Chapter 9 SSID Screen

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.

9.3.4.2 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 2-bit unused field and a 6-bit DSCP field which can define up to 64 service levels. The following figure illustrates the DS field.

Figure 85 DiffServ: Differentiated Service Field

DSCP Unused

(6-bit) (2-bit)

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 forwarding behavior, the PHB (Per-Hop Behavior), that each packet gets 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.

9.3.4.3 ToS (Type of Service) and WMM QoS

The DSCP value of outgoing packets is between 0 and 255. 0 is the default priority. WMM QoS checks the DSCP value in the header of data packets. It gives the traffic a priority according to this number.

In order to control which priority level is given to traffic, the device sending the traffic must set the DSCP value in the header. If the DSCP value is not specified, then the traffic is treated as best-effort. This means the wireless clients and the devices with which they are communicating must both set the DSCP value in order to make the best use of WMM QoS. A Voice over IP (VoIP) device for example may allow you to define the DSCP value.

 

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NWA-3500/NWA-3550 User’s Guide