Performance Improvement of TCP on Wireless Cellular Networks by Adaptive FEC Combined with Explicit Loss Notification

Transcription

1 erformane Improvement of TC on Wireless Cellular Networks by Adaptive Combined with Expliit Loss tifiation Masahiro Miyoshi, Masashi Sugano, Masayuki Murata Department of Infomatis and Mathematial Siene, Graduate Shool of Engineering Siene, Osaka University 1-3 Mahikaneyama, Toyanaka, Osaka , Japan Osaka refeture College of Health Sienes, Habikino, Habikino, Osaka , Japan Abstrat- We propose a new adaptive sheme ombined with ELN (Expliit Loss tifiation) that was proposed for improving TC performane in wireless ellular networks. In our method, transmission ors on the wireless link are measured at the paket level and the or status is notified the TC er with ELN. Aording to this information, an appropriate ode is determined in order to maximize the TC performane. We first ompare the TC performane using Snoop rotool, ELN and the fixed, through whih we find the appropriate ode against given BER (bit or ratio). We then show how the adaptive an be realized using our solution, and also examine the appropriate observation period of measuring BER enough for the fading speed on the noisy wireless link. We finally demonstrate that our method an ahieve better performane than the onventional fixed by using the Gilbert model as a wireless or model. I. Introdution A mobile Internet tehnology based on a wireless ellular network has been rapidly deployed in these several years. In the Internet, TC is used as a standard transport layer protool. TC reognizes a ongestion ourrene within the network by a paket, and performs ongestion ontrol by throttling the ongestion window. Then, we enounter the problem that the paket es due to the transmission ors ause unexpeted degradation of TC throughput in a wireless ellular network environment. Many approahes have been proposed to improve TC throughput in the wireless ellular network. The split onnetion approah suh as Indiret-TC [1] is an early proposal in the field. It involves splitting eah TC onnetions between a er and a reeiver into two separate onnetions. Snoop rotool [2] is an improved sheme of split onnetion approahes, and it retains end-to-end semantis; it uses Snoop Agent to ahe the TC segment and retransmits the segment only on the wireless link. ELN (Expliit Loss tifiation) [3] is a more preise approah with a apability of distinguishing the paket due to ongestion from the one by transmission ors. Another approah to improving the TC performane is to enhane the lower layer protool, i.e., the data link layer protool, by inorporating, e.g., ARQ (automati repeat request) and/or (forward or orretion) [4]. is a simplest solution to improve the bit or ratio seen by the higher layer protool. However, it is ineffiient if an or ondition of the radio hannel varies greatly. Aordingly, an adaptive or orretion sheme is proposed in [5] in order to ompensate suh a drawbak of. However, utilizing the adaptive or orretion solely is not a realisti solution beause it is ompliated to measure BER (bit or rate) for eah bit or the unit of several bits and hange the ode appropriately against irregular wireless ors. In referene [5], it is shown an adaptive or orretion method for wireless LAN where the paket size and the degree of redundany are adaptively ontrolled aording to the paket or rate. But it is needed speially-formatted UD datagrams as probed to measure the paket or rate, and that these datagrams ompress available bandwidth. In short, it has not been proposed Adaptive sheme using no probed. In this paper, we propose a new adaptive sheme ombined with ELN, and we show a method for deiding threshold value to hange adaptive ode, whih does not larified by [5]. In our method, transmission ors on the radio link are measured at the paket level and the or status is notified the TC er with ELN. Aording to this information, an appropriate ode is seleted. We first evaluate the TC performane using Snoop rotool, ELN and the fixed, through whih we find the appropriate ode against given BER. We will then show how the adaptive an be realized using our solution, and also examine the appropriate observation period of measuring BER enough for the fading speed on noisy wireless link. We will finally evaluate our proposed method by Gilbert model [6] as wireless or model, and show that our method ahieves better performane than the onventional fixed. The rest of this paper is organized as follows. In Setion II, we show the network model in our study. In Setion III, we evaluate the TC performane using Snoop rotool, ELN and the fixed. We desribe our proposed method in Setion IV, and evaluate it in Setion V. We finally desribe onluding remarks in Setion VI. II. Network Model We use the network onfiguration shown