Specifications

Appendix C Operating Environment Installation and Operation Manual
C-16 Ethernet Transmission Technology FCD-E1LC Ver. 1.0
0 4 8 12 16 20 24 28 31
Destination MAC Address (DA)
Destination MAC Address (DA) Source MAC Address (SA)
Source MAC Address (SA)
Tag Protocol Identifier (8100) PRI 0 VLAN ID
EtherType (IP = 0800) Ethernet Payload (42 to 1500 Bytes)
Ethernet Payload
.
.
.
CRC
Figure C-5. Structure of Ethernet Frame with VLAN Support
Ethernet frames with VLAN support include a tag header immediately after the
source MAC address (therefore, such frames are also referred to as
tagged
frames
).
The tag header comprises 4 bytes:
• Two bytes for the tag protocol identifier. For Ethernet-encoded tags in
accordance with IEEE802.1Q, these bytes carry the equivalent of 8100.
• Priority (PRI) specified by the user (3 bits: 7 is the highest priority and 0 is
the lowest priority). The priority field enables the user to instruct the network
to handle high-priority frames preferentially, for example, to reduce the delay
or minimize the chances for loss of frames in case of congestion.
The ToS field included in the IP packet headers (see Figure A-2) can also be used
to ensure preferential handling of certain types of packets.
• One bit for the canonical format indicator (always 0 as shown in Figure A-5 ).
• VLAN ID (12 bits), used to indicate the VLAN to which the frame belongs.
Transporting IP Traffic over Ethernet
Encapsulation in Ethernet Frames
IP traffic is carried in the LLC data field of the Ethernet frame (see Figure A-4 ).
This is called
encapsulation . The EtherType value for the IP protocol is 0800.
Whenever possible, the whole IP packet (including the header) is inserted in one
Ethernet frame. However, IP packets can be much longer than the LLC data field
of Ethernet frames: in this case, it is necessary to fragment the IP packets in
accordance with the desired size of data field, and transfer each fragment in a
separate frame. The receiving IP host then reassembles the original packet from
its fragments.
Note