Evaluation of End-to-End TCP performance over WCDMA

Evaluation of End-to-End TCP performance over WCDMA Liang Hu Department of Communications, Optics & Materials Technical University of Denmark, Lyngby, Denmark Email:{lh}@com.dtu.dk Abstract this article investigates the end-to-end TCP performance in the scenario where WCDMA is employed as the access link. Unlike previous work, we incorporate the impact of Internet loss rate and delay into the study. The performance of TCP over WCDMA is studied from several aspects: the TCP protocol configurations, the different variant of TCP protocols, the interaction of TCP and RLC layer protocols, and Impact of core network in terms of delay and loss rate. Finally, a novel TCP proxy concept is proposed in order to improve the TCP performance in large delay-bandwidth product scenario. The simulation results shows that: firstly, for small file size, such as web pages, the TCP initial slow start can harm the throughput of users who are allocated high channel bit rates DCH channels; Secondly, a large TCP slow start initial counter (larger than 1 maximum segment size) can improve the throughput of TCP in the slow start phase; Thirdly, there is strong correlation between TCP and RCL layers protocols: in general the more reliable is the RLC layer, the more benefit can be obtained from TCP throughput. However, a too large RLC MaxDAT will lead to TCP timeout thus harm TCP throughput. Finally, it is showed that the novel TCP proxy brings significant TCP performance gain in the high delay bandwidth product scenario while only a small improvement in low delay bandwidth product scenario. Keywords-component; TCP performance; WCDMA; RLC layer; TCP proxy I. INTRODUCTION Ubiquitous Internet access is regarded as a key success factor for third generation mobile communication system. WCDMA/UMTS [6, 7] shows this trend by providing efficient support for packet-switched data services with data rates up to 384 kb/s for wide area coverage and maximum 2M bits/s for hot spot areas. The mobile internet based WCDMA is expected to bring one or two order of magnitude higher data rates than the previous 2G cellular networks. However, it is important that the combination of Internet application and underlying transport layer can make good use of the large bandwidth brought by WCDMA. TCP is the dominant transport protocol for most popular Internet applications such as FTP, HTTP, email, and so on [5]. A number of studies can be found in the literature [1, 2] have shown TCP performs poorly over wireless links. However, those studies are primarily focus on wireless local area networks. In contrast, WCDMA provides additionally unique link layer RLC to try to hide the physical layer transmission error from TCP. Thus it is interesting to investigate how TCP performs over WCDMA air interface. This paper employs a simulation based approach to investigate the end-to-end TCP performance over WCDMA air interface for FTP traffic using common and dedicated transport channels (DCH). Unlike [3], our simulator incorporates the impact of core network and Internet to end-to-end TCP performance. Further, our simulator is based on the detailed implementation of RLC layer protocols including the parameters: MaxDAT, RLC window size and Status Prohibit Timer. The contributions of our work are expected to be as follows: Study the impact of TCP slow start to system performance under different bit rate DCH channels for different file sizes Study the impact of TCP initial counter to system performance under different bit rate DCH channels for different file sizes Compare the performance of variants of TCP: Reno, New Reno, SACK Study the interactions between RLC and TCP protocols Propose a split TCP proxy to enhance TCP performance in large delay bandwidth scenarios. (To Be done) II. WCDMA OVERVIEW A. WCDMA network architechture Figure 1 shows the WCDMA network architecture in packet switch operation [4]. The network functionality is divided into three groups: User Equipment (UE), UMTS Terrstrial Radio Access Network (UTRAN) and Core Network. UTRAN consists of Node B and Radio Network Controller (RNC). The Core Network comprises two basic

