Abstract

This paper details the development of an educational and research tool, the Network Performance Application (NPA), which provides real-time network performance measurement through a simulated voice over IP session using live real-time data transmission over packet networks. This application, written in Visual C++ for MS Windows environments, is used to collect Quality of Service (QoS) statistics such as packet loss, end-to-end delay, inter-arrival delay jitter, and out-of-order packet delivery. The paper also demonstrates a practical application using a government standard for VoIP transmissions.

Keywords network modeling; network performance; Quality of Service; Voice over IP

Modern communication applications and media types have challenged the capabilities and usefulness of the analog plain old telephone system (POTS). Cellular telephones and personal computers have created demand for real-time secure multimedia communications between heterogeneous clients. This has led to a new set of application possibilities with multimedia communications, and consequently new challenges. In contrast to the POTS, the Internet is a packet data oriented communication network. A continuous stream of voice data transmitted across a POTS connection/network is broken up into discrete packets when sent across an Internet connection. Real-time voice data is expected to arrive at the destination in sequence and on a timely basis if continuous playback and low latency is to be achieved. However, packet-based transmission can disrupt data flow if packets are delayed or lost.

Unlike connection oriented networks where data travels along a dedicated channel, packet networks comprise a collection of paths linked by routers and gateways. Therefore, packets belonging to the same conversation (in an audio application) might take different paths arriving out-of-order. Packets may also experience different queuing delays at network routers causing them to arrive at their destination with varying inter-arrival delays, referred to as jitter. Congestion at network nodes (routers) may impede packets during times of high network use which may lead to packets experiencing large end-to-end delays or being discarded in an attempt to maintain sending rates. These characteristics collectively contribute to what is generally known as Quality of Service (QoS) characteristics as they describe the quality of the network medium to reliably deliver data.

Purpose of study

This study details the design and development of an educational and research tool, the Network Performance Application (NPA), to observe and characterize the nonideal behavior of packet networks. This application simulates communication in a voice over IP (VoIP) system over an actual network connection (or concatenation of network connections) by implementing the transfer of real-time packet data and real-time collection of statistics to characterize the QoS of the underlying packet network(s). This application is useful in predicting and modeling of network behavior as it affects a voice packet stream.

Current problems associated with VoIP using similar or mixed networks demand investigation. Error management schemes and uninterrupted transmission flow require foreknowledge of patterns of Internet behavior. This study simulates communications scenarios to qualitatively analyze the impacts of specific application requirements such as large packet size or fast send rate on overall QoS. The development of this tool also facilitates the development of an information base with which to make educated decisions during the design of an Internet VoIP communication application, or during its refinement.

Preexisting network utilities

Several free utilities and commercial products exist for measuring aspects of network performance. These include NetWisdom,1 Network Performance Monitor,2 DBS,3 NetSpec,4 OPNET,5 Ping,6 netperf,7 TTCP,8 and others. The purpose and methods of these tools vary to suit network computing needs but they fail to provide features for measuring critical aspects of performance with respect to a VoIP system as found in NPA. Many of the tools are designed to measure large corporate network efficiency. They are not designed to measure performance for a modular real-time system across varied network segments. Ping, netperf, and TTCP are typical tools used to measure some aspects of network performance. All are free public utilities designed for UNIX systems of which some have MS Windows-based implementations.

The ping utility is a system administration tool used to check computer operation and network connection functionality. Ping is available from both DOS and UNIX terminal windows. Ping uses the Internet Control Message Protocol (ICMP) Echo function. A terminal sends a small packet to a specified IP address then listens for a return packet. The successful receipt of a packet indicates a good connection as well as provides information about the network. Timing information can be used to estimate latency on a given network path. While overall network latency information is useful, it is insufficient for analyzing QoS such as inter-arrival time, loss, and outof-order packets. Furthermore, it does not measure the QoS response to varied packet parameters such as size and send rate.