ror recoveries at RLP (Radio Link Protocol) for uplink and downlink channels are .... symmetric. Figure 2: Throughput over error-free asymmetric radio links.
TCP over Asymmetric CDMA Radio Links Yong Bai, and Andy T. Ogielski WINLAB, Rutgers University, 73 Brett Road, Piscataway, NJ 08854-8060 {yongbai, ato}@winlab.rutgers.edu
Abstract In TCP-supported traffic flows like file downloading and Web browsing, the connection is inherently asymmetric since the size of TCP data packets is different from that of TCP ACK packets. To exploit the asymmetry, different uplink and downlink channel data rates can be allocated in wireless Internet access through third-generation mobile cellular systems. In this paper, we investigate TCP performance over asymmetric CDMA radio links in cdma2000 system. It is shown that appropriate downlink/uplink data rate ratio should be determined in order to achieve comparable system performance as that of the symmetric radio links, and the required data rate ratio varies with radio channel conditions, especially Frame Error Rates (FERs). This phenomenon is mainly due to the fact that frame error recoveries at RLP (Radio Link Protocol) for uplink and downlink channels are dependent on each other. To use our results in practical systems, we propose a procedure on channel data rate allocations for TCP-supported data services in order to achieve acceptable system performance and use channel resource efficiently.
1 Introduction Asymmetry is exhibited in the infrastructure of some networks, such as cable modem networks, direct broadcast satellite networks, Asymmetric Digital Subscriber Loop (ADSL) networks, where bandwidth capability in the downstream (to user) direction is often orders of magnitude larger than that in the upstream direction. The main reason to use asymmetric connections is that various activities, like browsing World Wide Web, navigating and reading Usenet News, require the transfer of much more data to the user than back into the network. Regarding TCP-supported Internet data services, in addition to aforementioned asymmetric characteristic of upstream/downstream traffic flows, TCP connection itself exhibits asymmetry. TCP data segment size is determined by MSS (Maximum Segment Size) that normally defaults to 0-7803-6507-0/00/$10.00 ©2000 IEEE
536 octets for out-of-local connections [1], and therefore 556-octet TCP packets are formulated with 20-octet TCP header. In contrast, the size of TCP ACK packets is only 20 octets (size of TCP header) when TCP ACK is not piggybacked, which is common for asymmetric Internet traffic flows, such as file downloading, Web browsing, etc. In the third-generation mobile cellular systems, multi-rate data services are provided. For instance, implemented by various channel coding and spreading schemes, the traffic channel capacity in cdma2000 system is highly variable [2]. The cdma2000 traffic channels are classified as FCHs (Fundamental Channels) (up to 9.6 kbps in Rate Set 1) and SCHs (Supplemental Channels) (from 9.6 kbps up to 2.0 Mbps). The data rate of SCHs can be flexibly assigned to support different QoS (Quality of Service) requirements of various data services. With attention to save scarce radio channel resources, it is reasonable to allocate different SCH data rates at downlink (to user) and uplink channels to exploit the asymmetry of TCP packet delivery. However, the TCP behavior over asymmetric lossy radio channels needs to be investigated to ensure desirable system performance in such a context. The performance of TCP and its compatibility with wireless network protocols (especially IS-707 Radio Link Protocol (RLP) [3]) over symmetric CDMA radio links have been investigated in [4], [5], [6], and [7]. Radio Link Protocol (RLP) is a wireless network protocol to perform error recovery in RLP frame level (lower-level than TCP) to reduce exhibited error rate in the traffic channels. IS-707 RLP uses NAK (negative acknowledgment) based selective repeat ARQ protocol to retransmit erased data frames. In case of data frame losses, RLP performs partial data frame error recovery through limited retransmission attempts requested via NAK control frames. NAK control frames for downlink (to mobile) frame error recovery are delivered via uplink channels, and vice versa. Therefore, the uplink and downlink are dependent on each other in terms of frame error recovery. This raises more complicated interactions between uplink and downlink channels, and unequal channel data rates may adversely affects system performance if NAK control frames can not be delivered promptly over relatively
slower radio links. Previous study of effects of asymmetry on TCP performance is about reverse bottleneck link for TCP ACK packets [8], [9]. It has been shown that asymmetry increases TCP’s sensitivity to random losses, causing throughput deterioration. In these investigations, there are no RLP layer ARQ schemes involved, and appropriate channel data rate allocation to suit the asymmetry of TCP packets has not been studied. In this paper, we investigate overall system performance and interactions of TCP and RLP over asymmetric radio links and obtain appropriate downlink/uplink data rate ratios to couple them together. To take advantage of TCP asymmetry and avoid adverse effects of asymmetric radio links taking place, we proposed a channel data rate allocation procedure for TCP-supported data services. The rest of the paper is organized as follows. Simulation model for our investigation is shown in Section 2. Simulation results are presented in Section 3, and the channel data rate allocation procedure is proposed in Section 4. Section 5 gives our conclusions.
