User manual
21
new priority classes and Generic Attribute Registration Protocol (GARP) was not published separately but was
incorporated into a major revision of the standard, IEEE 802.1D-1998. It also required a short amendment
extending the frame size of the Ethernet standard by four bytes which was published as IEEE 802.3ac in 1998.
The QoS technique developed by the working group, also known as class of service (CoS), is a 3-bit field
called the Priority Code Point (PCP) within an Ethernet frame header when using VLAN tagged frames as defined
by IEEE 802.1Q. It specifies a priority value of between 0 and 7 inclusive that can be used by QoS disciplines to
differentiate traffic. Although this technique is commonly referred to as
IEEE 802.1p
, there is no standard or
amendment by that name published by the IEEE. Rather the technique is incorporated into IEEE 802.1Q standard
which specifies the tag inserted into an Ethernet frame.
802.1P defined 8 priorities
（
0-7
）
, corresponding 4 priority (High, Medium, Normal, Low)
Default priority mark 0 and 1 is low queue, 2 and 3 is normal queue, 4 and 5 is Medium, 6 and 7 is High.
DiffServ architecture provides each transport packets in the network are classified into different categories,
classified information is contained in the IP packet header, DiffServ architecture using the first 6 bits of IP packet
header TOS( Type of Service) to carry the packets' classified information. This definition is only for the lower 6
bits, one number does not exceed 63. This definition supports both IPv4 (ToS field) and IPv6 (Traffic Class field).
DSCP has 64 priority values (0-63), the lowest priority 0 and the highest priority 63. In fact, the DSCP field is a
superset of the IP precedence field, DSCP field definition is backward-compatible with IP precedence field.
So far, the defined DSCP with default DSCP, the value is 0; class selector DSCP defined as the
backward-compatible with IP precedence, the value(8,16,24,32,40,48,56); Expedited Forwarding (EF), generally
used for low latency service, the recommended value is 46 (101110); identified by forwarding (AF) defines four
service levels, each service level has 3 down process, so spent 12 DSCP values ((10,12,14), (18,20,22), (26,28,30),
(34,36,38)).
The priority value of the device (1-16) is defined as the lowest priority, as the first queue. Priority value
(17-32) is defined as the second queue, the priority value (33-48) is defined as the third queue, the priority value
(49-64 is defined as the fastest queue, the highest priority.
