Cerberus ADSL2+ Wi-Fi (P 6331-5)
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QoS
Quality of service allows network administrators to configure the routers to meet the real time
requirements for voice and video.
Different networks use different QoS markings like:
•
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.
There are some additional terms you should get familiarize with:
•
Ingress: Packets arriving into the router from a WAN/LAN interface.
•
Egress: Packets sent from the router to a WAN/LAN interface.
•
Trusted mode: Honors the domain mapping (ToS byte, WME, VLAN user priority).
•
Untrusted mode: Does not honor domain mapping. This is the default QoS setting.
•
Traffic Conditioning Agreement (TCA): The TCA needs to be defined for each interface:
•
Ingress mappings (Domain =>CoS)
•
Egress Mappings (CoS => Domain)
•
Untrusted mode (default)
•
Shaper
Summary of Contents for Cerberus ADSL2+ Lite
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