ZyXEL Communications X2002, User Manual

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Chapter 8 QoS
X2002 User’s Guide
168
ToS
The X2002 supports the following classes of service for outgoing VoIP packets.
• Minimize Delay - Use this when the time it takes for a packet to travel from
the source host to destination host (latency) is most important.
• Maximize Throughput - Use this when the volume of data transmitted in any
period of time is important.
• Maximize Reliability - Use this when it is important that you have some
certainty that the data will arrive at the destination without retransmission
being required.
• Minimize Cost - Use this when it is important to minimize the cost of data
transmission. Network providers may offer two types of service (for example
fiber and copper) and charge differently depending on which you use. In this
case, choosing “minimize cost” may inform the network provider to use the
lower-cost route to send your VoIP traffic.
Note: It is recommended to use the Minimize Delay class for VoIP traffic.
DiffServ
DiffServ (Differentiated Services) is a class of service (CoS) model that marks
packets (based on the application types and traffic flow) so that they receive
specific per-hop treatment at DiffServ-compliant network devices along their
route. 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.
DSCP and Per-Hop Behavior
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 forwarding treatment.
Resources can then be allocated according to the DSCP values and the configured
policies.
The X2002 allows you to mark outgoing frames with following PHB classifications:
Assured Forwarding (AF) - This is a PHB group allowing you to choose from one
of four classes of forwarding assurance. Each class has three choices of drop
precedence to choose from. When congestion occurs, the DS nodes on your
network treat all IP packets marked with the same class and drop precedence
identically. DS nodes can be configured to protect packets with lower drop
precedence value by discarding packets with a higher drop precedence value.