What company isn’t supporting or at a minimum thinking about Voice over IP (i.e. VoIP) on the network?
When it comes to voice over the network, three things related to performance must be taken into consideration:
| 1. | The traffic volume on the existing network. |
| 2. | The latency when packets traverse the infrastructure. |
| 3. | The ability to keep the network up and available at all times. |
 "The number one killer of voice traffic is network latency and jitter. Latency, jitter and packet loss cause poor audio quality and dropped calls," said Joel Trammell - Co-Founder and Chief Executive Officer of NetQos. |
"Latency caused by overloaded call managers or network congestion can be a major cause of poor VoIP performance."
Introducing Cisco IP SLA
Cisco IOS IP Service Level Agreements (Cisco IP SLAs) are a feature included in Cisco IOS Software that allow administrators the ability to to Analyze IP Service Levels for IP applications and services (e.g. Jitter).
Cisco IP SLA's uses active traffic-monitoring technology to monitor continuous traffic on the network.
This is a reliable method in measuring over head network performance. Cisco Routers provide IP SLA Responders that give accuracy of measured data across a network.
Cisco IOS IP SLAs Overview
Sample IP SLA Measurements:
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TCP Connect RTT. |
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UDP Echo measures the response time from the Cisco router to any IP device. |
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DLS+ measures the DLSw+ protocol stack and the network response time between DLSw peers. |
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DHCP measures the RTT taken to discover a DHCP server and obtain a lease from it. The IP address is then released. |
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HTTP measures Round Trip Time (RTT) taken to connect and access data from an HTTP server (i.e. transaction time RTT). |
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DNS measures the response time by taking the difference between the time taken to send DNS requests and receiving a reply. |
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Jitter measures direct packet loss and Jitter. Jitter is inter-packet delay variance. It is very important when utilizing VoIP/Video. |
Measurements between links
Most network management systems (NMS) packages measure latency as seen by the NMS.
IP SLA leverages the router as a probe where the router takes the IP SLA measurement and the NMS polls the router via SNMP for the measurement value.
This method offers a way to get measurements between links and not all from a central location.
NetFlow and IP SLA - a marriage made in heaven?
Like IP SLA, NetFlow allows the router to be utilized as a probe as well, however the difference with NetFlow is the router streams the information back to the collector (i.e. it isn’t polled via SNMP like IP SLA).
Both IP SLA and NetFlow complement each other nicely:
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IP SLA measures VoIP latency (i.e. Jitter). |
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NetFlow measures QoS (i.e. DiffServ and ToS) and traffic details similar to a packet analyzer. |
The following configuration is an example of IP SLAs operation utilizing the configuration available in Releases 12.4 Mainline and 12.4T.
Below will configure a jitter test to destination 172.29.139.134 with 64 byte packets and 20ms packet spacing sending 20 packets per measurement. The operation is scheduled to start in 5 minutes.
ip sla monitor 1 type jitter dest-ipaddr 172.29.139.134 dest-port 5000 num-packets 20 ip sla monitor schedule 1 life 300 start-time after 00:05:00
More details on the above can be found here.
 "It isn’t enough to recognize jitter as a problem in VoIP, that’s easy even with free tools," said Mike Patterson – CEO of Plixer International. |
"Understanding what’s causing the jitter is paramount."
"Sometimes it is caused by a switch, other times it’s exacerbated by other applications competing for the same or similar QoS. The PBX can be part of the problem as well."
Patterson was keen to make the following point:
"Questions regarding activating NetFlow and how using a NetFlow Analyzer can provide details on VoIP, get asked in a very large portion of our conferences."
Cisco has the solution and the price is right - FREE
The analysis of QoS (i.e. DiffServ and ToS) in NetFlow can clearly layout what priority applications are requesting, but latency measurements are generally not possible with NetFlow.
Because of this, a nice complement to NetFlow is IP SLA which allows for latency measurements on such issues as jitter.
What’s more, the Cisco NBAR and IP SLA technologies as well as NetFlow are available on most Cisco routers and they are all free!
Are you taking advantage of NetFlow and IP SLA?
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Network Performance Daily blog comments on NetFlow article
Brad - thanks for covering this topic. We added our own perspective on the article at Network Performance Daily http://www.networkperformancedaily.com/2007/12/cisco_ipsla_netflow_voip.html
NetQos Blog
You're welcome Pam.
By the way, added a direct link to Network Performance Daily from the Cisco Library.
Sincerely,
Brad Reese
http://www.BradReese.Com