Information Technology Reference
In-Depth Information
Voice Activity Detection
During a normal telephone conversation, typically only one person speaks at a time, and occasionally, there will also be complete
silence on the call (it usually constitutes between 35 percent to 50 percent of the call). Using VAD, when these periods of silence
occur, instead of sending packetized silence across the network, no packets are sent.
As an average over a period of time, VAD can save up to 35 percent of bandwidth on a call volume of 24 calls or more.
One problem with VAD is that it can lead to a situation where participants in a call can come to believe that the call has been
disconnected. This situation can be remedied by using comfort noise generation (CNG), which provides white noise and ensures that
participants in a call do not believe that the call has been disconnected.
VAD can be enabled on a dial peer using the
vad
command (the default) and disabled using the
no vad
command.
It is possible to configure VAD in CUCM by navigating to
Service, Service Parameters
, selecting the publisher, and then choosing
the CUCM service. The
SilenceSuppressionSystemWide
and
SilenceSuppressionWithGateways
parameters control VAD settings
for SCCP endpoints and MGCP gateways, respectively.
Classification and Marking
Classification is the process by which routers, switches, or other devices identify packet or frame types. Classification can be based on
a variety of criteria, such as protocol and port, and usually occurs (in depth) at the network edge (QoS trust boundary).
After packet types are identified, they can be given a marking so that other network devices do not have to reclassify them in depth.
(They do not have to reexamine IP addresses, TCP/UDP ports, and so on.) Packets can be marked using fields such as the ToS Byte or
Differentiated Services (DS) fields in the IP header, the 802.1p field in the 802.1Q tag, and so on. Marking typically also occurs at the
network edge.
Routers and switches might offer preferential QoS treatment in terms of queuing, assignment of bandwidth, selective dropping, and so
on based on packet and frame markings.
Figure 2-1
shows the 802.1Q tag and 802.1p field.
