4 Network Architecture - ST3...

UMTS Basics, Version 3.0

T.O.P. BusinessInteractive GmbH

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4 Network Architecture



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4.1 UMTS Network Architecture ........................................................3 4.2 UMTS Core Network (1/2).............................................................4 4.2 UMTS Core Network (2/2).............................................................5 4.3 Core Network Functions ..............................................................6 4.3.1 Connection Management (1/2)..................................................7 4.3.1 Connection Management (1/2)..................................................8 4.3.2 Session Management................................................................9 4.3.3 Mobility Management (1/2)......................................................10 4.3.3 Mobility Management (2/2)......................................................11 4.4 Functional Structure of the Core Networks (Phase 1) .............12 4.5 Universal Terrestrial Radio Access Network............................13 4.6 Functional Structure of the UTRAN ..........................................14 4.7 UTRAN Functions.......................................................................15 4.7.1 Radio Resource Control..........................................................16 4.7.2 Admission Control ..................................................................17 4.7.3 Congestion Control .................................................................18 4.7.4 Code Allocation .......................................................................19 4.7.5 Power Control (1/4).................................................................20 4.7.5 Power Control (2/4).................................................................21 4.7.5 Power Control (3/4).................................................................22 4.7.5 Power Control (4/4).................................................................23 4.7.6 Handover Control (1/4)............................................................24 4.7.6 Handover Control (2/4)............................................................25 4.7.6 Handover Control (3/4)............................................................26 4.7.6 Handover Control (4/4)............................................................27 4.7.7 Macrodiversity .........................................................................28



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4.1 UMTS Network Architecture

The UMTS Network Architecture consists of the Core Network (CN) and the Universal Terrestrial Radio Access Network (UTRAN). The interface between the Core Network and the UTRAN is called Iu. The Core Network (CN) is responsible for making connections in the UMTS network. The UTRAN provides the air interface Uu to the User Equipment. Now let us have a closer look at the Core Network.



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4.2 UMTS Core Network (1/2)

The Core Network consists of 3 domains: - the circuit-switched domain - the register and service domain - the packet-switched domain



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4.2 UMTS Core Network (2/2)

The circuit-switched domain is a modified version of the Network Switching Subsystem (NSS), which we know from GSM. It is circuit-switched and consists of the Mobile Services Switching Center (MSC), which has been adapted for UMTS, and the Visitor Location Register (VLR). The Register Domain consists of the Home Location Register (HLR), the Equipment Identity Register (EIR) and the Authentication Center (AuC). These are also used in GSM. The Service Domain consists of the Intelligent Network (IN) and other Service Development Platforms. The Packet Switched Domain is a packet-switched network based on the current GPRS architecture.



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4.3 Core Network Functions

The Core Network provides the following functions: - Connection Management (CM), which provides the bearer services and the procedures for circuit-switched connections - Session Management (SM), which is responsible for the set up, monitoring and release of a packet-switched connection - and - Mobility Management (MM), which is used to determine the location of a User Equipment so a connection can be set up.



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4.3.1 Connection Management (1/2)

Connection Management (CM) covers various functions: depending on the different services used, Bearer Management offers every type of data transfer. These services can be Real Time services (RT), with a fixed delay and regular bitrate, or Non-Real Time services (NRT), with a variable delay. Real time services need a fixed bitrate, for which a circuit-switched connection is typically used. The connection between the User Equipment and the Core Network (CN) is called Radio Access Bearer (RAB). The Core Network initiates the setup, modification, monitoring and release of an RAB. The UTRAN (Universal Terrestrial Radio Access Network) carries out these functions.



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4.3.1 Connection Management (1/2)

Call Control (CC) defines the procedures used for setup, monitoring and release of the mobile terminated and mobile originated calls in the circuit-switched domain, e.g. in the MSC. The supplementary services are not necessary for connection setup. They are directly related to a call and make network use more comfortable. Call forwarding and voice mail are typical supplementary services. The short message service is also a task of the Core Network and works the same way as in GSM.



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4.3.2 Session Management

Session Management defines the set up, monitoring and release of a packet-switched connection. For this purpose, information defined in the PDP Context is used, such as the quality of service. The PDP Context can be in two different states: - PDP Context Inactive - or - PDP Context Active - If the PDP Context is inactive, no PDP address is activated, in other words the User Equipment can not be contacted. - After a successful PDP Context Activation Request, the User Equipment switches to PDP Context Active. The User Equipment is assigned a PDP address. Now the User Equipment can be contacted, and the routing- and locating information is updated as soon as the User Equipment changes its location area. - In the PDP Context Active state the existing PDP Context can be modified. Further PDP Contexts can be activated in addition to the existing PDP Context. - The User Equipment can switch to the inactive state through a Packet Data Detach or after deactivation of the last PDP Context.



