Siemens SCALANCE XM-400 Configuration Manual page 39

The additional 4 bytes are located in the header of the Ethernet frame between the source
address and the Ethernet type / length field:
Figure 4-1 Structure of the expanded Ethernet frame
The additional bytes contain the tag protocol identifier (TPID) and the tag control information
(TCI).
Tag protocol identifier (TPID)
The first 2 bytes form the Tag Protocol Identifier (TPID) and always have the value 0x8100.
This value specifies that the data packet contains VLAN information or priority information.
Tag Control Information (TCI)
The 2 bytes of the Tag Control Information (TCI) contain the following information:
QoS Trust
The tagged frame has 3 bits for the priority that is also known as Class of Service (CoS), see
also IEEE 802.1Q.
CoS bits
000
001
010
011
100
101
110
111
The prioritization of the data packets is possible only if there is a queue in the components in
which they can buffer data packets with lower priority.
SCALANCE XM-400/XR-500 Web Based Management (WBM)
Configuration Manual, 05/2017, C79000-G8976-C248-12
Priority
0 (lowest) Background
1 Best Effort
2 Excellent Effort
3 Critical Applications
4 Video, < 100 ms delay (latency and jitter)
5 Voice (language), < 10 ms delay (latency and jitter)
6 Internetwork Control
7 (highest) Network Control
Technical basics
Type of the data traffic
4.4 VLAN
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