Westermo U200 Operator manal - Manual (Page 17)

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6 Deterministic Ethernet - QoS
6.1 Principles of Deterministic Ethernet
Westermo OnTime switches can operate in full duplex mode. This ensures that an Ethernet
controller will never see any collisions occurring when operated in such a manner. The core
section of the Network; the redundant ring topology always runs full duplex and at 100Mbit/s;
this cannot be altered.
In addition a very fast switching core is provided to ensure that the switch can handle full wire
speed on each port. Finally, a large buffer is available to store packets destined for a busy
port. However, it is very unlikely that the buffers are used during normal network operation.
It should be noted that if buffers are used in such a network then it is not viable to state that a
network is Deterministic. In practice, the only time such buffers maybe used is in 10M / 100M,
half duplex devices. Where such devices are in use, a feature called Head of Line Blocking
Prevention is automatically implemented to ensure critical data is received at the destination
node, see 6.5 for details.
The switch contains two priority queues. A packet that is identified as a high priority packet is
put in the high priority queue. The switch alternates between the two queues by using strict
priority. I.e. packets from the low priority queue are only sent if the high priority queue is
empty. A packet is identified as a high priority packet based on priority tagging according to
IEEE 802.1p (layer 2 priority) or IP Type of Service (ToS -layer 3 priority).
6.2 Layer 2 priority
The IEEE 802.1p and IEEE802.1q standards specify an extra field for the Ethernet MAC
header. This field is called Tag Control Info (TCI) field, and is inserted between the source
MAC address and the MAC Type/Length field of an Ethernet packet, see figure below.
Figure 8, MAC header with tag
This field contains a 3 bit priority field that is used for priority handling. The switch will put a
tagged packet with the priority field set to four or higher in the high priority queue, while all
other packets will be put in the low priority queue.