in Figure 1 with IMT-2 support [7] to investigate the TC performane. arameters are summarized in Table 1. Here, the bandwidth of the downlink is 384kbps for best effort servie in the first deployment in Japan. In this onfiguration, sine the major appliation of the urrent Internet is to download the Web douments from the fixed server to the wireless terminal, we assume that TC segments are transmitted towards the wireless terminal from the wired terminal, and ACK segments are in the reverse diretion. We onsider that paket is mainly aused by the transmission or on the wireless link, and the paket due to buffer overflow is assumed to be negligible. Wireless Terminal Wireless link Base 64Kbps, 3 Station 1Mbps, 4 384Kbps, 2 1Mbps, 1 1, 2, 3, 4: ropagation delay Figure 1 Network model Wired link Wired Terminal

2 Table 1 Network parameters TC segment size ACK size Buffer size ropagation delay (τ1,τ2,τ3,τ4) 1 byte 4 byte 5 kbyte 1ms III. erformane Comparison for TC on Wireless Cellular Network In this setion, we first desribe major shemes of improving for TC throughput on wireless ellular network, and then evaluate TC throughput for major shemes. TC provides reliable end-to-end data ommuniation using the following two main ongestion ontrol mehanisms. The one is Fast Retransmit and Fast Reovery mehanisms, whih throttle the ongestion window size to half, if the TC er detets triple dupliate ACKs ontinuously. The other is a retransmission timeout mehanism, whih draws bak the ongestion window size to 1 MSS (Maximum Segment Size), if the ACK for the TC segment is not before retransmission timeout timer expiration. Those ongestion ontrol mehanisms perform well on wired links sine the most of paket es ours due to ongestion. However, on wireless links, it does not beause the paket may our due to transmission ors and TC annot distinguish the paket due to ongestion and transmission ors. Therefore, it is well known that the paket es due to transmission ors ause unexpeted degradation of TC throughput in a wireless ellular network environment. Many approahes have been proposed to improve TC throughput in the wireless ellular network. These are summarized in the IETF (Internet Engineering Task Fore) [8]. Here, we desribe Snoop rotool [2], ELN [3], and the link-layer sheme as major shemes. Snoop rotool Snoop rotool is an improved sheme of split onnetion approahes. It retains end-to-end semantis of TC, suh that it uses Snoop Agent to ahe the TC segment and retransmits the segment only on the wireless link. Snoop rotool performs the following sequene in the ase of the wired terminal being the TC er. (1) The TC segments are ahed at the base station. (2) The TC segments are retransmitted at the base station, if paket is deteted on the wireless link. ELN ELN is an approah that enables distinguishing the paket due to ongestion from transmission ors. If the paket ours due to transmission ors, the TC er takes appropriate ations. That is, ELN performs the following sequene in the ase of the wired terminal being the TC er. (1) The TC sequene numbers are ahed at the base station. (2) The ACK paket is attahed ELN bit ative at the base station, if the paket, of whih sequene number is ahed, is lost. (3) The TC er takes an appropriate ation when reeiving ELN bit ative. One typial realization is that the TC er does not perform ongestion ontrol. The Data Link layer sheme Enhanements of the link-layer protool inlude ARQ (automati repeat request) and (forward or orretion) [4]. is a simplest solution to improve the bit or ratio seen by the higher layer protool. However, it is ineffiient if an or ondition of the radio hannel varies greatly. Aordingly, an adaptive or orretion sheme is proposed in [5] in order to ompensate suh a drawbak of. However, utilizing the adaptive or orretion solely is not a realisti solution beause it is ompliated to measure BER (bit or rate) for eah bit or the unit of several bits and hange the ode appropriately against irregular wireless ors. We evaluate TC throughput by using the TC Reno version, whih is a urrent de fato standard for TC implementations. For enhanement of the TC performane, we onsider the following three shemes. ELN, whih distinguishes between paket due to ongestion and others. Snoop+ELN, whih is a ombination of Snoop and ELN. as an enhanement of the data link layer protool. We used the ns-2 simulator [9]. Simulation results are shown in Figure 2, where the horizontal axis shows the bit or rate on the wireless link. We adopt BCH (127, 92) as the ode. It an be observed in the figure that is most effetive to prevent TC throughput degradation in the range where BER is greater than.1. On the other hand, in the lower BER region, TC throughput of beomes smaller than other shemes due to its overhead and Snoop+ELN ahieves the best performane. TC throughput (kbps) Reno ELN Snoop plus ELN 1.E-7 1.E-6 1.E-5 1.E-4 1.E-3 Bit or rate Figure 2 Comparison of TC throughput

