Siemens SCALANCE XB-200 - Page 32

Technical basics
4.6 VLAN tagging
SCALANCE XB-200 Web Based Management
32 Configuration Manual, 04/2015, C79000-G8976-C360-02
The additional 4 bytes are located in the header of the Ethernet frame between the source
address and the Ethernet type / length field:
Figure 4-3 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:
CoS prioritization
The tagged frame has 3 bits for the priority that is also known as Class of Service (CoS). The
priority according to IEEE 802.1p is as follows:
CoS bits
Type of data
000
Non time-critical data traffic (less then best effort [basic setting])
001
Normal data traffic (best effort [background])
010
Reserved (standard)
011
Reserved ( excellent effort )
100
Data transfer with max. 100 ms delay
101
Guaranteed service, interactive multimedia
110
Guaranteed service, interactive voice transmission
111 Reserved
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.