register 1_umts architecture (2)

I. UMTS Architecture

1 UTRAN and UMTS Radio Protocols

UMTS Architecture Module Objectives:

• Introduction to UTRAN, • UTRAN Identifiers (RNTI),

• UTRAN Functionalities,

• UTRAN Protocol Models and Protocol Stacks.


UTRAN and UMTS Radio Protocols 2



1 UMTS Network Architecture, Definitions



1.1 UMTS Network Domains With the success of GSM and the increasing demand for pure data transmission services, a new mobile communication system was necessary to support services with higher needs for network resources. For the ETSI (European Telecommunication Standards Institute) UMTS shall fulfill these demands. UMTS (Universal Mobile Telecommunication System) can be considered to be the successor of GSM/GPRS. As GSM also UMTS follows the basic structure of all communication networks. They can be divided into three parts: • User Equipment (UE): The user equipment consists of mobile equipment plus the

hard- and software that is necessary to provide services. The mobile equipment itself mainly has the task to provide access and transport services to the network.

• Access Network (AN): The Access Network is part of the fixed network. Any access

network AN has two tasks. First the AN is responsible to enable the UE (user equipment) to access the network (e.g. establishing radio links between AN and UE). The second task is to transparently transport information between UE and CN (core network). In UMTS the AN has a special name : UTRAN (UMTS Terrestrial Radio Access Network).

• Core Network (CN): The Core Network CN is the second big part of the fixed

network. The CN is the network that is responsible for the basic telecommunication service. This can be switching of circuits or routing of packets. The applications itself can reside within the core network, but can also be in an external network (e.g. a server in the internet).



UTRAN UTRANCN

User Data User Data

Access / TransportSignalling




Non Access Signalling Non Access Signalling

Application Data (User Data)

Menu

UE

M enu

UE

figure 1 Top level design of mobile communication network UMTS and types of information.



In this course we will deal with UMTS Release 3 (corresponds to UMTS Release 1999). In this case the CN (core network) is taken from GSM/GPRS. This means the core network of UMTS Release 3 contains a CS–CN–Domain (Circuit Switched Core Network Domain), which is in fact a standard GSM network, and a PS–CN–Domain (Packet Switched Core Network Domain), formed by the GSM–GPRS network part (SGSN, GGSN). These two parts, PS- and CS–CN–Domain, are independent of each other. For the UTRAN this will mean that it has to serve two core networks.



InternetCNServer

UTRAN

Video/Telephony

CS-CN

Internet Session PS-CN

Menu

UE 1

Menu

UE 2

Menu

UE 3

figure 2 Core network domains and corresponding services provided by UMTS.



To provide a consistent set of term definitions, the 3GPP/ETSI made the following definition, that can be found in TS 23.101. These definitions divide the different network blocks according to their function with respect to a provided service. We have the following so called domains: • User Equipment Domain: The user equipment domain is the mobile part of the

network. It represents the UE as physical entity. There are two sub-domains inside:

USIM domain: The USIM (User Services Identity Module) is the entity that contains the user identity and the user specific settings.

Mobile Equipment domain: This domain is the hard- and software in the

mobile phone, necessary to get access to the network and to support the core network services.

• Infrastructure domain: The infrastructure domain covers the fixed network part of

UMTS, that means all physical entities controlled by the network operators. There are two sub- domains:

Access Network domain: The access network domain is the UTRAN that

serves the user.

Core Network domain: The core network domain represent that part of the fixed network, that is responsible for the basic services (switching, routing, SMS). The core network itself is a physical entity. According to their function for a running service, the following logical domains can be distinguished:

o Serving CN domain: The serving CN domain represent that part of the

CN, that is currently serving the user.

o Home CN domain: The home CN domain is that part of the CN, the home operator of the subscriber controls. In the home CN domain the permanent subscriber data base for the user can be found (HLR).

o Transit CN domain: The transit CN domain covers all CN parts that do

not belong to the serving CN domain, but are used to transport user data.



USIM

MobileEquipment

Cu

UE UTRAN CN

Uu Iu ServingNetwork

UTRAN

TransitNetwork

HomeNetwork

Yu

Zu

Infrastructure

figure 3 Network functional domains of UMTS.



1.2 Communication between Network Functional Domains – Strata

The communication between the different domains can be divided according to their task. In this course we will restrict ourselves to the point of concern, UTRAN. As already mentioned UTRAN has to tasks : • access between UE and UTRAN, • transparent transport of signaling messages (not related to access) between UE

and CN. Therefore we can distinguish three types of signaling between UE, UTRAN and CN: • access stratum (AS) : The access stratum covers all signaling exchange used to

control the access of an UE to the network. Access stratum messages occur between UE and UTRAN and between UTRAN and CN. The difference between the access stratum UE-UTRAN and UTRAN-CN is, that the UTRAN-CN access stratum shall be independent of the radio technology used in UTRAN. This enables the CN to use several different radio access technologies.

