ZyXEL Communications VSG1432-B101 Series User Manual

Chapter 10 Quality of Service (QoS)

VSG1432-B101 Series User’s Guide

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 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.

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.