PT-7728 User’s Manual
Featured Functions
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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:
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Configure how you want your switch to treat selected applications and types of traffic by
assigning various grades of network service to them.
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No extra tags are required in the packet.
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DSCP uses the IP header of a packet and therefore priority is preserved across the Internet.
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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 PT-7728 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:
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A packet received by the PT-7728 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.
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Because 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 PT-7728 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 which traffic queue the packet is mapped to.
Traffic Queues
The PT-7728 hardware has multiple traffic queues that allow packet prioritization to occur. Higher
priority traffic can pass through the PT-7728 without being delayed by lower priority traffic. As
each packet arrives in the PT-7728, 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 PT-7728 supports two different queuing mechanisms:
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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 except the link capacity, lower priority
traffic is not blocked.
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Strict: This method services high traffic queues first; low priority queues are delayed until no
more high priority data nePT to be sent. This method always gives precedence to high priority
over low-priority.
