ZyXEL Communications P-660HN-F1A, User Manual

Page: 265 / 430

Chapter 16 Quality of Service (QoS)
P-660HN-F1A User’s Guide
265
16.7 Token Bucket Example
This is an example of how the token bucket works.
The algorithm works as follows (see also Figure 115 on page 265 ):
A: Assume that there are 2000 tokens in the bucket at the first moment (T = 0).
B: One hundred tokens are added to the bucket after one second. A packet of
1500 bytes arrives and the P-660HN-F1A transmits it directly as there are already
enough tokens in the bucket to cover the size of the packet. The P-660HN-F1A
then deducts 1500 tokens from the bucket leaving 600 tokens in the bucket
(2100-1500).
C: One hundred more tokens are added in the bucket after one second. A packet
of 500 bytes arrives and the P-660HN-F1A again transmits it directly and then
deducts 500 tokens from the bucket leaving just 200 tokens (700-500).
D: After one more second, one hundred more tokens are added to the bucket. A
packet of 1000 bytes flows in. The P-660HN-F1A holds the packet since the
number of tokens are insufficient.
E: After enough tokens (1000) are in the bucket, the P-660HN-F1A transmits it
and then deducts 1000 tokens from the bucket.
Figure 115 Token Bucket Scenario Example
Table 77 Example Data
b=125,000 bytes
(around 1 Megabit) This is the size of the bucket. The bucket holds up to 125,000
tokens.
t=100 This means a token is generated every 0.01 (=1/t) seconds. The
maximum instantaneous transmission rate for outgoing traffic is
(b’x8)/t Mbps where b’ is the number of tokens in the bucket at
that instant.
Time (sec.)
2000
1200
400 Av
ai
la
b
le
To
ke
n
s
0
2000
A
B
2100-1500
=600
700-500
C
=200
300
400
D
1000-1000
E
=0