Architecture and Design of the Rainbow Mobile Terminals, Base Stations and Mobility Servers

Transcription

1 Architecture and Design of the Rainbow Mobile Terminals, Base Stations and Mobility Servers G.Fleming 1, G.Nikolaidis 2, L.von Allmen 3, N.Alonistioti 2, M.Maraki 4,A.El-Hoiydi 3, B.Perrin 3 1 University of Limerick, Limerick, Ireland, 2 University of Athens, Athens, Greece, 3 CSEM, Neuchatel, Switzerland, 4 Intracom, Athens, Greece, Contact - Gary.Fleming@ul.ie, Tel , Fax : Abstract : The development and prototyping of a radio independent architecture for UMTS is one of the core objectives of the RAINBOW (Radio Independent Broadband On Wireless) project. The Rainbow mobility server is one of the key components in the Rainbow architecture used to achieve this objective. This is used to connect different radio access systems, via a standard local exchange, in a generic and radio independent manner. The other key components used to achieve this objective are the Mobile Terminal and Base Station. In Rainbow these are designed to be highly generic for different radio access systems and exhibit a high degree radio independence of transport and functionality. 1. Introduction In the development of the Rainbow architecture a division between and transport functions has been used in the design of the considered nodes. In line with this division, transport functions are considered to be relatively dumb functions which manipulate data in a pre-defined manner. Typical transport functions are segmentation and re-assembly and buffering of user data. To assist in the definition of the individual nodes, in this paper, the transport architecture is overviewed in terms of system wide groups of functionality which are then used to describe the Rainbow transport layers. The groups of entities required to configure and interact with these transport entities are then identified and discussed. Following this, the architecture of each of the nodes (i.e. the Mobile Terminal - MT, Base Station - BS and UMTS Mobility Server - UMS ) is treated separately, with emphasis being placed on the interactions between and transport in each node. 2. Overview of Rainbow Architecture The functions of the Rainbow architecture can be divided into 3 groups. These groupings which provide different levels of abstraction to the application (see Fig. 1) are the following : (1) Basic, (2) /Radio Independent, (3) /Mobility Independent. In Rainbow the basis transport functions encompass the radio dependent transport functions used across the radio interface and the transport medium used in the fixed network (i.e. ATM or AAL). In this section only the radio dependent transport layers will be described as the fixed network transport utilises standard B-ISDN/ATM technology. For the third generation systems the radio dependent transport layers are the RTE Enhanced Logical Layer (RELL) and the Radio Adaptation Layer (RAL). The RELL is a demonstrator specific layer which builds on the services provided by a radio interface emulator [1] to provide functionality similar to that provided by the third generation radio interfaces under investigation (i.e. CODIT and ATDMA). The RAL builds on this by abstracting away from the functionality provided by a given radio interface and providing a radio independent service interface. For second generation mobile

2 terminals and BSs (DECT), the DSAL (DECT Signalling Application Layer) abstracts the higher and transport functions away from the DECT lower layers and provides a service interface which is identical to that of the RAL. The second category of the transport layers are the mobility dependent layers. These layers encompasses functions which are not characteristic of a single radio interface but common to many. Typical functions of these layers are handover execution (i.e. switching and bridging), support for macrodiversity (i.e. hard combining and multicasting) and synchronisation. This category consists of two similar layers : the Mobility Adaptation Layer (MAL) and the Mobility InterWorking Unit (M-IWU). The functionality of the mobility dependent layers in DECT is minor as the capabilities for enhanced transport mobility procedures are limited. The third category of transport functions builds on the mobility independent functions of the underlying layers. Two layers exist in this category the Service Adaptation Layer (SAL) and the Signalling Network Layer (SNL). The SAL is a service specific layer which enhances the services of the MAL to provide a set of radio and mobility independent user plane services. This layer is located in the MT and the UMS and is responsible for ARQ between these nodes as well as speech coding, transcoding and interworking of u-plane data. The Signalling Network Layer (SNL) is present in all nodes and is used for the transparent routing of signalling messages [2]. The SNL is also present in DECT MTs and BSs as it provides homogenous handling of the application signalling for all radio access techniques. However, SAL functionality in DECT is limited to speech coding for the DECT MT. Each of these groupings of transport functions is described in detail in a companion paper [3] 3. ling the The functions which are most closely associated with transport can also be separated into three distinct groupings, similar to those used for the transport. These groupings and their relationship with transport can be seen in Fig. 2. The first grouping contains radio dependent entities. The components of this group exist in the MT and BS and are mainly radio interface specific. The most significant radio dependent entities are the Link (LC) and the Radio Bearer (RBC). These entities are responsible for configuring the radio dependent transport links between the MT and the BS. The of the transport group is performed by Q.2931 and is not considered in this paper. MT BS Radio Dependent The second group is mobility dependent. This is responsible for the connections necessary to support user traffic between the MT and the Mobility Server and caters for the handing over of these connections. The main entities in this group are the Resource (RC), HandOver Handler (HOH), the Switching and Bridging (SBC), the Combine and Multicast MT BS Basic Mechanisms UMS / Mobility Independent / Radio Independent Figure 1: Groups of transport functions UMS Mobility Dependent Figure 2 : Grouping of Functions

