Impact of Layer Two ARQ on TCP Performance in W-CDMA Networks

1. Impact of Layer Two ARQ on TCP Performance in W-CDMA Networks Hiroshi Inamura, Osamu Takahashi, Hirotaka Nakano, Taro Ishikawa, NTT DoCoMo, Inc. Hiroshi Shigeno, Department of Information and Computer Science, Keio University Presented by Ming-Yen Lai

2. Outline • Introduction • Overviews WCDMA Network Features • Related Works • TCP Performance with RLC Retransmission • Performance Evaluation via Emulation and Simulation • Conclusion

3. Introduction • The quality of transmission in the wireless networks that incur bit error can be improved with link layer retransmission. • This paper addresses the interaction between TCP and RLC (Radio Link Control) in evaluating TCP performance. • The paper examines the trade-off between link utilization and transport layer performance by changing the link control parameters.

4. Overviews WCDMA Network Features (cont.) • WCDMA, an IMT2000 was developed by 3GPP. • WCDMA has two types of services: voice service on switched circuits and data service created by packet transfer. • In packet service, WCDMA system controls transmission error by Radio Link Control (RLC).

5. Overviews WCDMA Network Features (cont.) • WCDMA protocol stack

6. Overviews WCDMA Network Features (cont.) • Retransmission in RLC

7. Overviews WCDMA Network Features • The following are the benefits of RLC retransmission above the end-to-end reliability offered by TCP. • The small PDU size used in RLC makes retransmission more efficient. • The response time on the feedback from the receiver is smaller than is possible with TCP’s end-to-end feedback.

8. Related Works (cont.) • The locations of producing significant delay in WCDMA Networks • RLC retransmission • Processing delay for FEC and interleaving • Buffer in RNC • Problems for TCP in WCDMA Networks • High bit error rates • Delay-jitter

9. Related Works (cont.) • Approaches for improving TCP performance over wireless & mobile networks. • Improvement of the existing TCP protocol • Split connection approach • Link layer solution

10. Related Works (cont.) • According to prior research, we made the following assumptions. • For optimizing TCP, only those techniques compatible with modern TCP implementations could be used. • To preserve the end-to-end communication model, we do not adopt the intermediate node approach. • A detailed model is needed that accounts for the use of selective repeat style ARQ.

11. Related Works • Based on the above assumptions, we examined the link characteristics of the WCDMA network while setting the following goals. • Suppress delay-jitter in link layer to avoid excess retransmissions • Clarify the trade-off between jitter-suppression and link utilization to improve TCP throughput • Optimize the link layer and TCP parameters

12. TCP performance with RLC retransmission (cont.) • Bandwidth-delay product • The optimal advertised receive window size is based on the product of the bandwidth and the delay of the link network.

13. TCP performance with RLC retransmission (cont.) • Persistence in link layer retransmission • Persistence in link layer retransmission involves a trade-off between IP packet loss and the efficiency of link utilization. • MAX_DAT : the maximum number of retransmission attempts for a single PDU

14. TCP performance with RLC retransmission • Suppressing delay-jitter and increasing the available bandwidth • RTO = SRTT + max(G, K*RTTvar) where K = 4 • Timer_Status_Prohibit (TSP) defines the delay imposed on the receiver before it can issue a STATUS PDU.

15. Performance Evaluation via Emulation and Simulation (cont.) • Evaluate TCP time out behavior of a representative operating system (OS) using the emulator. • Evaluate the optimal TSP value from simulations. • Change the maximum number of retransmission attempts (MAX_DAT) to change the link persistence to determine the relationship between TCP throughput and packet loss rate and thus obtain the optimal MAX_DAT. • Determine the appropriate advertised receive window for TCP reflecting the WCDMA link BDP with simulated TCP traffic.

16. Performance Evaluation via Emulation and Simulation (cont.)

17. Performance Evaluation via Emulation and Simulation (cont.) • The number of incidents of RTO vs BLER (using WCDMA emulator)

18. Performance Evaluation via Emulation and Simulation (cont.) • Throughput vs BLER (using WCDMA emulator)

19. Performance Evaluation via Emulation and Simulation (cont.) • Number of STATUS PDU and RTO vs Timer_Status_Prohibit (using Opent)

20. Performance Evaluation via Emulation and Simulation (cont.) • Throughput versus Timer_Status_Prohibit (using Opent)

21. Performance Evaluation via Emulation and Simulation (cont.) • Throughput and Down link segment loss rate vs MAX_DAT (using Opent)

22. Performance Evaluation via Emulation and Simulation • Throughput versus TCP Receive Window Size per BLER (using Opent)

23. Conclusion • Linux is more aggressive to triggering retransmission in response to jitter than BSD or Solaris; it allows RTO to occur more frequently. • For the trade-off between this suppression and link utilization, the optimum TSP value is 200ms. • PDU retransmission should attempted up to five times to hold the BLER 0~10% in simulated network. • To performance of high latency networks, the TCP receive window should be set at 48 – 64 Kbytes.