Universal Mobile Telecommunication System Computer Science Essay

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UMTS (Universal Mobile Telecommunication System) is third generation (3G) mobile communications system being developed within the framework defined by ITU. In order to provide high quality mobile communication at low cost, UMTS is used to accelerate convergence and integration between Telecommunication, Information Technology and service provider. UMTS support 2 Mbit data transfer rate per subscriber, facilitating, the deployment of mobile multimedia services such as downloading of MP3 audio and video streaming [1]. UMTS is continuously bringing the development of new techniques and wireless technologies around the world. These common developments need the common agreement towards the standardization. The main objective of this standardization is to confirm identical specification for different parts. The UMTS was specified to ensure equipment compatibility based on UTRAN standardization. 3GPP specifications are generally based on GSM specifications and generally known as UMTS system. [2][3]. Universal Mobile Telecommunications System (UMTS), also referred to as Wideband Code Division Multiple Access (WCDMA), is one of the most significant advances in the evolution of telecommunications into third-generation (3G) networks. W-CDMA is an asynchronous system where base stations do not have to maintain a system-wide reference time scale. However, each cell or each sector of a cell must now use a different scrambling code. Because there is no global timing reference, the time offsets between signals received from multiple users by a base station in such a system may be quite significant. Since the cross-correlation between scrambling codes assigned to different users is no longer zero, the received signal from any user depends not only on the signal from that user but also on the signals received from all other users over a number of consecutive symbol periods.[4].

3.2 Architecture of UMTS Core Network

The core network controls the connection when the user initiates or terminates the call to access packet switched PS or Circuit Switched CS services. The core network also provide interworking with external networks, It also manage mobility of the user in a home and visited networks, also responsible for location updating, authentication, control charging and accounting. The core network of the UMTS is the combination of GSM network subsystem (NSS) and backbone of GPRS with a complete new radio network (UTRAN).The main difference between the two UMTS & GSM is the radio interfaces and access technique.

Figure 3.1: UMTS Architecture []

3.2.1 Cirrcuit Switched and Packet Switched domains

The core network of UMTS is based on the two separate domains circuit switched CS and packet switched PS. The circuit switched domain provide real time constraints such as video telephony and voice while the packet switched domain provides services like web browsing, email, MMS/SMS. The core circuits switch and packet switch services can be accesses simultaneously. Before access the mobile equipment has to register with the required domain the phase called (IMSI attach) for registration with the CS domain and GPRS attach for registration with the PS domain .TMSI Temporary Mobile Subscriber Identity will be assigned for access to CS domain and PTMSI (Packet. TMSI) for packet services .The termination to the domains are done by IMSI detach and GPRS detach respectively. When the registration is occurred the user equipment is tracked by its location management procedures.LA location area in the CS domain and RA routing area in the PS domain. After registration, the CS & PS can be access in one of the following configuration.

1) Provision of CS & PS Services: The user equipment can access CS & PS services at the same time. Similar to class A GPRS Terminals.

2) Provision of PS services: The user equipment can access PS domain only at one time and no access to CS domain at the same time. It is similar to Class C in GPRS.

3) Provision of CS Services: The user equipment is only register to CS domain, and can access to CS domain.

The mobility management (MM) context is activated for the registered domain. The procedures like mobility, call connection and security management is involved in the context, which represent a logical connection with the associated domain. The information about temporary identifiers (TMSI or P-TMSI) and actual area LA and RA Two independent MM context are created in the user equipment when it is attached to both domains. NON-Access Stratum hands them. The distinction is not clear between two domains as far as radio interface is concerned. The data and signaling of CS and PS is conveyed via some physical connection under control of Access Stratum (AS).Registration and location updating is performed by user equipment for CS & PS domain. [5]. Network Elements in the UMTS core Network not

included in Reference Architecture

There are some network entities in the UMTS core network that can be integrated in the architecture of the UMTS network for the additional tele services or applications. There integration is optional and implementation does not affect the basic function of elements in the reference core network. These entities are below. [6].

ï‚· The Interworking Function (IWF):

It is basically located in the MSC, provides a data bridge between the CS domain and the fixed networks such as PSTN, ISDN or packet data networks. Its functionality comprises signaling control and protocol.

