FAQs on VoIP Monitor

Questions

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

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

  3. Why are the VoIP metrics shown as zero or 'Not available' in ManageEngine IT360?

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

  5. What is VoIP codec?

  6. How much bandwidth does each monitor occupy ?

  7. Why the Source Router field is empty?

  8. Why i am getting "Source router SNMP write cosmmunity may be wrong" message?

  9. Why i am getting "Source router SNMP read community may be wrong" message?


Answers

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 ManageEngine IT360 to gather performance data from the router.

2. Why should the IP 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.

3. Why are the VoIP metrics shown as zero or 'Not available' in ManageEngine IT360? 

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.

4. 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 reffered 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. ManageEngine IT360'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 ManageEngine IT360, 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).

5. 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

6. How much bandwidth does each monitor occupy ? 

The bandwidth occupied depends on the codec selected. Look at the above table for reference.

7. Why the Source Router field is empty?

In order to monitor VoIP between two locations You just need to tell ManageEngine IT360 which links you want to monitor i.e source and target. The source router(Cisco router enabled with IPSLA agent) should be discovered by the ManageEngine IT360. Once the Cisco router is discovered in the ManageEngine IT360 with valid SNMP read and write community strings it will be available in the Source Router select field to select as a source for adding VoIP Monitor.

Please follow these steps to discover a source router in ManageEngine IT360:

    1. Go to the source router telnet session and enable snmp read and write communities.

    2. Go to Admin --> Credential Settings --> New  
        Enter valid read and write communities which is enabled in the source device.

    3. Go To Admin -->Add Device    
        Enter the name or IP address of the source device, select the newly added credential and then click on Add Device.

    4. Go to VoIP Monitors --> Settings
        Select  newly added source router and proceed with the configuration.

8. Why i am getting "Source router SNMP write community may be wrong" message?

Possible reasons:

  1. Verify the device through telnet whether the write community is enabled or not.
  2. Make sure you enter a valid(which is enabled in source) write community string in the field.
  3. The device may not responded due to busy for this instance, please try again.

9. Why i am getting "Source router SNMP read community may be wrong" message?

Possible reasons:

  1. Verify the device through telnet whether the read community is enabled or not.
  2. Make sure you enter a valid(which is enabled in source) read community string in the field.
  3. The device may not responded due to busy for this instance,  please try again.


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