GE T1000, Technical Manual

Page: 92 / 302

Chapter 4
– Functions
GE Reason Switches
92 REASON SWITCHES-TM-EN-3
Figure 52: CoS bits inside and 802.1Q frame
There is a general agreement that QoS term is referred to means that equipment guarantee quality at determined kind of traffic
in a given network. Thus, Quality of Service can be performed in different layers and different ways. Besides, the CoS term which
is one of the QoS mechanism, the one that uses the priority information inside the 802.1Q Ethernet frame. At this manual, the
term QoS is used as means that the network performs quality at determined service and CoS is used as a way to implement QoS
in a given LAN by inspecting 802.1Q frames.
As this is a 3-bit field, there is the possibility to use eight values of priority, from 0
to 7. 802.1Q standard shows how these bits should be used to ensure
prioritization to traffic, shown in the figure below.
One important point is that the value 1 is the lowest priority mark. Legacy
equipment, which does not support 802.1Q frames, is understood as priority
number 0. Priority 0, at the standard, is mapped as Best-effort quality, above
the number 1. This ensures that legacy equipment traffic would not be always
treated as background traffic when mixed to 802.1Q aware equipment. In
addition, even though priority 7 is the highest, it is not recommended to use
this prioritization in traffic that does not belong to network control or
management.
Figure 53: Traffic type acronyms, show in section l.4 on the IEEE 802.1Q
Besides, IEC 61850-90-4 Technical Report recommend mapping the CoS bits as
shown below. In addition, the document specifies the usage of priority High for
Sampled Values data and priority Medium or High for synchronization messages.
MMS messages, which are used by IED communication in power system and
other traffics over TCP/IP networks should use priority to Low or Medium,
according to the technical report.