Multiservice Networking, IPX Networking, and Security - CCIE Routing and Switching Exam Certification Guide - page 549

If a group of users make 20 calls in an hour, and each call lasts 10 minutes, the erlangs are
calculated as follows:

20 calls/hour × 10 minutes/call = 200 minutes/hour

Traffic volume = (200 minute/hour) / (60 minutes/hour)
= 3.33 erlangs

Voice Activity Detection (VAD)
Because you listen and pause between sentences, typical voice conversations can contain up to
60 percent of silence. In plain telephone networks, all voice calls use fixed-bandwidth, 64-kbps
links, regardless of how much of the conversation is speech and how much is silence. In multi-
service networks, all conversation and silence is packetized. Using VAD, packets of silence
spurs can be suppressed. Instead of sending VoIP packets of silence, VoIP gateways can inter-
weave data traffic with VoIP conversations to more effectively utilize network bandwidth.
Bandwidth savings are at least 35 percent in conservative estimates.

VAD is enabled by default for all VoIP calls. Although VAD reduces the silence in VoIP
conversations, it also provides Comfort-Noise-Generation (CNG). Because you can mistake
silence for a disconnected call, CNG provides locally generated white noise so that the call
appears normally connected to both parties.

If VAD is not active, it can be enabled by using the vad command under the dial-peer
statement. Example 12-2 shows the vad command enabling VAD for the VoIP dial peer 100.

Example 12-2 Enabling VAD for a VoIP Dial Peer

dial-peer voice 100 voip
destination-pattern [phone]
vad
session target ipv4:1.1.1.1

RTP

In VoIP, RTP transports audio streams. RTP is defined in RFC 1889. RTP runs over User
Datagram Protocol (UDP), which has a lower delay than TCP. Because of the time sensitivity
of voice traffic and the delay incurred in retransmissions, you use UDP instead of TCP. Real-
time traffic is carried over UDP ports that range from 16384 to 16624. The only requirement is
that the RTP data is transported on an even port and RTCP is carried on the next odd port. RTCP
is also defined in RFC 1889. RTCP monitors the delivery of data and provides control and
identification functions.

Because voice applications are sensitive to the delay of packets, any quality of service (QoS)
techniques on the network need to prioritize these RTP/UDP IP packets over other traffic, such
as File Transfer Protocol (FTP) and Hypertext Transfer Protocol (HTTP). You can use the ip
precedence number subcommand under the dial-peer command to mark VoIP packets with a