A Service Frame in a Carrier Ethernet Network (CEN) is an Ethernet frame exchanged across UNIs from and to the service provider.
These could be either untagged or IEEE802.1Q Ethernet frame as shown below.
Total size of an Untagged Ethernet Frame varies between 64B and 1518B whereas that of an IEEE802.1Q Ethernet Frame varies between 68B and 1522B with an extra 4B of Customer Tag compared to Untagged Frame.
> C-Tag field represents the Customer Tag.
> TPID is the Tag Protocol Identifier, which is 0x8100 for C-Tagged Frame.
> PCP is the Priority Code Points, which is total 3 bits with different priority values (0 through 7).
> CFI is the Canonical Format Indicator, which represents the Little Endian Bit ordering of Ethernet Frame.
> VLAN ID is the Virtual LAN Identifier with 0 through 4095 values. VLAN ID 0 represents Priority Tagging and VLAN ID 4095 reserved. So possible values of VLAN IDs are from 1 through 4094.