Queue Scheduling - 3Com Baseline 2928 PWR Plus User Manual

Figure 2-4 An Ethernet frame with an 802.1Q tag header
As shown in Figure
bytes in length), whose value is 0x8100, and the tag control information (TCI, two bytes in length).
Figure 2-5 presents the format of the 802.1Q tag header.
Figure 2-5 802.1Q tag header
Byte 1
TPID (Tag protocol identifier)
1 0 0 0 0 0 0 1 0 0 0 0 0 0 0
7 6 5 4 3 2 1 0 7
The priority in the 802.1Q tag header is called 802.1p precedence, because its use is defined in IEEE
802.1p. Table 2-3
Table 2-3 Description on 802.1p precedence
802.1p precedence (decimal)
0
1
2
3
4
5
6
7

Queue Scheduling

In general, congestion management adopts queuing technology. The system uses a certain queuing
algorithm for traffic classification, and then uses a certain precedence algorithm to send the traffic. Each
queuing algorithm is used to handle a particular network traffic problem and has significant impacts on
bandwidth resource assignment, delay, and jitter.
In this section, two common hardware queue scheduling algorithms Strict Priority (SP) queuing and
Weighted Round Robin (WRR) queuing are introduced.
2-4, the 4-byte 802.1Q tag header consists of the tag protocol identifier (TPID, two
Byte 2
0
6 5 4 3 2 1 0
presents the values for 802.1p precedence.
802.1p precedence (binary)
000
001
010
011
100
101
110
111
Byte 3
TCI (Tag control information)
CFI
Priority VLAN ID
7 6 5 4 3 2 1 0 7
2-6
Byte 4
6 5 4 3 2 1 0
Description
best-effort
background
spare
excellent-effort
controlled-load
video
voice
network-management