Performance Modeling of Web Access Over HSPA Networks

2010 International Congress on Ultra Modern Telecommunications and Control Systems and Workshops (ICUMT) Performance Modeling of Web Access over HSPA Networks Manuel Alvarez-Campana, Enrique Vázquez, Joan Vinyes Departamento de Ingeniería de Sistemas Telemáticos Universidad Politécnica de Madrid Madrid, Spain {mac,enrique,vinyes}@dit.upm.es Abstract— HSPA (High Speed Packet Access) technologies have channel HS-DSCH (High Speed Downlink Shared Channel), become an attractive solution for broadband wireless Internet along with a number of features added to the Node-B which are access. This article evaluates the quality of service experimented grouped under a new layer of media access control MAC-hs. by one HSPA user in the presence of multiple terminals sharing These features include a fast packet scheduling mechanism the cell capacity. The study focuses on Web traffic and includes combined with the use of adaptive modulation and coding analytical and simulation results, providing a basis for (AMC) and a hybrid retransmission scheme (H-ARQ). dimensioning and admission control in HSPA networks given the intended user quality of experience. The results show that the Fig. 1 illustrates the traffic channels used in HSPA. In the downlink channel capacity can be shared by several tens of Web downlink, the HS-DSCH provides a high capacity channel users with a satisfactory quality. This is possible because the which is dynamically shared between the users in the cell. In activity factor of this application (i.e. the time spent in the uplink, HSPA terminals can use either a DCH (Dedicated downloading pages compared to the time spent in reading them) Channel) or an E-DCH (Enhanced DCH). The latter option is low. However, the results also indicate that the page download depends on whether the cell supports EUL or not. time is penalized by the delay associated with the initial procedure for allocating resources on the traffic channels. This The downlink shared channel HS-DSCH is a transport problem can be alleviated by an adequate configuration of the channel that may be associated to one or more physical timers associated to the resource allocation procedures in HSPA. channels, i.e. OVSF (Orthogonal Variable Spreading Factor) codes, with a fixed spreading factor value of 16. This value Keywords-component; HSPA, Web, Simulation, Quality of allows up to 15 codes for the HS-DSCH channel on a 5 MHz Experience W-CDMA carrier. An operator can devote a full carrier to HSPA, or share it between HSPA and UMTS Release 99 I. INTRODUCTION services. In this second option, quite common in initial HSPA deployments, the number of codes for high speed packet HSPA [1] (High Speed Packet Access) is an umbrella term services is typically 5 or 10. For areas with high service covering a set of extensions to the original UMTS (Universal demand, operators are currently considering the deployment of Mobile Telecommunications System) radio interface aimed to dedicated carriers for HSPA with 15 codes. improve the throughput for packet-switched services. The first two HSPA extensions are HSDPA (High Speed Downlink EUL improves the uplink performance by combining a Packet Access) and EUL (Enhanced Uplink), respectively number of techniques similar to HSDPA, including H-ARQ defined in 3GPP Release 5 and 6 specifications. The evolution retransmissions, a fast link adaption mechanism, and an of 3GPP systems continues and operators are already preparing advanced packet scheduler. Unlike HSDPA, the allocation of for the introduction of new extensions such as Evolved HSPA radio resources in the uplink is not set on the basis of a shared (also known as HSPA+) and LTE (Long Term Evolution). channel, but by controlling the distribution of power between the terminals. The packet scheduler operates on a request-grant This article focuses on the performance evaluation of principle where the UEs (User Equipment) request permission mobile Web access over HSPA from the user point of view. to send data and the scheduler decides when they can transmit Although UMTS operators have been introducing HSPA and how much power they can use on the E-DCH. extensions gradually since 2006, there is still little practical knowledge on how to properly dimension HSPA networks so In practice, the actual performance of HSPA depends on that users are guaranteed a given quality of experience. many factors: number of codes available in the cell, allocated power, propagation conditions, terminal category, etc. As an HSDPA [2] provides downlink throughputs up to 14.4 orientation, an HSPA cell with 10 codes provides a typical Mbit/s (theoretical) for packet-switched services. This contrasts aggregated download throughput at IP level between 2 and 3 with the