Dscp And Per-Hop Behavior; Tos (Type Of Service) And Wmm Qos - ZyXEL Communications NWA-3160 User Manual

Chapter 8 Wireless Screen

8.3.4.2 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 81 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 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.

8.3.4.3 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.
The following table lists which WMM QoS priority level the NWA uses for specific
DSCP values.
Table 29 ToS and IEEE 802.1d to WMM QoS Priority Level Mapping
DSCP VALUE
224, 192
160, 128
A
96, 0
64, 32
A. The NWA also uses best effort for any DSCP value for which another WMM
QoS priority is not specified (255, 158 or 37 for example).
130
DSCP Unused
(6-bit) (2-bit)
WMM QOS PRIORITY LEVEL
voice
video
besteffort
background
NWA-3160 Series User's Guide