3 IV. Adaptive Combined with Snoop+ELN In Figure 3, we present the TC throughput of Snoop+ELN ombined with. The ode is hanged as BCH (127, 92) as 5-bit orret ode, BCH (127, 64) as 1-bit orret ode and BCH (127, 36) as 15-bit orret ode. The figure learly shows an existene of the optimal parameter to ahieve the best performane dependent on BER. In short, an adaptive ombined with Snoop+ELN sheme is more effetive in order to prevent TC throughput degradation. In this paper, we propose a new adaptive sheme ombined with ELN, and we show a method for deiding threshold value to hange adaptive ode, whih does not larified by [5]. TC throughput (kbps) 2 18 without A: p D: p. 1 1.E-7 1.E-5 1.E-3 Bit or rate (5bit orret) (1bit orret) (15bit orret) B: p.3 C: p.3 Figure 3 TC throughput on various ode ombined with Snoop+ELN A without A (127,92) B (127,64) (127,36) B C D A: p >.3 (In the ase of two or more in a window of ten ) B: p >.3 (In the ase of two or more in a window of ten ) C: p >.3 (In the ase of two or more in a window of ten ) D: p <.1 (In the ase of two or more in a window of ten ) Figure 4 Method of hanging adaptive ode In the urrent Internet, network ongestion has been solved to improve network infrastrutures. We onsider that paket is mainly aused by the transmission or on the wireless link, and the paket due to network ongestion is assumed to be negligible. In this assumption, TC ers reognize the transmission or on the wireless link by ELN, and perform no ongestion ontrol. Of ourse, we should take aount of the fat that the paket or rate is dynamially hanged aording to the ondition of the wireless link. In the adaptive sheme, it is important how ode should be hanged aording to the paket or rate. In this paper, we adopt a reative-based adaptive algorism by observed ors. In the reative-based adaptive algorism, it is important that the algorism is robust C D in the sense that it is not influened by the sensitive ors. The authors in [5] proposed a tehnique of adaptively hanging ; the ode is hanged when two or more segments among ten segments enounter the transmission or. Our proposed method is based on that idea. Based on the above numerial results, we show a method for hanging the apability aording to p obtained by ELN as shown in Figure 4. Here, the urrent ode is hanged to the more or orretable ode when p exeeds the threshold value. Otherwise, it is hanged to the less overhead ode. In Figure 3, the symbol of the A, B, C, and D is the threshold value of p and orrespond to the same symbol in Figure 4. We finally examine the time period of measuring p. The smaller value of the time period is expeted so that it an be adapted to the time hanging paket rate. However, it must be large enough for the fading speed. The fading speed is given by f d (Doppler frequeny) [1] as: f d vf where f is the arrier frequeny, v is the vehile speed, and is the speed of light. For the 9-MHz arrier frequeny, the above formula yields a Doppler frequeny of 3Hz/m/se. Therefore, for the high-speed wireless transmission (e.g., in the order of Mbps), the fading speed an be viewed as a roughly onstant value for the time period of measuring p (e.g., 1 sampling with the frame length of 1-byte segment). V. Evaluation of Adaptive Combined with Snoop+ELN In this setion, we evaluate our proposed method by the Gilbert model [6] as a wireless or model. It has been widely used to model the noisy link with time-varying ors. In the Gilbert model, two states of Error-free and Error are expressed in terms of transition probabilities p GE and p EG, and average or-free length L G and or length L E. See Figure 5. Error Free p GE Error p EG Figure 5 Gilbert model 1 pge L (2) G 1 p EG (3) LE The Gilbert model is a two-stare Markov model and eah state is memoryless. Realling that the geometri distribution (1)

4 is a disrete equivalent of the exponential distribution [11], we determine the length G(p) of staying in eah state as follows. ln( u) G( p) (4) ln(1 p) where u is a random number uniformly distributed from to 1, and p is the leaving probability from the state. Using the above model, we generate wireless ors and simulate whether lost or suess for eah ing paket. See the flow hart illustrated in Figure 6. nn+1 n>1? analyses. In the table, the seond olumn shows the number of lost by wireless paket ors and the third olumn does the number of. The fourth olumn shows the number of overhead, whih is the ase with fixed obtained by Eq. (a-5). The final olumn shows the number of suessfully, in whih we omit the number of overhead from the number of totally. We an observe that our adaptive an ahieve the best performane. That is, by using the adaptive, the