User`s manual

TN-5516/5518 Series Featured Functions
3-39
Step 1: Assign a value to “Hello Time” and then calculate the left most part of Eq. 4 to get the lower limit of
Max. Age.
Step 2: Assign a value to “Forwarding Delay” and then calculate the right most part of Eq. 4 to get the upper
limit for Max. Age.
Step 3: Assign a value to Forwarding Delay that satisfies the conditions in Eq. 3 and Eq. 4.
Using Traffic Prioritization
The TN-5500’s traffic prioritization capability provides Quality of Service (QoS) to your network by making data
delivery more reliable. You can prioritize traffic on your network to ensure that high priority data is transmitted
with minimum delay. Traffic can be controlled by a set of rules to obtain the required Quality of Service for your
network. The rules define different types of traffic and specify how each type should be treated as it passes
through the switch. The TN-5500 can inspect both IEEE 802.1p/1Q layer 2 CoS tags, and even layer 3 TOS
information to provide consistent classification of the entire network. The TN-5500’s QoS capability improves
the performance and determinism of industrial networks for mission critical applications.
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 TN-5500 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.
The TN-5500 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 this 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.