• transport stratum : The transport stratum protocols and messages have the task to

transport higher layer PDUs (protocol data units) and user data. Because UTRAN has the task to transparently transport data between UE and CN, there will be transport stratum messages between UE and UTRAN and between UTRAN and CN.

• non- access stratum (NAS) : The non-access stratum covers all messages of

higher layers and user data, that do not deal with access or transport tasks. This covers pure application control (application stratum), service request and control (serving stratum), handling of subscription data and subscriber specific services (home stratum).

The strata are exactly defined in TS 23.101. It has to be noted, that a single protocol can belong to different strata (e.g. RRC belongs to AS and transport stratum).



Transport Stratum

UE UTRAN CN

Access Stratum Access Stratum

Transport StratumTransport Stratum

Non Access Stratum (NAS)

figure 4 Network strata relevant for UTRAN



2 UTRAN Architecture and UTRAN Identifiers



2.1 UTRAN Network Architecture and Entities The radio technology WCDMA, that is used in UMTS, in contained in the UMTS Terrestrial Radio Access Network UTRAN. Now UTRAN is a cellular radio system like GSM. This means that every coverage area of UTRAN is divided into cells. Every cell is served by an antenna providing radio coverage of this cell. The UTRAN architecture is strictly hierarchical. Every cell is served by one and only one so called Node B. The task of the Node B is to control the antennae of every cell and to perform the lowest layer tasks. This means the Node B handles the WCDMA physics. One Node B can handle several cells. The higher access and transport tasks of UTRAN are performed by a RNC (Radio Network Controller). Every Node B is connected to one and only one RNC. Again one RNC can support several Node Bs. The RNC provides for all cells of all connected Node Bs the access and transport tasks between UE and UTRAN, and the RNC is responsible to control the connection to the CN for the UE. One RNC together with all its Node Bs and their cells form a RNS (Radio Network Subsystem). The UTRAN itself consists of one or more RNS. UTRAN knows the following interfaces: • Uu : Interface between UE and Node B (cell). • Iub : Interface between Node B and its controlling RNC. • Iur : Interface between two RNC. This interface is optional, it is necessary to support

soft handover. The Iur interface can be implemented via virtual channels. • Iu : Interface between RNC and CN. In fact one RNC can have at most one Iu

interface to a SGSN (PS-CN domain), one Iu interface to a MSC (CS-CN domain), there can be multiple Iu interfaces to a broadcast domain (e.g. for cell broadcasting).



cell cell cellcellcell cellcell

Node B Node B

RNCRNS

Node B

RNC

Node B

RNS

IurIub Iub

Iub Iub

CS-CN PS-CN BC-CNCN

Iu

figure 5 UTRAN Architecture und functional entities.



2.2 Serving, Controlling and Drift RNC There is a very strict management principle in UTRAN. Together with these principles three terms, indicating the RNC functionality, are connected to. Every RNC can support all three different functionality. It depends on the situation, which functionality has to be applied. The first term, that is going to be discussed, is controlling RNC (C-RNC). Every cell has one and only one C-RNC. The C-RNC of a cell is exactly the RNC that is connected with the Node B serving the cell. The tasks of the controlling RNC covers the following areas: • admission control based on UL interference level and DL transmission power, • system information broadcasting, • allocation / de-allocation of radio bearers, • data transmission and reception. This means the controlling RNC of a cell is responsible for all lower layer functions related to the radio technology.



cell

Iub

Controlling RNC

-admission control

- system information broadcasting

- radio bearer allocation / release( code allocation / release)

- data transmission and reception

C-RNC

Node B

figure 6 Controlling RNC functionality.