3 (CMC) and Bearer InterWorking Unit (BC-IWU). In the following sections each of these entities is overviewed and the interaction between these entities and the transport functions in the MAL and M-IWU is considered. The final group identified is the service dependent group. This group is responsible for the of the service specific functions of the SAL. It includes configuration and of transcoders in the mobility server and vocoders in the mobile terminal. In addition this group contains InterWorking (IWC) which s the interworking of mobile to fixed calls and vice versa in the mobility server. 4. Architecture of the nodes Within this section of the paper each of the three considered nodes will be described separately. The design of the UMS is considered for both 2 nd and 3 rd generation systems while the design of the MT and BS is considered in terms of 3 rd generation systems only. This is due to the fact that although much of the modeling in the MT and BS is common for 2 nd and 3 rd generation, this pertains to the higher functions which are not described in detail in this paper. 4.1 Architecture of the MT The different transport layers in the MT and their relationship with the entities is shown in Fig 3. The UMTS Call Agent (UCCA) is responsible for configuring the SAL in the MT, while the MT RC is indirectly responsible for configuring the MAL. The RBC configures the U-plane component of the RAL, while the SBE configures the signalling plane part of the RAL. Finally the LC is responsible for interacting with the lower layers (RELL) to configure them to support specific radio channels. The UCCA is a service independent function responsible for interfacing with the user/application to initiate a call. Following successful interaction with the peer UCC in the UMS the UCCA proceeds to configure the transport plane in the MT. The UCCA also initialises the SAL, however, the function which the UCCA interacts with is dependent on the service type. For voice calls it interacts with Vocoder (VC) to initialise and configure the vocoder in the SAL, for data calls it interacts with the Terminal Adaptation (TAC) to initialise the appropriate data functions (refereed to as Terminal Adaptation Function - TAF) in the SAL. The Resource (RC) establishes a bearer connection between the MT and the UMS. After the bearer connection has been established the RC configures the MAL by initialising the SBF. To establish a connection the RC must interact with the Radio Bearer (RBC). During handover, the RC is triggered by the handover initiation entities to execute the handover. The actions taken by the RC are dependent on the type of handover being performed. The RBC gets radio resources off its peer in the BS and requests the underlying Link (LC) to open a traffic channel(s). The RBC can then initialise a (Multi) Bearer Function (M)BF in the RAL and allocates a RAL-SAP which is used to identify the instance of the (M)BF. This RAL-SAP is then passed to the RC. The purpose of (M)BF is to map one fixed network bearer to (multiple) radio bearer(s). It is configured at initialisation to support a buffer size and depth tailored for the current service and radio interface. To establish a traffic channel the LC communicates with the RELL and returns a RELL-SAP, for each channel, to the RBC. The SBC s the establishment and release of bridges between the application bearer and the radio bearers and the switching between the bearers of the BSs during handover. The SBC is also responsible for deactivating the old radio bearers, and reconfiguring the SBF in the MAL (by removing the binding between the SBF and the old RAL-SAP and binding the SBF to the new RAL- SAP) and then activating the new bearer(s).

