ZyXEL Communications VWG1312-B10A, User Manual

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Chapter 10 Quality of Service (QoS)
VMG1312-B10A User’s Guide
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The srTCM evaluates incoming packets and marks them with one of three colors which refer to
packet loss priority levels. High packet loss priority level is referred to as red, medium is referred to
as yellow and low is referred to as green.
The srTCM is based on the token bucket filter and has two token buckets (CBS and EBS). Tokens
are generated and added into the bucket at a constant rate, called Committed Information Rate
(CIR). When the first bucket (CBS) is full, new tokens overflow into the second bucket (EBS).
All packets are evaluated against the CBS. If a packet does not exceed the CBS it is marked green.
Otherwise it is evaluated against the EBS. If it is below the EBS then it is marked yellow. If it
exceeds the EBS then it is marked red.
The following shows how tokens work with incoming packets in srTCM:
• A packet arrives. The packet is marked green and can be transmitted if the number of tokens in
the CBS bucket is equal to or greater than the size of the packet (in bytes).
• After a packet is transmitted, a number of tokens corresponding to the packet size is removed
from the CBS bucket.
• If there are not enough tokens in the CBS bucket, the Device checks the EBS bucket. The packet
is marked yellow if there are sufficient tokens in the EBS bucket. Otherwise, the packet is marked
red. No tokens are removed if the packet is dropped.
Two Rate Three Color Marker
The Two Rate Three Color Marker (trTCM, defined in RFC 2698) is a type of traffic policing that
identifies packets by comparing them to two user-defined rates: the Committed Information Rate
(CIR) and the Peak Information Rate (PIR). The CIR specifies the average rate at which packets are
admitted to the network. The PIR is greater than or equal to the CIR. CIR and PIR values are based
on the guaranteed and maximum bandwidth respectively as negotiated between a service provider
and client.
The trTCM evaluates incoming packets and marks them with one of three colors which refer to
packet loss priority levels. High packet loss priority level is referred to as red, medium is referred to
as yellow and low is referred to as green.
The trTCM is based on the token bucket filter and has two token buckets (Committed Burst Size
(CBS) and Peak Burst Size (PBS)). Tokens are generated and added into the two buckets at the CIR
and PIR respectively.
All packets are evaluated against the PIR. If a packet exceeds the PIR it is marked red. Otherwise it
is evaluated against the CIR. If it exceeds the CIR then it is marked yellow. Finally, if it is below the
CIR then it is marked green.
The following shows how tokens work with incoming packets in trTCM:
• A packet arrives. If the number of tokens in the PBS bucket is less than the size of the packet (in
bytes), the packet is marked red and may be dropped regardless of the CBS bucket. No tokens
are removed if the packet is dropped.
• If the PBS bucket has enough tokens, the Device checks the CBS bucket. The packet is marked
green and can be transmitted if the number of tokens in the CBS bucket is equal to or greater
than the size of the packet (in bytes). Otherwise, the packet is marked yellow.