Connection Control Signaling in UMTS Access

Call/Connection Control Signaling in UMTS Access Network Nikos H. Loukas1, Marc de Lignie2, Konstantin Georgokitsos3, Toon Norp2 and Johan De Vriendt4 1

University of Athens, Department of Informatics, Greece 2 KPN Research, Leidschendam, The Netherlands 3 Alcatel Telecom, Stuttgart, Germany 4 Alcatel Telecom, Antwerp, Belgium

ABSTRACT: One of the challenges of the Universal Mobile Telecommunication System (UMTS) is a closer interaction between the mobile and the broadband fixed networks (B-ISDN/ATM), in order to use the same fixed network infrastructure and to offer mobile terminal users the same services as available to the fixed terminal users. In this context, the design of the UMTS pursues service, transport and signaling protocol integration with fixed B-ISDN. This paper elaborates only on the signaling integration aspects, where the main emphasis is put on the Call/Connection control signaling protocols. Guidelines and mechanisms towards the definition of a widely accepted signaling access system for the target UMTS are given, while a possible implementation scenario is discussed in the framework of a test-bed implementation of a UMTS access network for the ACTS RAINBOW project. 1. INTRODUCTION The new and emerging broadband and mobile communication networks will be capable of offering a rich set of services and features. Nextgeneration mobile telecommunication networks will be required to co-exist with fiber-optic broadband communication networks, which are expected to become far more ubiquitous during this decade, [1]. Currently, broadband and mobile network operators are already providing several features, most deployed by ad-hoc means, through the use of interworking functions that interwork specific networks, [2]. With the emergence of Broadband ISDN (B-ISDN) and public land mobile networks, and the type of services that these networks will be able to offer, the network interoperability problem has become a critical issue. The Universal Mobile Telecommunication System (UMTS), currently being specified within ETSI1, [3], will form the basis for third generation wireless systems in Europe, and is intended to consolidate today’s diverse and incompatible mobile environments into a seamless radio and network infrastructure, capable of offering a wide range of 1Future

Public Land Mobile Telecommunication System (FPLMTS), [4], is a parallel standard developed by ITU and it is closely aligned with UMTS.

telecommunication services on a global scale. The goal of UMTS is to support a large variety of advanced (constant and variable bit rate) services up to 2 Mbps, designed to support a range of voice, data, video and multimedia applications. In this respect, UMTS should be defined not as a separate overlay network, but as a system that allows true integration of mobile and fixed communication into a single, advanced telecommunications infrastructure, where the latter can handle both call control and mobility management procedures, [5]. The recognition, that ATM has recently emerged as the predominant switching and transport technology for wide and local area future broadband networks, implies that it is desirable for UMTS to provide similar ATM-type service capabilities to the extent possible on the radio interface. In this framework, one basic problem area, that must be addressed for the development of a multimedia-capable mobile system, refers to the signaling protocol requirements necessary to support advanced services in the wireless network part. This paper addresses the aspects related to the UMTS signaling protocol integration into B-ISDN/ATM, where the emphasis is put on the Call/Connection control signaling protocols for the access network. The rest of this paper is organized as follows: Section 2 presents the generic network configuration considered for the target UMTS. In Section 3, the UMTS signaling access control protocol requirements are outlined. Section 4 describes the signaling protocol model proposed for the UMTS access system. In Section 5, an implementation viewpoint of the described signaling protocol architecture in the context of the demonstrator for the ACTS AC015 RAINBOW project is given, which comply with the proposed signaling architecture. Finally, Section 6 contains our concluding remarks. 2. UMTS NETWORK ARCHITECTURE UMTS, aiming at the integration of the many current mobile markets, will have to implement specific technical solutions in different environments (public, business, domestic). Fig. 1 shows the generic, target network configuration, which is generally compliant with the different

application environments considered for UMTS, [5], [7]. UMTS Air Interface

UNI

MSDP

MSCP MSCP

MT Mobile Terminal

BTS

CSS

MSDP

ATM Signalling Network

B-ISDN Local Exchange

Radio Access System

MSCP

B-ISDN Transit Exchange

B-ISDN Fixed Network

= user plane communications = control plane communications MT: Mobile Terminal BTS: Base Tranceiver Station CSS:Cell Site Switch

MSCP: Mobile Service and Control Point MSDP: Mobile Service and Data Point LE: Local Exchange

