3Com 3824 Implementation Guide - Page 51

How Traffic Prioritization Works 51 determines the level of service that type of traffic should receive. Refer to Table 6 for an example of how different traffic types can be mapped to the eight IEEE 802.1p priority levels. Table 6 IEEE recommendation for mapping 802.1p priority levels to 802.1D traffic types IEEE 802.1p Priority Level 0 1 2 3 4 5 6 7 IEEE 802.1D Traffic Type Best Effort (default) Background Standard (spare) Excellent Effort (business critical) Controlled Load (streaming multimedia) Video (interactive media), less than 100 milliseconds latency and jitter Voice (interactive voice), less than 10 milliseconds latency and jitter Network Control Reserved traffic The traffic marking and prioritization supported by the Switch using layer 2 information is compatible with the relevant sections of the IEEE Std 802.1D, 1998 Edition (incorporating IEEE 802.1p). The IEEE 802.1D standard is the most widely used prioritization scheme in the LAN environment, but it does however have some restrictions: ■ It requires an additional 4-byte tag in the frame, which is normally optional in Ethernet networks. Without this tag, the scheme cannot work. ■ The tag is part of the IEEE 802.1Q header, so to implement QoS at layer 2, the entire network has to implement IEEE 802.1Q VLAN tagging. ■ It is only supported on a LAN and not across routed WAN links, because the IEEE 802.1Q tags are removed when the packets pass through a router. Differentiated Services (DiffServ) Traffic Marking DiffServ is a Layer 3 marking scheme that uses the DiffServ Code Point (DSCP) field in the IP header to store the packet priority information. DSCP is an advanced intelligent method of traffic marking because you can choose how your network prioritizes different types of traffic. DSCP