Diffserv; Dscp And Per-Hop Behavior; Tos (Type Of Service) And Wmm Qos; Figure 20 Diffserv: Differentiated Service Field - ZyXEL Communications G-3000H User Manual

G-3000H User's Guide

5.3.2.1 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.

5.3.2.2 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.
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.

Figure 20 DiffServ: Differentiated Service Field

DSCP
(6-bit)
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.

5.3.3 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 ZyAIR uses for specific DSCP
values.

Table 11 ToS and IEEE 802.1d to WMM QoS Priority Level Mapping

DSCP VALUE
224, 192
160, 128
56
Unused
(2-bit)
WMM QOS PRIORITY LEVEL
voice
video
Chapter 5 Wireless Configuration