Fig. 1 - Generic UMTS network architecture In this configuration, the Mobile Terminal (MT) comprises all the call, connection and mobility control functionality required from the user side. The Radio Access System (RAS) implements the radio related functions required to connect the MT to the B-ISDN context. The Cell Site Switch (CSS) comprises the basic switching functionality in the UMTS access network. This functionality may vary from simple interconnection (i.e., metropolitan and local area networks) to complex private branch exchanges (PBXs) in private environments, to complete exchange functionality in the public environment. Finally, the B-ISDN fixed network provides the switching and transmission functions required by UMTS, while the Intelligent Network (IN) part (MSCP, MSDP) provides additional service and control logic required to support mobile users. The basic idea behind integrating mobile and fixed communications is the commonality of the required functionality and the common use of (existing and forthcoming) infrastructure, [6]. In this context, beyond the UMTS/B-ISDN integration scenario of Fig. 1, other (technically feasible) integration scenarios, mainly based on integration with GSM and narrowband ISDN have also been proposed. Evolution from GSM and integration into N-ISDN are attractive solutions enabling the use of existing infrastructure. However, such integration scenarios would not support the advanced services and capabilities foreseen for UMTS due to the inherent limitations of the supporting networks. These solutions might serve as an intermediate step for the introduction phase of UMTS because B-ISDN might be unavailable in some areas. From the above considerations, it is concluded that integration with B-ISDN should be one of the primary goals for UMTS. 3. ACCESS SIGNALING PROTOCOL REQUIREMENTS In the task for full integration with B-ISDN, the UMTS signaling protocols are constrained by the

mobile-specific requirements, in particular those imposed by the radio interface. UMTS signaling requirements within and between the core broadband and intelligent networks do not significantly differ from those of B-ISDN and IN. This means that the signaling solutions that finally are employed by B-ISDN and IN will in general be suitable for UMTS as well, [5]. For the radio access system the situation is rather different. The flexibility that the UMTS RAS is expected to provide puts many requirements on the signaling protocol stack. The first requirement comes from the fact that UMTS is supposed to provide a wireless access to B-ISDN. From the signaling integration viewpoint, this means that it has to base the Call Control (CC) on the access signaling protocol standards that are available for the fixed ATM network. Via Call Control a call can be established between users, and connections can be set up within the call. The user can specify the type of service, the call configuration (addressed parties and number and type of connections) and the ATM transfer capability, the Quality of Service (QoS) requirements and end-to-end compatibility information for each connection. This approach implies that no changes to the existing UNI signaling for indicating the service is required, while permits the use of future extensions of BISDN signaling protocols to UMTS. In today’s wired ATM environment, the usernetwork interface is a fixed port which remains stationary throughout the connection life time. The current B-ISDN User-Network Interface (UNI) protocol stack, [8], [9], is targeted toward carrying a single protocol over a simple point-to-point or point-to-multipoint fixed interface. However, in the UMTS access system mobility causes the users’ Base Transceiver Station (BTS) to change constantly, and the MT’s connections must be transferred from BTS to BTS, through a handover process. The handover functionality assumes that the fixed network of the access part has the capability to dynamically set-up and release bearer connections during the call (Fig. 2). A wellaccepted methodology to support these features is the call and bearer separation at the UNI. In this case, the Call Control association should uniquely identify each active mobile call within RAS. In order to ensure the appropriate transmission quality, which in turn guarantees service quality to the UMTS users, it is often required to use macrodiversity; i.e, to transport the same information along a number of uncorrelated paths between the mobile terminal and a predetermined point of the fixed network. The common points in the mobile terminal and the fixed network must combine the information received from the different

paths and recover the information to be forwarded using suitable combining algorithms. From the access signaling viewpoint, the support of macrodiversity features implies the need of multicasting and "multipoint-to-point" functionality. Fixed Access Call Control association New Bearers establishment Old Bearers release

3

Mobile Terminal

Based on the generic network configuration of UMTS and the above requirements, it becomes easily apparent that more advanced signaling support is required than that offered by the currently available, fixed B-ISDN UNI standards [8], [9]. This is a result of the functional blocks included in each network element of the UMTS access network configuration. Fig. 3 illustrates the allocation of the main functional entities, that is the Call Control (CC) entity, the fixed Bearer Control (BC) entity and the Radio Bearer Control (RBC) entity. UMTS Access System

Handover to a new cell

Fig. 2 - Dynamic handling of bearers2 during handover On the other hand, one of the major enhancements, required in the current signaling standards, towards the definition of an accepted signaling protocol for UMTS access network, is related to the support of the transcoding functionality that the system has to provide. Existing speech, audio and video bearer services have to be extended to allow mobile specific source codings. This requires transcoding (service interworking) inside the network when a mobile and a fixed user communicate. Transcoding transforms the format (coding scheme) of the information transported between the service components used in different parts of the network. The control of transcoding involves the possibility for the user to indicate the source coding for a specific service at call set-up and for the network to transport the required control information towards the interworking unit. Finally, several application environments are expected for UMTS, and their characteristics strongly call for a flexible radio access architecture. The access part of the UMTS will incorporate a number of different radio access interfaces and the mobile users would expect the same or even better QoS, compared to the service quality provided by each system operating independently. In this respect, the signaling protocol stack must be flexible enough to support a number of different RAS topologies and radio interfaces with different allocations of the control functions for each case.

