When people talk about industry standards and Voice over IP (VoIP), the hot topic usually is Session Initiation Protocol (SIP), an Internet Engineering Task Force (IETF)-defined method of handling interactive, multimedia user sessions.

 

But, SIP isn’t the only standard that relates to VoIP. In particular, when it comes to VoIP Quality of Service (usually referred to as QoS), standards like Multiprotocol Label Switching (MPLS) and Diffentiated Services (DiffServ) are more applicable.

 

In a recent phone conversation, TMCnet asked Kentrox Senior Systems Engineer Randy Garrett to explain what these standards do and how his company has applied them in its development of the Q1300 QoS Appliance and QoS-enabled VoIP routers.

 

 

Kentrox, Garrett said, uses the DiffServ standard—a subset of MPLS, which is defined by IETF—to deliver VoIP Quality of Service.

 

In order to explain the value of DiffServ, Garrett first defined “classes of service.” These, he said, are identifiers used to determine privileges for a particular session (i.e. call).

 

MPLS provides only eight standard classes of service, he said. DiffServ expands on that, upping the number of classes to 64.

 

 

Delivering VoIP Quality of Service is a four-step process, Garrett told TMCnet.

 

 

The first task of QoS is to identify traffic. Based on parameters set by the user (e.g. IP address, protocol types, port numbers), a Differentiated Services Code Point (DSCP) is assigned to the session.

 

 

Next, the QoS system looks at all the different types of traffic, and puts them into appropriate buffers for servicing.

 

“VoIP is put into what's most commonly referred to as a low-latency queue,” Garrett said.

 

VoIP traffic—because it involves time- and quality sensitive voice signal—is considered high-priority. Other types of traffic (video over IP, for example) may get that designation as well, but it will be given slightly less priority than VoIP. A Weighted Fair Queue (WFQ) is used to achieve this.

 

 

This part of QoS applies especially to WANs, which have a limited amoung of bandwidth available--what's known in the industry as the "pinch-point," Garrett said. Traffic shaping utilizes that bandwidth efficiently.

 

Most manufacturers have two or three different techniques for achieving this, and those techniques typically are based on standards set by Cisco.

 

 

The final step of VoIP QoS is to determine how traffic should be handled if the parameters the user set are exceeded. For example, the system must determine if traffic should be queued up, or tossed into the bit bucket, Garrett told TMCnet.

 

 

At its core, VoIP Quality of Service is about prioritizing IP traffic and delivering it based on those priorities. There’s no real magic to it, Garrett admitted.

 

That being the case, Kentrox differentiates its products by focusing on easy-of-use and manageability.

 

Garrett, who used to work at Cisco, illustrated this point by comparing that company’s QoS support with Kentrox’s products.

 

All Cisco routers, he said, support QoS—but with a lot of “ifs.” A couple examples are the amount of memory needed, and the availability of competent IT staff needed to program the routers.

 

“QoS programming on a Cisco router is not a trivial thing,” Garret told TMCnet.

 

At Kentrox, out-of-the-box QoS functionality is a standard feature of the company's products.

 

Case-in-point: The Q1300 QoS Appliance, which is designed for customers who already have a network established, and, because they are adding VoIP, need to enable QoS.

 

These customers can work to get QoS set up on their existing routers, Garrett noted. Or, they can install the QoS Appliance, which sits behind the router and in front of the LAN and takes care of all QoS functions.

 

“QoS is not rocket science, because it's standards-based,” Garrett told TMCnet. “But we believe that we've made it a lot easier to program and manage, at an affordable price-point.”

 

 

VoIP Quality of Service