ï‚· The SMS Service Center (SC):

It is in charge of SMS message delivery and is common to CS and PS domain. If the mobile terminated SMS messages,(SMS-GMSC) SMS Gateway MSC is required to enable communication with the serving SGSN or MSC. In the same manner SMS-interworking MSC function will needed to route SMS messages from the core network to the SC

ï‚· The Cell Broadcast Centre (CBC):

It is responsible to deliver broadcast short messages in the service area. It is a part of the core network and it communicates with the UTRAN using the Iu-BC interface. The entities for location service (LCS) provision that will enable the system to determine the geographical position of the UMTS user.

The entities which are related to CAMEL which proposes a set of functions and procedures based on the intelligent network concept which make operator specific services available to UMTS (and GSM) users who roam outside their home network. The main entity with in the CAMEL architecture is the GSM Control Function (gsm SCF).

Figure 3.3 give more complete architecture of the UMTS core network. The figure shows the Charging Function (CGF) in the PS domain that collects and validates records traffic data information. The results are first formatted and then sent to the network billing system for processing. UMTS Protocol Architecture Based On "Stratum"

A set of protocols which have the same functional role in the network are called Stratum.

a) Access Stratum:

The function of the access stratum is to support the NAS non access stratum. This will includes the functions and protocols for the transport of information across the UMTS terrestrial access network (UTRAN and air interface).AS protocols are presented in the lower layer of IU and Uu interfaces. The transport of information between UE and Core network is enabled. The protocols in the Uu interface referred to radio protocols. Which enable setup and control the radio related information to coordinate the use of radio resources between UE and UTRAN In Iu interface AS Is referred as Iu protocols which enables the setup and control the of Iu bearers. In Iu interface AS applies mechanisms to convey data and signaling information from UTRAN to CN serving access node (MSc or SGSN) from functional point of view these protocols are divided into two plane.

b) Non Access Stratum:

The protocols outside the access stratum are included in non-access Stratum (NAS) is a functional layer running between user equipment and the core network CN. The layer supports traffic a signaling messaging between UE and CN. They concern all data and signaling messages between core network and user equipment on radio access technology.

3.2.2 Network Elements Of CS domain

The circuit switch domain of the core network consist of the following elements.[8].

ï‚· The Mobile-services Switching Center (MSC)

It connects the UTRN and other MSCs It manages the mobility and registration of the subscriber allocates the physical resources in combination with UTRN. Call routing; handover, roaming, charging and accounting are also control by MSC.

ï‚· The Visitor Location Register (VLR)

It is a database which store the information of the user equipment which is located in the local area LA covered by VLR. This stores MSNR (Mobile Subscriber Roaming Number), TMSI, LA and supplementary services like IMSI, MSISDN, and HLR address one or more MSc can be linked with VLR and may be embedded within the same MSC equipment

ï‚· The Gateway MSC (GMSC)

The GMSC collect location information of the UE call to the MSC serving the UE current instant. It also ensures the internet functionality with other networks like PSTN and N-ISDN.

ï‚· The Home Location Register (HLR):

This is the data base which is shared by CS and PS domains contain static information like MSISDN, IMSI, and UMTS subscription information. In the CS domain, dynamic information like current VLR address is used to rout incoming calls towards MSC. In the PS Domain the address of the serving SGSN contain in HLR.

ï‚· The Equipment Identity Register (EIR)

The EIR shared data base by CS and PS which maintain a list of mobile equipments to prevent calls from stolen or unauthorized mobile.

ï‚· The Authentication Center (AuC):

This protected data base accessed by HLR contains USIM secret keys of each subscriber.

3.2.3 Network Elements Of The PS domain:

The modified version of the GPRS network is UMTS PS domain. In the core network the PS domain co-exist with a CS domain and sharing HLR, EIR and Auc database. Two additional physical nodes will be required to support PS domain services that are serving GPRS Support Node (SGSN) and the Gateway GPRS Node (GGSN). The SGSN responsibility is to communication between UMTS user and PS domain within the serving area. It is also responsible for user authentication, Ciphering, Integrity, Charging and mobility management procedure for UMTS users. Besides of data transfer and routing the SGSN has embedded VLR functionality. The GGSN responsibility is logical interface to external packet data network (PDN).The protocol used by PDN is name as PDP. The data packet from SGSN is converted into PDP format by GGSN and is sent to external network. Network address (IP) can also be allocated dynamically by GGSN. The GGSN and SGSN in the core network are connected by IP based GPRS backbone which can be intra PMLN. Roaming agreement is required in case of connecting with different PMLN. If it connect the GGSN/SGSN of the same GPRS provider or inter-PMLN or when connecting GGSN/SGSN to different PLMNs. In the second case there will be roaming agreement between providers.

