Tech Talk Live Blog

Quality of Service

Frank Olshansky

Quality of Service, also known as QoS, is a very important topic to know and understand when working with modern networks. Networks are generally designed so that all traffic is treated in the same manner. However, this model does not work when voice, video, or other real-time traffic share the same network as data, because voice and video have stricter requirements regarding bandwidth, latency, jitter, and packet loss. As an example, an e-mail has almost no latency requirement and can handle packet loss. A voice call has a latency requirement of 150 milliseconds or less (per direction) and less than 1% packet loss. The primary goal of QoS is to provide preferential treatment to the traffic that requires it, while also ensuring that the other traffic can flow properly as well. This can be done through one of two QoS models: the Integrated Services model, also known as IntServ or the Differentiated Services model, also known as DiffServ.

The Integrated Services model uses the Resource Reservation Protocol, also known as RSVP, to reserve resources on the routers that the traffic needs to traverse, and thus guarantees a certain level of quality of service. The problem with this model is that it does not scale well, because when RSVP reserves the resources, no other traffic can use these resources. This causes a problem if you have a lot of traffic that requires reservations. This QoS model is not used much in large scale networks.

The Differentiated Services model is broken down into three main parts, classification and marking, policing, and scheduling. The DiffServ model classifies and marks the packets using a 6 bit Differentiated Services Code Point (DSCP) value in the Differentiated Services field (DS field) of the IP header. The router then uses this DSCP value to decide what to do with the packet based on what the administrator defines in the router.

In theory, an administrator can define up to 64 different classes for the traffic. However, in practice, an administrator will normally stick to either a five class, an eight class, or an eleven class QoS model, and classify traffic based on one of four defined Per Hop Behaviors (PHB). These Per Hop Behaviors are: the Default PHB, the Expedited Forwarding (EF) PHB, the Assured Forwarding (AF) PHB, and the Class Selector PHB.

The common DSCP values for the eleven class QoS model are listed below, along with their PHB names and what kind of traffic that value is most commonly used for:

DSCP value PHB name Application
48 CS6 IP Routing/Network Control
46 EF Voice
34 AF41 Interactive Video
32 CS4 Streaming Video
26 AF31 Mission Critical
24 CS3 Call Signaling
18 AF21 Transactional Data
16 CS2 Network Management
10 AF11 Bulk Data
8 CS1 Scavenger
0 0 Best Effort


The right column in the above table is meant as a guide only. There is no requirement to set specific traffic to the above values, it is based on what is most important in your organization and dependent largely on your network. This being said, I strongly recommend the following two points be adhered to, otherwise there might be some very undesirable results.

The DSCP value of 48 should never be defined by an administrator. This value is used by BGP, OSPF, and EIGRP packets and no other packets should be marked with this DSCP values.

The DSCP value of 46 should be used for voice traffic only.

Below are some links that explain QoS in depth:




I will be discussing the configuration of QoS in a later Tech Talk Live blog entry.

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