2 Simulation Model We established a simulation model to investigate the overall system performance and interactions of TCP and RLP in a wireless Internet access scenario, i.e., a mobile host communicates with a remote host over radio link, Internet access point (IAP), and wireline link, as shown in Fig. 1. Here IAP is an abstract node to ease the investigation. In the practical CDMA system, it may encompass several network components, such as base station (BS), mobile switching center (MSC), and internetworking function (IWF), etc. Mobile Host
Remote Host
TCP
Data Source
After transversing the Wireline Link, the TCP packet is delivered to the Interface at IAP. The Interface formulates a 582-octet PPP frame by adding a 20-octet IP header and a 6-octet PPP header to the TCP packet, and then fragments the PPP frame into fairly short RLP frames. The RLP frames are transmitted by the peer-to-peer RLP modules over the Radio Link. At the Mobile Host side, the Interface reassembles the incoming frames from RLP and passes the correctly reassembled TCP segments to the TCP module at the Mobile Host. The transfer of TCP ACK segments (encapsulated in 40-octet IP datagrams) from the Mobile Host to the Remote Host follows the reverse path. Since we observe the radio links from the view point of RLP, the Radio Link is characterized by three parameters, i.e., radio link data rate CRL , RLP frame error rate FE , and normalized Doppler frequency fd T . The RLP frame errors can be either i.i.d. (statistically independent and identically distributed) or correlated (by choosing different fd T ). In this paper, we only employ the i.i.d. frame error model for our investigation. The Wireline Link module represents a very simplified Internet cloud, characterized by one-way transmission delay τW L (set to 200 ms) and bandwidth CW L (set to 1.5 Mbps). This model is implemented in SSF (Scalable Simulation Framework), a new public-domain standard for discreteevent simulation of large, complex systems [10]. We use a TCP Reno variant ported to SSF from the LBNL’s Network Simulator ns [11]. The RLP entity implements the IS-707 ARQ protocol. In a high-bandwidth cdma2000 system, quite large Physical Layer frame size and high Physical Layer FER may result in unacceptable overall system performance, here we employ IS-707 RLP enhanced with LTU subdivision at Multiplex Sublayer and 20 ms Physical layer frame based (PHY-based) processing scheme presented in [7].
3 Simulation Results Interface TCP RLP
IAP (Internet Access Point) Radio Link , , FE , fdT ) (CRL
RLP
Interface
Wireline Link (CWL , τWL )
Figure 1: Simulation model of wireless Internet access In this model, the Mobile Host acts as a client to communicate with the Remote Host (as a server) for bulk data like FTP. 556-octet TCP packet (536-octet data payload and 20-octet TCP header) is generated by the Data Source (a bulk data pool) and sent to the TCP module for transmission.
Here performance is evaluated by throughput defined as delivered data in the Application layer (here is FTP) divided by the transmission time. In measuring throughput, TCP and RLP establishment and tearing-down phases are not taken into account. Let us first investigate system performance over error-free radio links. Fig. 2 shows throughput as a function of downlink and uplink data rates when there are no RLP frame errors. Throughput over symmetric radio links is also shown in this and following figures for comparison. It is seen in Fig. 2 that uplink data rate 19.2 kbps is capable to support a wide range of downlink data rates from 9.6 kbps up to 153.6 kbps. Here we say an uplink data rate is capable if it can result in a throughput ≥ 90% of the throughput that can be achieved over symmetric radio links at the same FER con-
ditions. Furthermore, we say that a capable link data rate ratio between downlink SCH and uplink SCH can be up to 8 (= 153.6/19.2) in this case. The large capability of errorfree radio link is consistent with our intuitive inference as the downstream TCP data packet size is quite larger than the upstream TCP ACK packet size. 120
uplink data rate = 9.6 kbps uplink data rate = 19.2kbps symmetric
100
Fig. 4, 5, and 6 show system performance over FE = 0.05, 0.10, and 0.15 asymmetric radio links, respectively. It is seen that the adverse effect is severer with the increase of FE , e.g., 38.4 kbps uplink data rate can support up to 153.6 kbps with FE = 0.01, but its capability reduces to 76.8 kbps with FE = 0.05. Thus, if the downlink/uplink data rate ratio is too large and FER is high, NAK control frames for the RLP entity at IAP can not be delivered promptly for its frame error recovery, leading to TCP timeouts and significant performance deterioration. 150
Throughput (kbps)
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uplink date rate = 9.6 kbps uplink date rate = 19.2 kbps uplink date rate = 38.4 kbps uplink date rate = 76.8 kbps symmetric
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Figure 2: Throughput over error-free asymmetric radio links 0
Next, we investigate the system performance with RLP frame errors. Fig. 3 shows system performance over asymmetric radio links with FE = 0.01. It can be seen in Fig. 3 that the capability of certain uplink data rate is less than that in the error-free case. For instance, 19.2 kbps uplink data rate can only support up to 36.8 kbps downlink data rate, far less than 153.6 kbps as in frame error-free case. Now the capable uplink data rate for 153.6 kbps downlink data rate is 76.8 kbps, resulting in a much smaller capable downlink/uplink data rate ratio equal to 4.