Figure 3.2: NETWORK ELEMENTS IN CS & PS DOMAIN Protocol Architecture in the PS domain

The interface between the physical nodes in the PS domain is identified by G and identical to one that used in the GPRS network. However the Gb interface is not used anymore and replaced by Iu-Ps interface which enables the UTRAN-SGSN interaction. In the CS domain the SS7 and MAP architecture are used to interface the SGSN with the EIR Gf interface) and the HLR (Gr interface).In contrast to the CS domain PS domain use GPT (GPRS tunneling protocol) for enabling communication between SGSN and GGSN Through Gn interface using tunneling principle. The vertical connection between GGSN and SGSN or between two GGSN or SGSN is a GTP Tunnel. All types of PDP (X.25 or IP) data are always encapsulated into IP datagram and then it is tunneled through GPRS backbone. As shown in the figure 3.5 this tunnel is also known as Tunnel Endpoint Identified (TIED). A similar tunneling procedure is use between the SGSN and UTRAN. [9] [10].


3.3 UMTS access network:

The radio access network of UMTS is called UTRAN/Universal Terrestrial Radio Access Network. Which is responsible for radio resource, data, signaling traffic exchange between UE and CN and handle withdrawal and allocation of radio bearers required for traffic support and control to some extent UE mobility and network access technology is based on WCDMA? Table3.5 shows the architecture of UTRAN. We have compared the component and interfaces of UMTS with that of GSM. These are open interfaces and interworking with different manufacture can be done. The mobility is handled by UTRAN at cell and URA (UTRAN Registration area) level. This is independent from mobility management handled by CN. [11][12].

Table 3.1: UMTS Interfaces and their equivalents in GSM []



Equivalent in GSM












Node B to RNC



3.3.1 The Radio Network Sub-System (RNS)

There are one or more radio sub systems known as radio network subsystem RNS and




RLC Sublayer

equivalent BSS in GSM. It is composed of one RNC radio network controller and one or more Node Bs Connected by Iub Interface. Radio Network Controller (RNC)

Radio network controller is control unit of UTRAN, performs various tasks and controls all radio resources within RNS. It is same as BSC in GSM. Most of protocols between RAN and UE are implemented in the RNC. It communicates over Iu Interface with a maximum of one fixed network nodes SGSN and MSC. Each RNC is allocated to an SGSN and MSC. It will also have the option of using Iur-interface to communicate over core network CN with neighboring RNCs. The RNC is responsible for the following Tasks.

a) Call Admission Control:

In contrast to GSM, CDMA in UMTS provide a large number of possible channels at the radio interface, but not all of these can be used at the same time. Because by increasing the

MAC Sublayer

Layer 1

number of channels interference can occur. So the RNC for individual cal must calculate traffic load for each cell. On the basis of this information Call Admission Control CAC decides whether the interference level after the channel request is occupied is acceptable and, if necessary, rejects the call.

b) Radio Resource management:

The RNC manages the radio resource in all the cells attached to it. Utilization level, priority control and interface calculation is responsibility of RNC.

c) Radio bearer set up and release:

The radio bearer in UMTS is the radio data channel within access stratum above the radio Link Control (RLC) sub layer. The RNC is setup. The responsibility of RNC is setting up, maintaining and ultimately releasing radio bearers as required.

d) CODE Allocation:

In UMTS CDMA codes are managed in code tree. The RNC allocates part of these codes to each mobile and during the course of a connection the allocation can be changed.

e) Power Control:

CDMA network will work efficiently when the transmitter power of all mobile users is controlled. The actual fast control process take place in Node B but the target control values are established in RNC. The RNC only perform (counter loop ) loose power control to minimize the cell interference between the adjacent cell with in the RNS and between nearby RNS.

f) Packet Scheduling:

The same resource at radio interface is share by the mobile users in the packet data transmission. The cyclical allocation transmission capacity to mobile station individually is the responsibility of RNC.

g) Handover control and RNS relocation:

On the signal strength of the Node B and UE. The RNC, decide another suitable cell connection. When the UE moved out of the range of one RNC .A new RNC is chosen for the user this called RNS allocation.

h) Encryption:

The data is encrypted in the RNC, which is arriving for transmission from fixed network.

i) Protocol conversion.

