Class of service.
I remeber last year back when i heard the term COS. I was dealing with a company and we were talking about QOS for their VoIP and he used the term COS. I knew what it meant, but what did it do?
The COS 3 bit field are defined by the IEEE 802.1p standard. The bits can be used to mark packets to a specific class of service. The markings use the three 802.1p priority bits and allows a L2 ethenet frame to be marked with eight different levels of priority, value 0 -7. The three bits allow for eight levels of classifaction, allowing a direct correspondence with IPv4 IP precedence TOS values. The disadvantage using COS marking is the frames lose their COS markings when transiting a non- 802.1q link, that which carries the 802.1p field in the trunk header.
Or you may see it written like below.
000 = Best effort
001 = Medium priority data
010 = High priority data
011 = Call signaling
100 = Video
101 = Voice bearer
101 = Reserved for management traffic and routing data
110 = Reserved for management traffic and routing data