What, (when) is QoS, CoS (happen)?

What is actually QoS (quality of service)? specially IP QoS, IP Network QoS. IP QoS is transport packet-data with special or custom requirement, capability to provide custom transport service. 

How about CoS. Juniper refers to QoS as CoS (class of service).

When is QoS happen? QoS will occur if congestion happen, Even you configure QoS in network equipment but congestion doesn't happen, QoS isn't occur.

What is requirement for QoS?
- Networks converged

- Classification traffic and marking

- Define policies

QoS is optional feature in IP network, if congestion doesn't happen in your network, rethink again to enable QoS, some of feature is CPU intensive and memory consumption. The network will transport and deliver packet-data with best-effort if QoS is disable.