Detailed Network Architecture - 1.3.1.2

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Starting with the user, we have the **User Equipment (UE)**. This is your mobile phone or data device, which consists of the **Mobile Equipment (ME)**, the hardware, and the **USIM (UMTS Subscriber Identity Module)**, an enhanced smart card holding your identity and subscription details. The UE connects wirelessly to the network via the **Uu interface**, which is the WCDMA radio link.

Next, we encounter the **UMTS Terrestrial Radio Access Network, or UTRAN**. This is the 3G-specific radio access part. It's composed of two crucial elements:

1.  **Node B:** These are the base stations, analogous to GSM's BTS. They manage the physical radio interface (Uu), handling the actual transmission and reception of WCDMA signals. Each Node B serves one or more cells. Node Bs connect to the next level, the RNC, via the **Iub interface**.
2.  **Radio Network Controller (RNC):** The RNC is the intelligent controller of the UTRAN. It manages a group of Node Bs, allocating radio resources (Radio Resource Management or RRM), controlling power, and performing crucial security functions like ciphering. A key feature of the RNC is its role in managing **handovers**, especially **soft handovers** where the UE temporarily connects to multiple Node Bs. The RNC communicates with other RNCs via the **Iur interface**, enabling seamless handovers even between areas controlled by different RNCs without involving the Core Network directly. The RNC is also the gateway from the UTRAN to the Core Network, connecting via the **Iu interface**. This Iu interface is logically split into two parts: the **Iu-CS** for circuit-switched traffic like voice, connecting to the MSC, and the **Iu-PS** for packet-switched data, connecting to the SGSN.

Finally, we have the **Core Network (CN)**. This is the central brain of the UMTS system, largely an evolution of the GSM/GPRS Core Network, designed to handle both circuit-switched and packet-switched traffic with high efficiency. The CN is divided into two distinct logical domains:

1.  The **Circuit-Switched (CS) Domain** is dedicated to real-time, connection-oriented services, primarily voice calls and older 3G video calls. Its main components are the **Mobile Switching Centre (MSC)**, which acts as the central switch for calls and manages mobility, the **Visitor Location Register (VLR)**, a temporary database for visiting subscribers, and the **Gateway MSC (GMSC)**, which handles incoming calls from external networks like the PSTN.
2.  The **Packet-Switched (PS) Domain** is optimized for connectionless data services, such as internet Browse, email, and streaming. It builds upon the GPRS architecture. Key elements here are the **Serving GPRS Support Node (SGSN)**, which tracks the mobile's location and manages packet data sessions within its service area, and the **Gateway GPRS Support Node (GGSN)**, which acts as the gateway to external IP networks like the Internet, handling IP address allocation and enforcing QoS.

Supporting both CS and PS domains are common core network elements like the **Home Location Register (HLR)**, the master database of all subscriber information; the **Authentication Centre (AuC)**, which handles security and authentication; and the **Equipment Identity Register (EIR)**, used to track mobile device identities and bar stolen phones.

This detailed and interconnected architecture enables UMTS to provide a rich array of services, from high-quality voice to high-speed mobile internet, defining the capabilities of the 3G era.

Key Concepts

• Three Layered Structure: UE, UTRAN, CN.

• UTRAN Components & Roles: Node B (radio), RNC (radio control, handover intelligence).

• Core Network Domains: CS for voice, PS for data.

• Key Interfaces: Uu, Iub, Iur, Iu (CS/PS) are crucial for understanding data/voice flow.

• Evolution from GSM/GPRS: Recognition that many CN elements are adapted from previous generations.

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• Examples

• Making a Voice Call: Your UE connects to a Node B (Uu). The Node B sends the call request to the RNC (Iub). The RNC forwards it to the MSC in the CS domain (Iu-CS). The MSC processes the call, possibly querying the HLR/VLR, and routes it to the destination.

• Browse the Internet: Your UE connects to a Node B (Uu). The Node B sends your data packets to the RNC (Iub). The RNC forwards the packets to the SGSN in the PS domain (Iu-PS). The SGSN manages your data session and forwards packets to the GGSN. The GGSN then sends your packets to the Internet.

• Soft Handover: As you drive and your phone's signal weakens from one Node B but strengthens from an adjacent one (both potentially under the same RNC, or different RNCs connected via Iur), your RNC manages the seamless transition by maintaining connections to both Node Bs for a brief period, ensuring no call drop or data interruption.

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• Flashcards

• Term: UE

• Definition: User Equipment (mobile phone + USIM).

• Term: UTRAN

• Definition: UMTS Terrestrial Radio Access Network (Node B + RNC).

• Term: Node B

• Definition: UMTS base station (radio transceiver).

• Term: RNC

• Definition: Radio Network Controller, manages Node Bs, handovers, radio resources.

• Term: Core Network

• Definition: Central part of UMTS, composed of CS and PS domains.

• Term: CS Domain

• Definition: Circuit-Switched domain for voice (MSC, VLR, GMSC).

• Term: PS Domain

• Definition: Packet-Switched domain for data (SGSN, GGSN).

• Term: Iur Interface

• Definition: Connects two RNCs, enabling inter-RNC soft handovers.

• Term: Iu Interface

• Definition: Connects RNC to Core Network (Iu-CS to MSC, Iu-PS to SGSN).

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• Memory Aids

• "UE Talks to UTRAN, UTRAN Talks to CN": The basic flow of the architecture.

• "Node B is the Base, RNC is the Boss": To remember their hierarchy in UTRAN.

• "CS for Calls, PS for Pages (and Packets)": To distinguish the Core Network domains.

• "Iu-CS for Circuit, Iu-PS for Packet": To remember the two parts of the Iu interface.

• "Iur for RNC-to-RNC": A simple way to remember the Iur interface.

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Examples & Applications

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