The terms “Bearer” and “Connection” are used with the same meaning throughout this paper. 2

4. CALL/CONNECTION CONTROL ACCESS SIGNALING MODEL

CC

CC

BC

BC

BC

LE

CSS

RBC

RBC

BTS

MT

Fig. 3 - Call/connection control model This architecture considers the MT acting as termination point for call and radio bearer control. Call Control signaling will be an enhanced version of the existing B-ISDN Call Control signaling protocols, [8], [9]. The major enhancements concern the ability to invoke the bearer control functions and the control of the mobile specific user plane functions such as transcoders and macrodiversity combiners. The UMTS BTS acts as a termination point for fixed and radio bearer control, in order to keep it as simple as possible. Since our explicit design objective is to define a generic signaling protocol architecture that is independent of the underlying radio technology, the study of the radio interfacerelated signaling is beyond the scope of this paper. The Local Exchange (LE) is a signaling point for both Call and Bearer Control signaling. The UMTS CSS may have full Call/Bearer Control functionality (especially in private networks), or only Bearer Control functionality (mainly in public access network configurations) depending on the environment considered. With this structure, maximum reuse of the B-ISDN protocols in the access network is pursued, since a direct call control association between the LE and the MT is accomplished, by extending the B-ISDNbased Call Control signaling to the wireless end-

user. This potentially has a better signaling performance at call set-up and handover, helps for an easier and unique identification of each mobile call and permits the support of maximum integration into B-ISDN, although it offers rather low utilization of radio resources. Such an approach, removes the need for a new control association to be set-up during handover, thus reducing processing power requirements and delay. In this way, handover can be performed in an efficient manner by means of signaling procedures i.e., by dynamically handling (establishment and release) the connections in the fixed and radio network part. Moreover, this approach provides the capability to the UMTS users to signal the requested UMTS service in terms of ATM layer parameters, such that no changes to the existing UNI signaling for indicating the service is required, while future extensions of B-ISDN signaling protocols directly apply to UMTS. A key problem in the access signaling model presented above, is the provision of a unique control or signaling channel between the MT and the LE, in order to be guaranteed that signaling sessions are maintained and uniquely identified in the LE and to ensure a proper signaling operation in the UMTS access network. For example, when during a handover a new connection leg is established in the access network, any existing signaling associations between the MT and other network elements have to be maintained. This conflict can be solved by dynamically allocating a different Signaling VPI/VCI (SVPI/SVCI) pair in the fixed network for each session between a particular MT and the LE (or the MSCP), using the services of the metasignaling protocol, [10]. Metasignaling is part of the ATM layer management entity and it is used for the assignment of a unique Signaling VC (SVC) between an end-system and the network. It performs the following functions: assignment, checking and removal of point-to-point and associated broadcast SVC. Metasignaling messages are transferred on the metasignaling channel identified by VPI=0 and VCI=1 into single-cell packets. In the network configuration considered in this paper, it is assumed that the ATM layer is present from the BTS on. In BTS, interworking between the radio link protocol and ATM is performed. For each MT entering in the coverage area of a BTS’s cell, an SVC between BTS and LE is established using the metasignaling channel. After the BTS has obtained the SVCI, it can exchange signaling information using the new SVC. It is assumed that the BTS has the capability to distinguish the signaling information from the data stream before forwarding it to the LE.

