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 stack.
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.