On a site where IP telephony is or will be deployed, the
Unified CM and the IP Phones are normally configured to use a Virtual Local
Area Network (VLAN) such that voice is logically separated from data. Although
both traffic types are carried on the same physical channel they are
transmitted on different VLANs, one for voice and other for data. This
configuration allows voice to be transmitted with higher priority than data.
In a call center that will use silent monitor you must connect the agent desktop system to the PC port on the back of the IP
phone, such that voice packets reaching the phone can be collected by the
silent monitor subsystem to then forward to the supervisor workstation. The
agent desktop system then uses one single physical channel to interact
with two different VLANs.
The agent desktop system accesses the physical channel via an
Ethernet Network Interface Controller (NIC). The NIC monitors the channel and
collects Ethernet frames addressed to the agent's computer. The NIC then runs a
preprocessing step to extract IP packets from the Ethernet frames and deliver
them to the TCP/IP stack on the operating system.
During internal testing Cisco identified that some Ethernet
NIC card drivers available in the market are not capable of pre-processing
Ethernet frames that have an IP packet encapsulated in a VLAN frame; that is
the NIC card driver discards the Ethernet frame altogether if the IP packet
is encapsulated in an 802.1Q frame. Some vendors can provide a configuration
setting that allows their NIC card driver to forward VLAN traffic to the TCP/IP
If an agent desktop's NIC card driver discards VLAN traffic,
then the silent monitor subsystem on that desktop cannot collect
and forward voice packets to the supervisor workstation and silent monitor cannot function properly. Cisco developed a procedure to determine if a
particular Ethernet NIC card driver works with the CTI OS silent monitor. The
procedure is described in the following sections.