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Figure 1. Cisco NAM Deployment

NAM can be deployed in various locations depending on the use case being addressed. For example, when

deployed in a branch, NAM can monitor the quality of all calls entering and exiting that branch office and monitor any

dips in quality levels in that location. Alternatively, a NAM in the data center can monitor signaling messages sent

from a phone to the Cisco Unified Communications Manager cluster and collect detailed information about the

calling and called parties. A NAM located at the edge of the main campus can raise alarms about poor call quality

from the main campus to a particular remote office location.

How Voice Monitoring Works

VoIP phone calls are set up using a signaling protocol, such as Session Initial Protocol (SIP), using which the phone

endpoints exchange information. Once setup is complete, voice traffic is sent through a streaming protocol such as

RTP. NAM detects both signaling messages and voice traffic over RTP streams. It also has the ability to link a set of

call setup messages to the RTP streams associated with them.

NAM monitors signaling messages for Skinny Call Control Protocol (SCCP), SIP, H.323, and Media Gateway Control

Protocol (MGCP). The information collected from signaling messages includes currently active calls, call statistics,

call history, detailed information about the calling and called parties, codecs used, port numbers used, and other

relevant information. These statistics can be collected both on a real-time and historical basis.

In order to measure the quality of the call, NAM detects and monitors RTP streams. First, NAM examines the packet

header and identifies whether it is an RTP packet. If so, it checks whether the packet belongs to a new or existing

RTP stream. Once the RTP packet is detected and associated with a stream, it is sent to the MOS process for

quality analysis. The MOS process performs real time computations to measure voice quality metrics such as jitter,

actual packet loss, adjusted packet loss, seconds of concealment, and severe seconds of concealment. Using the

aforementioned metrics, the NAM computes the R-Factor MOS based on the ITU-T recommendation G.107. The

best, worst, and average values for these metrics are reported every minute through GUI.

An important aspect of this real-time reporting is that voice quality metrics are available to users even when the call

is active. There is no need to wait for the end of the call before such statistics are collected. This real-time visibility is

a critical part of the solution and facilitates rapid responses to problems.