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.
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:
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.
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:
|4||4||VI||Video, < 100 ms latency and jitter|
|5||5||VO||Video, < 10 ms latency and jitter|
|7||7 (highest)||NC||Network Control|
Implementing QoS on EtherWAN 1.94/4.x Firmware Switches can be easily done using the web interface.
After enabling QoS, a Policy must be chosen from “Strict Priority (Queue 0-3),” “Strict Priority (Queue 3) + WRR (Queue 0-2)” or “WRR (Queue 0-3).”
The Weighted Round Robin (WRR) policy is an algorithm that performs polling scheduling between queues according to the assigned weight to ensure that each queue gets a certain service time. The weighted value indicates the proportion of the acquired resource.
Strict Priority (Queue0-3)
Packets are emptied from the queues in order, starting with queue 3 and ending with queue 0. When packets in each queue are completely emptied, next queue’s packets are set for transmission.
Strict Priority(Queue3) +WRR(Queue0-2)
Packets must be emptied from queue 3 first and the three remaining queues are emptied according the WRR weights in the Weighted Round Robin section (see below).
WRR (Queue 0 – 3)
Each queue is allowed to resolve a certain number of packets (according to the defined WRR weights) before moving to the next queue.
Use the 802.1p Priority page assign the queues to VLAN priorities. Higher VLAN priority values correspond to higher priority queues.
The DSCP page offers a wide range of DSCP priorities to set, and they are all assigned to the lowest-priority queue, 0. For each DSCP priority, the value of the queue can be set.
QoS is a feature of routers and switches that prioritizes network traffic according to defined parameters. This prevents packet loss for network traffic that is deemed critical. EtherWAN offers a wide range of managed and unmanaged switches that support QoS, for commercial, industrial, and hardened applications.