3.10 Quality of service - traffic shaping
Information
You can prioritize the order in which tasks are handled according to the following
methods:
• Based on the VLAN tag "COS" (Ethernet)
• Based on the VLAN tag "DSCP" (IP)
If both tags occur simultaneously in one telegram, the SCALANCE device will
decide which one has the higher priority.
PROFINET devices send time-critical data with the following VLAN tag:
• VLAN ID: 0
• Priority Code 6.
Pure PROFINET data traffic has no IP header and therefore no DSCP information.
Non-VLAN-capable switches such as the SCALANCE X-100 or X-200 forward data
using the "COS" priority in the VLAN tag.
VLAN-capable switches such as the XB/XC/XP-200, X-300, XM-400 and X-500
forward data traffic according to their configuration settings. In the default setting,
all ports are "untagged members" of "default VLAN 1".
This untagged setting causes the VLAN tag to be lost once the first switch has
forwarded it. The "COS" priority information is thereby removed as well.
COS and PROFINET
If the PROFINET data traffic passes the (VLAN-capable) device and advanced
VLAN isolation is not necessary, then use the settings below. These settings will
cause the VLAN tag to be retained:
• With an X-300, enable "VLAN 0 Aware mode" in the "Switch > Configuration"
menu.
• With MSPS, go to the menu "Layer 2 > VLAN" and set Bridged Mode to
"802.1D Transparent Bridge".
For an XM-400 or X-500, tick the "Transparent" box for VLAN1.
Recommendation for X-200 and X-300
The SCALANCE X-300/400 support "CoS to Queue" and "DSCP to Queue
Mapping", which enable forwarding of packets with different origins and different
priority levels. To allow backwards compatibility with earlier firmware versions,
DSCP Mapping is disabled by default.
You can find the QoS settings in the following menu: "Switch > QoS > QoS
Configuration".
Leave the default DSCP Mapping setting.
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