Table 6-1 Protocol Structure For Layer 2 Qos - NEC Sl2100 Networking Manual

SL2100 ISSUE 1.0
Layer 2 devices work with Ethernet frames (encapsulated IP packets) rather than IP addresses. Layer
2 QoS uses the Priority field of the Ethernet frame. This field has three bits and can have eight
possible values (000 to 111 in binary). Some switches can be configured to prioritize traffic based on
these values. This field is available only if the Ethernet device is configured for VLAN (IEEE 802.1q)
operation (VLAN is outside the scope of this document).
Protocol Structure - IEEE 802.1p: LAN Layer 2 QoS
Table 6-1 Protocol Structure for Layer 2 QoS on page 6-12 shows the format of an Ethernet frame
and the User Priority field that is used for Layer 2 QoS.

Table 6-1 Protocol Structure for Layer 2 QoS

IEEE 802.1Q Tagged Frame for Ethernet
7 Bytes 1 Byte 6 Bytes 6 Bytes 2 Bytes 2 Bytes 2 Bytes 42~1496 4 Bytes
Bytes
Preamble SFD DA SA TPID TCI Type Length Data CRC
Expanded View of TCI Field
3 Bits 1 Bit 12 Bits
User Priority CFI Bits of VLAN ID (VID) to identify possible VLANs
The following define the fields used for the protocol structure:
Preamble (PRE) - The PRE is an alternating pattern of ones and zeros that tells receiving stations a
frame is coming, and synchronizes frame-reception portions of receiving physical layers with the
incoming bit stream.
Start-of-frame delimiter (SFD) - The SOF is an alternating pattern of ones and zeros, ending with two
consecutive 1-bits indicating that the next bit is the left-most bit in the left-most byte of the destination
address.
Destination Address (DA) - The DA field identifies which station(s) should receive the frame.
Source Addresses (SA) - The SA field identifies the sending station.
Tag Protocol Identifier (TPID) - The defined value of SL2100 in hex. When a frame has the
EtherType equal to SL2100, this frame carries the tag IEEE 802.1Q / 802.1P.
Tag Control Information (TCI) - The field including user priority, Canonical format indicator and VLAN
ID.
User Priority - Defines user priority, giving eight priority levels. IEEE 802.1P defines the operation for
these three user priority bits.
CFI - Canonical Format Indicator is always set to zero for Ethernet switches. CFI is used for
compatibility reason between Ethernet type network and Token Ring type network.
VID - VLAN ID is the identification of the VLAN, which is basically used by the standard 802.1Q. It
allows the identification of 4096 VLANs.
Length/Type - This field indicates either the number of MAC-client data bytes that are contained in the
data field of the frame, or the frame type ID if the frame is assembled using an optional format.
Data - Is a sequence of bytes of any value. The total frame minimum is 64 bytes.
Frame Check Sequence (FCS) - This sequence contains a 32-bit cyclic redundancy check (CRC)
value, which is created by the sending MAC and is recalculated by the receiving MAC to check for
damaged frames.
6-12 Network Design Considerations