For UTRAN the following principle is applied. An UE that is attached to an UTRAN is served by one and only one RNC. This RNC is called the serving RNC (S-RNC). The existence of a serving RNC does not imply that the UE is camped on a cell belonging to the S-RNC. The serving RNC handles all higher layer functions related to radio access and information transport through UTRAN. In detail the S-RNC performs the following functions: • the S-RNC handles the Iu interface towards the CN for this UE, • the S-RNC handles the complete radio resource control for this UE, • location / mobility handling, • ciphering, • backward error correction (layer 2 functionality). In UMTS it is possible that one UE is connected to more than one cell, or connected to a cell, that does not belong to the S-RNC. This means the UE is connected with a cell controlled by a RNC different to the S-RNC. This foreign RNC is called drift RNC (D-RNC). In principle the D-RNC is the C-RNC of a cell the UE is connected to, but its not the S-RNC. Therefore the D-RNC performs the C-RNC functions for the cells not controlled by the S-RNC. When a D-RNC is involved for a UE, then the data streams between UE and UTRAN and UE-CN always pass the S-RNC. In the downlink the S-RNC sends the data to own cells and to the D-RNC (soft handover), this is called splitting. The UE receives all the data streams from the cells, it is connected to, and adds them together (RAKE receiver). In the uplink the S-RNC receives data from the own cells and from the D-RNC. Here the S-RNC combines the data streams. This combination is performed by the S-RNC in the following way : the S-RNC takes only the data frame with the smallest bit error rate, all other data frames will be discarded. The usage of a D-RNC requires a Iur interface between D-RNC and S-RNC. Because the implementation of an Iur interface is optional, it is a matter of network planning, whether the usage of D-RNC is allowed or not. The interface itself does not need to be a physical line, it can be implemented via virtual paths or virtual channels.



Iub

Serving RNC

-Iu interface controlling

- radio resource control

- location / mobility handling

- encryption / integrity check

- backward error correction

- combining / splitting of datastreams

Iub

CN

Iur

Iu

D-RNC S-RNC

Node B Node B

Menu

UE

figure 7 Serving RNC functionality.



2.3 Geographical and Entity Identifier As in GSM there is a need to address different physical, geographical or logical entities within UMTS. Here first of all the geographical and physical entities of UTRAN will be described. 1. PLMN Id :

The PLMN-ID is used to address a PLMN in a world wide unique manner. As in GSM the PLMN-ID consist of a MCC (mobile country code) and a MNC (mobile network code). MCC and MNC are allocated by ITU-T and are specified within ITU-T E.212.

PLMN-ID = MCC + MNC.

2. CN-Domain Ids : CS- and PS core network introduce their own regional area concept. This is the concept of Location Area for CS and the concept of Routing Area for PS. This exactly the same as in GSM/GPRS. We have:

LAI = PLMN-ID + LAC (Location Area Identity/Code) RAI = PLMN-ID + LAC + RAC (Routing Area Identity/Code)

3. RNC Id: Every RNC node has to be uniquely identified within UTRAN. Therefore every RNC gets a RNC-ID. Together with the PLMN-ID the RNC-ID is unique world wide. The RNC-ID will be used to address a RNC via Iu, Iur and Iub interface. For the serving RNC the identifier is called S-RNC-ID, for the drift RNC it is denoted as D-RNC-ID and the controlling RNC has a C-RNC-ID. For one RNC node these identifiers are always the same. The RNC identifier itself is allocated by O&M. Global RNC-ID = PLMN-ID + RNC-ID

4. Cell Id and UTRAN Cell Id: The cell ID C-ID is used to address a cell within a RNS. The cell ID is set by O&M in the C-RNC. Together with the RNC-ID the cell ID forms the UTRAN cell ID UC-Id. UC-ID = RNC-ID + C-ID

5. Local Cell Identifier The local cell identifier is used in the Node B to identify resources. There is a unique relation UC-Id to local cell identifier.



6. Service Area Id : Several cells of one location area can be defined to form a service area. Such a service area is identified with a SAI (service area id): SAI = PLMN-ID + LAC + SAC It can be used to support location based services.

7. URA ID : The UTRAN introduces its own are concept next to LA and RA. This is the UTRAN registration area

MNC2 6 2 0 1MCC

PLMN-ID

LAI PLMN-ID LAC (2byte)

RAI PLMN-ID LAC (2byte) RAC (1byte)

GlobalRNC-ID PLMN-ID RNC-ID (12 bit)

UC-ID CRNC-ID (12 bit) C-ID (28 bit)

SAI PLMN-ID LAC SAC (2 byte)

URA-ID URA ID (2 byte)

Location Area

Routing Area

RNC

UTRAN cell ID

Service Area

UTRAN registrationarea

Public LandMobile Network

figure 8 UTRAN identifiers for geographical areas and UTRAN entities.



2.4 UTRAN specific UE Identifiers The UE and the subscriber can have several identifiers for the PLMN. Typically we can distinguish two types of identifiers according to the point of generation of the identifier: • NAS (non access stratum) identifiers : These identifiers are allocated by the core

network. In detail there are IMSI, TMSI and P-TMSI (and IMEI). • UTRAN identifiers : UTRAN identifiers are always temporary. This means they are

allocated to the UE for the time of the need. After the last procedure the last procedure the identifiers are released.

