Quality Of Service Testing In The VoIP Environment: A Primer
BY BARBARA DUQUET
In recent years, the business world has reaped tremendous benefits from the many
exciting products and applications made possible by the marriage of data and voice
technologies. Now the efficiencies and enhanced services resulting from that revolution
are about to be eclipsed by the magnitude of change made possible as data networks become
the transport for voice. IP telephony, or Voice over IP (VoIP), is the exploding new
technology enabling voice to be carried over IP-based, packet-switched local- and
wide-area networks.
WHY VoIP?
Why is VoIP attracting so much attention? The advantages to a company adapting the
technology are significant. In traditional circuit-switched networks, when a connection is
established, a channel is dedicated end-to-end for the duration of the communication. This
means that any unused bandwidth (roughly 60 percent of speech is silence) remains
unavailable until the call is released. In the packet-switched world, many types of
communication share the bandwidth, which fills the available capacity much more
effectively. Speech compression technologies used in preparing voice signals for transport
on a packet network reduce bandwidth demands from the traditional 64 Kbps to as little as
6 Kbps -- a significant reduction. In addition to this economy of scale, combining all
traffic onto a single network represents an opportunity for major savings in the physical
plant. The advantage currently attracting the most attention is the ability to bypass the
PSTN and its toll charges, and applications made possible by merging voice with the
Internet will drive the level of interest even higher.
Because of this enormous potential, the market for VoIP solutions is ramping up
quickly. However, there remain significant quality issues that must be addressed before we
see widespread acceptance of VoIP as a mainstream business tool.
In the brief history of IP telephony, voice calls over the Internet have gotten a bad
reputation because of poor quality. Due to its real-time nature, an effective voice
conversation requires a reasonable level of continuity. That continuity can be negatively
impacted by the competition of large numbers of packets (representing other types of data)
with voice packets for network bandwidth, a situation that never existed in the
circuit-switched world. The voice quality issue is made more complex because of its
subjective nature. Equipment responsible for processing voice for transport over an IP
network must be able to retain all the nuance, inflection, and pauses that comprise
effective human communication - not always an easy task given the challenges mentioned
previously, and one whose capability must be verified using methods that take human
perceptual subjectivity into account.
It is important that service and equipment providers build into their VoIP solutions
the ability to test, measure, and evaluate the performance of the various elements needed
to create a VoIP transmission. This paper will identify those elements and suggest some
strategies for testing can help ensure the level of quality required to make VoIP a viable
service offering. This paper is intended for any manufacturer, system integrator or
service provider for whom guaranteeing solid voice quality performance is a critical
issue.
VoIP CALL ELEMENTS
A VoIP call can consist of several elements: endpoints; gateways; some type of
packet-switched network; and sometimes a circuit-switched network. Which of these elements
are present in a particular call depends on what types of endpoints are being used. An
endpoint in a VoIP scenario is either a PC with an Internet telephone application
installed, an Internet telephone itself, or a regular telephone. In a conference
situation, all types of endpoints could theoretically be participating.
The difference in what elements are present is determined by where the necessary voice
signal processing is done to package the voice for transport over the packet network. If
the endpoints are PCs or Internet telephones, the speech encoding and packetizing
functions are incorporated.
When a standard telephone is at one or both ends of the connection, an interface must
be provided between the voice network and the packet network. IP telephony gateways are
equipped with standard interfaces to the PSTN (analog, T1/E1) as well as interfaces to the
packet network. The necessary encoding/decoding, compression/decompression and
packetizing/depacketizing are done in between.
The processing of a voice signal into the format necessary for transport over a packet
network is performed in all cases by an encoding/decoding subsystem called a vocoder.
These systems encode, compress (usually), and packetize the signal. When the signal
reaches its destination, the process must be reversed by the vocoder on the destination
end, either in a gateway or the endpoint itself. The algorithms used for the vocoder
functions can differ from manufacturer to manufacturer -- a possible performance variable.
The final element is the packet-switched network itself - the "cloud" that
provides the data transport between the other elements. The network, consisting of various
physical media, network protocols, and the routers and switches controlling the flow of
traffic, is the most problematic of the connection elements. (Note: While the interface to
the PSTN is an important component and should be tested, it does not generally impact
voice quality and is not discussed here.)
