ZyXEL Communications VMG1312-B Series User Manual page 182

Chapter 10 Quality of Service (QoS)
Table 50 I nt ernal Layer2 and Layer3 QoS Mapping
PRIORITY
QUEUE
6
7
Token Bucket
The t oken bucket algorit hm uses t okens in a bucket t o cont rol when t raffic can be t ransm it t ed. The
bucket st ores t okens, each of which represent s one byt e. The algorit hm allows burst s of up t o b
byt es which is also t he bucket size, so t he bucket can hold up t o b t okens. Tokens are generat ed
and added int o t he bucket at a const ant rat e. The following shows how t okens work wit h packet s:
• A packet can be t ransm it t ed if t he num ber of t okens in t he bucket is equal t o or great er t han t he
size of t he packet ( in byt es) .
• Aft er a packet is t ransm it t ed, a num ber of t okens corresponding t o t he packet size is rem oved
from t he bucket .
• I f t here are no t okens in t he bucket , t he Device st ops t ransm it t ing unt il enough t okens are
generat ed.
• I f not enough t okens are available, t he Device t reat s t he packet in eit her one of t he following
ways:
I n t raffic shaping:
• Holds it in t he queue unt il enough t okens are available in t he bucket .
I n t raffic policing:
• Drops it .
• Transm it s it but adds a DSCP m ark. The Device m ay drop t hese m arked packet s if t he net work
is overloaded.
Configure t he bucket size t o be equal t o or less t han t he am ount of t he bandwidt h t hat t he int erface
can support . I t does not help if you set it t o a bucket size over t he int erface's capabilit y. The sm aller
t he bucket size, t he lower t he dat a t ransm ission rat e and t hat m ay cause out going packet s t o be
dropped. A larger t ransm ission rat e requires a big bucket size. For exam ple, use a bucket size of 10
kbyt es t o get t he t ransm ission rat e up t o 10 Mbps.
Single Rate Three Color Marker
The Single Rat e Three Color Marker ( srTCM, defined in RFC 2697) is a t ype of t raffic policing t hat
ident ifies packet s by com paring t hem t o one user- defined rat e, t he Com m it t ed I nform at ion Rat e
( CI R) , and t wo burst sizes: t he Com m it t ed Burst Size ( CBS) and Excess Burst Size ( EBS) .
182
LAYER 2 LAYER 3
IEEE 802.1P USER
PRIORITY TOS (IP
(ETHERNET PRECEDENCE)
PRIORITY)
6 4
5
7 6
7
IP PACKET
DSCP
LENGTH (BYTE)
100110
100100
100010
100000
101110
101000
110000
111000
VMG1312-B Series User's Guide