CCNA Quality of Services (QoS)

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8- QUALITY OF SERVICE
8.1 Basics Of QoS
8.2 QoS Mechanisms

8.1 Basics Of QoS

QoSOVERVIEW
• Quality Of Service
• Industry-wide set of standards and
mechanisms for ensuring high-quality
performance for critical applications
• Ability to provide different priority to
different applications, users, or data
flows, or to guarantee a certain level
of performance to a data flow

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PRIORITIZATIONOFNETWORKTRAFFIC
• Some traffic flows are more important
for the companies
• If the bandwidth demand exceeds the
available amount, traffic congestion
occurs
• Result = Loss / Delay of the critical
data

NETWORKCHARACHTERIST. MANAGEDBY QOS
• Bandwidth : The rate at which traffic
is carried by the network
• Latency : The delay in data
transmission from source to
destination
• Jitter : The variation in latency.
• Reliability : The percentage of packets
discarded by a router

VOICETRAFFIC
• Sensitive for delays / drops
• No retransmission if the packet is lost
(UDP Based)
• Smooth (Flows at even rate, doesn’t burst)
• Benign (Not greedy for lots of BW fairly
low per call)

VIDEOTRAFFIC
• Sensitive for delays / drops
• No retransmission if the packet is lost
(UDP Based)
• Bursty (Part of picture moves rest is
static on screen cause of keyframes)
• Greedy (More BW you define, better
the picture quality is)

DATATRAFFIC
• Insensitive for delays / drops
• Smooth/Bursty
• Benign/Greedy
• TCP-based retransmits.

QUEUINGALGORITHMS
• Instead of dropping traffic, routers
and switches queue packets into an
output queue and all new packets go
through the queuing system
1. FIFO
2. WFQ
3. CBWFQ
4. LLQ

FIFO(FIRSTINFIRSTOUT)
• Traffic departs from the interface as it
arrived
• No traffic priority
FIFO

WFQ(WEIGHTEDFAIRQUEUING)
• Flow-based queuing algorithm
• Schedules interactive traffic to the
front of the queue to reduce response
time
• Fairly shares the remaining bandwidth
between high bandwidth flows.

CBWFQ(CLASSBASEDWEIGHTEDFAIRQUEUIN)
• Extends the standard WFQ
functionality to provide support for
user-defined traffic classes
• A queue is reserved for each class, and
traffic belonging to a class is directed
to the queue for that class

LLQ(LOWLATENCYQUEUING)
• Brings strict priority queuing to class-
based weighted fair queuing
(CBWFQ)
• Allows delay-sensitive data (such as
voice) to be given preferential
treatment over other traffic by letting
the data to be dequeued and sent first

8.2 QoS Mechanisms

QoSTECHNOLOGIES
• Used to provide improved network
"service" to the applications at the
edges of the network
1. Best Effort
2. Integrated Services
3. Differentiated Services

BESTEFFORT
• Network does not provide any
guarantees that data is delivered
Users obtain unspecified variable bit
rate and delivery time, depending
on the current traffic load

INTEGRATEDSERVICES
• Provides a way to deliver the QoS that
real-time applications require by
explicitly managing network resources
to provide QoS to specific user packet
streams
• Uses "resource reservation" and
"admission control" mechanisms

INTEGRATEDSERVICES(cont…)
• Use the Resource Reservation Protocol
(RSVP) to signal the QoS needs of an
application’s traffic along devices in the
end-to-end path through the network

DIFFERENTIATEDSERVICES
• Architecture that specifies a simple,
scalable and coarse-grained mechanism
for classifying and managing network
traffic and providing QoS
• Packet's "class" can be marked directly
in the packet (No need for RSVP)
• Achieves better QoS scalability, while
IntServ provides a tighter QoS
mechanism for real-time traffic

DIFFERENTIATEDSERVICES(cont…)
• Architecture that specifies a simple,
scalable and coarse-grained mechanism
for classifying and managing network
traffic and providing QoS
• Packet's "class" can be marked directly
in the packet
• Achieves better QoS scalability, while
IntServ provides a tighter QoS
mechanism for real-time traffic

QoSTOOLS
• QoS technologies refer to the set of
tools and techniques to manage
network resources
• QoS Tools :
1. Classification & Marking Tools
2. Congestion Avoidence Tools
3. Congestion Management Tools

CLASSIFICATIONANDMARKING
• Traffic is analyzed to discover that
which class it belongs to and packets
are marked after discovery process
• Classification of the traffic is made via
ACLs or Class-Maps
• Marking network traffic allows to set or
modify the attributes for a specific class
of traffic ( Layer -2 or Layer-3 )

LAYER-2MARKING
• Implemented via 3 bit priority code
point field (which identifies class of
service markings) in TCI field of
Ethernet header
• Ethernet frames are marked with 8
levels of priority
VALUE DESCRIPTION
7 Reserved
6 Reserved
5 Voice
4 Video Conference
3 Call Signal
2 High Priority Data
1 Medium Priority Data
0 Best Effort Data

LAYER-3MARKING
• Implemented via 8 bit Type Of Service
Field (IPv4) or Traffic Class Field
(IPv6) in IP header
• In Diffserv model packet is marked
using DiffServ Code Point (DSCP) bits
(6 bits)
VALUE DESCRIPTION
7 Network
6 Internet
5 Critical
4 Flash-Override
3 Flash
2 Immediate
1 Priority
0 Routine

TRAFFICSHAPING&POLICING
• Traffic Policing: When the traffic rate
reaches the configured maximum rate,
excess traffic is dropped (or remarked)
• Traffic Shaping : Retains excess packets
in a queue and then schedules the
excess for later transmission over
increments of time

CCNA Quality of Services (QoS)

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