ZyXEL Communications XS3800-28, User Manual, Page: 357 / 789

Chapter 49 Differentiated Services
XS3800-28 User’s Guide
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49
Differentiated Services
49.1 DiffServ Overview
This chapter shows you how to configure Differentiated Services (DiffServ) on the Switch.
Quality of Service (QoS) is used to prioritize source-to-destination traffic flows. All packets in the flow are
given the same priority. You can use CoS (class of service) to give different priorities to different packet
types.
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.
49.1.1 What You Can Do
• Use the Diffserv screen ( Section 49.2 on page 358 ) to activate DiffServ to apply marking rules or IEEE
802.1p priority mapping on the Switch.
• Use the DSCP Setting screen ( Section 49.3.1 on page 360 ) to change the DSCP-IEEE 802.1p mapping.
49.1.2 What You Need to Know
Read on for concepts on Differentiated Services that can help you configure the screens in this chapter.
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 265 DiffServ: Differentiated Service Field
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
DSCP (6 bits) CU (2 bits)