Implementing Quality of Service for Prioritizing Network Traffic

Quality of Service (QoS) is a networking mechanism that helps control and prioritize traffic so that more critical traffic can be sent first on the network. This feature helps ensure performance for critical network traffic. QoS is especially useful in Intelligent Transportation Systems where it helps solidify Signal Controller communication in high volumes of local video traffic. It is also extremely useful in prioritizing other critical traffic such as VoIP phones for emergency call boxes. Other common applications of QoS include Video-on-demand. Voice over IP (VoIP), and Internet Protocol television (IPTV).

QoS allows for the optimization of the performance for specific applications on a network and acquire information about the network’s bit rate and packet rate. Packets routing can be fine-tuned to avoid transmission delays.

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When should QoS be used?
Use QoS when encountering bandwidth congestion issues, and when it becomes necessary to prioritize network traffic, (for example, prioritizing one VLAN over another). QoS can also ameliorate intermittent issues with network traffic. Occasionally, in ITS systems, surges in CCTV data can overwhelm other traffic. The controller and switch VLANs use QoS to prioritize that data above other VLANs and prevents critical data from being lost. QoS can also solve interference problems such as packet loss, jitter, and latency.

Traffic With and Without QoS
As the volume of data increases towards 100% utilization, the potential for loss of data increases exponentially. In this image all data is trying to fit through the pipe. In the process, some critical data is lost.

With QoS, critical traffic is protected, and passes through without loss as shown below.

QoS guarantees the most important traffic is passed first by optimizing it based on priority. This is accomplished by prioritizing the VLANs used for each of the devices:

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QoS Description and Features

• There are two types of QoS: CoS and DCSP.
• QoS is a broader term which covers features like policing, shaping, traffic classification, and queuing techniques.
• CoS (Class of Service) is limited to Layer- 2 Ethernet, and uses 3-bits of the 802.iQ.tag to differentiate your traffic.
• DSCP operates at Layer 3, and is found in 6-bits of the IP header. Advanced methods such as Best Effort, Expedited Forwarding, and Assured Forwarding are used.

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CoS (Class of Service)
Class of service is determined by a 3-bit field called the Priority Code Point (PCP) within an Ethernet frame header when using VLAN tagged frames as defined by IEEE 802.1Q, and allows for the managing of network traffic through the grouping together of similar types of traffic. CoS specifies a priority value between 0 and 7, that can be used by QoS disciplines to differentiate traffic. Although this technique is commonly referred to as IEEE 802.1p, there is no standard or amendment by that name published by the IEEE. Rather the technique is incorporated into the IEEE 802.1Q standard which specifies the tag inserted into an Ethernet frame.

Eight different classes of service are available as expressed through the 3-bit PCP field in an IEEE 802.1Q header added to the frame. The way traffic is treated when assigned to any particular class is undefined and left to the implementation. The IEEE however has made some broad recommendations: