Token Bucket - ZyXEL Communications SBG3500-N000 User Manual

Table 52 Internal Layer2 and Layer3 QoS Mapping
LAYER 2
PRIORITY IEEE 802.1P USER
QUEUE PRIORITY
(ETHERNET
PRIORITY)
5 5
6 6
7 7

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).
• After a packet is transmitted, a number of tokens corresponding to the packet size is removed
from the bucket.
• If there are no tokens in the bucket, the SBG3500-N Series stops transmitting until enough
tokens are generated.
• If not enough tokens are available, the SBG3500-N Series treats the packet in either one of the
following ways:
In traffic shaping:
• Holds it in the queue until enough tokens are available in the bucket.
In traffic policing:
• Drops it.
• Transmits it but adds a DSCP mark. The SBG3500-N Series may drop these marked packets if
the network is overloaded.
Configure the bucket size to be equal to or less than the amount of the bandwidth that the interface
can support. It does not help if you set it to a bucket size over the interface's capability. The smaller
the bucket size, the lower the data transmission rate and that may cause outgoing packets to be
dropped. A larger 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.
Chapter 10 Quality of Service (QoS)
LAYER 3
TOS (IP
PRECEDENCE)
3
4
5
6
7
SBG3500-N Series User's Guide
200
IP PACKET
DSCP
LENGTH (BYTE)
011110 <250
011100
011010
011000
100110
100100
100010
100000
101110
101000
110000
111000