III. SIMULATON SETTINGS A. Simulation Parameters We evaluate the end-to-end TCP performance over WCDMA DCH channels by OPENT simulator [8]. The parameters are given in the table below: Parameter Parameter Description Figure 1: WCDMA network architecture in PS domain nodes: Serving GPRS Support Node (SGSN) and Gateway GPRS support Node (GGSN). GGSN provides inter-working with external packet-switched networks such as IP networks via the GI interface. SGSN is connected to RNC via the IuPS interface. SGSN is connected to RNC via the lups interface. UE is connected to UTRAN over the WCDMA radio interface Uu. B. Maintaining the Integrity of the Specifications Figure 2 depicts the WCDMA protocol architecture for the transmission of user data plane data which is generated by TCP or UDP based applications. The applications as well as the TCP/IP protocol suite are located at the end-nodes: UE and host. The Packet Data Convergence Protocol (PDCP) provides header compression functionality which improves spectra efficiency for transmitting IP packets over the radio interface. The Radio Link Control (RLC) layer can operate in three different modes: The acknowledged mode, unacknowledged mode and transparent mode. The acknowledged mode provides reliable data transfer over the error-prone radio interface. This is accompanied by retransmitting erroneous RLC PDUs. In the unacknowledged mode, the data transfer over the radio interface is not error free but no additional delay due to retransmission. The functionality of transparent mode is similar to unacknowledged mode but no protocol information is appended to the PDU. Physical Layer Transport Channel DCH Type TTI 10 ms Channel Bite Rate Constant=[64 128 256] kbps Closed Loop Power Control Ideal (Independent erasures Block) Outer Loop Power BLER Target=10% Control RLC layer Operation Mode Acknowledged PDU Delivery In-Sequence PDU Size 320 bits RLC_Tx Window 1024 PDUs Size RLC_Rx Window 1024 PDUs Size SDU Discard Mode After MaxDAT MaxDAT 10 Polling Mechamism Not used currently Timer Status Prohibit (milliseconds) Timer STATUS Periodic (milliseconds) PDCP Layer TCP/IP Header Compression Header Fully Compressed TCP layer Version Reno MSS 536 B awnd 32768 B Initial cwnd 1 MSS Initial ssthresh awnd Initial RTO 3 sec Maximum RTO sec Minimum RTO 1 sec Duplicated ACKs for fast retransmit 3 Delay of internet and UMTS CN ms (one way) Figure 2: WCDMA protocol architecture-u-plane Application Layer FTP 300 kb Table 1 Simulation Parameters B. Perofmance Metrics RLC throughput at RNC, C. Simulation Models The simulation model is based on the system architecture discussed in the previous section. UE, Node B, RNC and host are modeled according to the aforementioned protocol stack as in figure 2. The applications in the simulator are HTTP and FTP which are the commonly used by internet users.

The file inter-request time is exponential distribution with average value 30 seconds. Thus we assume the network is not always saturated. Instead, user requests files in an exponential distributed inter-request time. Figure 3: Simulation Model IV. SIMULATION RESULTS A. Imapct of TCP slow start Impact of TCP slow start DCH File size 50k k 200k 64 20 kbps 35 kbps 42 kbps 128 28 kbps 58 kbps kbps 256 37 kbps 70 kbps 150kbps (Other simulation parameters: initial counter=1 Internet loss ratio=0, Internet delay=0) RLC Throughput (kbps) 1 140 120 40 DCH 128 kbps DCH 64 kbps RLC throughput VS File Size 20 50 150 200 File Size ( kbytes) Figure 4: the impact of TCP Slow Start -RLC throughput drops dramatically in while slightly in DCH 64 kbps From the efficiency point of view, the impact of the initial slow start after the TCP connection establishment depends on the overall amount of download data. For small files (like WWW pages) the document transmission is affected by the poor radio link utilization during the initial slow start, with the subsequent throughput degradation. On the other hand, for larger files (e.g. ftp downloads) the effect is expected to be minor. In the following, we aim to study the influence of slow start to the RCL throughput, with various file sizes and DCH bite rates. Figure 4 shows the RLC throughput as a function of the file size in a dedicated channel (DCH) for three different constant channel bit rates 64 kbps, 128 kbps and 256 kbps. We assume the loss ratio of Internet is 0 and delay of Internet is also 0. The MaxDAT parameters of RLC layer is 10 and TCP initial counter (initial congestion window) is equal to one. The file inter-request time is exponential distribution with average value 30 seconds. As shown in the graph, in general, for all DCH channel bit rates, small file size has a lower throughput than large file size. For a higher bit rate DCH channel, small file size degrades the RLC throughput significantly. For low speed DCH channel, the throughput of RCL decreases very little when the file size change from big size to small size. For the, the RLC throughput decrease 200% when the file size changes from 200 Kbytes to 50 Kbytes. In contrast, for DCH 64 kbps, the RLC throughput decreases only slightly when the file size becomes 50 Kbytes. The reason is that, assuming user continuous downing files, downloading small files involves more TCP slow start phases compare to large files. Thus, when the delay bandwidth product is large, the RLC throughput of small file decreases significantly due to not being able to fully utilize network capacity in the TCP slow start phase. While in a low delay bandwidth product environment, the decrease is not so obvious B. Impact of TCP slow start initial counter DCH Initial 1 2 4 counter 64 35 kbps 38 kbps 41 kbps 256 70 kbps 75 kbps 82 kbps (Other simulation parameters: file size= k bytes Internet loss ratio=0, Internet delay=0 ) RLC throughput (kbps) 85 75 70 65 55 50 45 40 RLC throughput VS TCP initial counter DCH 64 kbps 35 1 1.5 2 2.5 3 3.5 4 initial counter (MSS) Figure 5: The impact of TCP initial counter -large initial counter improve TCP slow start

