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Chapter 14 Quality of Service (QoS)
to handle the packets differently depending on the code points without the need to
negotiate paths or remember state information for every flow. In addition,
applications do not have to request a particular service or give advanced notice of
where the traffic is going.
14.7.2 IEEE 802.1Q Tag
The IEEE 802.1Q standard defines an explicit VLAN tag in the MAC header to
identify the VLAN membership of a frame across bridges. A VLAN tag includes the
12-bit VLAN ID and 3-bit user priority. The VLAN ID associates a frame with a
specific VLAN and provides the information that devices need to process the frame
across the network.
IEEE 802.1p specifies the user priority field and defines up to eight separate traffic
types. The following table describes the traffic types defined in the IEEE 802.1d
standard (which incorporates the 802.1p).
Table 78 IEEE 802.1p Priority Level and Traffic Type
PRIORITY
LEVEL
Level 7
Level 6
Level 5
Level 4
Level 3
Level 2
Level 1
Level 0
14.7.3 Token Bucket
The token bucket algorithm uses tokens in a bucket to control when traffic can be
transmitted. The bucket stores tokens, each of which represents one byte. The
algorithm allows bursts of up to b bytes which is also the bucket size, so the
bucket can hold up to b tokens. Tokens are generated and added into the bucket
at a constant rate. The following shows how tokens work with packets:
• A packet can be transmitted if the number of tokens in the bucket is equal to or
greater than the size of the packet (in bytes).
174
TRAFFIC TYPE
Typically used for network control traffic such as router configuration
messages.
Typically used for voice traffic that is especially sensitive to jitter (jitter is the
variations in delay).
Typically used for video that consumes high bandwidth and is sensitive to
jitter.
Typically used for controlled load, latency-sensitive traffic such as SNA
(Systems Network Architecture) transactions.
Typically used for "excellent effort" or better than best effort and would
include important business traffic that can tolerate some delay.
This is for "spare bandwidth".
This is typically used for non-critical "background" traffic such as bulk
transfers that are allowed but that should not affect other applications and
users.
Typically used for best-effort traffic.
P-794H User's Guide
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