9 Case Study: Service Quality Support in an IP-based Cellular RAN In this chapter, we shall study an IP-based Radio Access Net- work (RAN) as an example of applying the technologies of the preceding chapters. In the framework of preceding chapters, an IP RAN can be considered to be a multi-service Internet access domain supporting endpoint mobility. From the viewpoint of Diff- Serv, mobility is handled on the link layer. The traffic engineering framework of IETF is used to structure the example. In what follows, we shall first study the motivation for using IP-based transport in cellular radio access network. Next, IP RAN transport architecture is explained. After that, an implementation of service quality support in IP-based RAN is accounted for using IETF’s traffic engineering framework. The technologies listed here are generic to use of DiffServ-based transport in IP based RAN. DiffServ transport will be used in cellular backbones as well. Concentrating on IP-based RAN in this chapter has the advantage of providing a relatively well-defined case study of a multi-service access network. The same principles can be applied in the backbone network, too, but QoS management there is easier due to higher aggregation level of flows. Implementing Service Quality in IP Networks Vilho R ¨ ais ¨ anen  2003 John Wiley & Sons, Ltd ISBN: 0-470-84793-X 276 SERVICE QUALITY SUPPORT IN AN IP-BASED CELLULAR RAN Further optimizations for service quality support in IP-based RANs are possible, but are not discussed here. The goal of the present chapter is to provide a case study showing how traffic engineering and other advanced IP technologies reviewed in pre- vious chapters can be put together to implement service quality in an IP-based RAN. 9.1 MOTIVATION FOR USING IP-BASED TRANSPORT IN CELLULAR RAN The primary aim of QoS mechanisms in any cellular RAN trans- port is providing of quality support for services mapped to the four 3GPP traffic classes described in Chapter 5. Additionally, the QoS mechanisms used must also provide an adequate quality sup- port control layer and management layer traffic. The easiest way of supporting services with varying quality requirements is to reserve capacity in the network according to the sum total of all traffic classes during peak hour. Traditionally, Radio Access Networks of GSM, GPRS, and 3G networks have been built using Time Division Multiplexing (TDM) links, based on an Erlang calculation for the peak hour traffic. This has been an adequate solution for networks in which the end user traffic consists almost solely of circuit-switched voice tele- phony. Interactive voice has strict delay requirements, whereby the network needs to be dimensioned to accommodate bursty data traffic demand during peak hours. The dimensioning of telephony networks for aggregated speech traffic, taking into account Pois- son process-like arrival of connections in individual servers (base stations) is an established discipline within traditional telephony (cf., e.g., [McD00]). The problem with applying this approach to a multi-service network domain is that it does not make best use of the operator’s investment in transport capacity, especially con- cerning the variety of services that need to be supported by third generation mobile networks. For this reason, new alternatives have been evaluated. In a study made by the Mobile Wireless Internet Forum (MWIF), using IP-based transport in the RAN of different third generation networks was found to be a viable option [IRT01]. Data-type applications have been adopted in the mobile net- work, including browsing the Internet using Wireless Application 9.1 MOTIVATION FOR USING IP-BASED TRANSPORT IN CELLULAR RAN 277 Protocol (WAP). With my GPRS phone, I can initiate a connec- tion and start browsing the news titles from Financial Times or Helsingin Sanomat, latter being the largest daily newspaper in Fin- land, within a few seconds. I have ample time to check for latest news headlines during a 5-minute bus trip to work from home. Othertraffictypes,whicharemadepossiblebythe3GPPQoS framework for mobile networks, are streaming and data transfer, such as uploading or downloading of digital photographs. Adding pictures to text using Multimedia Messaging Standard (MMS) is possible already with the “2.5G” GPRS networks. The set of sup- ported end user services for 3G will consist of not only speech and short message service (SMS), but to include also MMS, data, and real-time content. As discussed in Chapter 3, the differences in service quality requirements in a heterogeneous traffic mix can be used in network dimensioning. An IP-based transport