Chapter 10 Quality of Service (QoS)
VMG1312-B Series User’s Guide
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10.2 What You Need to Know
The following t erm s and concept s m ay help as you read through this chapt er.
QoS versus Cos
QoS is used to priorit ize source- t o- destination t raffic flow s. All packets in t he sam e flow are given
the sam e priority. CoS ( class of service) is a way of m anaging t raffic in a network by grouping
sim ilar types of t raffic together and treating each t ype as a class. You can use CoS to give different
priorit ies t o different packet types.
CoS t echnologies include I EEE 802.1p layer 2 t agging and DiffServ ( Differentiat ed Services or DS) .
I EEE 802.1p tagging m akes use of t hr ee bit s in t he packet header, while DiffServ is a new protocol
and defines a new DS field, which replaces t he eight - bit ToS ( Type of Service) field in t he I P header.
Tagging and Marking
I n a QoS class, you can configure whet her to add or change t he DSCP (DiffServ Code Point ) value,
I EEE 802.1p priorit y level and VLAN I D num ber in a m atched packet . When t he packet passes
through a com patible network, t he networking device, such as a backbone switch, can provide
specific treat m ent or service based on the t ag or m arker.
Traffic Shaping
Burst y traffic m ay cause net work congest ion. Traffic shaping regulates packet s to be transm it ted
wit h a pre- configur ed dat a t ransm ission rat e usin g buff er s ( or queues) . You r Device uses t he Token
Bucket algorithm t o allow a certain am ount of large burst s while keeping a lim it at the average rat e.
Tr a f f i c
Tim e
Traffic Rat e
Tr a f f i c
Tim e
Traffic Rat e
( Before Traffic Shaping)
( Aft er Traffic Shaping)
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