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QoS General Overview
Quality of Service (QoS) refers to transmission quality and available service that is measured and some-
times guaranteed in advance for a particular type of traffic in a network. QoS lends itself to circuit-
switched networks like ATM, which bundle traffic into cells of the same length and transmit the traffic
over predefined virtual paths. In contrast, IP and other packet-switched networks operate on the concept of
shared resources and best effort routing, using bandwidth as needed and reassembling packets at their
destinations. Applying QoS to packet-switched networks requires different mechanisms than those used in
circuit-switched networks.
QoS is often defined as a way to manage bandwidth. Another way to handle different types of flows and
increased bandwidth requirements is to add more bandwidth. But bandwidth can be expensive, particu-
larly at the WAN connection. If LAN links that connect to the WAN are not given more bandwidth, bottle-
necks can still occur. Also, adding enough bandwidth to compensate for peak load periods mean that at
times some bandwidth is unused. In addition, adding bandwidth does not guarantee any kind of control
over network resources.
Using QoS, a network administrator can gain more control over networks where different types of traffic
(or flows) are in use or where network congestion is high. Preferential treatment can be given to individ-
ual flows as required. Voice over IP (VoIP) traffic or mission-critical data can be marked as priority traf-
fic and/or given more bandwidth on the link. QoS can also prevent large flows, such as a video stream,
from consuming all the bandwidth on a link. Using QoS, a network administrator can decide which traffic
needs preferential treatment and which traffic can be adequately served with best effort.
QoS is implemented on the switch through the use of user-defined policies, port-based QoS configuration,
and integration with virtual output queuing to manage egress congestion. The following simplified illustra-
tion shows an example of how video traffic can receive priority over email traffic.
OmniSwitch
The Internet
Prioritization
Policy
video feed
Best Effort
email server
Sample QoS Setup
QoS sits in the ingress and egress software path. IP calls QoS to validate packets destined for the switch.
IP also calls QoS to validate and/or prioritize packets originating from the switch.
The OmniSwitch 10K integrates traffic management with QoS scheduling. A virtual output queue (VOQ)
architecture is implemented to funnel traffic from ingress to egress, requiring only one stage of packet
buffering. Deep ingress buffers facilitate holding ingress traffic when congestion occurs on the egress,
helping to provide delivery with little or no packet loss.
The general order of events with respect to the OmniSwitch 10K implementation of QoS are as follows:
page 21-4
OmniSwitch AOS Release 7 Network Configuration Guide
March 2011
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