4 For the scenarios considered in RAINBOW the MT Combining and Multicast and Function (CMC / CMF ) are located in the radio lower layers (i.e below the RELL). Thus the higher transport layers do not need to be reconfigured during a macrodiversity handover. The Signalling Bearer Entity (SBE) shown in Fig. 3 is responsible for configuring the transport layers to support a dedicated signalling channel (i.e. DCCHs). When the SNL receives a message from the application layer it asks the SBE for the identifier of the RAL-SAP to which the message needs to be delivered. If a DCCH does not exist the SBE interacts with the LC to open a signalling channel. The LC allocates a RELL-SAP for the DCCH and asks the RELL to open a signalling channel. Once the channel is open the LC passes the RELL-SAP to the SBE. The SBE obtains a RAL-SAP and initialises a Segmentation And Re-assembly Function (SARF) specifically for the DCCH. Rainbow 3 rd Generation Mobile Terminal HOC RC / HOH Rainbow 3 rd Generation Base Station RBC RBC BC-IWU UCCA SBC SBE SNL SAL TAF Vocoder TAC VC SBE SNL MAL SBF M-IWU IWSF IWSC Q.2931_SAAL (FBC) LC RAL SARF RELL (M)BF LC RAL SARF RELL (M)BF Figure 3: Architecture of the Mobile Terminal Figure 4 : Architecture of the Base Station 4.2 Architecture of the BS The BS architecture, shown in Fig. 4, and the MT architecture have many similarities. The radio dependent entities are common to the BS and MT. The significant difference between these nodes occurs at the higher layers of transport and. Neither the SAL nor the MAL layers are present in the BS. However, the MAL is replaced by a related layer called the M-IWU, which is led by the BC-IWU and the Interworking and Synchronisation (IWSC). As the BS is connected to ATM it has an layer and an associated Q.2931_SAAL entity. The RBC in the BS is invoked by the receipt of a radio bearer establishment message from its peer in the MT. It interacts with the LC, and the RAL, as described in 4.1 and returns a RAL-SAP to the BC-IWU. The BC-IWU waits for a bearer establishment message, which contains a unique identifier of the ATM connection (e.g VCI/VPI), from the Q.2931_SAAL module. On receipt of this message the BC-IWU triggers the establishment of the radio bearers by interacting with the local RBC. The IWSC initialises the IWSF and binds it to the RAL-SAP and the VCI/VPI. All data coming from the RAL-SAP is then mapped to the given ATM connection and vice versa. The configuration of the signalling transport by the SBE is the same as was performed in the MT. However signalling in macrodiversity requires the SBE to operate somewhat differently, these differences are considered outside of this paper and can be found in [3]. 5. Architecture of the UMS The UMS performs all mobility transport functions together with functions like UMTS call, B-ISDN call and connection and UMTS bearer. The central unit of the UMS is the RC. This is responsible for the co-ordination of the different modules which interact with the transport functions, residing in the User Plane Handler (UPH). The main tasks of RC is to (set-up and release) the bearers during a call. The HOH co-ordinates the handover procedures

