Transcript Presentation3

Department of Computer and IT Engineering
University of Kurdistan
Computer Networks II
Quality of Service (QoS)
By: Dr. Alireza Abdollahpouri
What is QoS?
 Some applications require “deliver on time” assurances
 must come from inside the network
Microphone
Sampler
A D
converter
,
Buffer
D
A
Speaker
 Example application (audio)
 sample voice once every 125us
 each sample has a playback time
 packets experience variable delay in network
 add constant factor to playback time: playback point
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What can go wrong with IP?
F ile Edit L ocate View
Help
500
E
D
C
B
A
400
300
Help
200
Network Traffic
100
0
IP network
1) IP packet is lost
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2) IP packet is delayed
Best effort is not enough
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QoS Parameters
In packet-switched networks, QoS is affected by various
factors including
Bandwidth
Delay
Jitter
Packet loss
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Bandwidth
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Delay
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Delay
 Delay is the time required for a signal to
traverse the network.
Sent
Latency
Received
Data Packet
Time
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Jitter
High jitter
Low jitter.
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Jitter
Variable interpacket timing caused by the
network a packet traverses.
Sent
Jitter
Received
Time
Data Packet
‫ میتواند به دلیل صف بندی در مسیریابها‬Jitter
‫و یا عبور بسته ها از مسیرهای مختلف اتفاق بیفتد‬
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‫‪Packet loss‬‬
‫دالیل اتالف بسته‬
‫تاثیراتالف بسته‬
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‫)‪QoS vs. QoE (Quality of Experience‬‬
‫تصویر دریافتی‬
‫‪ :QoS‬کیفیت از لحاظ پارامترهای شبکه‬
‫‪ :QoE‬کیفیت از دید کاربر‬
‫‪11‬‬
‫تصویر ارسالی‬
QoS requirements for different applications
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Traffic descriptors
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Traffic types
Constant-bit-rate traffic
(CBR)
Variable-bit-rate traffic
(VBR)
Bursty traffic
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Applications
 Elastic (delay-tolerant)
 Tolerate delays and losses
 Can adapt to congestion
 Non-elastic (Real-Time)
 Needs some kind of guarantee from network
 Main Question? How guarantee Delay and losses
 End to End, is it enough?
 In the Network
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Techniques to Improve QoS
• Packet scheduling techniques (e.g., FIFO, Priority
Queuing, WFQ)
• Traffic Shaping techniques (e.g., Leaky Bucket,
Token Bucket)
• Resource reservation techniques (e.g., RSVP)
• Admission Control techniques (mechanisms used
by a router or a switch to accept or reject a flow
based on flow specifications)
• Traffic monitoring and feedback
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Scheduling
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Scheduling
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Sequence number
Playback Buffer
Packet
arrival
Packet
generation
Network
delay
Playback
Buffer
Time
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Scheduling
FIFO queue
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Scheduling
Priority queuing
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Scheduling
Weighted fair queuing
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Traffic
shaping
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Traffic Shaping
b bits
r b/s
Leaky bucket
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Note:
A leaky bucket algorithm shapes
bursty traffic into fixed-rate traffic by
averaging the data rate. It may drop
the packets if the bucket is full.
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Leaky bucket drawback
Leaky bucket does not credit an idle host.
If a host is not sending for a while, bucket
becomes empty.
The idle time of a host is not considered
in leaky bucket.
In token bucket, idle hosts accumulate
credit for the future in the form of tokens.
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Traffic Shaping
Token bucket
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Token Bucket Algorithm: Calculating the length of
the maximum rate burst.
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Burst length S second.
Token bucket capacity C bytes
The token arrival rate p bytes/sec
The maximum output rate M bytes/sec
S=C/(M-p)
 Example: C=250KB, M=25MB/sec,
p=2MB/sec. In this case S=11msec.
Token bucket - example
Question:
A computer on a 6-Mbps network is regulated by a token
bucket. The token bucket is filled with a rate 1 Mbps. The
bucket is initially filled to capacity with 1 Mb. How long
can the computer transmit at the full 6 Mbps?
Answer:
The net outflow from the token bucket is 5Mbps. As a
result, the time it takes for the full bucket to empty is
1Mb/5Mbps=0.2sec. Thus, during the first 0.2 seconds the
computer transmits at the maximum 6-Mbps rate and then
it switches to 1-Mbps.
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The Leaky and token Bucket example
(a)
Input to a leaky bucket.
(1 MB burst). Network
speed is 200 Mbps
(b)
Output from a leaky
bucket (2MB/sec), 1 MB
bucket size
Output from a token bucket
with capacities of
(c) 250 KB,
(d) 500 KB,
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Note:
The token bucket allows bursty traffic
at a regulated maximum rate.
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Resource
reservation
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Resource reservation
 “A flow of data needs resources such as buffer,
bandwidth, CPU time..”
 The quality can be improved by reserving
these resources in beforehand
 The flow doesn’t need to compete with other flows
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Integrated Service
 Enhancing IP Service Model
 Add QoS service classes
 Explicit resource management at IP level
 Per flow state maintained at routers which is
 used for admission control and scheduling
 set up by signaling protocol, users explicitly request
their needs.
 This is done with RSVP protocol
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Resource reservation protocol
RSVP
RSVP is not a routing protocol. It relies on a routing
protocol to provide a route/tree along which it sends
control messages to make reservation.
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Basic Operations of RSVP
 Sender sends PATH message via the data
delivery
 Each router adds its state and the address of
the pervious hop
 Receiver sends RESV message on the
reverse path
 Specifies the reservation style, QoS desired
 Set up the reservation state at each router
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Integrated Services Example (RSVP)
Achieve per-flow bandwidth and delay guarantees
Example: guarantee 1Mbps and < 100 ms delay to a flow
Sender
Path RSVP Message
Receiver
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Integrated Services Example (RSVP)
Allocate resources - perform per-flow admission
control
Sender
RESV RSVP Message
Receiver
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Integrated Services Example (RSVP)
Install per-flow state
Receiver
Sender
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Integrated Services Example (RSVP)
Install per flow state
Sender
RESV RSVP Message
Receiver
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Integrated Services Example (RSVP)
Per-flow classification
Receiver
Sender
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Integrated Services Example (RSVP)
Per-flow buffer management
Receiver
Sender
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Integrated Services Example (RSVP)
• Per-flow scheduling
Receiver
Sender
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Admission
control
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‫‪Admission control‬‬
‫‪ mechanism used by a router or a switch to‬‬
‫‪accept or reject a flow based on flow‬‬
‫‪specifications‬‬
‫احتمال بروز تراکم در شبکه ای که ا ز ‪ AC‬استفاده می کند پایین تر‬
‫است‪ .‬چرا که این مکانیزم ‪ ،‬جامعیت ترافیک موجود در شبکه را با‬
‫جلوگیری از پذیرش سایر ترافیکها حفظ می نماید‪.‬‬
‫‪45‬‬
QoS Issues on wireless
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Dynamically varying network topology
Imprecise state information
Lack of central coordination
Error-prone shared radio channel
Hidden terminal problem
Limited resource availability
Insecure medium
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Questions
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