network in the RAN still has to be dimen- sioned to support delay-critical traffic during peak hours, in the same way as with traditional telephony. For the less urgent traffic types, however, no fixed capacity needs to be reserved, but instead the multiplexing gain of packet switching can be leveraged to obtain high utilization level in the network. The enabling technol- ogy here is DiffServ, which makes possible sharing a single capac- ity “pipe” by diverse class of applications with varying service quality support requirements. Implementation of service quality with IP and differentiated treatment is advantageous when com- pared to the traditional concept of reserving capacity for different services in the network. The exact benefits from using DiffServ- based transport vary according to the precise combination of end users’ services in the mobile network, as well as the access network topology, but analyses indicate that savings can be up to tens of percent as compared to traditional network dimensioning. Radio access network being a major factor in cellular network CAPEX, this fact translates to monetary savings for the operator. In general, a Radio Access Network transport based on IP brings with it several benefits: • Less transport capacity is needed in the RAN. • There are fewer protocol layers to be managed. • Same type of transport hardware can be used in RAN as in Internet backbone. 278 SERVICE QUALITY SUPPORT IN AN IP-BASED CELLULAR RAN Leveraging the benefits of IP in a best possible manner requires further technologies, which are not within the scope of this chap- ter. Discussion about some of the related issues can be found in [IRT01]. An IP-based RAN also has the benefit of allowing the natu- ral incorporation of other access technologies apart from cellular radio into a single multi-service, multi-access network. In such a network, IP is the protocol tying different access technologies together. This concept, sometimes called the All-IP network, is illustrated in Figure 9.1. In addition to the WLAN access shown in Figure 9.1, other possible access methods include ADSL/SDSL and wireline Ethernet. The usability of IP-based RAN is not limited to 3GPP networks, but can be used in other mobility networks as well. Indeed, the analysis of MWIF referred to previously covers both 3GPP and 3GPP2 networks, the latter being CDMA2000 variant of the International Mobile Telecommunications 2000 (IMT2000) third generation mobile framework. In the MWIF study, also IntServ has been included as a potential IP service quality support technology in the RAN. More generically, service quality support in mobile networks has been discussed recently in [CZ02]. In that scheme, bandwidth brokers in radio network access network border routers perform admission control to SLAs in a DiffServ network, based on effec- tive bandwidth approximation. Adaptive applications have been found to make it possible to raise the utilization level of an access network [CZ02], and indeed the adaptive bit rate AMR codec that is used in the 3GPP systems is able to adjust to available bit rate. Irrespective of the radio access technology used, having a proper Operator backbone IP-based RAN WLAN access Public internet Figure 9.1 A High-level view of an All-IP network 9.2 IP RAN TRANSPORT ARCHITECTURE 279 service quality support model is important for providing proper service quality support in a multi-service environment [GC02]. 9.2 IP RAN TRANSPORT ARCHITECTURE We shall next take a brief look at 3GPP IP RAN transport architec- ture for WCDMA radio access. The architecture will be extended to cover evolution versions of GPRS radio technologies as well, but these are not covered here. Before taking a look at architectures, let us make a short excursion to the world of generic transport architecture of a mobile network. 9.2.1 PLMN transport architecture A generic cellular network, called Public Land Mobile Network (PLMN) in GSM parlance, consists of three distinct transport parts: • long-haul backbone; • medium-capacity fibre transport; • bandwidth-limited access links. The high-level transport architecture domains are shown in Figure 9.2. The long-haul backbone is typically a general-purpose transport network having traffic engineering capabilities adequate for high traffic aggregation levels. The backbone connects the medium- capacity access part of the mobile network to mobility servers GPRS roaming exchange High-capacity backbone Medium-capacity fibre transport BTS/ Node B BTS/ Node B Internet domain GGSN SGSN Mobility servers Cellular network Other PLMN Bandwidth-limited access links Figure 9.2 Transport