User`s manual

Quality of Service (QoS)
Quality of Service (QoS) is the ability of a device to reserve sufficient bandwidth for a particular transmission.
QoS is important to companies with critical and bandwidth-intensive applications such as e-commerce web
servers, multimedia applications, corporate accounting systems and voice over IP.
Ethernet and QoS
Ethernet networks deliver traffic on a "best effort" basis. All traffic has equal priority and an equal chance of being
delivered in a timely manner. As the amount and complexity of traffic increases, network access is not denied but
performance can suffer.
To help alleviate this situation, the IEEE originally outlined the 802.1p standard to define how Ethernet switches
can classify traffic. Simply stated, 802.1p standardizes a switch's ability to prioritize traffic across the LAN and all
802.1p compliant devices. 802.1p is now officially incorporated into the 802.1D standard.
Priority values and traffic types
The 802.1p standard specified eight priority levels from 0 to 7, with 7 being the highest, that can be used to
classify different types of traffic. The standard also offered a list of suggested traffic types.
Starting with the
highest priority first, these suggested types are:
Network Control - "must get there" to maintain and support the network infrastructure
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Voice - less than 10 millisecond delay, and therefore, maximum jitter❍
Video - less than 100 millisecond delay❍
Controlled Load - important business applications subject to some form of admission control❍
Excellent Effort - or "CEO's best effort," the best effort type of services that an MIS organization
would deliver to its most important customers
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Best Effort - Ethernet LAN traffic as we know it today❍
Background - bulk transfers and other activities that are permitted on the network but which should
not impact network use by other users and applications
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The Switch's QoS capabilities
The Switch supports eight incoming priority levels, four outgoing traffic classes and configurable queue sizes on
Gigabit ports.
Prioritizing traffic
The Switch prioritizes traffic using the three User Priority bits—also known as the 802.1p priority field—in
the 802.1Q tag header. These bits give the packet a priority value ranging from 0 to 7.
Using these priority values both incoming and outgoing, the Switch forwards higher priority traffic before
lower priority traffic. This priority information is carried from one Ethernet LAN to another or across a WAN
or ISP connection.
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Traffic classes outgoing
Packets are placed into outgoing traffic classes and placed on separate queues based on their priority
value. The Switch can have up to four traffic classes. Each traffic class has a separate queue. The Switch
empties queues based on the priority of the traffic class.
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Priority queuing incoming
10/100 ports
On each 10/100 port, the Switch holds all incoming priority-tagged packets on one queue,
forwarding packets from this single queue across the switching fabric in the order of the packet's
priority value. Outgoing, the Switch automatically creates an outgoing packet queue for each of the
four traffic classes, again forwarding packets starting with Traffic Class 3, the highest priority.
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Gigabit ports
On each Gigabit port, the Switch automatically creates up to eight incoming and four outgoing
packet queues. An incoming queue is created for each of the eight priority levels. An outgoing
queue is created for each of the four traffic classes.
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Queue sizes
By default, each priority and traffic class queue size is set to use 100% of a port's memory. This is
the most efficient setting. This means that packets of any priority or traffic class can always be
queued up as long as there is any port memory available. But, this also means that memory could
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