The communication between RNCs, CN and connected Node B must handle by RNC.

j) ATM Switching

The communication path between RNC and Node Bs and between CN and RNC normally based on ATM router. The RNC will be to connect and switch ATM connection to communicate between different nodes. Iu Core network (Iucs MSC/GSM, Iups SGSN/GPRS).Iur Other RNCs, Iub: Base station is the different interfaces.

k) Operation and Maintenance (O&M)

The data available must be transmitted over interface defined to an OMC operations and maintenance centre. Node B:

In UMTS the node B corresponds to the BTS Base Transceiver Station. Node B can manage one or more cells connected to RNC over interface Iub. It include CDMA receiver which convert the radio interface Iub. It include CDMA Receiver which convert the radio interface signal into data stream and then forward to RNC over Iub Interface. While in contras direction the CDMA transmitter prepare incoming data for transport over radio interface and routes it to power amplifier. Some particularly time critical tasks cannot be stored in RNC due to large distance between Node B and RNC. The RNC have exact picture of the cell current situation so that can make sensible decision on power control, handover and call admission control. The mobile station and node B continuously measure the quality of connection and interference level and the result is transmitted to RNC. In some special case the splitting and combining data stream of different sectors is handle by Node B.

3.4 UMTS Protocol Stack

The UMTS protocol stack can be divided into

1) Control Plane

In control plane the control information's are exchanged

2) User Plane

Actual data is broadcasted between the users in user plane.

3) Management Plane

In management plane the individual layers configuration is done

Control Plane User Plane

Network Layer

Data Link Layer

Physical Layer


In OSI model the UMTS is put into operation on three layers.

ï‚· Physical Layer

ï‚· Data Link Layer

ï‚· Network Layer

3.4.1 Physical Layer:

The purpose of the physical layer is to condition the digital data from higher layers so that it can be transmitted over a mobile radio channel reliably. In the transmit direction, it performs such functions as channel coding, interleaving, scrambling, spreading, and modulation. In the receive direction, these functions are reversed so that the transmitted data is recovered at the receiver. The MAC layer delivers user data and signaling over a number of transport channels. The physical layer maps each of these channels into a physical channel and transmits the information over the radio interface. A physical channel is characterized by its associated carrier frequency, scrambling, and channelization codes, the radio frame length, and the relative phase angle when meaningful. A radio frame is 10 ms long and consists of 15 time slots. Because the chip rate is 3.84 Mc/s, there are 38,400 chips in a frame and 2,560 chips in a slot. Except where otherwise indicated, the description of this section applies only to the FDD mode of UMTS.


3.4.2 Data Link Layer:

The MAC layer, as the name implies, determines how different types of information coming from the higher layers over different logical channels should be transmitted over a physical channel on a radio frame (that is, the medium), and controls the timing of those transmissions [7], [15]. It provides the following services to the upper layers: data transfer, reallocation of radio resources and redefinition of MAC parameters, and measurement of the traffic volume and signal quality, and reporting the results to the RRC layer. The MAC layer interacts with the RLC sub layer over a number of logical channels. Data flows on each logical channel are associated with a certain priority based on the attributes of the radio bearer service and the RLC buffer status. For example, if a particular UE is running two applications simultaneously, say, voice and a file transfer in the background, the RRC sub layer may assign a different priority to each of the two applications. Similarly, multiple UEs may be assigned relative priorities as well. To meet these priority requirements, the MAC layer may use some scheduling algorithms and map, say, high-priority data to a high-bit-rate transport format and low-priority data to a low-bit-rate transport format. Thus, the responsibility of the MAC layer is to map each logical channel onto a transport channel, selecting, on the basis of the associated priorities, an appropriate transport format within a Transport Format Combination (TFC) set that is assigned by the RRC layer. When multiple users access a RACH, the MAC layer informs the physical layer of the RACH resources assigned to each user (such as access slots, channelization codes, back-off parameters, and so on). Other functions of the MAC sub layer include the following:

ï‚· Multiplexing higher layer protocol data units (PDUs) onto transport blocks and delivering them to the physical layer on a common transport channel or a dedicated channel

ï‚· Demultiplexing the transport blocks received from the physical layer into higher-layer PDUs and presenting them to the higher layer

ï‚· Measuring traffic volumes on a logical channel and reporting the information to the RRC layer so that it can control the admission of new users and provide required QoS

ï‚· Ciphering of data for transparent mode operation of the RRC layer

3.4.3 Network Layer RLC Functions

As shown in Figure 3.5, the RLC layer interfaces the RRC, PDCP, and BMC layers on one side and the MAC layer on the other. Its main functions are

ï‚· Data transfer

ï‚· Segmentation and reassembly

ï‚· Error detection and correction

ï‚· Flow control

ï‚· Ciphering for the purpose of providing security Packet Data Convergence Protocol (PDCP)

The packet data convergence protocol helps achieve this goal by adapting different network layer protocols to the RLC layer so that the user data can be transferred across the UTRAN transparently. Another function of PDCP is to improve the transmission efficiency for delay-sensitive information such as voice or video. PDCP receives the user data from the higher layers, performs the header compression if requested, and sends the resulting PDCP PDUs to the RLC layer. Broadcast/Multicast (BMC) Protocol

This layer 2 sub-layer is responsible for transferring messages from the network that are to be broadcast or multicast to all mobile stations in a cell. The user data that comes from the higher layers are temporarily saved in the BMC, if necessary, until they are scheduled for transmission. The transfer takes place using the unacknowledged mode service of the RLC on a common traffic channel to the MAC layer.22 User data that is not intended to be broadcast over a cell is passed transparently by the BMC. As for other functions performed, this entity on the network side periodically estimates the volume of the cell broadcast traffic and forwards the information to the RRC layer using an indication primitive. There are two types of messages in BMC-the cell broadcast message from the network and the schedule message that gives the location of broadcast messages in the next schedule period as well as the location of the schedule message for the following period. The BMC layer, on the UE side, determines which messages to forward to its upper layer and sends an indication to the RRC layer. Radio Resource Control Protocol

Radio resources comprise W-CDMA frequencies, different channel types, channelization codes, spreading factors, scrambling codes, the capability to control transmitter power, and so on. The RRC layer manages these radio resources so as to establish, maintain, and release connections between UTRAN and the UE.

ï‚· Broadcasting non-access stratum user data such as cell broadcast messages to UE that are transferred transparently through the UTRAN.

ï‚· Broadcast of system information. The system information includes, among other things, NAS system information, timers and counters used by the UE in the idle mode, user registration area (URA) identity and information for periodic cell and URA updates, parameters for cell selection and reselection, configuration parameters for the common physical channels in the cell or for the common and shared physical channels in the connected mode, uplink interference and dynamic persistence levels for PRACHs, and CPCH parameters to be used in the cell.

ï‚· Paging and notification. Paging messages are usually sent by higher layers to establish a connection. However, paging may also be initiated by the UTRAN to force UEs to read the latest system information or transition, if necessary, to a known state.

ï‚· Initial cell selection and reselection. The UE is required to initiate cell selection on a designated UTRA carrier and camp on it.

ï‚· Management of RRC connections between UE and UTRAN and associated radio resources.

ï‚· Management of radio bearers. A radio bearer is an RLC layer service for transferring user data between UE and the serving radio network controller. The UTRAN may establish a new radio bearer by sending, to a UE, a RADIO BEARER SETUP message from its RRC layer indicating, among other things, the desired uplink and downlink transport channels, the frequency, and the maximum allowed uplink transmit power.

ï‚· Support of mobility functions.

ï‚· Routing higher-layer PDUs.

ï‚· Management of links between opportunity-driven multiple access (ODMA) relay nodes.

ï‚· Providing an interworking capability between UTRAN and a gateway node.

ï‚· Contention resolution in the TDD mode.

ï‚· Arbitration of radio resources on the uplink dedicated control channel.

ï‚· UE measurements and their reporting.