In this chapter only the UTRAN identifier are of interest. It is a typical principle in communication and computing systems that every entity working on a specific task, allocates its own identifier and handler. This is also the case for UTRAN. Every UTRAN entity like RNC and Node B will provide their special identifier for the UE. These identifiers are called Radio Network Temporary Identifier (RNTI). There are four types of RNTI:

• s-RNTI : The s-RNTI is allocated by the serving RNC. The S-RNC uses the s-RNTI

to address the UE. The D-RNC uses the s-RNTI to identify the UE to the S-RNC. The s-RNTI uniquely addresses the UE in the S-RNC.

• d-RNTI : The d-RNTI is allocated by a D-RNC, but the d-RNTI is never used on the

air interface Uu. Instead the S-RNC uses the d-RNTI to identify the UE to the D-RNC. The d-RNTI uniquely identifies the UE in the D-RNC.

• c-RNTI : The c-RNTI is allocated by a controlling RNC when the UE accesses a new

cell of this C-RNC. The c-RNTI is unique in the cell. The corresponding C-RNC shall be able to translate the c-RNTI into s-RNTI (if C-RNC=S-RNC) or into d-RNTI (if C-RNC=D-RNC). The c-RNTI is used by UE to identify itself to the C-RNC, and is used by the C-RNC to address the UE.

• u-RNTI : The u-RNTI (UTRAN – RNTI) consist of RNC-Id and s-RNTI

u-RNTI = RNC-ID + s-RNTI.

So the u-RNTI is unique world wide. The u-RNTI will be used by UE and S-RNC to identify the UE on common radio channels and during paging and cell access.



IubIub

CN

Iur

Iu

D-RNC S-RNC

Node B Node B

Menu

UE

RNTI

- allocated by RNCs

- 16 bit length (u-RNTI 32 bit)

- used within UTRANand on Uu only

s-RNTI

d-RNTI

u-RNTI orc-RNTI B, C

u-RNTI orc-RNTI A

s-RNTId-RNTI

c-RNTI B, Cd-RNTIs-RNTI

c-RNTI A

figure 9 Radio Network Temporary Identifiers RNTI and their usage and allocation.



3 UMTS Protocol Stacks – Overview



3.1 Protocols UE – UTRAN – CN for CS Domain In this section the protocol stack used for circuit switched services is discussed. The protocols between UE – UTRAN and UTRAN – CN for the control plane are shown in the first figure. There are the following important protocol layer : • Physical Layer (PHY): The physical layer on the air interface provides access to the

WCDMA radio interface. Therefore it performs spreading, scrambling, modulation, channel coding, rate matching, … .

• Medium Access Control (MAC): The MAC protocol belongs to layer 2. The tasks of

MAC are the control of random access and the multiplexing/de-multiplexing of different UE onto shared radio resources.

• Radio Link Control (RLC): As MAC also the RLC protocol is a layer 2 protocol.

RLC provides three reliability modes for every radio bearer. These modes are : Acknowledged (AM), Unacknowledged (UM) and Transparent (TM).

• Radio Resource Control (RRC): The RRC protocol is the first protocol of layer 3.

The RRC protocol performs all higher layer tasks related to the access stratum on the air interface (e.g. radio bearer setup).

• NAS Protocol: On top of RRC there are the control protocols for the non access

stratum (NAS). For the circuit switched services these are : MM (mobility management), CC (Call Control), SS (Supplementary Services) and SMS (Short Message Service), if it is not provided by the packet switched protocol stack.

• Radio Access Network Application Part (RANAP): RANAP is between UTRAN

and CN. It performs all tasks related to transport stratum for control signaling and access stratum between UTRAN and CN. It is the counterpart to RRC.

• Signaling Connection Control Part (SCCP): The SCCP has mainly transport

tasks. It is used to establish a signaling connection for a UE. So the UE can then be identified by the signaling connection and not by an explicit identifier.

• MTP3B, SAAL, AAL5, ATM: These protocols belong to the transport network

(ATM). They provide a signaling bearer to transport SCCP and RANAP.



RNS MSC

PHY

MAC

RLC

RRC

MM, CC, SS, SMS

PHY

MAC

RLC

RRC

SAAL / AAL5

MTP 3 B

SCCP

RANAP

ATM

Layer 1

Relay

SAAL / AAL5

MTP 3 B

SCCP

RANAP

ATM

Layer 1

MM, CC, SS, SMS

UE

SAAL : Signalling ATM Adaptation LayerATM : Asynchronous Transfer ModeAAL 5 : ATM Adaptation Layer type 5MTP 3B : Message Transfer Part level 3 for Broadband

figure 10 Control protocols between UE-UTRAN-CN for CS domain.



