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Quality of Service (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 enters 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