Dimensioning of the LTE access network

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<ul><li><p>Telecommun Syst (2013) 52:26372654DOI 10.1007/s11235-011-9593-2</p><p>Dimensioning of the LTE access network</p><p>Xi Li Umar Toseef Dominik Dulas Wojciech Bigos Carmelita Grg Andreas Timm-Giel Andreas Klug</p><p>Published online: 3 September 2011 Springer Science+Business Media, LLC 2011</p><p>Abstract This paper proposes efficient analytical models todimension the necessary transport bandwidths for the LongTerm Evolution (LTE) access network satisfying the QoStargets required by different services. In this paper, we con-sider two major traffic types: elastic traffic and real time traf-fic. For each type of traffic, individual dimensioning modelsare proposed for both the S1 interface and the X2 interface.For elastic traffic the dimensioning models are based on theProcessor Sharing models; while for real time traffic the di-mensioning models are based on the fundamental queuing</p><p>X. Li () U. Toseef C. GrgTZI-ikom, Communication Networks, University of Bremen,Otto-Hahn Allee NW1, 28359 Bremen, Germanye-mail: xili@comnets.uni-bremen.de</p><p>U. Toseefe-mail: umar@comnets.uni-bremen.de</p><p>C. Grge-mail: cg@comnets.uni-bremen.de</p><p>D. Dulas W. BigosNokia Siemens Networks Sp. z o.o., Strzegomska 56A,53-611 Wroclaw, Poland</p><p>D. Dulase-mail: dominik.dulas@nsn.com</p><p>W. Bigose-mail: Wojciech.Bigos@comarch.pl</p><p>A. Timm-GielInstitute of Communication Networks,Hamburg University of Technology, Schwarzenbergstr. 95E,21073 Hamburg, Germanye-mail: timm-giel@tuhh.de</p><p>A. KlugNokia Siemens Networks GmbH &amp; Co. KG,Lise-Meitner-Str. 7/2, 89081 Ulm, Germanye-mail: andreas.klug@nsn.com</p><p>models. For validating these analytical dimensioning mod-els, a developed LTE system simulation model is used. Ex-tensive simulations are performed for various traffic and net-work scenarios. The analytical results derived from the pro-posed dimensioning models are compared with the simula-tion results. The presented results demonstrate that the pro-posed analytical models can appropriately estimate the re-quired performances for different service classes and priori-ties. Hence they are suitable to be used for dimensioning ofthe LTE access network with different traffic and networkconditions.</p><p>Keywords LTE access network S1 Interface X2Interface Handover Dimensioning QoS</p><p>1 Introduction</p><p>The roadmap of Next Generation Mobile Network (NGMN)is to provide mobile broadband services. Services like Mo-bile TV, multimedia online gaming, Web 2.0, and high-speed Internet will produce tremendous traffic in the futuremobile networks. To make this happen, 3GPP introduces anew radio access technology, known as Long Term Evolu-tion (LTE) to ensure the competitiveness of the 3GPP tech-nology family for the long term. LTE supports extensivelyhigh throughput and low latency, improved system capacityand coverage performance.</p><p>LTE introduces a new air interface and radio access calledas Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), which is specified in the new 3GPP Releases8 and 9. To support the LTE radio interfaces and the E-UTRAN, 3GPP also specifies a new Packet Core, the En-hanced Packet Core (EPC) network architecture. This paperis only focused on dimensioning the transport network of</p></li><li><p>2638 X. Li et al.</p><p>Fig. 1 LTE E-UTRAN</p><p>the E-UTRAN (i.e. the LTE access network), which is basedon IP. E-UTRAN is designed to support high data rates, lowlatency, and hence to bring improved user experience withfull mobility. This is achieved by introducing a new, fully IP-based flat architecture with the enhanced Node B (eNode B)directly connected to the access gateway (aGW). The logicalarchitecture of the LTE access network is shown in Fig. 1.The eNode B (denoted as eNB in this paper) is responsi-ble for Radio Resource Management (RRM) decisions, han-dover (HO) decisions, scheduling of users as well as radioand transport bearers, etc. The aGW provides terminationof the LTE bearer and acts as a mobility anchor point forthe user plane. The eNB is connected to the aGW with theS1 interface. Between the eNBs the X2 interface is defined,which is used to connect the eNBs with each other in thenetwork. The X2 interface is needed for the case of HO toforward the traffic from a source eNB to its target eNB.</p><p>Due to a significantly improved air interface providingmuch higher throughput and radio interface capacity, LTEwill result in a much higher demand on the transport capac-ities in the access network than 3G UMTS networks. Thus,how to properly dimension the transport resources for a cost-efficient LTE access network (on the S1 and the X2 inter-faces), considering the fast growing traffic and new servicesprovided by the LTE, becomes a critical network planningproblem.