IE-SW-VL08M Series User’s Manual
Featured Functions
3-42
y
IEEE 802.1D
—a layer 2 marking scheme.
y
Differentiated Services (DiffServ)
—a layer 3 marking scheme.
IEEE 802.1D Traffic Marking
The IEEE Std 802.1D, 1998 Edition marking scheme, which is an enhancement to IEEE Std 802.1D,
enables Quality of Service on the LAN. Traffic service levels are defined in the IEEE 802.1Q 4-byte
tag, which is used to carry VLAN identification as well as IEEE 802.1p priority information. The
4-byte tag immediately follows the destination MAC address and Source MAC address.
The IEEE Std 802.1D, 1998 Edition priority marking scheme assigns an IEEE 802.1p priority level
between 0 and 7 to each frame. This determines the level of service that type of traffic should receive.
Refer to the table below
for an example of how different traffic types can be mapped to the eight
IEEE 802.1p priority levels.
IEEE 802.1p Priority Level
IEEE 802.1D Traffic Type
0 Best
Effort
(default)
1
Background
2
Standard (spare)
3
Excellent Effort (business critical)
4
Controlled Load (streaming multimedia)
5
Video (interactive media); less than 100 milliseconds of
latency and jitter
6
Voice (interactive voice); less than 10 milliseconds of
latency and jitter
7
Network Control Reserved traffic
Even though the IEEE 802.1D standard is the most widely used prioritization scheme in the LAN
environment, it still has some restrictions:
y
It requires an additional 4-byte tag in the frame, which is normally optional in Ethernet
networks. Without this tag, the scheme cannot work.
y
The tag is part of the IEEE 802.1Q header, so to implement QoS at layer 2, the entire network
must implement IEEE 802.1Q VLAN tagging.
It is only supported on a LAN and not across routed WAN links, since the IEEE 802.1Q tags are
removed when the packets pass through a router.
Differentiated Services (DiffServ) Traffic Marking
DiffServ is a Layer 3 marking scheme that uses the DiffServ Code Point (DSCP) field in the IP
header to store the packet priority information. DSCP is an advanced intelligent method of traffic
marking because you can choose how your network prioritizes different types of traffic. DSCP uses
64 values that map to user-defined service levels, allowing you to establish more control over
network traffic.
Advantages of DiffServ over IEEE 802.1D are:
y
Configure how you want your switch to treat selected applications and types of traffic by
assigning various grades of network service to them.
y
No extra tags are required in the packet.
y
DSCP uses the IP header of a packet and therefore priority is preserved across the Internet.
