ZyXEL GS1500-24P User Guide - Page 80
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Chapter 10 QoS Use the Priority to Queue Mapping screen (Section 10.7 on page 86) to configure the priority level-to-physical queue mapping. Use the Packet Scheduling screen (Section 10.8 on page 87) to set priorities for the Switch queues. This can help distribute bandwidth across the different traffic queues. 10.3 What You Need to Know Read on for concepts on QoS that can help you configure the screens in this chapter. 10.3.1 DiffServ DiffServ is a class of service (CoS) model that marks packets so that they receive specific per-hop treatment at DiffServ-compliant network devices along the route based on the application types and traffic flow. Packets are marked with DiffServ Code Points (DSCPs) indicating the level of service desired. This allows the intermediary DiffServ-compliant network devices to handle the packets differently depending on the code points without the need to negotiate paths or remember state information for every flow. In addition, applications do not have to request a particular service or give advanced notice of where the traffic is going. 10.3.2 DSCP and Per-Hop Behavior DiffServ defines a new DS (Differentiated Services) field to replace the Type of Service (ToS) field in the IP header. The DS field contains a 6-bit DSCP field which can define up to 64 service levels and the remaining 2 bits are defined as currently unused (CU). The following figure illustrates the DS field. Figure 35 DiffServ: Differentiated Service Field DSCP (6 bits) CU (2 bits) DSCP is backward compatible with the three precedence bits in the ToS octet so that non-DiffServ compliant, ToS-enabled network device will not conflict with the DSCP mapping. The DSCP value determines the PHB (Per-Hop Behavior), that each packet gets as it is forwarded across the DiffServ network. Based on the marking rule different kinds of traffic can be marked for different priorities of forwarding. Resources can then be allocated according to the DSCP values and the configured policies. 80 GS1500-24P User's Guide