ï‚· Admission control or, its broader equivalent, QoS control.

3.5 UMTS Interface Types

Iu Interface

In UTRAN this interface is used to connect the Core network node with Radio Network Controllers.

IuPS Interface

In Iu interface IuPS (Iu Packet Switched) connects the RNC nodes with the SGSN.

IuCS Interface

In Iu interface (Iu Circuit Switched) is used to connect the RNC node with MSC/VLR.

Iub Interface

This interface is used to connect the RNC with Node B. In UTRAN this interface is used to combine the different protocol of NBAP over SAAL-UNI.

Iur Interface

This interface connects the RNC with other nodes. In URTAN Iur interface combines the SCCP and RNSAP, both are implemented in Broadband SS7 or SIGTRAN.

Uu Interface

The Uu interface makes the connection between the user terminal and RNC through Node B. This interface also supports the combination of Iub Framing Protocol, RRC and RLC/MAC [15].



Before transmission, the voice/data sender spreads data using spreading codes. UMTS uses so called orthogonal variables spreading factor (ovsf).Orthogonal codes are generated by doubling a chipping sequence with and without the flipping. The sign of the chips doubling also result in spreading a bit twice as much as before the code as shown in the figure.

These codes are orthogonal having different spreading factor. One code is never part of the code. The spreading codes are used for transmission separation while scrambling code for user separation.

3.6.2 Spreading Codes (OVSF)

UMTS support multiple services and applications using same connection. It has multiple logical connections on a single physical connection. The services and applications have different performance requirements uses different spreading codes. OVSF codes offer separation of logical connections (channels) on single physical connection (single sender's uplink). Services and applications using different SF receive different bitrates, services/applications=logical channels. The OVSF-codes do not offer separation among physical channels (Users in a cell) Complex (non-scalable) to manage multiple unique OVSF-codes for each user, coordination, signaling, negotiation. Identical OVSF-codes can be used simultaneously by several users in the cell at a time selection is based on the desired quality, radio conditions etc.


3.6.3 Scrambling codes

The main purpose of the scrambling code is the separation of users within cell (uplink, UTRA-FDD), each user have a unique scrambling code (UTRA-TDD) scrambling codes are cell-specific, separation cells. Scrambling do not contribute to further spreading .XORs final chip-sequence with quasi-orthogonal codes 'gold codes'. Quasi-orthogonal have not tight synchronization requires among users UEs for codes to remain orthogonal UMTS do not require synchronization among users, only user and base station. [16].


3. 7 Power Control in UMTS:

The power control is necessary to compensate attenuation and path loss. When the mobile equipment moves toward or away from the base station, the increase or decrease in the signal level occurs respectively. Which is critical for all mobile system? The control of power is also necessary to reduce the C/I co-channel interference. The base station should receive all signal at equal power .The control of power is necessary for reducing noise and interference in a cell. We have different types of power control described below.

3.7.1 Open Loop Power Control:

This is the capability of UE to set initial transmit power. UE measures down link signal (pilot signal from the base station)and set signal power proportional receive power signal. It is used for setting initial uplink and downlink transmitting power at a time of accessing the network. This control is between the RNC and the UE. [4].

3.7.2 Close-Loop Power Control:

In close loop power control base station measures uplink signal from the UE. It analyze and then issues power control command to UE.UE measures down link signal and forwards data to base station. It is more accurate and more sophisticated to processing in a base station. UTRA-FDD used close loop control on both up and down link while UTRA-TDD on downlink. UE measures and reports base station process and adjust.

3.7.3 Inner Loop Power Control:

This is also known as close loop power control. This control is between UE and Node B. The effect of this control Is that the received power is maintained constant even in the fading channel. It reduces ―near-far-terminal and all the signal should be received with equal strength. Distribute interference resource among active UE.

3.7.4 Outer Loop Power Control:

The outer loop power control is related to long-term variations of the channel. Target signal-to-interference SIR is specified. If in case, the received SIR is less the target specified. Then the transmit power will need to be increased. Otherwise, it will need to be decreased. This control is between the RNC and the UE. This is also known as slow close loop power control and happens at the rate of 10-100HZ.it share interference level in the cell (RNS).The RNC order the Node B to lower interference level in it cells. The Node B executes the order via inner loop power control.