domains in a GPRS/3GPP network 280 SERVICE QUALITY SUPPORT IN AN IP-BASED CELLULAR RAN (including SGSN and GGSN), to other cellular networks via GPRS Roaming Exchange (GRE) network via SGSN, and to non-cellular Internet domains via GGSN. As the name implies, GRE network facilitates roaming between operators by providing service qual- ity enhanced connectivity between mobility servers in different PLMNs. We shall return later in this chapter to the topic of service quality support towards Internet domains. Medium-capacity fibre transport network delivers the traffic between access links to the long-haul backbone. (Node B is the canonical name for base station in WCDMA.) The medium- capacity fibre transport can often be dimensioned with sufficient capacity so that it will not be congested. Bandwidth-limited access links connect BTSs and Node B’s to fibre-based transport. The link layer technology used in this part of the transport architecture depends on the environment. Typical technologies include, leased fibre links and microwave links. The bandwidth-limited part of RAN transport can be hierar- chical in nature, resembling a tree with Node Bs as leaves, and DiffServ-capable routers making up the branches of the tree trunk (see Figure 9.3). For resiliency purposes, also loops may be used. As the name implies, the bandwidth-limited links are typically of limited capacity, and interface to the medium-capacity fibre links. A single PLMN may encompass thousands or tens of thousands of Node Bs, and an accordingly large number of narrow-bandwidth links in RAN branches. The cost of entire RAN equipment makes up the largest part of the cost of an entire PLMN, part of that Node B Node B Node B Node B DiffServ router DiffServ router DiffServ router Fibre transport Figure 9.3 An illustration of RAN transport hierarchy Note : Frame combiner not shown 9.2 IP RAN TRANSPORT ARCHITECTURE 281 being attributable to the transport used. Even though the cost of the actual transport may not be percentually the largest part of the total cost, transport capacity can be limited by the avail- ability of leased capacity or licences for the microwave transport. Thus, transport capacity savings of tens of per cents are made possible multiplexing gain using differentiation-based transport inside RAN branches instead of per-aggregate capacity reserva- tions translates significant transport cost savings for the mobile network operator. 9.2.2 IP RAN transport architecture IP-based RAN is an evolution of the 3GPP mobile network archi- tecture, extending the scope of use of IP transport (Figure 9.4). In GPRS and Release 99 3GPP architectures, the “core network” inter- faces between GGSN and SGSN (including GRE interface) have been based on IP. In Release 4, a 3GPP standardization successor of Release 99, IP transport option up to Radio Network Controller (RNC) was made possible. In the IP option of Release 4, the basic R99 architecture is not modified, the only change being the trans- port technology used beneath the GTP tunnel between the Node B’s and RNC. An IP RAN is a logical next step in this evolution, integrating the IP-based transport more closely into the RAN architecture. It turns out that for overall efficiency, it is beneficial to redesign the radio network control layer, replacing a single RNC with distributed radio resource control architecture. The explanation of the entire IP RAN radio resource control architecture is not within the scope of the book. For the purposes of this chapter it is sufficient to know that from the viewpoint of mobile terminals, the 3GPP service quality support control provides the same service quality support User IP layer Application Radio tunneling RAB Link Radio tunneling RAB Link GTP IP Link GTP IP Link GTP IP Link GTP IP Link Link User IP layer User IP layer Terminal IP base transceiver station & radio network gateway Serving GPRS support node Gateway GPRS support node Figure 9.4 IP RAN architecture protocol stacks for user layer traffic Note : The architecture has been simplified to make the role of bear- ers clearer 282 SERVICE QUALITY SUPPORT IN AN IP-BASED CELLULAR RAN as the preceding 3GPP network variants. An overview of the IP RAN protocol stacks on a logical level is shown in Figure 9.4. From the viewpoint of protocol stacks, the only difference in the actual stacks is that the GTP tunnel uses IP-based bearer up to basis station. Please note that in Figure 9.4, only user layer traffic protocol stack is shown. 