User Documentation

User Manual Managed Switches

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

that allows you to 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.

The advantages of DiffServ over IEEE 802.1D are:

 You can 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 to preserve priority 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

Weidmüller managed Switches classify 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 switch 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.

 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 Weidmüller Switch 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 to.

Traffic Queues

The hardware of Weidmüller switches has multiple traffic queues that allow packet prioritization to

occur. Higher priority traffic can pass through the Weidmüller switch without being delayed by lower

priority traffic. As each packet arrives in the Weidmüller switch, 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 Weidmüller switches support two different queuing mechanisms:

 Weight Fair: This method services all the traffic queues, giving priority to the higher priority

queues. Under most circumstances, the Weight Fair method gives high priority precedence over

low priority, but in the event that high priority traffic does not reach the link capacity, lower priority

traffic is not blocked.