User`s manual

TN-5516/5518 Series Featured Functions
3-40
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.
Traffic Prioritization
The TN-5500 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:
A packet received by the TN-5500 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.
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 TN-5500 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.