9.2.3 Handover traffic Mobile networks based on Code Division Multiple Access (CDMA), of which Wideband CDMA (WCDMA) used in 3GPP networks is a subspecies, use the so-called soft handover for handling terminal mobility. This means that a terminal may communicate with the mobile network through more than one base station (called “Node B” in CDMA) at a time. A mobile terminal does not have to detach from the previous Node B before commencing communication with the next one. This arrangement is necessary due to CDMA power control. Due to this, traffic to a particular mobile participating to soft handover is being transmitted via multiple Node Bs in both uplink and downlink directions. A practical implementation of soft handover requires the function of a frame selector function in the network, which – for uplink direc- tion – receives radio frames from different soft handover “legs”, and combines the signals arrived via different routes into a single PDU stream towards the core network (see Figure 9.5). The frame selector is also known as a macrodiversity combiner, with the term “microdiversity” being reserved for mobility handled by individ- ual Node Bs. For downlink direction, the signal is split into different paths and frame combining is performed in the terminal. In 3GPP Terminal Node B Node B Frame selector Core network Figure 9.5 Principle of soft handover Note : Transport of radio frames is shown by dashed lines, and normal GTP tunnelled traffic by a solid line 9.2 IP RAN TRANSPORT ARCHITECTURE 283 Release 99 networks, frame combining for uplink and frame split- ting for downlink is done in RNCs. The importance of the soft handover concept for the present discussion is that in addition to 3GPP user layer and signalling traffic, radio frames being part of soft handover traffic need to be transported in the links of bandwidth-limited RAN. Soft handover traffic has typically high forwarding priority, the frame-combining algorithm having limitations for the allowed delay difference on the different paths. In addition to soft handover traffic, WCDMA networks in gen- eral can also carry so-called drift traffic, consisting of non-processed radio frames. In the case of 3GPP R99 network, drift traffic can be transported between a drift RNC and a serving RNC.Inthis case, the drift RNC forwards non-processed radio frames to the serving RNC. Drift traffic, too, has high priority. In what follows, generic references to handover traffic are made, covering both soft handover and drift traffic. 9.2.4 Service mapping in IP RAN The 3GPP QoS model, reviewed in Chapter 5, applies indepen- dently of the transport technology used in the RAN. Thus, the user layer QoS model used in IP RAN is the same as in Release 99 networks. What is different between IP RAN and R99 UTRAN is the implementation of the transport part of the radio access bearer. The implementation of service quality support in IP RAN transport is described below on a generic level. As in 3GPP R99, each application flow is associated with a PDP context describing the negotiated QoS support for the flow. The QoS attributes of a PDP context have been described in more detail in Chapter 5, and for the present purposes we are interested in the QoS attributes of the PDP context which may affect IP RAN transport. These include: • Traffic class (conversational, streaming, interactive, or back- ground); • Traffic Handling Priority (THP); • Allocation/Retention Priority (ARP). These QoS attributes are used in mapping the PDP context asso- ciated with the application flow onto a Radio Bearer (RB) for the 284 SERVICE QUALITY SUPPORT IN AN IP-BASED CELLULAR RAN Application PDP context Managed mapping IP transport bearer Radio access bearer Figure 9.6 Service mapping for user layer traffic in IP RAN radio interface, and a Radio Access Bearer (RAB) in RAN trans- port. Service quality support in the backbone transport can be based on RAB used in the RAN (Figure 9.6). Thus, as in R99, the PDP context is a service quality abstraction layer between the application, and the different link-layer technologies (radio inter- face, IP transport). Seen from end-to-end viewpoint, service quality support in an All-IP network for a terminal communicating with a host in the Internet includes the following steps: 1. Terminal maps application flow requirements into PDP context QoS attributes. 2. PDP context of appropriate type is initiated. a The QoS parameters of the PDP context map onto a suitable RAB and CN bearer. b RAB maps to specific WCDMA radio interface parameters. c RAB in the RAN maps onto suitable IP transport parame- ters (PHB). d QoS treatment for packets belonging to the PDP context is based on the RB service quality support in the radio interface, and DSCP marking in the RAN and backbone transport. 