The control plane discussed before is used to exchange signaling for access, transport and service related control. Like all modern communication system also UMTS transports the control signaling and the user data over the same transport network. This immediately implies, that there have to be protocols supporting the user data transfer. In the next figure the protocols used for this purpose are shown. In the lowest layers there are the same protocols as for the control plane. This results from the fact, that user data and control signaling use the same transport system. In detail there are the following protocols involved into the user data transport: • PHY, MAC, RLC: The air interface transport system is built out of PHY, MAC and

RLC as for the control plane. The same basic stack is used for the user plane. • user data stream: The user data streams are generated by the applications using

the circuit switched core network services (switched channels). These data streams are directly input to the RLC.

• ATM: The transport system for the Iu interface between UTRAN and CN is again

ATM. • AAL 2: To provide a circuit switched like transport bearer on Iu, the AAL 2 protocol

is used. This adaptation layer provides a bearer channel (virtual channel of AAL type 2) with certain QoS guarantees. Additionally the any AAL 2 virtual channel includes time stamps in the transport frames. This allows synchronization and timing control between sender and receiver.

• Iu User Plane protocol (Iu UP): The Iu User Plane protocol is on top of AAL2. This

protocol can provide different stages of user data stream support. So the Iu UP protocol can perform backward error correction, data rate controlling and can be used to optimize the buffer sizes for transmission to minimize the delay jitter. This protocol transports the user data and can create own signaling messages. The signaling messages of Iu UP protocols are transmitted as in-band signaling within the AAL 2 virtual channels used for the user data.



RNS

MSC

PHY

MAC

RLC

PHY

MAC

RLC

Relay

SAAL / AAL5

MTP 3 B

SCCP

RANAP

ATM

Layer 1

MM, CC, SS, SMS

UE RNS MSC

PHY

MAC

RLC

PHY

MAC

RLC

AAL 2

ATM

Layer 1

Relay

AAL 2

Iu UP

ATM

Layer 1

UE

Iu UP

User datastreams

User datastreams

figure 11 User plane protocols between UE-UTRAN-CN for CS domain.



3.2 Protocols UE-UTRAN-CN for PS Domain For packet switched service there are different procedures. So there is a need for special protocols for packet switched services. In fact these special protocols are on the higher layers, so that the lower layer will prove to be the same as for the circuit switched services. The packet switched control plane consists of: • PHY, MAC, RLC, RRC: The transport and access stratum protocols on the air

interface are the same for PS and CS. UMTS has been designed to support both types of services, so that there are no special protocols.

• ATM, AAL 5, SAAL, MTP 3B: Also the transport and access stratum on the Iu-PS

interface is similar to the Iu interface towards the MSC. • SCCP, RANAP: SCCP and RANAP are the same as for CS. The SCCP is mainly

used to set up a signaling connection to the SGSN in the core network. RANAP handles all signaling transport and access related tasks.

• NAS protocols: The only special protocols for the packet switched service are the

non access stratum protocols. Because there are essential differences how to handle a packet switched service request, the PS core network has its own mobility management GMM (GPRS Mobility Management). To set up a data session the SM (Session Management) protocol is used. The SMS is in fact the same as for CS.



RNS SGSN

PHY

MAC

RLC

RRC

GMM, SM, SMS

PHY

MAC

RLC

RRC

SAAL / AAL5

MTP 3 B

SCCP

RANAP

ATM

Layer 1

Relay

SAAL / AAL5

MTP 3 B

SCCP

RANAP

ATM

Layer 1

GMM, SM, SMS

UE

SAAL : Signalling ATM Adaptation LayerATM : Asynchronous Transfer ModeAAL 5 : ATM Adaptation Layer type 5MTP 3B : Message Transfer Part level 3 for Broadband

figure 12 Control protocols between UE-UTRAN-CN for PS domain.



In contrast to the control planes, that look very similar for PS and CS, the user plane has important differences. This is clear, because circuit switched data needs other transport mechanisms (switching) as packet switched data (routing). In detail there are the following protocols involved: • user data: The user data for packet switched services is usually dedicated to

external packet data networks (e.g. internet). These external data network have their own special network protocols (e.g. TCP/IP). When a UMTS user wants to be connected with such an external network, the UE has to send packets of this special network protocol, for the UMTS network this is only data. But because of its special role, the network protocol of the external network is called Packet Data Protocol (PDP). It is the task of the UMTS network to provide a tunnel (PDP context) for transparent transport of the PDP packets.

• Packet Data Convergence Protocol (PDCP): This protocol performs header

compression of the PDP packet header. This shall increase the efficiency of the air interface usage.

• RLC, MAC, PHY: The transport layers are the same as for control plane. • GPRS Tunneling Protocol User plane (GTP-U): The PDP packets are transported

in a GTP-U frame on Iu. GTP-U organizes addressing and identification of the originator and destination of the data between RNC and SGSN.

• UDP / IP: To route from RNC to SGSN the standard UDP / IP protocol stack is used.

