ZyXEL Communications P-870HN-5xb User Manual page 214

Chapter 16 Quality of Service (QoS)
transmission rate requires a big bucket size. For example, use a bucket size of 10
kbytes to get the transmission rate up to 10 Mbps.
Single Rate Three Color Marker
The Single Rate Three Color Marker (srTCM, defined in RFC 2697) is a type of
traffic policing that identifies packets by comparing them to one user-defined rate,
the Committed Information Rate (CIR), and two burst sizes: the Committed Burst
Size (CBS) and Excess Burst Size (EBS).
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.
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P-870HN-5xb User's Guide