</p><p>This paper is aimed to propose efficient analytical mod-els to dimension the bandwidths of the transport links at theeNB side (called eNB transport link in this paper) requiredfor the S1 and the X2 interfaces. The objective of the di-mensioning is to minimize the transport network costs (forleasing IP bandwidth) while being able to fulfill the QoS re-quirements of various services. In this paper, we considertwo fundamental types of traffic: elastic traffic and real timetraffic. Elastic traffic is generated by non real time (NRT) ap-plications and is typically carried by the TCP protocol. Typ-ical applications are Internet services like web browsing andFTP. Real time (RT) traffic is associated with real time appli-cations, which are delay-sensitive and have strict packet de-</p><p>lay requirements over the transport networks. Typical appli-cations in this traffic class are VoIP, streaming or video con-ferencing. In this work, for dimensioning the defined QoSrequirement for elastic traffic is the end-to-end applicationthroughput or transfer delay (which specifies the amount ofdata that can be transferred in a certain time period); whilefor real time traffic the considered QoS is the transport net-work delay, i.e. the end-to-end packet delay through the ac-cess network: S1 delay on the S1 interface and X2 delay onthe X2 interface.</p><p>In this paper, we propose two individual kinds of ana-lytical models for each traffic type for dimensioning of theLTE S1 and X2 interfaces to meet their individual QoS re-quirements. The proposed analytical dimensioning modelfor elastic traffic is based on the M/G/R-Processor Shar-ing (M/G/R-PS) model, which characterizes TCP traffic atflow level and is often used to calculate the mean transac-tion time or throughput for TCP flows. For real time traf-fic, we propose basic queuing model on the packet level toestimate the transport network delay performance. On de-veloping these analytical methods, special efforts are madeto properly model the main features and functionalities ofthe LTE radio interface, the IP-based LTE access transportnetwork using the Differentiated Service (DiffServ) QoSscheme, and model the impact of HO (for dimensioning ofthe X2 interface). Furthermore, we present how to apply theproposed analytical models for carrying out the bandwidthdimensioning for the S1 and X2 interfaces. For validatingthe applicability of the proposed analytical models, in thiswork a LTE simulation model is developed to verify the an-alytical results with the simulation results. This LTE simu-lator models in detail the important LTE network entities,protocol layers, required radio and transport scheduling andQoS functions, etc.</p><p>The remainder of the paper is organized as follows:Sect. 2 gives an overview of the developed LTE simula-tor. Section 3 describes the main dimensioning tasks andpresents a general dimensioning framework. Section 4 in-troduces the proposed analytical models for dimensioningthe S1 interface for elastic traffic and real time traffic, andSect. 5 presents their extensions for dimensioning the X2interface. Section 6 shows the detailed validation results anddimensioning results. At the end, conclusions and futurework are given.</p><p>2 LTE simulation model</p><p>The LTE simulator is implemented using OPNET simu-lation software. The developed LTE simulation model isshown in Fig. 2. It includes all basic E-UTRAN and EPCnetwork entities. The main focus of this simulation model ison the LTE access network.</p></li><li><p>Dimensioning of the LTE access network 2639</p><p>The LTE access transport network consists of eNBs, anda number of routers which connect the eNBs with the aGWand with each other. Figure 2 shows an example scenariowith two eNBs that connect with each other via an in-termediate router. The EPC user-plane and control planenetwork entities are represented by the aGW network en-tity. The aGW includes the functionalities of the eGSN-C(evolved SGSN-C) and eGSN-U (evolved SGSN-U). The re-mote node represents an Internet server or any other nodesthat provide the corresponding data services. To model thepossible delays between the aGW and the remote node,the simulation model allows configuration of certain delaydistributions for the data transmission between them (e.g.20 ms between an Internet server and the aGW). We assumethat the delays between the core network entities are negli-gible.</p><p>Figure 3 shows the LTE user-plane protocol structurewhich is developed within this LTE simulator. The proto-</p><p>Fig. 2 LTE simulation model</p><p>cols are categorized into three groups: radio (Uu), transport,and end-user protocols.</p><p>The radio (Uu) protocols include the peer to peer pro-tocols, such as PDCP (Packet Data Convergence Protocol),RLC (Radio Link Control), MAC (Medium Access Con-trol) and PHY (Physical), between the UE entity and theeNB entity. The PDCP, RLC and MAC (including air inter-face scheduler) layers are modeled in detail according to the3GPP specifications in this simulator. But the PHY (phys-ical) layer is not detailed modeled. However the effect ofthe radio channels and PHY characteristics are modeled atthe MAC layer in terms of the data rates of individual userperformance. For the UE mobility, general mobility modelssuch as random directional and random way points are used.