ interface. Its responsibilities are vast and complex:

• Radio Resource Management (RRM): This is the RNC's most critical function. It includes:
  • Admission Control: Deciding whether to accept a new user into a cell based on the current network load and the user's required Quality of Service (QoS). This prevents the cell from becoming overloaded and degrading service for existing users.
  • Code Management: Allocating and managing the limited pool of orthogonal OVSF codes within each cell it controls.
  • Power Control Management: The RNC runs the power control algorithms, analyzing measurement reports and sending commands to Node Bs and UEs to maintain optimal power levels across the cell.
• Mobility Management (Handover Control): The RNC is solely responsible for managing user mobility within the UTRAN. It continuously processes signal strength measurements reported by the UE and decides when a handover is necessary. It orchestrates all types of handovers, including soft handovers between its own Node Bs and inter-RNC handovers by communicating with other RNCs over the $Iur$ interface.
• Data Encryption and Decryption: While the security keys are generated in the Core Network, it is the RNC's job to perform the actual ciphering (encryption) of user data before sending it down to the Node B for transmission, and deciphering data coming from the UE.
• Protocol Conversion: The RNC acts as a translator, converting data between the protocols used on the radio access interfaces ($Iub$, $Iur$) and the protocols used to communicate with the Core Network ($Iu-CS$ and $Iu-PS$).

These elements manage voice calls and other real-time services. In later releases of UMTS, the monolithic MSC from GSM was often split into two more specialized components for better scalability:

• MSC Server: The control part. It handles all the signaling related to call control, such as processing dialed numbers, managing supplementary services (like call waiting), and communicating with the HLR. It makes the decisions but does not touch the actual voice data.
• Media Gateway (MGW): The data plane part. The MGW acts as a simple but high-capacity switch that handles the actual bearer traffic (the digitized voice stream). It sets up the physical path for the voice data based on instructions from the MSC Server. It also performs media conversion when connecting to other networks, such as converting UMTS voice packets to the format used on the traditional PSTN.

These elements are the foundation of UMTS mobile internet access.

• Serving GPRS Support Node (SGSN): This is the workhorse of the PS domain. It is responsible for a wide range of functions for data users within its service area. It authenticates users for data services, tracks their location at a level called a Routing Area, manages billing information, and routes packets between the RNC and the GGSN.
• Gateway GPRS Support Node (GGSN): This is the final gateway between the mobile operator's data network and the public internet. When a user starts a data session, the GGSN allocates them a dynamic IP address. It serves as the logical anchor point for the user's internet connection; all their data traffic to and from the internet passes through the GGSN. It is also responsible for enforcing policies, collecting charging data, and acting as a security firewall.