As it is shown in figure 5, a large TCP initial counter can improve the TCP performance for the Slow Start phase. Because a larger TCP initial counter can enable the sender to fill up the TCP pipe much more quickly than a small one in the slow start phase thus the utilization of radio link is increased. This is especially the case, when the delay bandwidth product is large. As shown in the figure, for DCH 256 kbps, the RLC throughput increases more than 30% when the initial counter increases from 1 MSS (Maximum Segment Size) to 4 MSS. In contrast, for DCH 64 kbps, the RLC throughput has a small increase less than 20% when the initial counter is set to 4. C. Impact of Internet loss rate DCH (kbps) Internet Loss Rate 1% 5% 10% 128 kbps 81 kbps 79 kbps 65 kbps 256 kbps 150 kbps 125 kbps 112 kbps RLC throughput (kbps) 150 140 130 120 110 90 70 Imapct of Internet Loss Ratio DCH 128 kbps 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 Internet Packet Loss Ratio Figure 6: the impact of Internet Loss Ratio -the throughput decreases dramatically in while drops only slightly in DCH 128 kbps The impact of Internet packet loss ratio is studies in this section. We assume the following simulation setting: 128 kbps and 256 at radio link, Internet loss rate (1%, 5%, and 10%); Inter-request time Exponential with mean 30s, file size 200k bytes. The conclusion we get from the simulation is that: 1) for high bandwidth radio link, the impact of Internet loss ratio is significant to the RLC throughput; As shown in the figure 6, the RCL throughput drops 36% when the Internet loss ratio increases from 1% to 10%. 2) For low bandwidth radio link, the impact of Internet loss ratio to the RLC throughput is minor. As shown in the figure 6, for DCH 128 kbps, the throughput degradation is only 23% when the internet loss ratio becomes 10%. The reason is that a high bandwidth radio link indicates a large Bandwidth Delay Product (BDP) and thus the throughput has more degradation each time when the segment is retransmitting in the TCP slow start phase. In contrast, a small bandwidth radio link indicates a small BDP, which minimize the effect of TCP retransmission when the packet loss ratio is high in the internet. D. RLC interaction with TCP protocol RLC throghput (kbps) 150 50 Influence of RLC MaxDAT to TCP performance BLER 10% BLER 20% BLER 40% 0 3 4 5 6 7 8 9 10 MaxDAT Figure 7: the impact of RLC MaxDAT Figure 7 plots the RLC throughput as function of RLC parameter MaxDAT for DCH data rate (64,128,256 kbps) respectively. The results clearly show that the more reliable the RLC, the higher RCL throughput can be achieved. In order to test the reliability of RLC, the MaxDAT is set to 3, 5 and 10. It is shown that the more reliable the RLC, the better RLC throughput can be achieved. In order to reach a high degree of reliability, different MaxDAT is needed for different BLER targets. As indicated in figure 7, for BLER 10%, when MaxDAT is set to 5, the RLC throughput is max while BLER 40% requires a MaxDAT larger than 10 to achieve max RLC reliability. CONCLUSION We address the end-to-end TCP performance over WCDMA networks. Firstly, TCP slow start does dramatically decrease the system performance when the file size is small and DCH has high data rate. In low bit rate DCH, the impact of slow start is very small. Secondly, a larger TCP initial counter can to some extent mitigate the throughput degradation in TCP slow start, especially in large delay bandwidth product case. Thirdly, the Internet loss ratio degrade a lot the TCP performance when the DCH bit rate is high while the impact is minor in a low bit rate DCH channel. Finally, regarding the interaction between the RLC and the TCP layer, the reliable the RLC layer, the higher network throughput can be achieved. The error recovery at RLC layer can hide the low layer radio transmission errors from TCP such that TCP endto-end recovery mechanisms can be less triggered and TCP congestion window can be less frequently reduced to half. For a given BLER, a sufficient large MaxDAT can bring the maximum RLC reliability, in order to achieve the maximum RLC throughput. For the further work, we plan to implement a TCP proxy at WCDMA core network to improve the TCP performance in large delay bandwidth scenario.

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