NEC SL1100 Issue 6.0Networking Manual 6 - 17Protocol Structure - IEEE 802.1p: LAN Layer 2 QoSFigure 6-6 Protocol Structure for Layer 2 QoS illustrates the format of anEthernet frame and the User Priority field that is used for Layer 2 QoS.The following define the fields used for the protocol structure:Preamble (PRE) - The PRE is an alternating pattern of ones and zeros thattells receiving stations a frame is coming, and synchronizes frame-receptionportions of receiving physical layers with the incoming bit stream.Start-of-frame delimiter (SFD) - The SOF is an alternating pattern of onesand zeros, ending with two consecutive 1-bits indicating that the next bit is theleft-most bit in the left-most byte of the destination address.Destination Address (DA) - The DA field identifies which station(s) shouldreceive the frame.Source Addresses (SA) - The SA field identifies the sending station.Tag Protocol Identifier (TPID) - The defined value of SL1100 in hex. When aframe has the EtherType equal to SL1100, this frame carries the tag IEEE802.1Q / 802.1P.Tag Control Information (TCI) - The field including user priority, Canonicalformat indicator and VLAN ID.Figure 6-6 Protocol Structure for Layer 2 QoSIEEE 802.1Q Tagged Frame for Ethernet7Bytes1Byte6Bytes6Bytes2Bytes2Bytes2Bytes42~1496Bytes4BytesPreamble SFD DA SA TPID TCI TypeLengthData CRCExpanded View of TCI Field3 Bits 1 Bit 12 BitsUser Priority CFI Bits of VLAN ID (VID) to identify possible VLANs
