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Quality of Service

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 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, WLAN 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:

o

Ingress mappings (Domain =>CoS)

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o

Egress Mappings (CoS => Domain)

o

Untrusted mode (default)

°

Shaper

Egress

For packets going out of the router, the markings (CoS) need to be translated to the

mappings understood by the network domains. The reverse CoS and domain mapping is

configured using the Egress. To access Egress

Egress

Egress

Egress, select the Advanced Menu

Advanced Menu

Advanced Menu

Advanced Menu and then select

QoS > Egress

QoS > Egress

QoS > Egress

QoS > Egress.

There are three Egress modes:

°

No Egress mode

°

Layer 2

°

Layer 3

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No Egress Mode

The default Egress page setting for all interfaces is No Egress. In this mode, the domain

mappings of the packets are untouched.

Egress

Egress

Egress

Egress

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Layer 2

The Egress Layer 2 page allows you to map the CoS of an outgoing packet to user priority

bits, which is honored by the VLAN network. Again, this feature is only configurable on

the WAN interfaces as VLAN is only supported on the WAN side in the current release.

Laye

Laye

Laye

Layer 2

r 2

r 2

r 2

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Layer 3

Egress Layer 3 enables you to map CoS to ToS so that the priority marking of outgoing

packets can be carried over to the IP network.

Layer 3

Layer 3

Layer 3

Layer 3

Ingress

Ingress enables you to configure QoS for packets as soon as they come into the router.

The domain mappings are converted to CoS (the common language) so that the priority

marking is carried over.

There are four Ingress modes:

°

Untrusted mode

°

Layer 2

°

Layer 3

°

Static