On playback delay in streaming communication

We consider the problem of minimizing playback delay in streaming over a packet erasure channel with fixed bandwidth. When packets have to be played in order, the expected delay inherently grows with time. We analyze two cases, namely no feedback and instantaneous feedback. We find that in both cases the delay grows logarithmically with the time elapsed since the start of transmission, and we evaluate the growth constant, i.e. the pre-log term, as a function of the transmission bandwidth (relative to the source bandwidth). The growth constant with feedback is strictly better that the one without, but they have the same asymptotic value in the limit of infinite bandwidth.Lincoln LaboratoryUnited States. Air Force Office of Scientific Research (Grant FA9550-11-1-0183)Hewlett-Packard Compan

On playback delay in streaming communication

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2012.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. 87-88).In this thesis, we consider the problem of minimizing playback delay in streaming over a packet erasure channel with fixed bandwidth. In recent years, there has been a rapid increase in live streaming applications where packets have to be played back at the receiver in order. With instantaneous feedback, the automatic-repeat-request (ARQ) protocol is delay optimal. However, with no feedback or delayed feedback, there is a trade-off between transmitting new packets and retransmitting old packets, to reduce the playback delay. Existing erasure codes such as Reed-Solomon codes and fountain codes that operate without feedback have delay proportional to the length of the stream, and hence are not suitable for streaming applications. Other coding schemes specifically de- signed for delay-constrained packet transmission aim to minimize the decoding delay. However, playback delay is a more natural metric for applications requiring in-order playback at the receiver. We aim to find good streaming codes that minimize playback delay for such channels with limited or no feedback. We analyze three cases, namely no-feedback, delayed feedback and broadcast with instantaneous feedback. We find that in all cases the playback delay grows logarithmically with the time elapsed since the start of trans- mission, and we evaluate the growth constant, i.e. the pre-log term, as a function of the transmission bandwidth (relative to the source bandwidth). The main tool used in the analysis of delay in all cases is to model packet decoding in terms of threshold crossing of a random walk. We can show that the expected playback delay is asymptotically equal to 1=z[lambda] log n where [lambda] is referred to as the growth constant. For the no-feedback case, the optimal value is [lambda] = D(1/b [rho]) where b is the bandwidth in packets per slot and [rho] is the success probability of the erasure channel. We prove that the simple coded repetition scheme where the source transmits combinations all packets generated so far in every slot achieves this optimal growth constant. With instantaneous feedback, the ARQ scheme is optimal and we can determine the exact expression for [lambda]. For the delayed feedback case we propose a greedy coding scheme and use it to determine a lower bound on [lambda]d as a function of feedback delay d. We can prove that the growth constant with feedback is strictly better that the one without, but they have the same asymptotic value in the limit of infinite bandwidth. We further extend the analysis to a broadcast streaming scenario with instantaneous feedback where the source is transmitting a common packet stream to N users over independent erasure channels. We determine how the growth constant [lambda]N scales with the number of the users N. It can be shown that greedy coding is optimal for the without feedback and instantaneous feedback cases, however we have not yet proved its optimality for the delayed feedback and broadcast streaming. This is the major part of ongoing research efforts. Other future research directions include extending the results to packet networks and considering more general channel models.by Gauri Joshi.S.M

Throughput-Smoothness Trade-offs in Multicasting of an Ordered Packet Stream

An increasing number of streaming applications need packets to be strictly in-order at the receiver. This paper provides a framework for analyzing in-order packet delivery in such applications. We consider the problem of multicasting an ordered stream of packets to two users over independent erasure channels with instantaneous feedback to the source. Depending upon the channel erasures, a packet which is in-order for one user, may be redundant for the other. Thus there is an inter-dependence between throughput and the smoothness of in-order packet delivery to the two users. We use a Markov chain model of packet decoding to analyze these throughput-smoothness trade-offs of the users, and propose coding schemes that can span different points on each trade-off.Comment: Accepted to NetCod 201

In-Order Delivery Delay of Transport Layer Coding

A large number of streaming applications use reliable transport protocols such as TCP to deliver content over the Internet. However, head-of-line blocking due to packet loss recovery can often result in unwanted behavior and poor application layer performance. Transport layer coding can help mitigate this issue by helping to recover from lost packets without waiting for retransmissions. We consider the use of an on-line network code that inserts coded packets at strategic locations within the underlying packet stream. If retransmissions are necessary, additional coding packets are transmitted to ensure the receiver's ability to decode. An analysis of this scheme is provided that helps determine both the expected in-order packet delivery delay and its variance. Numerical results are then used to determine when and how many coded packets should be inserted into the packet stream, in addition to determining the trade-offs between reducing the in-order delay and the achievable rate. The analytical results are finally compared with experimental results to provide insight into how to minimize the delay of existing transport layer protocols

The effect of block-wise feedback on the throughput-delay trade-off in streaming

Unlike traditional file transfer where only total delay matters, streaming applications impose delay constraints on each packet and require them to be in order. To achieve fast in-order packet decoding, we have to compromise on the throughput. We study this trade-off between throughput and in-order decoding delay, and in particular how it is affected by the frequency of block-wise feedback, whereby the source receives full channel state feedback at periodic intervals. Our analysis shows that for the same throughput, having more frequent feedback significantly reduces the in-order decoding delay. For any given block-wise feedback delay, we present a spectrum of coding schemes that span different throughput-delay tradeoffs. One can choose an appropriate coding scheme from these, depending upon the delay-sensitivity and bandwidth limitations of the application

Tiny Codes for Guaranteeable Delay

Future 5G systems will need to support ultra-reliable low-latency communications scenarios. From a latency-reliability viewpoint, it is inefficient to rely on average utility-based system design. Therefore, we introduce the notion of guaranteeable delay which is the average delay plus three standard deviations of the mean. We investigate the trade-off between guaranteeable delay and throughput for point-to-point wireless erasure links with unreliable and delayed feedback, by bringing together signal flow techniques to the area of coding. We use tiny codes, i.e. sliding window by coding with just 2 packets, and design three variations of selective-repeat ARQ protocols, by building on the baseline scheme, i.e. uncoded ARQ, developed by Ausavapattanakun and Nosratinia: (i) Hybrid ARQ with soft combining at the receiver; (ii) cumulative feedback-based ARQ without rate adaptation; and (iii) Coded ARQ with rate adaptation based on the cumulative feedback. Contrasting the performance of these protocols with uncoded ARQ, we demonstrate that HARQ performs only slightly better, cumulative feedback-based ARQ does not provide significant throughput while it has better average delay, and Coded ARQ can provide gains up to about 40% in terms of throughput. Coded ARQ also provides delay guarantees, and is robust to various challenges such as imperfect and delayed feedback, burst erasures, and round-trip time fluctuations. This feature may be preferable for meeting the strict end-to-end latency and reliability requirements of future use cases of ultra-reliable low-latency communications in 5G, such as mission-critical communications and industrial control for critical control messaging.Comment: to appear in IEEE JSAC Special Issue on URLLC in Wireless Network