Editing Class of service (section)

==Data services==
{{see also|IEEE P802.1p|Type of service}}
As related to network technology, COS is a 3-[[bit]] field that is present in an [[Ethernet]] frame header when [[802.1Q]] VLAN tagging is present. The field specifies a priority value between 0 and 7, more commonly known as CS0 through CS7, that can be used by [[quality of service]] (QoS) disciplines to differentiate and shape/police network traffic.

COS operates only on 802.1Q VLAN Ethernet at the [[data link layer]] (layer 2), while other QoS mechanisms (such as [[DiffServ]], also known as DSCP) operate at the IP [[network layer]] (layer 3) or use a local QoS tagging system that does not modify the actual packet, such as Cisco's "QoS-Group".

Network devices (i.e. routers, [[network switch|switches]], etc.) can be configured to use existing COS values on incoming packets from other devices (trust mode), or can rewrite the COS value to something completely different.  Most Internet Service Providers do not trust incoming QoS markings from their customers, so COS is generally limited to use within an organization's intranet.{{citation needed|date=December 2015}}

Service providers offering private-line WAN services will typically offer services which can utilize COS/QoS.