Preparing the Network for VoIP
When adding voice or video to an existing network, you should examine
several things in advance to provide the high level of availability users
expect in their phone system:
■ What features are needed?—Power for IP phones, voice VLANs on the
switches, network redundancy for high availability, security for voice
calls, and Quality of Service (QoS) settings.
■ The physical plant—Cabling at least CAT-5.
■ Electrical power for the IP phones—Use either inline power from
Catalyst switch or power patch panel. Need uninterruptible power
supply (UPS) with auto-restart, monitoring, and 4-hour response
contract. May need generator backup. Maintain correct operating
temperatures.
■ Bandwidth—Commit no more than 75 percent of bandwidth. Consider
all types of traffic—voice, video, and data. Have more than enough
bandwidth if possible. Include both voice and call-control traffic in
your planning.
■ Network management—Need to monitor and proactively manage the
network so that it does not go down.
Network and Bandwidth Considerations
The network requirements for VoIP include:
■ Maximum delay of 150–200 ms (one-way)
■ No more than 1 percent packet loss
■ Maximum average jitter of 30 ms
■ Bandwidth of 21–106 kbps per call, plus about 150 bps per phone for
control traffic
A formula to use when calculating bandwidth needed for voice calls is as
follows:
(Packet payload + all headers) * Packet rate per second
Auxiliary (or Voice) VLANs
Cisco switches can be configured to dynamically place IP telephones into
a VLAN separate from the data VLANs. They can do this even when the
phone and PC are physically connected to the same switch port. This is
called an auxiliary VLAN or a voice VLAN. Voice VLANs allow phones
to be dynamically placed in a separate IP subnet from hosts, to have QoS
(using 802.1Q/p headers) and security policies applied, and makes troubleshooting
easier.