Another alternative is to employ a mobile specific Signaling Network Layer (SNL), [11]. The SNL can be used to address the UMTS requirements (e.g., support for a large number of different environments, re-use of fixed network protocols, signaling efficiency etc.) in an effective manner, while resolving the possible contention for particular signaling VCs. The main task of the layer is to hide mobile specific aspects from the signaling applications and the Application layer protocols used by these applications. The function of SNL in the proposed access signaling model is to provide the means to make transparent the presence of a mobile environment to Call Control. To implement these requirements, the SNL has the capability of connectionless operation and routing, dynamic and distributed route updating and support for typeaddressing. A detailed description of the SNL can be found in [11]. 5. CALL/CONNECTION CONTROL IN RAINBOW The generic UMTS Call/Connection control model described above is highly reflected on the signaling protocols that are under specification for the ACTS AC015 Radio Access INdependent Broadband On Wireless (RAINBOW) project. The basic objectives of the project are to demonstrate, through a test-bed implementation of a UMTS access network, the feasibility of a radio interface independent UMTS access system and to study possible solutions for the integration of the UMTS RAS in the B-ISDN and IN context. A general description of the RAINBOW demonstrator can be found in [12]. An important boundary condition in specifying the Call/Connection Control signaling protocols in the RAINBOW demonstrator is to reuse an existing ATM switch without modifying it. This means that all the UMTS-specific functions, including the Call Control functions described above, have to be implemented in additional, enhanced, network elements to minimize the changes to control and transport functionality of the B-ISDN components. The UMTS Mobility Server (UMS), introduced in [13], may be considered for taking care of the special functions, which are required but cannot be easily provided within a standard ATM switching network. The UMS is considered as a solution to support third generation mobility in B-ISDN, [13]. The resulting configuration for the fixed UMTS access network part is as shown in Fig. 4. In this architecture, the switching elements of access part (LE, CSS) should not be modified to follow the service, transport and signaling protocol integration requirements.

- standard ATM transport and switching

BB LE

CSS BTS

BTS

CC MSCP

Mobility Server

Fixed Terminal

ATM Switch CC

BTS

CC

IN CONTROL

Mobility Server

CSS

UMS

UCC BC

BC

BTS

- BB LE: BroadBand Local Exchange - BTS: Base Transceiver Stations (GSM, DECT, ATDMA, CDMA air interfaces)

BTS1

BC

BC

RBC

RBC

BTS2

Fig. 4 - UMS-based architecture of UMTS access network part RBC

The UMS functionalities could be restricted to transport functionalities, so that only the software of the LE needs to be changed when introducing UMTS services. In this case the UMTS Call Control located at the LE hides the presence of UMS from the signaling protocol between MT and LE. The latter intends to be the target solution for a UMS-based access configuration of UMTS, [13]. The UMS-based approach has been employed for the support of mobile specific transport and control functions for the RAINBOW demonstrator access network configuration. In RAINBOW, it is possible to demonstrate an access network with the desirable feature of having separate signaling protocols for Call and Bearer Control. The ATM switch with Q.2931 signaling is used as a general bearer control node, i.e., the same switch functions for both CSS and LE. In combination with the UMS(access) node it operates as a UMTS CSS; in combination with the UMS(core) node it operates as a UMTS LE (Fig. 5). Fixed Terminal ATM Switch MSCP Core

LE

UMS Core

MSCP MSCP Access

UMS CSS1

CSS2

BTS1

UMS Access

BTS2

MT

Fig. 5- RAINBOW demonstrator architecture Analogously, the BTS connection handling receives and processes the primitives from the fixed and radio connection control protocol entities. Its main task is the interworking between the fixed network connection control and the radio interface connection control. Based on the above considerations, Call Control signaling in RAINBOW will not take place between the MT and the LE, but between the MT and the UMS only, as shown in Fig. 6.

UCC

MT

Fig. 6- Call and Bearer Control in RAINBOW Between the MT and the UMS a new Call Control signaling protocol, based on existing B-ISDN UNI standards and satisfying most of the requirements described above, is under specification. The term UMTS Call Control (UCC) is used for the Call Control part. UCC is very similar to Q.2931, because all basic Call Control functions of Q.2931 are also required in the UCC. Such a similarity is in fact a UMTS requirement, so that new signaling capabilities (modules) in the fixed ATM network can be applied in the UMTS access network without change. Moreover, the UCC signaling protocol includes the procedures to invoke the Bearer Control functions during the establishment of a new call, while on the other hand controls the mobile specific user plane functions inside the UMS such as transcoders and macrodiversity combiners. The Bearer Control protocol is based also on Q.2931 and uses the services of the UNI SAAL. Each node along the route of a connection contains Bearer Control. Although the Bearer Control can not be changed because of restrictions put by the ATM switch, it is possible to specify RAINBOWspecific procedures that assign new meanings to existing end-to-end information elements such as the Broadband low layer information (B-LLI), Broadband high layer information (B-HLI) and subaddress information elements. This may be helpful in providing interworking units at the BTS level with the required, possibly radio access specific information. To guarantee proper signaling operation UCC uses the services of the SNL. The function of SNL is to provide the means to make transparent the presence of a mobile environment to the UCC. This function is carried out by intercepting and filtering the messages submitted by the UCC state machine to the SAAL (and vice versa) and by properly interacting and cooperating with the entities that comprise the (IN-based) mobility control.

