Part V Appendices Service M odelling: Principles and Applications Vilho Räisänen 2006 John Wiley & Sons, Ltd ISBN: 0-470-01807-0
A 3GPP Bearer Concepts In the following text, we shall review 3GPP (Third Generation Partnership Project) bearer concepts and their relation to quality provisioning. There are minute variations of the details of how exactly this takes place, but we shall describe here a streamlined version that roughly corresponds to 3GPP Release 5. The description is based on 3GPP as described in (3GPP TS 23.107, 2004) and (3GPP TS 23.207, 2004). The 3GPP architecture is based on end-to-end bearers between the end-user and the. The architecture has been designed to support different kinds of s having specific quality requirements and characteristics, and supports the simultaneous use of multiple s. In principle, the high-level 3GPP architecture supports end-to-end negotiation involving signalling external to the mobile network also. The high-level reference picture for 3GPP is shown in Figure A.1. In practice, bearer related signalling takes place only within the network and external quality is handled with Service Level Agreements (SLAs). UE GGSN Remote AP Remote host Local UE IP bearer GGSN Backbone IP network Remote access point Remote host IP bearer layer SGSN Gn/Gp Scope of PDP context Access bearer layer (e.g. UMTS bearer) Figure A.1 High-level end-to-end reference model for 3GPP QoS architecture. From (3GPP TS 23.107, 2004) Service quality within a cellular domain is based on negotiated bearers between enduser equipment and the network. In the current embodiment of the standard, the end-user Service M odelling: Principles and Applications Vilho Räisänen 2006 John Wiley & Sons, Ltd ISBN: 0-470-01807-0
238 Service Modelling: Principles and Applications equipment is responsible for activating the bearer. The range of allowable bearer parameters is provisioned by the network provider. In principle, provisioning could be performed on user and granularity, but in practical implementations the range is provisioned for a class of s and for a group of end-users. For accessing a particular, a new bearer can be activated, or a can be used by modifying an existing one. It is possible to modify the properties of the bearer for the latter purpose. It is useful to note that using an existing bearer may involve multiplexing of multiple non-related flows onto the same bearer. In order to understand 3GPP quality support adequately, it is useful to have a basic understanding of the 3GPP Quality of Service (QoS) architecture. Consequently, we shall review the central architectural concepts prior to discussing bearers and provisioning. Architecture The top-level conceptual illustration of the 3GPP QoS architecture is shown in Figure A.2. It consists of a hierarchy of bearers between functional blocks of the architecture. We shall summarize functional blocks and bearers in the following text, and proceed to describe how different bearers relate to each other. UMTS TE MT RAN CN edge node CN gateway TE End-to end-s TE/MT local bearer UMTS bearer External bearer Radio access bearer CN bearer Radio bearer RAN access bearer Backbone bearer Physical radio bearer Physical bearer Figure A.2 UMTS QoS architecture. From (3GPP TS 23.207, 2004)
3GPP Bearer Concepts 239 The functional blocks in the figure are as follows: TE (left-hand side): Terminal equipment. Can be, for example, laptop computer, Internet tablet, or cellular phone itself MT: Mobile terminal. Functionality which terminates the Universal Mobile Telephony System (UMTS) bearer. MT handles one end of the UMTS bearer negotiation RAN: Radio Access Network. This functionality s access to radio interface resources and handles most of the mobility support such as handovers between base stations (called Node Bs for Wideband Code Division Multiple Access (WCDMA) CN Edge Node: Terminates Radio Access Bearer Service. In practice means Service Gateway GPRS Support Node (SGSN) in the current architecture Core Network (CN) Gateway Node: Endpoint of UMTS bearer in the network. The master node for UMTS bearer negotiation in the network. In practice, GPRS Gateway Support Node (GGSN) fulfils this function TE (right-hand side): Another terminal or. The topmost horizontal entity in the figure is end-to-end, and describes the conceptual bearer between end-user and the other end of the communication. It is supported by three lower-level bearers, namely, TE/MT Local Bearer Service, UMTS Bearer Service, and External Bearer Service. These relationships mean that the lower level bearer s are instantiated in such a way as to produce the desired properties for the higher-level bearer using it. In the same way, UMTS bearer is supported by Radio Access Bearer Service and CN Bearer Service, which in turn can be analysed in terms of lower-layer bearers. Eventually, the bearers map to resources which provide adequate quality support. We shall not need to go through all of the bearers; for the present purpose it is sufficient to understand the principle. We shall, however, study the second-highest level since there are certain important aspects related to it. The UMTS bearer provides the mobile network support for quality. This is the bearer which is negotiated in practice. To achieve adequate end-to-end quality, the rest of the bearers need to match the end-to-end performance targets, too. In particular, this is relevant for the external bearer linking the external (or other terminal) and the mobile network. In practical implementations, multiple quality support classes can be implemented with DiffServ towards non-cellular networks and with General Packet Radio System (GPRS) roaming exchange mechanisms towards cellular networks. The UMTS bearer resulting from negotiation is mapped to a suitable external quality support class. We shall not discuss the local TE/MT bearer here, since it is typically outside of operator. The 3GPP architecture document provides an overview of the logical functionalities involved in UMTS bearer negotiation in the form of two pictures which are shown in the following text. Figure A.3 shows the layer, or functionalities which are involved in setting up the UMTS bearer. Each of the architectural functional blocks involved in UMTS bearer is associated with admission and bearer s. Conceptually, multiple levels of bearers are associated with a of functionalities, so that the UMTS bearer is connected to the RAB and core network bearer, for
