NEC UNIVERGE SV9100 Manual page 14

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addresses, and is usually handled by Routers. However, sometimes it is necessary to implement
Layer 2 QoS – usually in large LAN environments with many IP phones.
Layer 2 devices work with Ethernet frames (encapsulated IP packets) rather than IP addresses.
These devices are usually Switched Hubs (Switches). As the IP header information is encapsulated,
the Switched Hubs are not able to reference the Type of Service (see: Layer 3 QoS) field in the IP
header to determine the priority of a frame
Layer 2 QoS uses the Priority field of the Ethernet frame. This field is three bits long and can have 8
possible values (000 to 111 in binary). Some switches can be configured to prioritise traffic based on
these values.
This field is only available if the Ethernet device is configured for VLAN (IEE802.1q) operation (VLAN
is outside the scope of this document).
Protocol Structure - IEEE 802.1p: LAN Layer 2 QoS
The diagram below shows the format of an Ethernet frame, and the User Priority field that is used for
Layer 2 QoS.
7 1
bytes byte
Preamble SFD
3bits
User Priority
Preamble (PRE) - The PRE is an alternating pattern of ones and zeros that tells receiving stations
that a frame is coming, and that provides a means to synchronize the 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.
TPID - defined value of 8100 in hex. When a frame has the EtherType equal to 8100, this frame
carries the tag IEEE 802.1Q / 802.1P.
TCI - Tag Control Information 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 3 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.
IEEE 802.1Q Tagged Frame for Ethernet:
6 6
bytes bytes
DA SA TPID
Expanded view of TCI Field
1bit
CFI Bits of VLAN ID (VID) to identify possible VLANs
2 2
bytes bytes bytes
Type
TCI
Length
12bits
42 - 1496
2 4
bytes bytes
Data CRC