This is a connection less, unreliable transport service. In principle only routing is performed with UDP / IP.

• AAL 5 / ATM: The UDP / IP datagrammes (packets) are transmitted on ATM using

the adaptation layer 5.



RNS SGSN

PHY

MAC

RLC

PDCP

Application,PDP

PHY

MAC

RLC

PDCP

AAL5

IP

UDP

GTP - U

ATM

Layer 1

Relay

AAL5

IP

UDP

GTP – U

ATM

Layer 1

UE

figure 13 User plane protocols between UE-UTRAN-CN for PS domain.



4 UTRAN Protocol Model and Protocols



4.1 UTRAN Protocol Model for Iu Interfaces The transport system used within UTRAN is ATM. There is an essential difference between the usage of ATM and the use of PCM lines in a GSM – BSS. ATM supports different types of bearer service labeled AAL type 1, AAL type 2, AAL type 3/4 and AAL type 5. In UTRAN only AAL type 2 and AAL type 5 are used. Bearers of AAL type 2 have to be set up with explicit signaling. This means before a AAL type 2 virtual channel can be used, there has to be signaling between the corresponding ATM switches. This behavior results in a new protocol model, where protocols for user bearer set up and release occur. The model is built out of two layers: • Transport Network Layer : The transport network layer consists of all protocols

used for the transport network solution. This includes the physical layer and its transport frame layer, also the bearer service protocols are included.

• Radio Network Layer : The radio network layer contains all protocols, that are

specific to the radio access and transport stratum. Also all other data streams, to be transported through UTRAN, belong to this layer.

This division into layers is also called horizontal structure. There is also a vertical structure. The elements of this vertical structure are planes. A plane is principle nothing else than a protocol stack. More than one plane can coexist next to each other. In detail there are the following planes: • Control Plane : The control plane consists of all application protocols that are used

for radio network controlling. To transport the messages of an application protocol, one or several signaling bearers, provided by the transport network, are necessary.

• User Plane : The user plane supports the data streams for user data. Therefore the

data streams are packed into frame protocols. These frame protocols will be transmitted via data bearers. In contrast to the signaling bearers of the control plane, the data bearers can require to be set up with explicit signaling.

• Transport Network Control Plane : The transport network control plane contains

the ALCAP (Access Link Control Application Part). The ALCAP protocols are used to set up and release the data bearers of the user plane. Also ALCAP messages require a signaling bearer for transmission. It is not necessary to use the ALCAP for



all data bearers. Especially the transport network control plane is not necessary, when pre-configured bearers only are used.

Transport Frame and Physical Layer

SignallingBearer

SignallingBearer

DataBearer

ALCAP

ApplicationProtocols

DataStreams

ControlPlane

UserPlane

Transport NetworkControl Plane

TransportNetwork

Layer

RadioNetwork

Layer

figure 14 UTRAN protocol model for Iu, Iub and Iur interfaces.



The use of the ALCAP is dependent on the type of bearer to be used. The signaling bearers are usually pre-configured. This means there is no dynamical set up and release for signaling bearers. Data bearers have to be set up and released with ALCAP, when they are not pre-configured. In this case the set up runs in the following manner: 1. The set up or release of a bearer is always controlled by an application protocol.

But to avoid the restriction to a single transport system, the application protocols shall not be specific to a certain transport solution. Therefore the application protocol can control the bearer via abstract parameters (QoS parameters) only. This principle is the same as for BICC (Bearer Independent Call Control). To trigger the set up of a bearer first the application protocol starts a procedure to the destination node.

2. After the application protocol triggered the procedure, the ALCAP, that is specific to

the bearer to be set up, performs all necessary procedures to configure the bearer. 3. When the application part receives the notification of a successful bearer set up, the

application protocol procedure can be finished, and the application can be informed to start the data stream transmission.



Transport Frame and Physical Layer

SignallingBearer

SignallingBearer

DataBearer

ALCAP

ApplicationProtocols

DataStreams

Application Protocol : Bearer Setup Request

ALCAP : Bearer Establishment Request

ALCAP : Bearer Establishment Confirmed

Application Protocol : Bearer Setup Complete

1a

1b

2a

2c

2b

3a

3b

NetworkElement A

NetworkElement B

figure 15 Inter-working between control, network control and user plane in UTRAN protocol model.



4.2 Iur Interface Protocols between S-RNC and D-RNC The discussed protocol model is applied to the UTRAN interfaces Iur, Iub and Iu. The application protocol on the Iur interface between two RNCs is the RNSAP (Radio Network Subsystem Application Part). All in all there are the following protocols on the control plane: • RNSAP (Radio Network Subsystem Application Part): The RNSAP protocol is

responsible for the communication between S-RNC and D-RNC. This covers resource allocation for a UE in a cell of the D-RNC, soft handover procedures and procedures to transfer the S-RNC functionality to a D-RNC (SRNS relocation).