19.2
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Figure 4: Throughput with FE = 0.05 150
uplink date rate = 9.6 kbps uplink date rate = 19.2 kbps uplink date rate = 38.4 kbps uplink date rate = 76.8 kbps symmetric 100
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120 uplink date rate = 9.6 kbps uplink date rate = 19.2 kbps uplink date rate = 38.4 kbps uplink date rate = 76.8 kbps symmetric
100
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Figure 5: Throughput with FE = 0.10
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4 Channel Data Rate Allocation Mechanism 0
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Figure 3: Throughput with FE = 0.01
153.6
From the above investigation, it is demonstrated that we can take advantage of the asymmetry of TCP packets by allocating different radio link channel rate accordingly. How-
150
asymmetry of the sizes of TCP data and TCP ACK packets. To use radio resource efficiently, it is desirable to allocate lower uplink data rate if it can support relatively higher downlink data rates. It was shown that a lower uplink data rate is capable to support a wide range of higher downlink data rates over error-free radio links, however, the capability is reduced with lossy radio links, and it is worse with increase of radio link FERs as RLP NAK control frames can not be delivered promptly under these conditions. Appropriate downlink/uplink data rate ratios were determined for different radio link FERs, and a channel data rate allocation procedure was proposed to take advantage of the asymmetry of TCP connection and avoid the adverse effects taking place.
uplink date rate = 9.6 kbps uplink date rate = 19.2 kbps uplink date rate = 38.4 kbps uplink date rate = 76.8 kbps symmetric
Throughput (kbps)
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Figure 6: Throughput with FE = 0.15 ever, the adverse effects can take place when the data rate ratio of downlink/uplink channels is not appropriate. Thus, in data rate allocation for the third-generation cellular systems, upgrade of radio link data rate at one direction may not help under certain channel conditions if the data rate at the opposite direction becomes bottleneck. Now, we propose a procedure to allocate channel data rates with both the beneficial and adverse effects of asymmetric radio links in mind. First of all, only FCH is allocated during new data call setup. The allocation of the downlink SCH is triggered when backlogged data reaches a predefined threshold, and the allocated downlink SCH data rate is determined based on the estimated arriving transmission data rate calculated by backlogged data divided by the building-up time. Note that the determination of channel data rate is not just based on the queuing size of back-logged data. The associated capable uplink data rate is determined simultaneously by taking into account of both the downlink SCH data rate and channel FERs based on our simulation results. Then a Supplemental Channel Notification Message (SCNM) is sent to MH to invoke its SCH upgrade. For the reverse direction, MH follows the same rule to trigger SCH allocation, and it sends a Supplemental Channel Request Message (SCRM) to BS for its upgrade. The radio link data rate combination is determined at BS, and sent to MH by SCNM. Furthermore, during the data transferring, if the FER of the radio links deteriorates, the corresponding capable channel data rates should be determined promptly and notify each other between MH and BS for their updates.
5 Conclusions We investigated TCP/RLP performance over asymmetric cdma2000 radio links in order to exploit the inherent
References [1] W. R. Stevens, TCP/IP Illustrated Volume I. AddisonWesley, 1994. [2] “The cdma2000 ITU–R RTT Candicate Submission,” July 1998. [3] TIA/EIA/IS–707–A.2, “Data services option standard for spread spectrum digital cellular systems: Radio Link Protocol,” March 1999. [4] A. Chockalingam, and G. Bao, “Performance of TCP/RLP Protocol Stack on the Correlated Fading DS-CDMA Wireless Links,” in Proc. VTC’98, pp. 363–367, May 1998. [5] Y. Bai, G. Wu, and A.T. Ogielski, “TCP/RLP Coordination and Interprotocol Signaling for Wireless Internet,” in Proc. of VTC’99 Spring, pp. 1945–1951, May 1999. [6] Y. Bai, A.T. Ogielski, and G. Wu, “TCP over IS-707,” in Proc. of PIMRC’99, pp. 1253–1257, Sept. 1999. [7] Y. Bai, P. Zhu, A. Rudrapatna, and A.T. Ogielski, “Performance of TCP/IP over IS-2000 Based CDMA Radio Links,” in Proc. VTC’00 fall. [8] H. Balakrishnan, V. Padmanabhan, and R.H. Kartz, “The Effects of Asymmetry on TCP Performance,” ACM MONET, vol. 4, pp. 219–241, Oct. 1999. [9] T.V. Lakshman, U. Madhow, and B. Suter, “Windowbased Error Recovery and Flow Control with a Slow Acknowledgement Channel: a Study of TCP/IP Performance,” in Proc. Infocom’97, pp. 1199–1209, 1997. [10] http://www.ssfnet.org/. [11] http://www-mash.cs.berkeley.edu/ns/.