5 within the UMS with the support of RC for the transport aspects of the involved bearers. Both, RC and HOH, are designed in such a way, that they can cope with the requirements for Call Setup and execution of Handover for both 2 nd and 3 rd generation radio interfaces. UCC Rainbow Mobility Server CH CCSupA RC HOCA HOH SNL SAL MAL UPH CMF SBF IWF TCF TCC IWC SBC CMC Q2931_SAAL (BC) AAL5_ATM Figure 5 : Architecture of the Rainbow UMS. The Q.2931_SAAL Module corresponds to the lowest level group (i.e. ATM/AAL ) discussed in section 3. The SBC and CMC correspond to the mobility dependent group. The TCC and IWC correspond to the service dependent. The involvement of the above entities during the execution of call setup and handover execution are illustrated in Fig. 6. The RC is triggered by call related entities to establish the necessary bearers for a UMTS call (here a MT to FT speech call is assumed). The RC establishes ATM bearers through the fixed ATM network towards the FT and towards the BS. The RC now has to invoke the appropriate user plane functions, which are required to support the UMTS call (e.g. switching functionality and transcoding). This is achieved through interactions with the respective lers (SBC, TCC) which interact directly with the UPH. At this point the call setup phase is completed and user data can be exchanged end to end. As was mentioned previously, the HO initiation and decision phase are not considered here, therefore description of the HO execution phase is presented. The handover entities are externally triggered for the establishment of the new bearer (towards BS new). The RC is responsible for the establishment of the ATM bearers. The event, that triggers the switching of bearers, is the reception of the HOProceed_req message. This message is received by HOH, which in turn co-ordinates the required actions within the UMS. The HOH first requests bearer related information from the RC, and then requests the establishment of the bridge from the SBC. The aforementioned procedure is generic in the sense that is applied for all the underlying radio interfaces and is differs only in the messages parameters used for each radio interface. The real differences between the various underlying radio technologies and handover algorithms are hidden from the HOH and the other modules. This is achieved through the interactions of SBC-SBF or CMC-CMF (e.g DelimiterDetected is a message, that signals a successful execution of HO for a 3 rd generation MT). A more detailed description of these algorithms and interfaces are provided in [3], [5]. The design of the UMS has followed an evolutionary path and the functionality of the basic entities (e.g. RC, HOH) provide support for multibearer connections. This is achieved by flexible invocation of the respective entities and bearers. The design of the aforementioned functionality has a large impact on the protocols and negotiation mechanisms developed for ling the mono or multi bearer connections required by the various kinds of calls anticipated.

6 CC entities HOC entities UMS internal Information Flows RC HOH IWC SBC Q.2931 UPH Trigger Estab. to FT ActivateSB_req ActivateSB_conf Invoke SBF Estab. to BTSo NewSpeechEP_req NewSpeechEP_conf Invoke TCF HOBearerSetup_req HOBearerSetup_conf Estab. to BTSn HOProceed_req Get Information SetBridge_req Activate Bridge HOProceed_conf SetBridge_conf DelimiterDetected SwitchingCompleted Trigger Release to BTSo Fig. 6 Information Flows within UMS for call setup and handover execution 6. Conclusion The transport and architecture used in MT, BS and UMS shows a high degree of radio independence with the vast majority of the and transport functions being applicable to ATDMA, CODIT and DECT. A division of mobility dependent and service dependent functions was introduced as many transport functions are dependent on user mobility but not tied to any individual radio access technology. The components of the Mobile Station, Base Station and UMTS Mobility Server were identified and the node internal interfaces described. The design and architecture presented has close correspondence to the SDL specification of each node. Most of the components have already been implemented, and initial testing has been performed. It is apparent that expending significant effort on design, architecture and specification, has lead to a very powerful and flexible demonstrator. It exhibits a high degree of future proofness and can be easily extended beyond it initial goals. 7. Acknowledgements This work is part of the RAINBOW Project (AC105), partially funded by the European Commision under the Advanced Communications Technology and Services (ACTS) Programme. 8. References [1] F.Casadevall et al.: A Real Time Emulator for the RAINBOW demonstrator, ". ACTS Mobile Communication Summit, Granada, Spain, November 1996; [2] D Zeller et al.: "Protocols to implement the SNL for COBUCO and Rainbow". ACTS Mobile Communication Summit, Aalborg, Denmark, 7-11 October 1997; [3] A.Saidi et al.: RAINBOW demonstrator transport chain, ". ACTS Mobile Communication Summit, Aalborg, Denmark, 7-11 October 1997; [4] N.H. Loukas et al.: "Call and Handover Protocols for UMTS". ACTS Mobile Communication Summit, Aalborg, Denmark, 7-11 October [5] I. Modeas, G. Nikolaidis, E. Zervas : Handover Switching Scenarios in UMTS. International Workshop on Mobile Communications, Thessaloniki, Greece, September 1996.