, Diff- number of other prioritization/CoS schemes are outlined Serv will support CoS on the Internet, extranets, and here. intranets. MAC layer prioritization. In a shared LAN environ- Priority settings may be made in several places. The most ment such as Ethernet, multiple stations may con- logical place is the application running in the end user's tend for access to the network. Access is based on system. But applications may not support the various first-come, first-serve. Two stations may attempt schemes that are available, meaning that edge switches simultaneous access, causing both stations to back off may need to infer priority levels for frames or packets by and wait before making another attempt. This is examining the contents of the packets. This is now easily minimized for switched Ethernet where only one done with so-called "multilayer switches" based on pol- station is connected to a switch port. A number of icies that are defined in policy-based management vendor-specific Ethernet priority schemes have been systems. developed. Token ring networks have a priority mechanism in which a reservation bit is set in tokens Latency to indicate priority. The average time delay experienced between an audio VLAN tagging and 802. 1p The IEEE 802.1Q frame- source (speaker) and destination (listener) does not tagging scheme defines a method for inserting a tag affect the quality of speech directly, but it affects the into an IEEE MAC-layer frame that defines mem- speaker's perceptions of echo from the listener's end, bership in a virtual LAN. Three bits within the tag and it affects the quality of a conversation (the ability of define eight different priority levels. The bit settings the listener to "turn the speech path around" and CHAPTER 3.2 Network infrastructure respond). The most practical means of treating the echo Table 3.2-7 problem is to cancel it at the endpoint closest to the cause of the echo. For an IP endpoint (e.g., telephone Application layer User & application access, profile set), this means wi