User`s manual

NPort S8000 Series Switch Featured Functions
6-23
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
The NPort S8000 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 NPort S8000 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 NPort S8000 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.
Traffic Queues
The NPort S8000 hardware has multiple traffic queues that allow packet prioritization to occur. Higher priority
traffic can pass through the NPort S8000 without being delayed by lower priority traffic. As each packet arrives
in the NPort S8000, it passes through any ingress processing (which includes classification,
marking/re-marking), and is then sorted into the appropriate queue. The switch then forwards packets from
each queue.
The NPort S8000 supports two different queuing mechanisms:
• Weight Fair: This method services all the traffic queues, giving priority to the higher priority queues.
Under most circumstances, this method gives high priority precedence over low-priority, but in the event
that high-priority traffic exceeds the link capacity, lower priority traffic is not blocked.
• Strict: This method services high traffic queues first; low priority queues are delayed until no more high
priority data needs to be sent. This method always gives precedence to high priority over low-priority.
Configuring Ethernet Traffic Prioritization
Quality of Service (QoS) provides a traffic prioritization capability to ensure that important data is delivered
consistently and predictably. The NPort S8000 can inspect IEEE 802.1p/1Q layer 2 CoS tags, and even layer 3
TOS information, to provide a consistent classification of the entire network. The NPort S8000’s QoS capability
improves your industrial network’s performance and determinism for mission critical applications.