ZyXEL Communications VMG1312-B Series, Руководство пользователя

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Chapter 10 Quality of Service (QoS)
VMG1312-B Series User’s Guide
180
10.8 Technical Reference
The following sect ion cont ains addit ional t echnical inform at ion about t he Device feat ures described
in t his chapt er.
IEEE 802.1Q Tag
The I EEE 802.1Q st andard defines an explicit VLAN t ag in t he MAC header t o ident ify t he VLAN
m em bership of a fram e across bridges. A VLAN t ag includes t he 12- bit VLAN I D and 3- bit user
priorit y. The VLAN I D associat es a fram e wit h a specific VLAN and provides t he inform at ion t hat
devices need t o process t he fram e across t he net work.
I EEE 802.1p specifies t he user priorit y field and defines up t o eight separat e t raffic t ypes. The
following t able describes t he t raffic t ypes defined in t he I EEE 802.1d st andard ( which incorporat es
t he 802.1p) .
DiffServ
QoS is used t o priorit ize source- t o- dest inat ion t raffic flows. All packet s in t he flow are given t he
sam e priorit y. You can use CoS ( class of service) t o give different priorit ies t o different packet
t ypes.
DiffServ ( Different iat ed Services) is a class of service ( CoS) m odel t hat m arks packet s so t hat t hey
receive specific per- hop t reat m ent at DiffServ- com pliant net work devices along t he rout e based on
t he applicat ion t ypes and t raffic flow. Packet s are m arked wit h DiffServ Code Point s ( DSCPs)
indicat ing t he level of service desired. This allows t he int erm ediary DiffServ- com pliant net work
devices t o handle t he packet s different ly depending on t he code point s wit hout t he need t o
negot iat e pat hs or rem em ber st at e inform at ion for every flow. I n addit ion, applicat ions do not have
t o request a part icular service or give advanced not ice of where t he t raffic is going.
DSCP and Per-Hop Behavior
DiffServ defines a new Different iat ed Services ( DS) field t o replace t he Type of Service ( TOS) field
in t he I P header. The DS field cont ains a 2- bit unused field and a 6- bit DSCP field which can define
up t o 64 service levels. The following figure illust rat es t he DS field.
Table 49 I EEE 802.1p Priorit y Level and Traffic Type
PRIORITY
LEVEL
TRAFFIC TYPE
Level 7 Typically used for net w or k cont r ol t raffic such as rout er configurat ion m essages.
Level 6 Typically used for voice t raffic t hat is especially sensit ive t o j it t er ( j it t er is t he
variat ions in delay) .
Level 5 Typically used for v ideo t hat consum es high bandw idt h and is sensit ive t o j it t er.
Level 4 Typically used for cont r olled load, lat ency- sensit ive t raffic such as SNA ( Sy st em s
Net work Archit ect ure) t ransact ions.
Level 3 Typically used for “ excellent effort ” or bet t er t han best effort and w ould include
im port ant business t raffic t hat can t olerat e som e delay.
Level 2 This is for “ spare bandw idt h”.
Level 1 This is t ypically used for non- crit ical “ background” t raffic such as bulk t ransfers t hat
are allow ed but t hat should not affect ot her applicat ions and user s.
Level 0 Typically used for best - effort t raffic.