</p><p>The LTE transport network is based on IP technology.The user-plane transport protocols as shown in Fig. 3 areused at both S1 interface and X2 interfaces. It mainly in-cludes the GTP (GPRS Tunneling Protocol), UDP, IP andlayer 2 protocols. Ethernet is used as the layer 2 pro-tocol for the current implementation. IP protocol is theone of the key protocols which handles routing, security(IPsec), services differentiation and scheduling functional-ities. The LTE transport network applies the DiffServ-basedQoS framework and it is established by connecting a numberof IP DiffServ routers between the eNBs and the aGW. Diff-Serv is developed by the IETF [1], which defines the threemost common Per Hop Behavior (PHB) groups correspond-ing to different service levels: Expedited Forwarding (EF),Best Effort (BE), and Assured Forwarding (AF). In the LTEtransport network, each PHB is assigned to a transport prior-ity and has its own buffer in the transport scheduler. To servedifferent PHBs, Weighted Fair Queuing (WFQ) schedulingis used. The definition of WFQ discipline is given in [2]. Letwk be the weight of the kth PHB queue and BW the totalavailable IP bandwidth. If there are in total N PHB queuesand all queues are transmitting data, then the kth queue ob-</p><p>Fig. 3 LTE protocol structure(user-plane)</p></li><li><p>2640 X. Li et al.</p><p>Fig. 4 LTE access networkdimensioning framework</p><p>tains a fraction of the total capacity BWk as calculated in (1).It shall be noticed that if one priority queue is empty (i.e. notutilizing its allocated bandwidth) then its bandwidth shall befairly shared by the other queues according to their weights.</p><p>BWk = wkNi=1 wi</p><p>BW (1)</p><p>For modeling the end-user protocols, the standard OP-NET protocols such as application and TCP/UDP are used.They are located at the remote Internet server and each UEentity. Furthermore, the control-plane is not directly mod-eled within the LTE simulation model. However the effect ofsignaling such as their overhead and delays are consideredat the respective user-plane protocols upon specific require-ments.</p><p>3 LTE access network dimensioning framework</p><p>In the framework of this paper, the main task of dimension-ing is to decide minimum required bandwidths for the trans-port links of the S1 and X2 interface in the LTE access net-work, for given traffic load of various services and definedQoS targets. For carrying out the dimensioning, we proposea general framework for the LTE access network dimension-ing as shown in Fig. 4.</p><p>As illustrated in Fig. 4, this framework includes threetypes of input parameters:</p><p>(1) User parameters define (i) the traffic demand whichspecifies the total amount of the offered traffic, the traf-fic classes with respective to different services, and traf-fic mix; (ii) user or service priority classes and their dis-tributions; (iii) mobility and handover parameters.</p><p>(2) QoS targets that need to be satisfied with the dimen-sioning.</p><p>(3) System parameters such as radio configurations (e.g.cell capacity), transport network functions (e.g. Diff-Serv QoS scheme, WFQ scheduling), resource controlfunctions (e.g. admission control), etc. The configuredsystem parameters will have considerable impact on theperformance of the networks and on the end users, andthus on the dimensioning results.</p><p>The dimensioning process is the core of this dimension-ing framework. In this paper, we design the dimensioningprocess for elastic traffic and real time traffic individually(will be explained in Sects. 4 and 5). The output of dimen-sioning is the required transport bandwidths for the S1 andX2 interfaces, which should support the offered traffic de-mand of all traffic types and meet the defined QoS targets.In this paper we derived the bandwidth in kbps, however inpractice the dimensioned link capacity will be mapped toavailable physical line(s).</p><p>4 Dimensioning model for the S1 interface</p><p>In this section, we present two analytical dimensioning mod-els for the dimensioning of the S1 interface: one for elastictraffic and one for real time traffic [8]. For elastic traffic, theconsidered QoS is the end-to-end application delay and theproposed dimensioning models are based on Processor shar-ing models. For real time traffic the QoS target is the trans-port network delay (i.e. S1 delay and X2 delay), accordinglywe propose essential queuing models for the dimensioning.</p><p>4.1 S1 dimensioning model for elastic traffic</p><p>The elastic traffic is typically carried by the TCP protocol.Due to TCP flow control, the rate of TCP flow adjusts itselfto adapt to the available bandwidth in the network. If TCPworks ideally (i.e. instantaneous feedback), all elastic traffic</p></li><li><p>Dimensioning of the LTE access network 2641</p><p>flows going over the same link will share the bandwidth re-sources equally and thus the system is essentially behavingas a Processor Sharing (PS) system [3]. This important prop-erty enables the applicability of M/G/R-PS model for esti-mating the end-to-end application performance of the elas-tic traffic. The M/G/R-PS model characterizes the TCP traf-fic at flow level. It is assumed that there are a l...</p></li></ul>