TN-5516/5518 User’s Manual Featured Functions
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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:
y IEEE 802.1D—a layer 2 marking scheme.
y 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.
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:
y It requires an additional 4-byte tag in the frame, which is normally optional in Ethernet
networks. Without this tag, the scheme cannot work.
y 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 across routed WAN links, since the IEEE 802.1Q tags are
removed when the packets pass through a router.
Differentiated Services (DiffServ) Traffic Marking
DiffServ is a Layer 3 marking scheme that uses the DiffServ Code Point (DSCP) field in the IP
header to store the packet priority information. DSCP is an advanced intelligent method of traffic
marking because you can choose how your network prioritizes different types of traffic. DSCP
uses 64 values that map to user-defined service levels, allowing you to establish more control over
network traffic.
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