User`s manual

EDS-828 Series Featured Functions
3-37
How Traffic Prioritization Works
Traffic prioritization uses the four traffic queues that are present in your EDS-828 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 EDS-828’s 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 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.
Advantages of DiffServ over IEEE 802.1D are:
• Configure how you want your switch to treat selected applications and types of traffic by assigning various
grades of network service to them.
• No extra tags are required in the packet.
• DSCP uses the IP header of a packet and therefore priority is preserved across the Internet.
• DSCP is backward compatible with IPV4 TOS, which allows operation with existing devices that use a layer
3 TOS enabled prioritization scheme.