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Priority Mapping
Packets of different types have different priorities. For example, the 802.11 packets sent by STAs carry user priorities, VLAN packets on the wired networks carry 802.1p priorities. Priority mapping must be configured on network devices to retain priorities of packets when the packets traverse different networks.
As shown in Figure 5-4:
- After receiving the upstream 802.11 frames from the STA, the AP maps the user priorities of 802.11 packets to the 802.1p priorities of 802.3 packets.
- After receiving the downstream 802.3 frames, the AP maps the 802.1p priorities of 802.3 packets to the user priority of 802.11 packets.
802.1p Field
Layer 2 devices exchange ethernet frames. As defined in IEEE 802.1Q, the PRI field (802.1p field) in the ethernet frame header identifies the Class of Service (CoS) requirement. Figure 5-5 shows the PRI field in ethernet frames.
The 802.1Q header contains a 3-bit PRI field, representing eight service priorities 7, 6, 5, 4, 3, 2, 1 and 0 in descending order of priority.