Time control

This subsection discusses the difference between single-access and 802.11 net-works at the MAC-layer. In 802.11 netnet-works the sender at the application layer does not know when a packet is transmitted on the link. Packets that are sent from a node traverse the network protocol stack from the top layer down to the lower layers. The top layers (application, transport and network layer) are the same regardless if a single-access or a 802.11 network is used by the node sending the packet. However, the lower layers (MAC and physical) differ.

When sending a packet using single-access networks we can approximate the time between sending the packet from the application layer and transmitting the packet onto the link. The time spent in the protocol stack is more or less constant for each packet, assuming that the send rate is not higher than the capacity of the network itself. That is, we can calculate the offered dispersion between two successive probe packets.

68 Paper B

Time 2) Busy channel, backoff and

contention phase

3) Probe packet 4) ACK

1) Probe packet ready to be transmitted

5) Ready to send next probe packet

Figure 6.4: The MAC-layer scheme when sending a packet using a 802.11 network.

However, in a 802.11 wireless network the lower layers will induce time jitter between successive probe packets. This jitter depends on, among other things, cross traffic using the same radio frequency channel. In Figure 8.11 the jitter effect is illustrated. A probe packet is sent in the application layer and traverses down through the protocol stack to the MAC layer. The probe packet is ready to be transmitted on the shared medium, indicated by (1) in the figure. Because of other traffic the probe packet has to wait until the shared medium is idle. The time a probe packet is in the contention phase (shown by (2) in the figure) depends on the cross traffic. When the probe packet is actually successfully transmitted (3) the node has to wait until an acknowledgment is received (4). The time to receive an acknowledgment depends on other com-peting packets and the network latency. That is, the dispersion between two successive probe packets is not deterministic.

When the analysis phase is performed, the offered dispersion values ob-tained from the data collection phase is one important component to produce a good estimate of the measured network path characteristic. Probe-packet dis-persion jitter will make the estimate of the network characteristic less accurate.

To get more accurate estimates, the analysis requires more dispersion samples which means that more probe packets have to be sent. However, this should be avoided since the bandwidth in wireless networks is usually the bottleneck.

Exactly how the dispersion jitter will affect current end-to-end dispersion-based measurement methods is a subject of further research.

6.3 Conclusions

In this paper we have analyzed properties in IEEE 802.11 ad-hoc networks that differs from single-access networks, from the view of dispersion-based measurement methods.

We have performed ns-2 simulations to show that the link capacity vary de-pending on the cross traffic load. Further, we have extended the term available bandwidth to cope with packet losses in wireless networks. In the article we raised the thought that variable link capacity and loss rate may cause problems to dispersion-based measurement methods.

We have also discussed the problems of moving nodes, route changes and dispersion time jitter at the probe sender.

In future research, we intend to study how the link capacity vary in larger non-simulated topologies. Further we will study how to detect and handle route changes in an ad-hoc network when using dispersion-based methods. Our ex-tended definition of available bandwidth is also subject of further study. We will investigate to what extent our extended definition is overly pessimistic. Fi-nally, we will explore how time jitter will affect contemporary available band-width measurement methods.

Bibliography

Bibliography

[1] Van Jacobson. Pathchar - a tool to infer characteristics of Internet paths.

Presented at the Mathematical Sciences Research Institute (MSRI), April 1997. Slides available from ftp://ftp.ee.lbl.gov/pathchar/.

[2] Bruce Mah. pchar: A tool for measuring Internet path characteristics, June 2001. http://www.employees.org/^~bmah/Software/pchar/.

[3] Bob Melander, Mats Bj¨orkman, and Per Gunningberg. Regression-based available bandwidth measurements. In Proceedings of the 2002 Interna-tional Symposium on Performance Evaluation of Computer and Telecom-munications Systems, San Diego, CA, USA, July 2002.

[4] Manish Jain and Constantinos Dovrolis. End-to-end available bandwidth:

Measurement methodology, dynamics, and relation with TCP throughput.

