On a site with IP telephony, the
Unified CM and the IP Phones normally use a Virtual Local
Area Network (VLAN) that logically separates voice from data. Although
both traffic types are on the same physical channel, they are
sent on different VLANs, one for voice and other for data. This
configuration enables you to send voice with higher priority than data.
In a call center with silent monitoring, the
agent desktop system uses one single physical channel to interact
with two different VLANs.
You connect the agent desktop system to the PC port on the back of the IP
phone. Then, the
silent monitor subsystem can collect the voice packets reaching the phone and forward the packets to the supervisor workstation.
The agent desktop system accesses the physical channel through an
Ethernet Network Interface Controller (NIC). The NIC monitors the channel and
collects Ethernet frames addressed to the agent's computer. The NIC 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 cannot preprocess Ethernet frames that have an IP packet encapsulated in a VLAN frame. The NIC card driver discards the Ethernet frame 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. Silent monitor cannot function properly on such a NIC. 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.