UMTS Architecture

Core Network (CN), UTRAN structure, Node B, RNC, and network interfaces.

A Tale of Two Networks: The Evolutionary Design of UMTS

The Universal Mobile Telecommunications System (UMTS) was conceived as a bridge between the voice-centric world of 2G/GSM and the data-driven future. To achieve this, its designers made a crucial strategic decision: instead of starting from scratch, they would build upon the successful and globally deployed foundation of the GSM network. This evolutionary approach allowed for a smoother, more cost-effective transition for mobile operators.

The result is a sophisticated architecture that is best understood as a combination of the old and the new. It consists of three fundamental parts:

User Equipment (UE): The 3G-capable mobile device, an evolution of the 2G Mobile Station.
The Core Network (CN): This is the "brain" of the network, responsible for high-level tasks like call routing, user authentication, and connecting to other networks. For UMTS, the Core Network was largely an evolution of the existing GSM Core Network, enhanced to handle high-speed data.
The UMTS Terrestrial Radio Access Network (UTRAN): This was the revolutionary new component of UMTS. The UTRAN is the complete set of network elements responsible for managing the radio interface. It replaced the GSM radio network (the BSS) and introduced the new, powerful WCDMA air interface technology.

This deliberate separation of the Radio Access Network (UTRAN) from the Core Network (CN) is a key design principle. It allows the radio technology to advance independently of the core switching and service infrastructure, a concept that has been central to the evolution of mobile networks ever since.

The UMTS Core Network (CN): An Enhanced Brain

The Core Network is the backbone of the UMTS system, handling subscriber management, billing, and connectivity to the outside world. It evolved directly from the GSM core, but was significantly upgraded to handle the new demands of mobile data. The most important change was its formal division into two parallel sub-domains, allowing it to handle voice and data traffic in the most efficient way possible.

A. The Circuit-Switched (CS) Domain

The CS Domain is the part of the Core Network dedicated to handling traditional, real-time services, most notably voice calls and video calls. It operates on the principle of , where a dedicated end-to-end connection is established and maintained for the entire duration of a call. This domain is essentially a direct evolution of the GSM Network and Switching Subsystem (NSS) and uses very similar components:

Mobile Switching Center (MSC) Server: The central component of the CS domain. It functions as a powerful telephone exchange, responsible for setting up, routing, and managing voice and video calls to and from UMTS users. It connects to the UTRAN on one side and the public telephone network on the other.
Gateway MSC (GMSC): This is a specialized MSC that acts as the entry and exit point for calls originating from or terminating in external circuit-switched networks, like the traditional Public Switched Telephone Network (PSTN).
Visitor Location Register (VLR): A temporary database that is always paired with an MSC. It stores a copy of the profiles for all subscribers who are currently located within the MSC's service area, allowing for rapid call setup without constantly querying the main HLR database.

B. The Packet-Switched (PS) Domain

The PS Domain was the game-changing enhancement for UMTS. It was specifically designed to handle all non-real-time data traffic, such as internet browsing, email, and file downloads. It operates on the principle of , which is far more efficient for the bursty nature of internet traffic. The key components of the PS domain evolved from the GPRS architecture that was added to later GSM networks:

Serving GPRS Support Node (SGSN): The SGSN can be thought of as the "MSC for data". It is the primary node in the PS domain for managing data sessions. Its responsibilities include authenticating the data user, managing their mobility (tracking which cell they are in), establishing and tearing down data connections (known as PDP contexts), and routing data packets to and from the correct RNC in the UTRAN.
Gateway GPRS Support Node (GGSN): The GGSN is the gateway to the outside world for all packet data. It acts as the bridge between the mobile operator's PS domain and external packet data networks, most importantly the internet. Its key roles include assigning an to the user's device, acting as the anchor point for the data session (even as the user moves between SGSNs), enforcing security policies, and routing outgoing packets to the internet and incoming packets to the correct SGSN.

C. Shared Databases of the Core Network

Several critical databases are shared by both the CS and PS domains, providing a unified and consistent view of the subscriber and network equipment.

