Quality of Service. The idea that packets on a network have a number in the header which defines a class, which then allows different classes to be treated differently as they pass through the network.
Not having QoS is called Best Effort.
QoS can be done at different network levels:
Layer 2 / Ethernet: Using 802.1p. This is often called CoS (Class of Service)
Layer ‘2.5’: Using MPLS (Multi Protocol Label Switching)
Layer 3 / IP: Using the DSCP in an IP packet header (Differentiated Services Code Point). This method is called DiffServ
Often the term QoS is taken to mean doing QoS at Layer 3 and above.
A modern PC or other device can set the DSCP bits based on application needs, for instance mark video packets with a higher priority.
The PC could also set the Ethernet frame priority using 802.1p frame priorities, and the switches on the LAN could also prioritise the Ethernet packets.
Instead of being done by end devices, routers at the network edge can set the DSCP bits based on 802.1p values, or on other criteria (such as by looking at the content type). The following routers can be configured to prioritise the video packets.
Most broadcast video and audio over IP systems specify the use of QoS.
See: CoS, Diff Serv, IEEE 802.1p
