Quality of service (QoS) is an advanced traffic prioritization feature that preserves network performance as the amount of traffic increases by enabling you to assign various grades of service to different types of traffic.

QoS is featured in H3C® premium switches and routers, including members of the SR6600, MSR 50, MSR 30 and MSR 20 router series, as well as the S12500, S9500, S7500E, S7500, S5800, S5820X, S5500-EI, S5500-SI, S3600-EI and S3100-EI switch series.

QoS enables you to apply a security policy and manage network congestion by defining exactly how you want your H3C network devices to treat selected applications and types of traffic, including multimedia, video, protocol-specific, time-critical and file-backup traffic. This increases reliability of data delivery and enables you to prioritize certain applications across your network. QoS also reduces bandwidth limitations, delay, loss and jitter.

Benefits of QoS:

• Provides predictable throughput for multimedia applications such as videoconferencing or VoIP while eliminating delay and jitter
• Improves performance for specific types of traffic and preserves performance as the amount of traffic grows
• Reduces the need to constantly add bandwidth to the network
• Manages network congestion

QoS achieves this by slowing down unimportant packets or, in some cases, eliminating them completely, leaving ample bandwidth for important packets to reach their destination as quickly as possible. QoS cannot speed up a packet, so it calculates how much high-priority traffic is in the pipeline, compares that to available upstream bandwidth, queues traffic in the buffer in order of importance, and continues to do so until it runs out of data to send or the buffer fills up. Excess data is held back or re-queued at the front of the line, where it is evaluated in the next pass.

In this way, QoS shapes traffic by delaying unimportant packets and sending priority packets first, then using any leftover space to send packets in descending order of importance. QoS achieves this through traffic classification, traffic policing, traffic shaping, congestion management and congestion avoidance, as described in more detail below:

• Traffic classification — A prerequisite for differentiated service, traffic classification identifies objects based on certain matching rules.
• Traffic policing — Polices the specification of traffic entering the router; enacts restrictions when traffic exceeds the specification to prevent network resources from being damaged.
• Traffic shaping — Actively adjusting the output speed of traffic, traffic shaping enables traffic to adapt to the network resources supplied by the downstream router, preventing packet dropping and congestion.
• Congestion management — Handles resource competition during network congestion by storing packets in a queue and using a dispatching algorithm to assign the forwarding sequence for each.
• Congestion avoidance — Exceeding congestion consumes network resources; congestion avoidance monitors the usage status of network resources, and as congestion worsens, enacts a policy of dropping packets to resolve network overload.