Analyze your Link's Behavior

Do you know which applications are using most of your bandwidth? Do you know your network's patterns of bandwidth usage? Do you know how much of your bandwidth goes to waste from retransmissions? The steps below show you techniques to help answer these questions.

   Steps:

1. Display your Network Performance Summary and change the interval at the top to cover the last week. Examine the Top 10 Classes pie chart. It shows you the ten classes on your managed link that use the most bandwidth.
   
   
   In the Top 10 chart above, NetNews (NNTP), web page downloads (HTTP), email downloads (POP3), inbound mail to your mail server (SMTP), file sharing with Gnutella, file downloads with FTP, real-time audio with Shoutcast, MPEG-Audio, and real-time video (MPEG-Video) are the most used applications.
   
   Are there any surprises in your group of ten? Are these the applications you would expect to see as the top bandwidth consumers? Are they the group you want to see? You can use PacketWise control features to restrict access for any traffic class whose traffic doesn't warrant the bandwidth it takes. Also, you can use control features to give bandwidth preference to critical applications that are not on your list, but should be.
   
   
2. Now examine the Utilization graphs for your inbound and outbound links. The graph helps you analyze how heavily your WAN link is loaded relative to its capacity, aggregating data from all applications. You can then go into a more detailed analysis of specific applications to look at their bandwidth-usage characteristics and compare them to the link's patterns.
   
   
   From this chart you can see that the unit has been connected to a network link with maximum capacity of about 4.5 Mbps, perhaps a network location in where three T1/E1 lines connect to a LAN.
   
   You can also see that both average (red) and peak (blue) traffic utilization was lower during two days than the remaining five days. Perhaps it was a weekend or any other interval of lower usage.
   
   Looking at just the red, average-rate line in the graph, note that average utilization doesn't exceed 1 Mbps on a sustained basis, even though a few peaks do go higher. Without seeing the blue, peak-rate line, this average might lead you to think that there is plenty of bandwidth to spare and congestion is never a problem. However, as you can see, peak rates frequently spike to near capacity.
   
   Examining the last two days on the graph, more traffic (and probably longer response times) are evident. Even the average line stayed up near 1 Mbps on 08/06. Perhaps one of the T1 (1.5 Mbps) lines was saturated for brief periods.
   
   Validating these guesses would require more detailed analysis for those specific dates. When a network link has multiple lines, sometimes traffic for specific applications only travels across one of the lines. Although there is a total of 4.5 Mbps capacity, there can be cases where the maximum throughput is actually 1.5 Mbps if the traffic is not evenly shared across the three lines.
   
   
3. Graph Class Utilization with Peaks for individual classes. Investigating the traffic classes that were the top consumers in the pie chart is a good start. Adjust the graphing interval to cover one week. Consult examples of applications' bandwidth utilization graphs and their interpretations.
   
   
4. Using your original Network Performance Summary, look at the Network Efficiency graphs for your link. The graph shows the percentage of traffic that was new, first-time traffic (not retransmissions). Low efficiency means high retransmission rates, a sign of congested router queues, delays, and time-outs. (Note: The formula for efficiency is bytes - tcp-retx-bytes / bytes.)
   
   The chart below shows almost optimum efficiency, meaning that very little bandwidth was devoted to retransmissions.
   
   
   
   
5. Graph Network Efficiency for individual classes. Examine the same traffic classes you investigated for bandwidth utilization, probably those that were the top consumers in your Top 10 pie chart. Adjust the graphing interval to cover one week. Consult examples of applications' efficiency graphs and their interpretations.
   
   For more on evaluating network efficiency, see the Assess Wasted Capacity recommendation.
   
   

Other Ideas:

If you collect a history of the Utilization graph from Network Performance Summaries over time (or the Link Utilization with Peaks graphs), you can estimate if and when you will outgrow your WAN capacity. Are you continually using more and more of your WAN capacity — not just with peak rates, but with sustained averages, and even with appropriate policies and partitions in effect? As your usage approaches the size line, you will need to add capacity or change management strategy. For example, perhaps you've been overly generous in allowing employees unrestricted access to Internet radio, but if that means an expensive bandwidth upgrade is imminent, then you'd rather restrict access.