• SCCP (Signaling Connection Control Part): The SCCP is used to set up a

signaling connection between S-RNC and D-RNC for the UE. This means the S-RNC sets up one SCCP signaling connection for every D-RNC and UE. The signaling connection will be used for fast identification of the UE in signaling messages.

• MTP 3B, SAAL, AAL 5, ATM: These protocols form the signaling bearer used for

the RNSAP protocol messages. The user plane of the Iur interface has the tasks to transport uplink and downlink data for the UE connected to a drift RNC. This tasks requires the following protocols: • Frame Protocols: The data to and from the UE will be encapsulated into a frame.

These frames are defined by so called frame protocols. These frame protocols also allow traffic management with in-band signaling.

• AAL 2, ATM: The frame protocols, that encapsulate the UE data, are transported

over AAL 2 virtual channels of ATM. These AAL 2 virtual channels have to be set up first.

Because the AAL 2 virtual channels require a dynamical set up, there is a need for a transport network control plane. This plane contains the following protocols: • AAL type 2 signaling protocol: This protocol is an ITU-T protocol, used to set up,

release and modify AAL 2 virtual channels. This is the Iur ALCAP.



• STC, MTP 3B, SAAL, AAL 5, ATM: These protocols provide the signaling bearer for the AAL type 2 signaling protocol. The STC (Signaling Transport Converter) provides functionality for congestion handling and load control. The protocol suite MTP 3B, SAAL, AAL5 and ATM can be shared with the signaling bearer of RNSAP of the control plane.

The physical layer used for to transport the ATM cells on Iur is not specified. It is up to the operator to choose an appropriate physical transmission system (e.g. STM-1, STM-4 or SONET).

Physical Layer

ATM

AAL 5 AAL 5 AAL 2

SAAL

MTP 3-B

SCCP

SAAL

MTP 3-B

STC

AAL type 2 SP

RNSAP Frame Protocols

figure 16 Iur protocol stack.



4.3 Iub Interface Protocols between Node B and C-RNC

The second UTRAN internal interface is the Iub interface between Node B and its controlling RNC. Also on this interface ATM with AAL 2 virtual channels is used. Therefore the transport network control plane is necessary again. The control plane of the Iub interface contains the following protocols: • NBAP (Node B Application Part): The NBAP protocol is the application protocol of

the Iub interface. It organizes all controlling tasks between RNC and Node B (e.g. code allocation, transceiver configuration).

• SAAL, AAL 5, ATM: These protocols constitute the signaling bearer for the NBAP

messages. The user plane of the Iub interface has to transfer the downlink and uplink data to and from the UE. Therefore different frames are defined in the same way as on the Iur interface. In detail the user plane consists of: • Frame Protocols: The Frame Protocols encapsulate the UE data (down- and

uplink) on the Iub interface. • AAL 2, ATM: The frame protocol packets are transmitted via Iub using AAL 2 virtual

channels. So AAL 2, ATM form the data bearer on the Iub interface. As on the Iur interface the Iub interface uses AAL 2 virtual channels for data stream transport. This means that the transport network control plane is necessary for set up, release and modification of AAL 2 virtual channels. The Iub transport network control plane looks similar to Iur: • AAL type 2 signaling protocol: The AAL type 2 SP provides the messages and

function to set up, release and modify AAL 2 virtual channels. • STC, SAAL, AAL 5, ATM: The STC (Signaling Transport Converter), SAAL, AAL 5

and ATM provide the signaling bearer for the AAL type 2 signaling protocol. (Note: The STC here is different to the STC on Iur.)



The physical layer is not standardized. It is up to the operator and vendor to choose an appropriate physical transmission system.

Physical Layer

ATM

AAL 5 AAL 5 AAL 2

SAAL SAAL

STC

AAL type 2 SP

NBAP Frame Protocols

figure 17 Iub interface protocol stack between controlling RNC and Node B.



4.4 Iu-CS Interface Protocols between S-RNC and MSC

In the last section the protocol model for Iur and Iub interface has been discussed. The Iu interface, connecting UTRAN and CN, is also built according to this model. But there are differences between Iu-CS towards the CS- core network domain and Iu-PS towards the PS- core network domain. The control plane for Iu-CS is formed out of the following protocols: • RANAP (Radio Access Network Application Part): The RANAP protocol is

responsible for all access and signaling transport related tasks. It is the application protocol of the Iu-CS interface.

• SCCP (Signaling Connection Control Part): The SCCP is used to set up signaling

connection between RNC and MSC. There will be one and only one SCCP connection UTRAN-MSC for every UE using circuit switched services.

