FAQs on VoIP Monitor

 

  1. Why do i need to set SNMP write community on the Source Router ?
  2. Why I am getting 'Source router SNMP write community may be wrong' error message?
  3. Why should the SLA Responder be enabled on the destination device ?
  4. Why are the VoIP metrics  shown as zero or 'Not available' in OpManager?
  5. What are all the VoIP QoS metrics measured by OpManager ?
  6. How do i choose the codec ?
  7. How much bandwidth does each monitor occupy ?


1. Why do i need to set SNMP write community on the Source Router ?

Both, the SNMP read and write community string needs to be set on the source router. The write community is used to configure the IPSLA on the device while the read community is used by OpManager to gather performance data from the router. [back to top]

2. Why I am getting 'Source router SNMP write community may be wrong' error message?

OpManager uses SNMP to gather data from the Cisco IP SLA agent. This error is displayed when wrong SNMP read / write community string is configured for the Source router of the VoIP Monitor in OpManager. 

To configure the correct SNMP write community string in OpManager, go to the snapshot page of the source router and change the SNMP credentials by clicking on the 'Click here to change' corresponding to the "Passwords" field. In the pop-up enter the appropriate credentials and submit it. After successfully submitting the correct SNMP credentials, try to add the VoIP Monitor again for the Source device (Maps > VoIP Monitor > Settings)
. [back to top]


3. Why should the SLA Responder be enabled on the destination device ?

Enabling the IP SLAs Responder provides the details of packet loss statistics on the device sending IP SLAs operations. IP SLAs Responder is enabled on the target router (rtr responder) before configuring a Jitter operation. [back to top]

4. Why are the VoIP metrics shown as zero or 'Not available' in OpManager?

You will see zero or 'not available' values when data is not collected for the monitored metrics. This can be either due to incorrect SNMP read community configured, or of the Responder is not enabled on the destination device. Make sure that the correct SNMP read community is configured and the SLA Responder is enabled. [back to top]

5.What are the critical parameters monitored to determine the VoIP QoS performance?

The monitored parameters include Latency, Jitter, Packet Loss, and MOS. The parameters are described below for reference:

Jitter : Jitter is defined as a variation in the delay of received packets. Users often experience disturbing sounds over a conversation coupled with loss of synchronization at times and is referred to as jitter. High levels of jitter can result in some packets getting discarded and thereby impact the call quality. Ensuring a jitter-free transmission to provide qualitative service depends on identifying the bottle-neck responsible for the jitter, and acting on it to eliminate it. OpManager's VoIP monitoring feature helps you find the problem and ensures maximum QoS on your VoIP network.

Packet Loss : Packet loss is a measure of the data lost during transmission from one resource to another in a network. Packets are discarded often due to network latency. Using OpManager, you can monitor the packet loss and take corrective actions based on the information.

One way Latency: Latency (delay) is the time taken for a packet to reach the destination device. When monitoring latency over VoIP, the delay measured is the time taken for a caller's voice at the source site to reach the other caller at the destination site. Network latency contributes to delay in voice transmission, resulting in huge gaps between the conversation and interruptions.

Round Trip Time: Round Trip Time is the time taken for a packet to reach the destination and again comes back to the source device. The total time it takes for the round trip is measured in milliseconds.

MOS: The Mean Opinion Score is the key quality indicator of VoIP traffic quality. And is measured in the scale of 1 to 5 (poor to excellent quality). [back to top]


6. What is VoIP codec?


Codecs (Coder/Decoder) serve to encode voice/video data for transmission across IP networks. The compression capability of a codec facilitates saving network bandwidth and it is therefore appropriate that you choose the correct codec for your IP network. Here is a quick reference to the codecs with the corresponding packets size and bandwidth usage:
 

Codec & Bit Rate (Kbps)

Operation Frequency

Default number of packets

Voice Payload Size

Bandwidth
MP or FRF.12
(Kbps)

Bandwidth
w/cRTP MP or FRF.12
(Kbps)

Bandwidth
Ethernet
(Kbps)

G.711a/u
(64 kbps)

60 msecs by default. You can specify in the range of 0 - 604800 msecs.

1000

160 + 12 RTP bytes

82.8 kbps

67.6

87.2

G.729
(8 kbps)

1000

20 + 12 RTP bytes

26.8 kbps

11.6

31.2

[back to top]

7. How much bandwidth does each monitor occupy ?

The bandwidth occupied depends on the codec selected. Look at the above table for reference. [back to top]
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