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Prestige 2802HW(L)-Ix Support Notes
All contents copyright (c) 2007 ZyXEL Communications Corporation.
111
Voice - QoS setup
DSCP and Per-Hop Behavior
Diffserv is a class of service (CoS) model that marks packets so that they receive specific per-hop treatment at
DiffServ-compliant network device along the route based on the application types and traffic flow. Packets are
marked with DSCP 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.
Diffserv defines a new DS(Differentiated Services) field to replace the TOS(Type of Service) field in IP header.
The DS field contains a 2-bit unused field and a 6-bit DSCP filed which can define up to 64 service levels. The
following figure illustrates the DS field.
DiffServ: Differentiated Service Field
Diffserv Code Point
6-bit
Unused
2-bit
The DSCP value determines the forwarding behavior, the PHB(Per-Hop Behavior), that each packet gets across the
Diffserv network. The PHB value is define in the RFC2597 and introduced where classes are developed such as
Business, Telecommuter, Residential etc. that can be offered by an ISP as different levels of service. The following
is the table illustrates the DSCP values:
Per Hop Behaviour
(PHB)
DiffServ Code Point
(DSCP)
IP
Precedence
Default
0
000000
Assured Forwarding
Low Drop
Probability
Medium Drop
Probability
High Drop
Probability
Class AF11 AF12 AF13 1