Network congestion: it’s a problem in today’s computing world. How does it manifest itself? Perhaps you’ve called a toll-free number to order a plane ticket or make a hotel reservation and the agent you speak with says, “Please bear with me. The computer is running slowly today.”

 

That’s one manifestation of network congestion, the result of too much digital traffic all trying to reach a particular destination at the same time. The result is not unlike too many cars on the interstate—everything slows down. Sometimes, it even stops.

 

In a new application note out this month (November, 2006), VoIP Quality of Service specialist Kentrox addresses the network congestion problem and explains how traffic shaping techniques can be used to get things running smoothly, and quickly, again.

 

This article presents an executive summary of Kentrox’s (News - Alert) application note. The full document is available to download here.

 

Not all Internet connection services are the same. They range in cost from about $30 a month to as much as $500, depending on bandwidth (how much traffic can be carried at a given time).

 

Local Area Networks, or LANs, Kentrox says, are capable of carrying 100 megabits per second, or more. On the Wide Area Network (or WAN) side, though, T1 connections can handle only about 1.544 megabits per second.

 

Maybe you can see it coming. What happens when 100 megabits per second of traffic from the LAN heads out onto the WAN (via a router)? You guessed it—a bottleneck occurs.

 

“This traffic, referred to as a burst, must be stored in the router’s buffers until it can be placed on the WAN,” Kentrox explains. “When the buffers are filled, any new traffic that tries to get onto the WAN will be discarded.”

 

If the traffic happens to be VoIP signal, the bottleneck can cause more than just a slowdown—it can result in severe quality issues such as unintelligible conversations and dropped calls.

 

Perhaps the easiest and most cost-effective way to manage network congestion is to install a traffic shaping device that controls which applications are prioritized as traffic is directed to the WAN.

 

One way to prioritize traffic is to use a set of standards called Differentiated Services (DiffServ for short), which allows traffic to be divided into seven priority levels: Expediated Forwarding (for VoIP), Assured Forwarding 4 (for video), Assured Forwarding 3 (for high-priority business apps), Assured Forwarding 2 (for priority business apps), Assured Forwarding 2 (for regular business apps), Best Effort (for all other apps) and Network Control (for network management). 

 

“These different priority levels help control network bandwidth and minimize the amount of lost data,” Kentrox explains in its application note.

 

While this type of prioritizing, which can also be referred to as basic Quality of Service (or QoS for short), is useful, it’s only part of the solution to network congestion.

 

To keep networks running smoothly for bandwidth-intensive applications, it is also necessary to perform traffic shaping—a method of controlling the flow of packets as they travel through the network.

 

“There are many applications competing for the limited WAN bandwidth, and shaping gives network managers the opportunity to decide how much bandwidth a given application should consume at any point in time,” Kentrox explains in its application note. “By controlling the amount of bandwidth allocated to each application, the rate an application’s packets arrive at their destination is smoother and much more predictable.”

 

Three types of traffic shaping exist: Weighted Random Early Discard, guaranteed minimum bandwidth, and bandwidth limitations.

 

Weighted Random Early Discard – In this method, which takes effect when there is congestion, DiffServe categories are used to weight packets based on their importance.

 

Guaranteed Minimum Bandwidth – A minimum amount of bandwidth is assigned to each

application to help prioritize traffic for bandwidth-intensive services.  

 

Bandwidth Limitations – Policies are created that cap the kbps a given application can consume.

Quality of Service is important, but it cannot be maintained when congestion occurs without traffic shaping, Kentrox says in its app note. Traffic shaping ensures that, when packet loss does occur, the least important packets are discarded first.

 

VoIP Quality of Service