User guide

ADSL2+ 4-Port Ethernet USB Wireless-G Router - User Guide
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4.5.25 QoS
QoS stands for Quality of service. The QoS framework allows network administrators to
configure the routers to meet the real time requirements for voice and video.
Different QoS marking is used in different network:
• ToS network: ToS bits in the IP header
• VLAN network: priority bits in the VLAN header
• DSCP network: uses only 5 bits of the CoS
• WLAN: WLAN QoS header
The QoS framework is supported on all the above domains. How do you make them talk to each
other? How can you make sure the priority from one network is carried over to another network?
Class of service (CoS) is introduced as the common language for the QoS mappings. When QoS
is enabled, the router has full control over packets from the time they enter the router till they
leave the router. This is how it works: The domain mapping (ToS bits, priority bits, etc.) of a
packet needs to be translated to CoS when the packet enter the router, and vice versa, the CoS of
a packet needs to be translated back to the domain mapping when the packet leaves the router.
There are 6 types of CoS (in descending priority):
• CoS1
• CoS2
• CoS3
• CoS4
• CoS5
• CoS6
The rules are:
1. CoS1 has absolute priority and is used for expedited forwarding (EF) traffic. This is always
serviced till completion.
2. CoS2-CoS5 are used for assured forwarding (AF) classes. They are serviced in a strict round
robin manner using the following priority scheme:
CoS2 > CoS3 > CoS4 > CoS5
3. CoS6 is for best effort (BE) traffic. This is only serviced when there is no other class of
service. If QoS is not enabled on your router, all traffic will be treated as best effort.