EDS-510A Series User’s Manual
Featured Functions
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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.
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 as 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.
Traffic Prioritization
EDS-510A classifies traffic based on layer 2 of the OSI 7 layer model, and the switch prioritizes
received traffic according to the priority information defined in the received packet. Incoming
traffic is classified based upon the IEEE 802.1D frame and is assigned to the appropriate priority
queue based on the IEEE 802.1p service level value defined in that packet. Service level markings
(values) are defined in the IEEE 802.1Q 4-byte tag, and consequently traffic will only contain
802.1p priority markings if the network is configured with VLANs and VLAN tagging. The traffic
flow through the switch is as follows:
1.
A packet received by the EDS-510A may or may not have an 802.1p tag associated with it. If
it does not, then it is given a default 802.1p tag (which is usually 0). Alternatively, the packet
may be marked with a new 802.1p value, which will result in all knowledge of the old 802.1p
tag being lost.
2.
As the 802.1p priority levels are fixed to the traffic queues, the packet will be placed in the
appropriate priority queue, ready for transmission through the appropriate egress port. When
the packet reaches the head of its queue and is about to be transmitted, the device determines
whether or not the egress port is tagged for that VLAN. If it is, then the new 802.1p tag is
used in the extended 802.1D header.
The EDS-510A will check a packet received at the ingress port for IEEE 802.1D traffic
classification, and then prioritize it based upon the IEEE 802.1p value (service levels) in that tag.
It is this 802.1p value that determines to which traffic queue the packet is mapped.