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4.3.3 Mobility Management (1/2)

Mobility Management serves to locate a User Equipment so a connection can be set up. The circuit-switched Core Network (CN) recognizes three states for a User Equipment. - Detached: The User Equipment is switched off. - Idle: The User Equipment is still not connected, but a signaling connection can be activated if required. - Connected: An active connection exists. The location area of a User Equipment is stored in the Register Domain (HLR and VLR) and must be updated as soon as the User Equipment changes its location while in idle mode. This procedure is called location update. If the User Equipment changes its location while there is an active connection, the connection can be taken over by a different cell. This procedure is triggered by changes in the quality of the radio link and is called handover. The handover is part of the Radio Resource Management in the UTRAN.



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4.3.3 Mobility Management (2/2)

The packet-switched Core Network (CN) recognizes three states for a User Equipment: Idle, Ready, Standby First, the User Equipment is in the idle state. It is not connected to the packet-switched Core Network. In order to change to Ready State, the User Equipment must be logged on to the network. In this case, the procedure is called GPRS Attach. Now the User Equipment has been recognized by the network and can send as well as receive data. Cell and Routing Area Updates are also performed in this state. After a certain period without any data being transferred, a timer switches the User Equipment to Standby. This switching can also be forced by the network. In standby, the User Equipment only performs Routing Area Updates. As soon as any data is transferred from or to the User Equipment, it automatically switches from standby to ready. Logging a User Equipment off the network is called GPRS Detach. After that, the User Equipment returns to the idle state, in other words it is no longer attached to the GPRS network. Switching directly from standby to idle is another possibility. This happens after a special timer expires. In addition, the "delete" command allows switching from the standby or the ready state to the idle state.



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4.4 Functional Structure of the Core Networks (Phase 1)

In the first phase of UMTS, modified network elements from the GSM network will be used in the circuit-switched domain of the Core Network. In the packet-switched domain of the Core Network, the GPRS network will be used. This consists of the network elements Gateway GPRS Support Node and a Serving GPRS Support Node that has been modified for UMTS.



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4.5 Universal Terrestrial Radio Access Network

The UTRAN is similar to the Base Station Subsystem (BSS) in the GSM network. It consists of the network elements responsible for Radio Resource Management. The UTRAN consists of several Radio Network Subsystems (RNS), which are connected to the Core Network via the Iu interface. Each RNS manages the radio resources of all its connections. The RNS consists of a Radio Network Controller (RNC), which is similar to the GSM Base Station Controller (BSC), and of one or more Node Bs, which can be compared to the Base Station (BS) of GSM. The RNC is connected to the Node B via the Iub interface.



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4.6 Functional Structure of the UTRAN

There is a Serving RNS (SRNS) for the connection between the User Equipment and the Core Network. The User Equipment, however, can also be connected to further RNSs, if additional radio resources are required, e.g. for a soft handover. These are then called Drift Radio Network Subsystems (DRNS). The combination of the data, as well as the signaling between the Radio Network Controllers is done by the Iur interface. As the Iub, Iur and Iu interfaces require great capacities, the very powerful Asynchronous Transfer Mode (ATM) is used as a transmission protocol.



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4.7 UTRAN Functions

The UTRAN provides the following functions: - Radio Resource Control - Admission Control - Congestion Control - Code Allocation - Power Control - Handover Control - Macrodiversity



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4.7.1 Radio Resource Control

The Radio Resource Control, that is the management and release of radio resources, is a function of the Radio Network Controller. When a Mobile Originated Call (MOC) is set up, the connection is initiated by the User Equipment. After the Radio Network Controller has made the required resources available, this is notified to the User Equipment. In the case of a Mobile Terminated Call (MTC), the connection is initiated by a paging procedure. This function is part of Mobility Management. Among other things, the Radio Resource Control is required when resources are to be made available for macrodiversity or for improving the quality of the bearer service.



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4.7.2 Admission Control

Admission Control serves to avoid overload situations in the radio network. Based on measurements of the interference and the net load within the concerned cell, the RNC decides whether or not to allow further connections. Each new connection to a user equipment occupies some of the available resources. If there are no more resources available, then the RNC denies the new user equipment access to the network.



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4.7.3 Congestion Control

If the subscribers active in a cell cause an overload situation, Congestion Control provides functions that bring the system back in to a stable and manageable state. Congestion Control can, for example, - force a handover to a different Node B - force a handover to the GSM system - reduce the datarate of individual active subscribers - perform a controlled teardown of active connections.