3. Between GGSN and external network, possible QoS interwork- ing is based on PDP context QoS profile. a In uplink direction, mapping to QoS mechanism beyond GGSN is based on PDP context properties. [...]... implementation of service quality mapping aspects in IP RAN Further enhancements are possible, but are not central to the current discussion 9.3 TRAFFIC ENGINEERING IN ALL -IP RAN In this chapter the technologies used in IP- based RAN transport are presented within the framework of IETF traffic engineering The “full loop” of traffic engineering process in IP RAN includes policy-based management for the configuration... geographical subscriber base as well as anticipated service usage patterns as inputs to the process The basic task of capacity planning in IP RAN is multi-service IP network dimensioning using the above inputs, and based on the techniques discussed in Chapter 5 The basic planning tool is based on information about subscriber count growth predictions, anticipated set of service types supported, and technical... [MNV02] 9.6 SUMMARY IP- based Radio Access Networks have the potential for allowing more efficient utilization of transport capacity for multi-service support, as well as using IP- based network technologies across different access network technologies In addition to monetary benefits, also the management interface to IP- based networks can be made simpler due to lighter protocol stacks All -IP RAN is an evolution... addition to basing of RAN transport on IP, also radio network architecture is tailored specifically to IP environment In the model described here, service quality in the actual transport is based on DiffServ framework and traffic aggregates The total set of service quality support functions related to IP transport is divided between radio network control (“3GPP layer”) and IP network functions (“IETF layer”)... bandwidth broker The 3GPP layer also provides transparent mobility for the end user, with user IP address staying the same Aside from the functions performed by the 3GPP layer, IP RAN elements are purely IP devices, an as such can be managed using the IETF traffic engineering process The DiffServ transport in the IP RAN is beneficially optimized for the properties of the mobile network traffic types, making... the bandwidth-limited part of IP RAN benefits from DiffServ transport implementation, tailored to support 3GPP traffic classes The tailoring refers both to structure of queueing system in view of the supported traffic types, and the complexity of management interface used in applying traffic engineering to the routers of IP RAN Management of 292 SERVICE QUALITY SUPPORT IN AN IP- BASED CELLULAR RAN service... mapping in the IP RAN access domain can be made configurable The “lower part” of traffic mapping, mapping RAB parameters to DSCP, can be accommodated by policy management Being based on traffic aggregates, the IP level mapping is not overly complex to be managed and optimized An example of such a mapping, one aspect of service quality requirements of 3GPP traffic classes can be supported in IP RAN by mapping... mapping of traffic classes onto QoS support schemes in radio interface and IP transport is controlled by the mobile network The latter mapping can be implemented in many different ways In what follows, the simple solution of predefined mapping onto RAB and further from RAB to IP QoS parameters in the transport is assumed All told, IP RAN transport needs to be able to deal with the traffic types listed in... from DiffServ BAs in IP RAN access to backbone LSPs can make use of BA mapping of services in the former 9.3.2 Capacity management Capacity management in IP RAN means making the most out of the built capacity In what follows, the traffic engineering process of Chapter 4 is assumed, that is, optimization of the behaviour of the network based on feedback from the network In the case of IP RAN, capacity management... reflected in relative forwarding prioritization of traffic aggregates in the IP transport The drop precedences of AF classes operate most efficiently with service instances utilizing closed-loop end-to-end transport quality control such as TCP IP transport parameters that can be controlled include 9.3 TRAFFIC ENGINEERING IN ALL -IP RAN 291 • per-PHB parameters in DiffServ routers, such as rate limiters, . User IP layer Application Radio tunneling RAB Link Radio tunneling RAB Link GTP IP Link GTP IP Link GTP IP Link GTP IP Link Link User IP layer User IP layer. a proper Operator backbone IP- based RAN WLAN access Public internet Figure 9.1 A High-level view of an All -IP network 9.2 IP RAN TRANSPORT ARCHITECTURE