User`s manual

NPort S8000 Series Switch Featured Functions
6-22
The Traffic Prioritization Concept
What is Traffic Prioritization?
Traffic prioritization allows you to prioritize data so that time-sensitive and system-critical data can be
transferred smoothly and with minimal delay over a network. The benefits of using traffic prioritization are:
• Improve network performance by controlling a wide variety of traffic and managing congestion.
• Assign priorities to different categories of traffic. For example, set higher priorities for time-critical or
business-critical applications.
• Provide predictable throughput for multimedia applications, such as video conferencing or voice over IP,
and minimize traffic delay and jitter.
• Improve network performance as the amount of traffic grows. This will save cost by reducing the need to
keep adding bandwidth to the network.
How Traffic Prioritization Works
Traffic prioritization uses the four traffic queues that are present in your NPort S8000 to ensure that high
priority traffic is forwarded on a different queue from lower priority traffic. This is what provides Quality of
Service (QoS) to your network.
NPort S8000 traffic prioritization depends on two industry-standard methods:
• IEEE 802.1D—a layer 2 marking scheme.
• Differentiated Services (DiffServ)—a layer 3 marking scheme.
IEEE 802.1D Traffic Marking
The IEEE Std 802.1D, 1998 Edition marking scheme, which is an enhancement to IEEE Std 802.1D, enables
Quality of Service on the LAN. Traffic service levels are defined in the IEEE 802.1Q 4-byte tag, which is used to
carry VLAN identification as well as IEEE 802.1p priority information. The 4-byte tag immediately follows the
destination MAC address and Source MAC address.
The IEEE Std 802.1D, 1998 Edition priority marking scheme assigns an IEEE 802.1p priority level between 0
and 7 to each frame. This determines the level of service that that type of traffic should receive. Refer to the
table below for an example of how different traffic types can be mapped to the eight IEEE 802.1p priority levels.
IEEE 802.1p Priority Level IEEE 802.1D Traffic Type
0
Best Effort (default)
1 Background
2 Standard (spare)
3 Excellent Effort (business critical)
4 Controlled Load (streaming multimedia)
5 Video (interactive media); less than 100 milliseconds of latency and jitter
6 Voice (interactive voice); less than 10 milliseconds of latency and jitter
7 Network Control Reserved traffic
Even though the IEEE 802.1D standard is the most widely used prioritization scheme in the LAN environment,
it still has some restrictions:
• It requires an additional 4-byte tag in the frame, which is normally optional in Ethernet networks. Without
this tag, the scheme cannot work.
• The tag is part of the IEEE 802.1Q header, so to implement QoS at layer 2, the entire network must
implement IEEE 802.1Q VLAN tagging.
It is only supported on a LAN and not routed across WAN links, since the IEEE 802.1Q tags are removed when
the packets pass through a router.