WHAT NEEDS TO BE EVALUATED?
In the previous section we identified the elements that comprise a VoIP transmission. This
section will discuss the potential problems these elements can introduce, usually when
trying to perform under heavy traffic demands: connection failure, latency (delay), jitter
(variable delays), and dropped packets.
Connection failure: The endpoint applications and devices discussed
above need to be able to place and receive calls, so this capability needs to be verified.
A gateway needs to be able to receive and send circuit-switched traffic on one side and
packet-switched traffic on the other, and this basic functionality needs to be verified as
well.
Latency: Voice signals need to be processed for transport over a
packet-switched network. The necessary compression and packetizing (and the reverse of
these processes) is done either by the intelligent endpoints or a gateway. Execution of
these functions requires a small amount of time, which can vary depending on the
architecture of the device (DSPs, compression algorithms) and the amount of traffic to be
processed. This processing time introduces delay, which is called latency. The human ear,
being a subjective evaluator, can tolerate some latency, usually up to around 250 ms,
before perceiving a drop in the quality of a connection. So, knowing how much latency an
endpoint or gateway introduces, especially when traffic load is high, is important to test
in order to ensure the 250-ms threshold is not exceeded. As it happens, the majority of
the delay is introduced after the packets leave the endpoint or gateway. Depending on how
busy each successive router in the network is, it can introduce another few milliseconds
or more into the cumulative latency. Outside of a carefully managed intranet, there is no
control over the number of router-to-router legs (hops) a packet takes. Therefore,
monitoring the total end-to-end latency that packets are experiencing is necessary in
maintaining a good quality VoIP transmission.
Jitter: Not only is it impossible to predict or control (using current
networks) how many hops packets from a VoIP call will traverse, packets from the same call
can be assigned different routes, with varying numbers of hops and different traffic
volumes along the way. Because of this, packets from the same conversation can experience
different amounts of delay on their way to their destination. These variable delays
produce a condition called jitter, where packets arrive at their destination at different
intervals. Most gateways have buffers to collect packets and return acceptable continuity
to the data, and these must be tuned so that the process itself does not create excessive
delay. So, another area of testing would involve monitoring jitter to make sure it is
being dealt with effectively.
Dropped packets: When a router becomes overloaded with traffic, it may
intentionally drop packets to relieve the congestion. With traditional data traffic, for
which these networks are optimized, there are error-checking methods built into the
protocols to address these situations and maintain data integrity. These methods require
some overhead not conducive to real-time traffic, and were not implemented for voice
transport. Again, the human ear can forgive a certain number of missing packets (generally
between 1 and 3 percent, depending on the data represented). Beyond this, the call quality
can degrade to unacceptable levels, so it is important to monitor and test for dropped
packets.
TESTING & MEASUREMENT
We have identified several conditions, which, if they occur, can negatively impact a
user's perception of the quality of the VoIP transmission: connection failure, latency,
jitter, and dropped packets. The failure of a call to connect is an obvious and easily
measured call control problem, but the effect the other conditions have on voice quality
is more difficult to quantify - how humans perceive an audio signal is very subjective.
Because of this, it is important to closely simulate "real world" conditions so
that testing is done on what people are actually hearing. Please see the sidebar entitled
A Real-World Example for specific information regarding VoIP testing using the Hammer VoIP
Test System.
CONCLUSION
There are many methods currently under discussion by VoIP equipment and service providers
for improving QoS and even providing customers with QoS guarantees. If implemented, these
methods should help in improving how conversations in the VoIP environment sound, adding
some consistency to quality performance. This is necessary before general business
acceptance outside enterprise intranets will occur. In the final analysis, the success of
the industry hinges on the positive perception of human beings using telephones.
Barbara Duquet is market segment manager for VoIP at Hammer Technologies, a leader
in computer telephony integration (CTI) application testing. Hammer's Windows NT-based
family of products includes the Integrated Telecommunications (Hammer IT) and Integrated
Stress Generator (ISG). Hammers are in use today by developers of computer telephony,
advanced switching, and enhanced services systems. For more information, visit the
company's Web site at www.hammer.com.
|