The issue of useless packet transmission for multimedia over the Internet

Abstract

When packet loss rate exceeds a given threshold, received audio and video become unintelligible. A congested router transmitting multimedia packets, while inflicting a packet loss rate beyond a given threshold, effectively transmits useless packets. Useless packet transmission wastes router bandwidth when it is needed most. We propose an algorithm to avoid transmission of useless multimedia packets, and allocate the recovered bandwidth to competing TCP flows. We show that the proposed algorithm can be easily implemented in well-known WFQ and CSFQ fair packet queueing and discarding algorithms. Simulation of a 15-s MPEG-2 video clip over a congested network shows that the proposed algorithm effectively eliminates useless packet transmission, and as a result of that significantly improve throughput and file download times of concurrent TCP connections. For the simulated network, file download time is reduced by 55% for typical HTML files, 36% for typical image files, and up to 30% for typical video files. A peak-signal-to-noise-ratio (PSNR) based analysis shows that the overall intelligibility of the received video is no worse than that received without the incorporation of the proposed useless packet transmission avoidance algorithm. Our fairness analysis confirms that implementation of our algorithm into the fair algorithms (WFQ and CSFQ) does not have any adverse effect on the fairness performance of the algorithms.

Introduction

Internet voice and video applications, such as IP telephony and video streaming, continue to gain popularity. A direct consequence is the increased congestion in the Internet routers. In the routers, multimedia packets, i.e. IP packets carrying voice or video traffic, compete head to head with traditional data packets for link bandwidth. Two important issues that need to be addressed when multimedia traffic is multiplexed with data traffic are: (i) fairness, and (ii) useless packet transmissions (UPT). The fairness issue arises due to the fact that the data applications use TCP, which cuts back the transmission rate when packets are discarded at the router due to congestion, but most multimedia applications, use UDP, which does not cut back transmission rates. As a result, multimedia flows unfairly gets more bandwidth than the congestion-responsive TCP flows. The fairness problem in the Internet is now well recognised. Several packet queueing and discarding algorithms, such as WFQ [1], CSFQ [2] and FRED [3], have been proposed in the last few years to effectively address the issue of fairness. Performance results confirm that these algorithms can fairly distribute link bandwidth among competing multimedia and data flows. Some router vendors have already started incorporating these algorithms in their latest products [4].

The issue of UPT, however, is less understood. UPT is based on the fact that for packetised audio and video, packet loss rate must be maintained under a given threshold for any meaningful communication [5], [6], [7]. When packet loss rate exceeds this threshold, received audio and video become useless. Thus a router transmitting multimedia packets at a fair rate (using WFQ for example), while inflicting a packet loss rate beyond the threshold actually transmits useless packets. These packets are useless, because they do not contribute to any meaningful communication. UPT effectively reduces the available bandwidth to competing TCP flows, which in turn increases file download times. Longer download times increase power consumption of battery-powered devices, and hence, have direct impact on mobile computing.

The UPT problem will be more significant in future due to the following trends: (i) rising popularity of voice and video applications, (ii) increasing deployment of fair packet queueing algorithms, and (iii) rapid proliferation of battery-powered mobile devices. Hence, there is a need to investigate mechanisms to effectively address the UPT issue in fair packet queueing algorithms. In this paper, we propose a UPT avoidance algorithm (we call it UPTA) that can be easily implemented in existing fair packet queueing and discarding algorithms. We have evaluated its performance using simulation of an MPEG-2 video stream competing with a TCP connection in a congested router. Simulation results show that with the proposed UPTA algorithm in place, we can effectively eliminate UPT, and significantly reduce file download times without deteriorating the perceived quality of the video stream.

The remainder of the article is organised as follows. We discuss related work in Section 2. Section 3 formally defines the UPT problem and derives the performance parameters. In Section 4, we propose an UPTA algorithm and show its incorporation in two well-known fair packet queueing algorithms, WFQ and CSFQ. In Section 5, we derive the packet loss rate threshold for intelligible communication for the MPEG-2 video clip used in the simulation. The simulation experiment is presented in Section 6, followed by the results in Section 7. Finally, we present our conclusions and discuss open issues in Section 8.