Terry Slattery
Terry Slattery, is a senior network engineer with decades of experience in the internetworking industry. Prior to joining Chesapeake NetCraftsmen as...
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Terry Slattery | November 27, 2012 |


Video and Voice: Is Your Network Ready?

Video and Voice: Is Your Network Ready? Identifying real or potential problems is our goal: Showing customers what is happening in a way that everyone understands.

Identifying real or potential problems is our goal: Showing customers what is happening in a way that everyone understands.

As a consultant at Chesapeake Netcraftsmen, I get to see a lot of interesting network problems and get access to some interesting tools that help us diagnose those problems. We often get called to perform network assessments, which typically need to be performed quickly and accurately. Using manual methods does not work well in a network assessment, due to the requirement for fast, accurate network discovery and assessment. Automated tools allow us to quickly collect important information about the configuration and operation of the network. We're then left with the task of analyzing the data that is collected by the tools, and reporting our findings to the customer.

Voice and video assessments are particularly interesting because they are often performed prior to the deployment of a voice or video system. However, if an enterprise has skipped the pre-deployment assessment, the job becomes even more critical when the assessment finally does get performed, post-deployment--because the voice or video system is already in place and is not performing acceptably.

It is important to identify single-instance problems as well as systemic problems. A single-instance problem might be a congested interface, while a systemic problem might be caused by an incorrect QoS configuration that is applied across the network.

Our data network assessments often see problems similar to those experienced in voice and video deployments. Two assignments that I've done in the past year have resulted in the discovery of significant data network problems caused by excessive buffering in routers. I wrote about one of them in a series of blog entries that are referenced in my post, Defending the Network from Applications.

When producing reports for customers, making them easily understood is important, and the key factor in achieving this is your choice of tool. There are multiple tools on the market; we use PathView from Appneta.

How It Works
PathView uses a set of small probes to generate network test packets. I like to call this approach Active Path Testing because test packets are injected into the network and the performance of these packets is monitored to determine various network characteristics. (See my blog on Network Management Architecture for the full suite of network management functions.) Using a sequence of test packets allows the tool to measure and report on the path bandwidth, path utilization, latency, jitter, and packet loss. All these factors are critical to voice and video, as well as many other applications. The figure below shows an example output during some tests that I ran for a customer.

As seen in the blue-tinted section of the graphic, the path capacity matches the theoretical maximum for a 10-Mbps path. The utilization of the path is near zero except for the time of the tests, during which I generated 9.1 Mbps of traffic. You can easily see the correlation between the utilization and the changes in latency and packet loss during the tests, as shown in the lower section of the graphic. When you're working within the tool's interface, placing the cursor on one of the graphs shows the corresponding data points on the other graphs.

In this screenshot, I'm focused on the peak data loss point to see when it occurred (around the 22:03 mark on the horizontal axis of all the graphs). The latency at this point is 240 ms, which is significantly above the 20-ms baseline latency. Jitter is also high at 90 ms and the measured path utilization is 7.6 Mbps. This is near the end of the traffic test, and all the metrics quickly ramp back down to their normal levels when the test traffic stops. The red sections of the Jitter and Latency charts show the alarm thresholds that were set prior to running the tests.

(Click here for larger view)

Next Page: A Typical Deployment


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