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4.7.4 Code Allocation

The Radio Network Controller is responsible for Code Allocation, i.e for assigning the codes to the individual connections. The RNC constantly monitors the codes used in its Node Bs. The codes must be unique within a single cell and its neighboring cells.



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4.7.5 Power Control (1/4)

There are several reasons why Power Control is so important in a UMTS network. In UMTS, the same frequencies are used in the active cells and their adjacent cells. Unnecessarily high transmit powers increase the level of interference within a cell, and thus reduce the network capacity. Further information about this can be found in module 5. If two User Equipments at different distances to the Node B transmit with the same power, the User Equipment nearby will drown out the weak signal from the distant User Equipment. This is also called the near-far-problem. Power Control takes over the task of adapting the transmit power so the signals are transmitted with just enough power to be received by the Node B. To this end, UMTS uses three different kinds of Power Control: - Open Loop Power Control - Closed Loop Power Control and - Outer Loop Power Control.



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The Node B uses certain channels to broadcast information about the transmit power that a User Equipment must use when performing its first network access. Since, in CDMA systems, each subscriber represents a source of interference for the other connections - we will speak about that more in module 5 - the random access to the network must be performed with the lowest transmit power possible. Accessing the network with the maximum transmit power, like in GSM, would unduly reduce the UMTS network performance.



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As soon as the connection has been set up, the Node B takes over Power Control. Based on the Signal-to-Interference Ratio (SIR) and the power received, the Node B makes the User Equipment adapt its transmit power. This happens every 0.667 ms. In comparison, the power in GSM is adjusted every 480 ms.



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With Outer Loop Power Control, the RNC adapts the target SIR value in the node B. The basis for this is a constant transmission quality with a determined target bit error rate or frame error rate. But why is it necessary to adapt the target SIR? The constant control of a cell's target SIR is very important, since the target SIR is crucial for the network performance in CDMA systems. A decreasing SIR would manifest itself, for example, in a shrinking cell size or in an impaired network capacity. If the RNC detects that a connection is deteriorating, the target SIR in the node B is increased by a certain amount. The user equipment will gradually increase its transmit power until it has achieved the new value. In the reverse case, the target SIR will be reduced by a certain amount and the user equipment will reduce its transmit power.



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4.7.6 Handover Control (1/4)

In UMTS, as elsewhere, there is the possibility of switching the connection if the transfer quality worsens. This process is called handover. There are two types of handover in UMTS: - Soft Handover (Soft - and Softer Handover) - Hard Handover (Inter-Frequency or Inter-System Handover)



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In the case of a soft handover, the User Equipment is connected to two or more cells using the same frequency. The data streams received are combined in the Radio Network Controller. The connections may even belong to different RNCs. In the opposite direction, the RNC duplicates the data and sends it to the User Equipment via different routes. Transmitting the data over a variety of routes increases the quality of the transmission after the data has been combined at the receiving end. The User Equipment looks for adjacent cells, which are communicated to it by the active Node B. It stores the codes identifying the different cells in a list. While connected, the User Equipment monitors the signal levels of the adjacent cells and communicates these to the active Node B. Based on the signal level and the quality of the information, the RNC instructs the User Equipment to change its Active Set. An "Active Set" includes all the cells the User Equipment is currently connected to. The softer handover is similar to the soft handover, only that the procedure refers to different sectors of one Node B. The data streams can be combined in the Node B.



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A hard handover is performed between two cells using different bearer frequencies. It can also be performed within a Node B in order to switch to another frequency. In this procedure, the User Equipment looks for new bearer frequencies without letting go of the active connection. To achieve this, in the FDD mode, the Wideband CDMA uses a compressed mode to measure other frequencies. The compressed mode creates gaps in the connection, without losing any data. When the handover is performed, the old radio link is released while the connection is taken over by a new cell with a new frequency. There is no interruption in the transmission during this procedure.



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Since UMTS and 2nd generation systems are designed to be interoperable, a handover between the different systems must be possible. If the User Equipment leaves the coverage area of a UMTS network, or if the quality of the signal worsens, the User Equipment switches to a different radio system.



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4.7.7 Macrodiversity

The macrodiversity function allows the User Equipment to be connected to several cells simultaneously. The User Equipment receives the data via different connections, which increases the quality of the communication. In the opposite direction, several Node Bs receive the data stream from one User Equipment. The data streams received are brought together again in the Radio Network Controller. The macrodiversity function allows the User Equipment to transmit with less power, as several paths are available. Thus there is less interference in the individual cells.

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