• MTP 3B, SAAL, AAL 5, ATM: These protocols provide the signaling bearer for

RANAP/SCCP messages. The user plane on Iu-CS has to support the transfer of real time circuit switched data streams. Therefore the Iu-CS user plane has the following protocols: • Iu UP (User Plane) protocol: The Iu UP protocol is used to provide additional

support functions for CS data streams on Iu. These functions can be : timing control, data rate control, backward error correction.

• AAL 2, ATM: For the data bearers to transport the data streams the AAL 2 virtual

channels are used. Again the transport network control plane is necessary, because AAL 2 virtual channels need to be set up and released. So the protocol suite on the transport network control plane is the already known stack, consisting of: • AAL type 2 signaling protocol: Used to set up, modify and release AAL 2 virtual

channels.



• STC, MTP 3B, SAAL, AAL 5, ATM: These protocols provide the signaling bearer for

the AAL type 2 signaling protocol messages. The physical layer is not standardized.

Physical Layer

ATM

AAL 5 AAL 5 AAL 2

SAAL

MTP 3-B

SCCP

SAAL

MTP 3-B

STC

AAL type 2 SP

RANAP Iu UP protocol

figure 18 Iu-CS protocol model between serving RNC and MSC.



4.5 Iu-PS Interface Protocols between S-RNC and SGSN

The Iu-PS interface is the interface between RNC and SGSN. The control plane of Iu-PS is similar to the Iu-CS control plane. It consists of: • RANAP: The application protocol for Iu-CS and Iu-PS. • SCCP: Provides signaling connection on Iu-PS. There will be one and only one

SCCP connection between RNC and SGSN for every UE using packet switched service. SCCP connections on Iu-PS and Iu-CS do not affect each other.

• MTP 3B, SAAL, AAL5, ATM: The signaling bearer for SCCP/RANAP. The user plane on Iu-PS is completely different to the user plane of Iu-CS. This is because the traffic to and from SGSN is packet switched, so routing layer are necessary. The UTRAN provides the following protocols on the Iu-PS user plane : • Iu UP protocol: As for Iu-CS the Iu UP protocol can provide additional support

functions for the data stream. In the moment (2001) the packet switched services do not use this protocol.

• GTP-U (GPRS Tunneling Protocol - User plane): GTP-U provides a frame for the

user data to be transported. In a GTP-U frame UE identifiers (IMSI) and other reference number and sequence numbers are contained.

• UDP / IP: The UDP / IP protocol suite is used as network layer between RNC and

SGSN. The main task of these protocols is to route from RNC to SGSN and vice versa.

• AAL 5, ATM: The ATM adaptation layer of type 5 (connection less, variable bit rate,

no synchronization support) is used as bearer for the packets of IP / UDP / GTP-U. The AAL 5 virtual channels do not need to be set up in a dynamical manner. Rather the operator is expected to pre-configure the AAL 5 bearer to be used for the packet transfer. Therefore on Iu-PS there is no need for a transport network control plane, no



bearer set up with explicit signaling is necessary.

Physical Layer

ATM

AAL 5

IP

AAL 5

SAAL

MTP 3-B

SCCP

UDP

GTP-U

RANAP Iu UP protocol

NoTransportNetworkControlPlane

figure 19 Iu-PS protocol stack between serving RNC and SGSN.



5 Protocol Standardization



5.1 Protocol Specifications UMTS and UTRAN is specified by 3GPP (third Generation Partnership Project), which is a project group of ETSI (European Telecommunication Standards Institute). All protocols will be standardized in a set of technical specifications TS available from 3GPP (ftp://ftp.3GPP.org/Specs) or from ETSI – CD – ROMs. For the UTRAN related protocols the 3GPP working group RAN (Radio Access Network) is responsible. The following set of recommendations are interesting for this course :

Topic

3GPP recommendation number

UTRAN General overview TS 25.401 WCDMA physics TS 25.2xx Radio interface protocols TS 25.3xx Iu interface TS 25.41x Iur interface TS 25.42x Iub interface TS 25.43x The non access stratum (NAS) protocols GMM, MM, CC and SM can be found in the recommendation TS 24.008. The recommendation TS 24.007 contains an overview about the protocol stack and the inter-working between NAS protocols and the radio protocols.



Menu

UE

UTRAN CN

Radio ProtocolsTS 25.3xx

Iu InterfaceTS 25.41x

Iur InterfaceTS 25.42x

Iub InterfaceTS 25.43x

NAS protocols (GMM, SM, MM, CC)TS 24.007, TS 24.008

figure 20 Radio and NAS protocol specifications and specification series.

register 1_umts architecture (2)

Documents