In Proceedings of ACM SIGCOMM, Pittsburg, PA, USA, August 2002.

[5] Vinay Ribeiro, Rudolf Riedi, Richard Baraniuk, Jiri Navratil, and Les Cottrell. pathchirp: Efficient available bandwidth estimation for network paths. In Passive and Active Measurement Workshop, 2003.

[6] Charles Perkins. Ad hoc networking. Addison-Wesley, 2001.

[7] ISO/IEC. Ieee 802.11 standard. ieee standard for information technology.

ISO/IEC 8802-11:1999(E), 1999.

[8] ISO/IEC. Corrigendum to ieee 802.11b standard 1999. ieee standard for information technology. ISO/IEC 8802-11:1999/Cor 1-2001(E), 2001.

[9] Robert Carter and Mark Crovella. Measuring bottleneck link speed in packet-switched networks. Technical Report 1996-006, Boston Univer-sity Computer Science Department, Boston, MA, USA, March 1996.

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[10] Constantinos Dovrolis, Parameswaran Ramanathan, and David Moore.

What do packet dispersion techniques measure? In Proceedings of IEEE INFOCOM, pages 905–914, Anchorage, AK, USA, April 2001.

[11] Ningning Hu and Peter Steenkiste. Evaluation and characterization of available bandwidth probing techniques. IEEE Journal on Selected Areas in Communication, 2003.

[12] The network simulator - ns-2. http://www.isi.edu/nsman/ns/.

Chapter 7

Paper C:

DietTopp: A first implementation and

evaluation of a simplified bandwidth measurement method

Andreas Johnsson, Bob Melander and Mats Bj¨orkman

In proceedings to the Second Swedish National Computer Networking Work-shop (SNCNW), Karlstad, 2004

73

Abstract

This paper describes the active available bandwidth measurement tool Diet-Topp. It measures the available bandwidth and the link capacity on an end-to-end path having one bottleneck link. DietTopp is based on a simplified TOPP method. This paper describe and motivate the simplifications and assumptions made to TOPP. Further, the paper describes some of the DietTopp implemen-tation issues.

A first evaluation of DietTopp in a testbed scenario is made. Within this evaluation the performance and measurement accuracy of DietTopp is com-pared to the state-of-the-art tools Pathload and Pathrate.

We show that DietTopp gives fast and accurate estimations of both the available bandwidth and the link capacity of the bottleneck link.

7.1 Introduction 75

7.1 Introduction

Measurements in best-effort networks are important for network error diagno-sis and performance tuning but also as a part of the adaptive machinery of user applications such as streaming video. Within our research we have focused on actively measuring available bandwidth between two network end-points.

Such active measurements are done by injecting probe packets (with a pre-defined separation) into the network. The probe packets are time stamped at the receiver end. These time stamps are then used to form an estimate of the available bandwidth. This is discussed in more detail in Section 7.2.

State-of-the-art bandwidth measurement tools and methods are for example TOPP [1], Pathload [2], Pathchirp [3], Delphi [4] and Spruce [5]. An overview of methods and tools in this area can be found in [6].

Within the scope of this paper we have developed, implemented and eval-uated a new available bandwidth measurement tool called DietTopp. This tool relies on a simplification of the TOPP bandwidth measurement method [1].

We show that DietTopp gives fast and accurate estimates of both the available bandwidth and the measured link capacity when there is one congested link in the end-to-end path.

This paper is organized as follows. The TOPP measurement method is briefly described in Section 7.2. The simplifications and assumptions made by DietTopp are discussed and motivated in Section 8.2.1. DietTopp implemen-tation issues are described in Section 7.4. Section 8.2.2 describes the testbed that has been used to evaluate our DietTopp implementation, while Section 8.3 shows and discusses the obtained results. Section 8.3 also compare DietTopp to other state-of-the-art measurement tools.