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When considering the VoFR equipment on the
market, look for vendors who offer the following capabilities:
Compression:
Unless you have access to cheap bandwidth, your VoFR equipment
should compress your voice signal from 64K bit/sec to at least
32K bit/sec. In most cases, compression to 16K or even 8K bit/sec
is possible and preferred. Some equipment vendors support dynamic
compression options. When bandwidth is available, a higher voice
quality is achieved using 32K bit/sec, but as other calls are
placed or other traffic requires bandwidth, an 8K or 16K bit/sec
compression algorithm is implemented.
Fragmentation:
VoFR equipment must segment data and voice packets into small
ones. This allows the voice quality to remain high. If fragmentation
is not done, a large data packet may "block" or delay
voice packets behind it. By fragmenting the packets, the voice
and data packets will have a more constant delay and a reduced
possibility of being temporarily blocked by a large data packet.
Prioritization:
Real-time voice traffic is very sensitive to delay, while many
data applications such e-mail are more delay-tolerant. Your
vendor should support voice prioritization. If you are running
delay-sensitive data applications and voice, you need to ensure
that both applications perform as needed.
Echo cancellation:
Due to the delay introduced by such processes as compression,
decompression, packetizing and transmission, users may hear
their own voice (an echo) across the frame relay network. This
problem must also be dealt with by the public switched telephone
network, or PSTN. VoFR equipment should have echo cancellation
capabilities to prevent a noticeable echo.
Jitter buffer:
Public frame relay networks are shared among many customers.
No matter how much control you exert over your own traffic prioritization
and fragmentation, you have no control over other customer traffic.
This can lead to some of your voice packets experiencing 50-msec
network delay, while others may get delayed during times of
congestion. The jitter buffer temporarily holds incoming packets
in order to assemble them in the correct order and recreate
a high-quality voice signal. Like dynamic compression, some
jitter buffers are also dynamic. This allows them to "sense"
variable delay and provide a higher quality voice signal.
This article discusses voice over frame relay
(VoFR). Keep in mind that there are other voice over data technology
options, such as voice over IP and voice over ATM, that can
do many of the same functions as VoFR.
This article appeared in Network
World: "What to look for in voice over frame relay CPE".
See the original and a number of related articles in Network
World. This article was produced in association with TeleChoice,
Inc. You can contact TeleChoice at its corporate headquarters
at (973) 239-0700 for more information.
View more information in protocols.com about
Frame Relay.
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