CoS...

class of service

specifically used at the datalink layer when 802.1q vlan tagging is present
a 3 bit header is represented in an ethernet frame specifying a priority value of between 0 and 7
this is a classification of network traffic at layer 2 known as the 802.1p standard
the class of service is 0-3 lower priotrity
and 4-7 higher priority
think QoS for layer 2

see table below from wiki... http://en.wikipedia.org/wiki/IEEE_P802.1p

IEEE P802.1p From Wikipedia, the free encyclopediaJump to: navigation, search IEEE P802.1p is the name of a task group active during 1995–98 responsible for adding traffic class expediting and dynamic multicast filtering to the IEEE 802.1D standard. Essentially, they provided a mechanism for implementing Quality of Service (QoS) at the Media Access Control (MAC) level. The group's work with the new priority classes and Generic Attribute Registration Protocol (GARP) was not published separately but was incorporated into a major revision of the standard, IEEE 802.1D-1998. It 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. Although this technique is commonly referred to as IEEE 802.1p, there is no standard or amendment by that name published by the IEEE. Rather the technique is incorporated into IEEE 802.1Q standard which specifies the tag inserted into an Ethernet frame.[1]

Priority levelsEight different classes of service are available as expressed through the 3-bit PCP field in an IEEE 802.1Q header added to the frame. The way traffic is treated when assigned to any particular class is undefined and left to the implementation. The IEEE however has made some broad recommendations:[2]




 0 (lowest) BK Background

 1 BE Best Effort

 2 EE Excellent Effort

 3 CA Critical Applications

 4 VI Video, < 100 ms latency

 5 VO Voice, < 10 ms latency

 6 IC Internetwork Control

 7 (highest) NC Network Control