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5.3QoS & bandwidth control
Covers why QoS is needed to protect the quality of voice and video traffic, priority control (priority queueing), which separates queues by priority, shaping and policing, which control transmission rate, the standardized priority-marking schemes DiffServ (DSCP) and IntServ, and congestion control, which prevents network-wide collapse.
When file transfers and VoIP calls share the same network link, large volumes of file-transfer traffic can delay VoIP packets, risking dropped or garbled calls. QoS (Quality of Service) is a set of technologies for preferentially processing the traffic that should be prioritized when traffic of differing characteristics coexists like this. This section focuses on the design decisions behind guaranteeing voice/video quality within limited bandwidth.
5.3.1Why QoS is needed
- A best-effort IP network is inherently designed to make no guarantee about the order or timing of delivery, so during congestion all traffic is affected equally by delay and loss. However, real-time traffic such as voice and video is sensitive to latency, jitter, and packet loss (delays or losses of a few hundred milliseconds severely degrade perceived quality), whereas file transfer tolerates some delay (it only needs to complete eventually). Ignoring this difference in characteristics and treating all traffic equally sacrifices the quality of real-time communication.
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