Normally, both the BTS, CSS/LE may implement the SNL to route the signaling traffic to either the UMS (access or core), the MSCP (access or core) or local application parts. In the demonstrator, however, the MT, BTS, UMS and MSCP will implement the SNL (because of the use of the ATM switch). This still allows to demonstrate location transparency, because some functional entities can be located in either the BTS, UMS(access or core) and MSCP(access or core). The Call/Bearer protocol stacks for the UMTS access network, which come as the result of the above discussion and are in conformance with the Call/Connection control model of Section 3, are shown in Figs. 7 and 8. It has to be noticed that since the fixed terminal of the RAINBOW demonstrator supports standard B-ISDN Call Control, interworking is needed between fixed BISDN Call Control and the UCC. However, this topic will not be covered in this paper. The same applies for the signaling protocol interaction between B-ISDN and IN-based Mobility Control. MT

BTS

CSS/LE

UCC

UMS UCC

SNL

SNL

RADIO LOWER LAYERS

SNL

SNL

RADIO SAAL LOWER ATM LAYERS PHY

SAAL ATM

ATM

ATM

PHY

PHY

PHY

Fig. 7 - UMTS Call Control Protocol stacks MT

BTS

CSS/LE

UMS

BC

BC

(Q.2931)

(Q.2931)

(Q.2931)

(Q.2931)

LOWER

SAAL

SAAL

SAAL

SAAL

LAYERS

ATM

ATM

ATM

ATM

PHY

PHY

PHY

PHY

BC RADIO

RADIO

LOWER LAYERS

BC

Fig. 8 - Bearer Control Protocol stacks 6. CONCLUSIONS In this paper the UMTS to B-ISDN signaling protocol integration aspects were discussed, towards the definition of a widely accepted wireless signaling access system for UMTS. The main focus was put on the design of a Call/Connection control signaling architecture for the UMTS access network. The implementation feasibility of the proposed model was investigated in the context of call/connection control signaling protocol architecture, currently being specified, for the ACTS AC015 RAINBOW project.

7. REFERENCES [1] D. Raychaudhuri, N. D. Wilson, “ATM-Based Transport Architecture for Multiservices Wireless Personal Communication Networks”, IEEE JSAC, vol.12, no.8, pp. 1401-1414 , Oct. 1994. [2] V. Raju, “Intelligent Networking in Broadband and Mobile Networks”, Proc. ICC ’96, pp. 245-249, Dallas, June 1996. [3] ETSI DETR SMG-503-01, “Framework for Network Requirements, Interworking and Integration for UMTS”, Version 1.5.1, June 1995. [4] M.H. Calendar, “Future Public Land Mobile Telecommunication Systems”, IEEE Pers. Comm., Special Issue, Vol. 1, No. 4, fourth quarter 1994. [5] “The European Path Toward UMTS”, IEEE Pers. Comm., Special Issue, Vol. 2 , No. 1, Feb. 1995. [6] T. Norp, “Protocol Stacks for an integrated UMTS User Access Network”, Proc. RACE Mobile Telecommunications Workshop, pp. 310-313, June 1993. [7] “UMTS System Structure Document”, MONET deliverable 70, R2066/BT/PM2/DS/P/070/b2, December 1994. [8] “B-ISDN User Network Interface Layer 3 Specification for Basic Call / Bearer Control”, ITUT Recommendation Q.2931, Dec. 1993. [9] ATM Forum, “UNI Specification”, Version 4.0, 1995. [10] ITU-T Recommendation Q.2120, “B-ISDN Metasignaling Protocol”, May 1993. [11] H. Mitts et al, “Connectionelss Signaling Network Layer for UMTS”, IEEE Pers. Comm. Mag., Vol.3, No.3, June 1996. [12] “RAINBOW demonstrator architecture and services”, AC015/BELL/CIT/DS/P/005/b1, RAINBOW deliverable 01, December 1995. [13] Johan De Vriendt et al, “The UMTS Mobility Server: a Solution to Support 3rd Generation Mobility in ATM”, Proc. Int. Zurich Seminar on Digital Communications, pp. 251-262, Springer, Ed. B. Plattner, Feb. 96. ACKNOWLEDGEMENTS This paper is based on work performed in the framework of the Radio Access INdependent Broadband On Wireless project, funded by the European Commission under project AC015/RAINBOW of the ACTS program. We would like to thank all members of this project for their cooperation.