Home Location Register (HLR): This is the master database of the network. The HLR stores the permanent profile for every subscriber. This includes their unique identities (IMSI), phone numbers (MSISDN), the services they are subscribed to (for both voice and data), their security credentials, and their current general location (the address of the VLR for voice and the address of the SGSN for data).
Authentication Center (AUC): A highly secure database that stores the secret key ( $K_i$ ) for each subscriber's USIM card. It is used by both domains to perform the authentication and key generation procedures.
Equipment Identity Register (EIR): A database containing the unique IMEI numbers of mobile handsets, used to track and block stolen or unauthorized devices from accessing the network.

The UMTS Terrestrial Radio Access Network (UTRAN): The Revolution in the Air

The UTRAN is the completely new part of the UMTS architecture. It represents the entire radio infrastructure and is responsible for managing the sophisticated WCDMA air interface. Its job is to provide the actual wireless connection to the user's device and to efficiently manage the radio spectrum. It consists of two new types of network nodes.

A. Node B

Node B is the UMTS term for what we commonly call a cell tower or base station. It is the functional equivalent of the BTS in a GSM network. A Node B is responsible for the radio transmission and reception in one or more cells. It contains the power amplifiers, transceivers, and antennas that communicate directly with the User Equipment (UE) over the $Uu$ interface. Compared to a GSM BTS, a Node B is often considered a "simpler" device in terms of high-level logic, as much of the complex control and resource management intelligence has been moved up to the Radio Network Controller (RNC). Its main tasks include WCDMA physical layer processing, modulation, and power control as directed by the RNC.

B. Radio Network Controller (RNC)

The RNC is the true brain of the UTRAN. It is a powerful network node that controls and manages a group of Node Bs connected to it. One RNC might control hundreds of Node Bs, overseeing the radio resources for a large geographic area. The RNC is a significant evolution from the GSM Base Station Controller (BSC), taking on a much wider range of complex tasks. Its key responsibilities are:

Radio Resource Management (RRM): This is its most critical role. The RNC manages the WCDMA radio resources, including admitting new users to the network (admission control), managing the power levels of all devices to minimize interference, allocating WCDMA codes to users, and controlling handovers.
Mobility Management: The RNC is the master of handovers. It processes measurement reports from the UE and Node Bs to decide when and where a handover should occur. It is the entity that manages the sophisticated procedure, which is a key advantage of WCDMA.
Data Management: The RNC manages the user data flowing between the Core Network and the Node Bs. This includes ciphering and deciphering data for security on the radio link and managing the data flow through buffers.
Interface Management: The RNC is the central hub of communication within the UTRAN and between the UTRAN and the Core Network, managing the standardized $Iu$ , $Iub$ , and $Iur$ interfaces.

Standardized Interfaces: The Glue Holding UMTS Together

The modular nature of the UMTS architecture is enabled by a set of well-defined, open interfaces that specify exactly how the different network components should communicate with each other. These standardized interfaces are crucial as they allow network operators to build their networks using equipment from multiple different vendors.

A. The Air Interface: $Uu$

This is the radio interface between the User Equipment (UE) and the Node B. It is where the WCDMA radio signals are transmitted and received.

B. The Node B to RNC Interface: $Iub$

This is the communication link between a Node B and its controlling RNC. It is used to carry both user traffic (voice and data packets) and the control signaling messages that the RNC uses to manage the Node B's radio resources.

C. The RNC to RNC Interface: $Iur$

The $Iur$ interface allows two different RNCs to communicate directly with each other. This interface is not always required in small networks, but it is critically important for enabling efficient soft handovers. When a user is at the edge of the coverage areas of cells controlled by two different RNCs, the $Iur$ interface allows these RNCs to coordinate the handover without having to involve the Core Network, which would introduce significant delays. This helps to maintain seamless mobility across large areas.

D. The UTRAN to Core Network Interface: $Iu$

The $Iu$ interface is the vital link connecting the entire UTRAN to the Core Network. Recognizing the dual nature of the CN, this interface is logically split into two separate connections:

The $Iu-CS$ Interface: Connects the RNC to the MSC Server in the Circuit-Switched domain. All traffic for voice and video calls flows over this interface.
The $Iu-PS$ Interface: Connects the RNC to the SGSN in the Packet-Switched domain. All internet and other data traffic flows over this interface.