240 Service Modelling: Principles and Applications TE MT RAN CN edge Gateway Ext. netw. Local Transl. UMTS BS RAB UMTS BS Subscr. UMTS BS Transl. Ext. Local BS Radio BS Radio BS RA BS RA BS CN BS CN BS Ext. BS RAN ph. BS M RAN ph. BS M RA NS RA NS BB NS BB NS Protocol interface Service primitive interface Figure A.3 Control layer functionalities for UMTS bearer. From (3GPP TS 23.207, 2004) TE MT RAN CN edge Gateway Ext. netw. Classif. Classif. Cond. Cond. Cond. Mapper Mapper Mapper Local BS External BS RAN phys. BS RAN access network BB network Data flow with indication of direction Figure A.4 User layer functionalities for UMTS bearer. From (3GPP TS 23.207, 2004) example. This figure illustrates nicely the central role played by GGSN in UMTS bearer negotiation. Figure A.4, in turn, illustrates the user layer functionalities belonging to the UMTS bearer, corresponding to treatment packets that are subjected along the UMTS bearer data path. The central idea here is providing of quality support between the functional blocks. Within each block, the following operations can be applied to packets making up the user layer traffic: Classification: detect suitable quality support for the packet in question Conditioning: application of operations such as traffic shaping and/or buffering for ling token bucket parameters Mapping: mapping packetto adequate traffic aggregate corresponding to the classification. takes care of low-level access to transport resources. Note that in the preceding text we have spoken of per-packet operations in the spirit of Internet routing. In practice, one needs to take into account flow orientation.
3GPP Bearer Concepts 241 As discussed in Chapter 1, one can differentiate between static and dynamic provisioning for s. 3GPP architecture supports both, providing the ability to dynamically link bearer properties to usage session as shown in Figure 1.4. Bearer negotiation Bearer negotiation takes place between the terminal and network, and is associated with an Access Point Name (APN). An APN is essentially a Service Access Point (SAP). Bearer negotiation may result from bearer activation or bearer modification. Bearer activation procedures are somewhat different for static and dynamic provisioning modes described in Chapter 1. We shall discuss them next. In static mode, bearer activation for a specific APN is requested by the terminal. The terminal may provide QoS attributes as part of the bearer activation or modification request. If attributes are not provided, network will fill them in. Network may downgrade requested QoS attribute values, but cannot upgrade them. In dynamic provisioned mode, some of the QoS attributes are determined by a session, for example, media stream belonging to a Session Initiation Protocol (SIP) multimedia session. In this example case, IP Multimedia Subsystem (IMS) provides a special authorisation token to be supplied as part of bearer activation request. GGSN uses the token to check that requested attributes indeed do correspond to session parameters. The QoS attributes that can be associated with a bearer are shown in Table A.1. Some of the attributes are relevant only for some of the traffic classes, as indicated in the table. For example, guarantee-type attributes are only relevant for real-time traffic classes (conversational and streaming) for the reason that they need constant token rate. Interactive traffic class is suitable for browsing, and does not provide bandwidth guarantees in the standard implementation. Traffic Handling Priority (THP) is provided as a means of prioritizing Service Data Units (SDUs) within the interactive traffic class. Table A.1 3GPP bearer QoS attributes and their relevance to the four 3GPP traffic classes (conversational, streaming, interactive, and background) Attribute Conv Str Int Bg Maximum bit rate X X X X Delivery order X X X X Maximum SDU size X X X X SDU format information X X SDU error ratio X X X X Residual bit error ratio X X X X Delivery of erroneous SDUs X X X X Transfer delay X X Guaranteed bit rate X X Traffic handling priority X Allocation/retention priority X X X X Source statistics descriptor X X Signalling indication X
242 Service Modelling: Principles and Applications Service provisioning Static and dynamic provisioning modes were described in Chapter 1. They correspond to primary and secondary APNs in GGSN. We shall concentrate on primary APN provisioning in the following text. An APN represents a provisioning point for network operator, allowing for application of class specific policies mapped to an APN. One of the most important policies is quality. Maximum quality level is provisioned for end-users per APN in Home Location Register (HLR). In the context of bearer negotiation, in addition to APN-specific maximum quality, resource availability is taken into account. In addition to quality, an APN can be associated with other parameters as well, including security and IP tunnelling related ones. For network operator, an APN is a central means of affecting quality and security support for s. From the viewpoint of IP-based end-user s, it could be viewed as being associated with logical resource processes and having the nature of an enabler. It is useful to keep in mind that access to Internet could also be viewed as a product-facing.