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• 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 Zyxel 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 Zyxel 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.
Chapter 10 Quality of Service (QoS)
VMG/XMG Series User's Guide
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