The task group developed a mechanism for implementing quality of service (QoS) at the media access control (MAC) level.
[3] The work also required a short amendment extending the frame size of the Ethernet standard by four bytes which was published as IEEE 802.3ac in 1998.
The QoS technique developed by the working group, also known as class of service (CoS), is 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.
It specifies a priority value of between 0 and 7 inclusive that can be used by QoS disciplines to differentiate traffic.
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.