suessfully are improved by 2% in Table 3, and by 1% in Table 4. It is beause our adaptive enounters less paket es than without, and needs less overhead than the fixed. Table 3 Comparison of in the ase of lower BER (pb.15, p.9) aket? _ounter _ounter+1 p (n) _ounter/1 p (n) >threshold(k)? lost over head (available) without (127,94) (127,64) (127,36) Adptive kk+1 kk-1 k Table 4 Comparison of in the ase of higer BER (pb.75, p.445) k>3? k<? k3 ode(k) k ode(): without ode(1): (127,72) ode(2): (127,64) ode(3): (127,64) threshold().3 threshold(1).3 threshold(2).3 threshold(3).1 lost over head (available) without (127,94) (127,64) (127,36) Adptive Figure 6 Flow of Adaptive Combined with Snoop+ELN In simulation, the number of ing is 5,, and length is fixed at 8 bits, whih is ommonly seen in the mobile phone data transfer servies. The parameter set of the Gilbert model is shown in Table 2. Table 2 Gilbert model parameters p GE p EG lower BER ase.6.5 higer BER ase.6.7 Here, we assume that bit ors in eah paket our randomly. We show the simulation results in Table 3 in the ase of a rather lower value of BER (p b.15, p.9), and in Table 4 in the ase of the higher BER (p b.75, p.445), respetively. We also show the values obtained through the simple analysis for the ase without and for the one with fixed. See appendix for the VI. Conluding Remarks In this paper, we have proposed a new adaptive sheme ombined with ELN. In our method, transmission ors on the radio link are measured at the paket level and the or status is notified the TC er with ELN. Aording to this information, an appropriate ode is seleted. We have evaluated the TC performane using Snoop rotool, ELN and the fixed, through whih we found the appropriate ode against given BER. We have also shown how the adaptive an be realized using our solution. We have finally evaluated our proposed method by the Gilbert model as the wireless or model, and show that our method ahieves better performane than onventional fixed.

5 ACKNOWLEDGMENT This work was partly supported by Researh for the Future rogram of Japan Soiety for the romotion of Siene under the rojet Integrated Network Arhiteture for Advaned Multimedia Appliation Systems, Speial Coordination Funds for promoting Siene and Tehnology of the Siene and Tehnology Ageny of the Japanese Government, Teleommuniation Advanement Organization of Japan under the rojet Global Experimental Networks for Information Soiety rojet, a Grant-in-Aid for Sientifi Researh (A) (2) from The Ministry of Eduation, Siene, Sports and Culture of Japan. AENDIX In this appendix, we show how the numbers of overhead and suessfully are alulated in the ase of without and fixed. [Without ] By assuming that the bit or in eah paket ours randomly, paket or rate p is represented by using the bit or rate p b and the paket length l as: l p 1 (1 ) (a-1) Using p, the number of lost,, is determined as: (a-2) Similarly, the number of suessfully, rv, is given by: (a-3) [Fixed ] If the bit ors in eah paket our randomly, p is represented by the following binomial distributed expression by using bit or rate p b, paket length l and or orret ability. p p b p rv l i ( l, ) 1 pb p i (1 ) i l i b (a-4) Using p and overhead H, the numbers of lost, overhead head, and suessfully rv are obtained by the following expressions: p rv head H head REFERENCES (a-6) (a-7) (a-8) [1] A. Bakre and B. R. Badrinath, I-TC: Indiret TC for mobile hosts, ro. of IEEE International Conferene on Distributed Computing Systems (ICDCS 95), pp , May [2] H. Balakrishnan, S. Seshan and R. H. Katz, Improving reliable transport and handoff performane in ellular wireless networks, Wireless Networks, vol.1, De [3] H. Balakrishnan and R. H. Katz, Expliit notifiation and wireless web performane, ro. IEEE GLOBECOM '98 Internet Mini-Conferene, v [4] E. Ayanoglu, S. aul, T. F. Laorta, K. K. Sabnani, and R. D. Gitlin. AIRMAIL: A link-layer protool for wireless networks, Wireless Networks, vol.1, pp.47-6, Feb [5] D. A. Ekhardt and. Steenkiste, A trae-based evaluation of adaptive or orretion for a wireless loal area network, Mobile Networks and Appliations, vol.4, no.4, pp , De [6] E. N. Gilbert, Capaity of a burst-noise hannel, Bell Systems Tehnial Journal, vol. 39, pp , Sept [7] Available at [8] G. Montenegro, S. Dawkins, M. Kojo, V. Magret and N. Vaidya, Long Thin Networks, RFC2757, Jan. 2. [9] Available at [1] A. Kumar and J. Holtzman, Comparative performane analysis of versions of TC in a loal network with a Mobile Radio Link, IEEE/ACM Transations on Networking, vol. 6, no. 4, pp , Aug [11] R. Jain, The Art of Computer Systems erformane Analysis: Tehniques for Experimental Design, Measurement, Simulation, and Modeling, Wiley, Using paket length l and data length d, overhead by fixed is alulated by following expression: d H l (a-5)