Voice-Over-Frame Relay Equipment
Vendors offering equipment capable of integrating voice and data traffic
over frame relay must consider how they will address issues such as compression,
echo cancellation, delay and delay variation, frame loss, and traffic prioritization.
Each of these, and other considerations, can affect voice quality. While
vendors offering voice-over-frame relay capable equipment may have similar
objectives regarding quality and performance, each vendor may choose to
pursue these objectives through different hardware and software implementations.
Common considerations and a few of the many potential methods used to provide
voice-over-frame relay are presented below. Voice Compression
Compression of voice is a result of removing the silent periods and
redundant information found in human speech. Voice compression is used
to reduce the amount of information needed to recreate the voice at the
destination end. Uncompressed digitized voice and fax require a large
amount of bandwidth. This often makes it impractical to transmit these
signals over low-speed access links. The use of low bit rate voice compression
algorithms can make it possible to provide high quality speech while using
bandwidth efficiently.
Echo Cancellation
Echo is a phenomenon found in voice networks. Echo occurs when the
transmitted voice is reflected back to the point from which it was transmitted.
In voice networks, echo cancellation devices are used within a carrier's
network when the propagation delay increases to the point where echo results.
The longer the distance, the more the delay, and the more likely that
echo will result. Voice transmitted over a frame relay network will also
face propagation delays. As the end-to-end delay increases, the echo will
become noticeable to the end user if it is not canceled. Since carriers
do not use echo cancellation equipment in their frame relay networks,
it is up to the CPE vendor to address echo cancellation in the CPE.
Delay and Delay Variation
The burst nature and variable frame sizes of frame relay may result
in variable delays between consecutive packets. The variation in the time
difference between each arriving packet is called "jitter".
Jitter can impede the ability of the receiving end CPE to smoothly regenerate
voice. Since voice is inherently a continuous waveform, a large gap between
the regenerated voice packets will result in distorted sound.
Frame Loss
Compressed voice can usually withstand infrequent packet loss better
than data can. If a voice packet is lost, the user will most likely not
notice. If excessive frame loss occurs, it is equally unacceptable for
VoFR and for data traffic.
Traffic Integration - Fax and Modem Support
Vendors implementing VoFR technology appear to be mimicking switched
public voice services. Since VoFR supports fax and data modem services
as well, end users that have high fax traffic volumes between branches
and headquarters will find this ability beneficial.
Prioritization
Voice, fax and some data types are delay sensitive. This means that
if the end-to-end delay or the delay variation exceeds a specified limit,
the service level will get degraded. To minimize the potential for service
degradation, vendors can employ a variety of mechanisms and techniques.
Fragmentation
Fragmentation is used to break up large blocks of data into smaller,
less delay creating frames. This is another means used to ensure the
highest
level of voice quality possible. Fragmentation attempts to ensure an
even flow of voice frames into the network, thus minimizing delay jitter
across
circuits that carry both packet voice and data.
Digital Speech Interpolation
Digital speech interpolation addresses silence suppression. The nature
of speech communication entails pauses between words and sentences. Advanced
voice compression algorithms, which identify and remove these redundant
patterns, effectively reduce the amount of speech information to be transmitted.
DSI uses advanced voice processing techniques to detect silence periods
and suppress transmission of this information. By taking advantage of
this technique, bandwidth consumption may be reduced.
next page ::
|