maximum of 384 kbit/s usually supported in UMTS Mbit/s. Additionally, a key aspect to consider is the number of Release 99. Other advantages of HSDPA are lower latency and users simultaneously accessing the cell since, as noted above, more efficient use of spectrum. These improvements are the radio resources are shared dynamically either in terms of achieved by introducing a number of enhancements in the radio codes (downlink) or power (uplink). access network. They include the use of a downstream shared 978-1-4244-7286-4/10/$26.00 ©2010 IEEE 710 HSPA Downlink shared channel (HS-DSCH) bits, with mean reading time Toff seconds between consecutive terminals pages. L is assumed to be the sum of the sizes of the HTML page plus all objects it includes (e.g. images). The analysis is UE 1 made at the IP layer. Thus, C is the total capacity available for transmission of IP packets in the downlink direction, and L includes the overhead due to IP and higher layer protocols. UE 2 At any given time, C is shared among the UEs (User TTI = 2 ms Equipment) that are concurrently downloading Web pages. Uplink dedicated channels (DCHs or E-DCHs) UMTS/HSPA However, when a user requests a new page download, the UE UE 3 Base Station cannot immediately utilize the HS-DSCH channel. To do so, (Node-B) the UE must transition from its current RRC (Radio Resource Control) state, for example “idle”, to the state “dedicated” in UE 4 which the UE is allowed to send information on the HS-DSCH channel. Fig. 1. Overview of HSPA traffic channels More details on the RRC states and the delays associated to Focusing in the downlink, the capacity of the HS-DSCH transitions among those states are provided in section III and channel is shared dynamically between the users of the cell section IV, respectively. In the analysis, this effect is modeled under the control of the fast scheduling algorithm in the Node- by introducing a delay Δ from the instant a new page is B. Intuitively, this means that the capacity per user is inversely requested to the instant the download on the HS-DSCH channel proportional to the number of users in the cell. This is the key starts. By choosing an adequate value for Δ we can consider not aspect investigated in this article. only the time required to transition to the RRC dedicated state, More specifically, the paper focuses on the problem of but also the delays introduced by upper protocol layers (e.g. TCP connection setup and slow start) . assessing the equivalent dedicated capacity (Ceq) effectively utilized by a user, given the total capacity of the downlink After the initial delay Δ, the page is downloaded at Cav bit/s, shared channel and the number of users in the cell. Ceq is which is defined as the fraction of the shared downlink capacity defined as the capacity value exclusively dedicated to one user C that is available to each user. Note that the average time that would provide the same quality of experience as on the spent downloading a Web page on the HS-DSCH channel is shared HSPA channel. only L/Cav, but the download time observed by the user is Δ+L/Cav. Therefore, the equivalent capacity Ceq observed by In the case of Web navigation, Ceq is the capacity that each the user will be smaller than Cav. user has for downloading pages. Thus Ceq determines the waiting time until Web pages are downloaded and displayed on According to the above described behavior, each user the browser, which is the typical performance indicator for generates L bits in a time equal to Δ+L/Cav+Toff s. The measuring the quality of experience of Web access. In the utilization factor (U) of the downlink shared channel can then literature, there are similar studies for Web access over other be calculated as the aggregated traffic generated by the N users shared media access technologies, such as cable networks and divided by the total available capacity C: GPRS [3][4]. N · L C To evaluate the equivalent capacity and the Web page U = (1) Δ + T + L / C download time, we present results obtained with an analytical off av model, and compare them with results obtained by simulation. Let α be the activity factor of a user assuming no one else Given the global capacity of the HS-DSCH channel, we is using the downlink capacity C. Note that α is an ideal estimate the maximum number of simultaneous users that parameter, independent of N, which characterizes the behavior guarantees a minimum equivalent capacity, so that the mean of a single user on a channel of capacity C. In this case, the page download time is kept under a given value. average page download time is L/C s, so α is given by: The rest of the article is organized as follows. Section II describes the analytical model and the results obtained with it.

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