Download ATM Networks

Asynchronous Transfer
Mode (ATM)
and QoS
COMP680E by M. Hamdi
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ATM Era : Multiservice Networks
Departure from Service Specialization
bulk
data
video
Multiservice
Network
voice
interactive
data
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Why ATM Did Not Make it the Way it was
Initially Envisioned
Advantages of
ATM
Disadvantages
of ATM
• Single network
optimized for everything
(Data, phone, TV)
• Large overhead for
packets
• Same technology for
WAN, MAN, LAN
(Seamless integration)
• QoS oriented and highspeed oriented
• Fast Hardware
• Tremendous amount of
research has been done
Commercial
Factors
• Millions of networks
already installed
• QoS is a bit complicated • Lack of applications
from the applications
• Expensive at the LAN
point of view and network (where it really matters)
management point of
• No strong business
view
incentive for QoS (even
• Not that great from web up to now)
browsing (which is one
• Can achieve similar
killer application)
speed with an IP router as
compared to an ATM
switch (May be)
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What is ATM?
ATM is packet switching!
• Switched or permanent connections
• Traffic type independent (voice, data, interactive video)
• Fixed length packet - 53 bytes (cell)
header
payload
Fixed length packet = cell
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ATM Cell Relay:
The Underlying Technology
Cell Features
Benefit
Small
Low latency to support real-time
services like audio and video (What is an
appropriate size?)
Fixed Length
Fast hardware switching and scalability
Standardized
Usable in all networks (LAN and WAN)
Voice
Data
Cells
Video
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Without Short Cells
Router
I
n
p
u
t
p
o
r
t
s
O
u
t
p
u
t
Voice
packet (50)
Data packet
(2000)
p
o
r
t
s
A voice packet waits behind a
large data packet
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With Short Cells
Router
I
n
p
u
t
p
o
r
t
s
O
u
t
p
u
t
Data #40
(50)
Data #2
(50)
Voice
(50)
Data #1
(50)
p
o
r
t
s
Voice packet to be transmitted after Data #1
•Voice packet can go immediately after
data packet #1
•Waiting for voice is reduced
significantly
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Virtual Paths & Virtual Channels
A Virtual Path (VP) describes the semi-permanent route between two
end points.
 A Virtual Channel (VC) describes a cell transmission channel inside a
virtual path

VCs
VP
VCs
VP
Physical
Transmission
Link
VP
VCs
VP
VCs
• Unique on a link-by-link basis
• Virtual channels are contained within virtual paths
• Interpreted at each switch to:
• determine output link
• determine outgoing VPI/VCI
• Two-level structure:
• allows “trunking” of virtual channels as one virtual path
• virtual path can be switched
• both used to route cells through network
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Connection Identifiers
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ATM switch routing
Virtual
Paths
ATM
Switch
ATM
Switch
ATM
Switch
ATM
Switch
ATM
Switch
Virtual
Circuits
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ATM Switches
Input
Output
Port VPI/VCI Port
45
VPI/VCI
1
29
2
45
2
45
1
29
1
64
3
29
3
29
1
64
29
64
2
1
3
29
• ATM switches translate VPI/VCI values
• VPI/VCI value unique only per interface—
eg: locally significant and may be re-used elsewhere in network
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ATM Switching
Connections (routes) set up by software
• Routing (path through multiple-switch network)
and resource allocation is performed once per
connection by switch control CPU
Cells are switched by hardware
• Hardware (table lookup + switching fabric)
switches each incoming cell to appropriate output
port
• Once a connection is established, cells are not
touched by software
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VP and VC Switch
• Two types of ATM switch
• VP switch does not look at VCIs, switching is based
on VPIs only
• VCI does not change when passing through a VP
switch; VPI may change
• VC switch looks at both VPI and VCI
• VCI (as well as VPI) may change when passing
through a VC switch
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Routing with a VP Switch
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A Conceptual View of a VP Switch
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Routing with a VC Switch
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A Conceptual View of a VC Switch
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ATM Protocol Stack
Upper Layers
ATM Adaptation Layer
ATM Layer
Physical Layer
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ATM Architecture
Application
Upper Layer Protocols
Presentation
Session
ATM Adaptation Layer
Transport
Network
ATM Layer
Data Link
Transmission-convergence
physical medium dependent
Physical
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Adaptation Layers: Service Classes
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Service Classes and Capacity of Network
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QUEUES and PRIORITY
CBR Traffic
Priority 2
Priority 3
Output
ABR Traffic
Classifier
VBR Traffic
Priority 1
Priority 4
UBR Traffic
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ATM Adaptation Layer: Summary
Class
ATM
Adaptation Layer
(AAL)
A
Service
Categories
AAL1
AAL2
Connection Timing
Mode
Concern
Yes
• Bandwidth and
throughput guaranteed
• Good for voice
and video
Connection(Variable)
Oriented
Yes
• Best effort bandwidth
and throughput
• Good for live video,
multimedia,
LAN-to-LAN
ConnectionOriented
No
• Best effort with
congestion feedback
• Reliable delivery
of bursty traffic if
latency okay
Connectionless
No
• No guarantee
• For SMDS/LAN
VBR
ATM Layer
VBR-RT
and
VBR-NRT
C
AAL5
ABR
(Available)
D
AAL3/4
Application
Examples
ConnectionOriented
CBR
(Constant)
B
Physical Layer
Bit Rate
UBR
(Unspecified)
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QUALITY OF SERVICE
– Max CDT, Mean CTD, CDV, CLR, CER, SECBR,
CMR
CBR
rt-VBR nrt-VBR ABR
UBR
PCR
Yes
Yes
Yes
Yes
Yes
SCR
N/A
Yes
Yes
N/A
N/A
MBS
N/A
Yes
Yes
N/A
N/A
MCR
No
No
No
Yes
No
CDVT(PCR) Yes
Yes
Yes
Yes
Yes
CDVT(SCR) N/A
Yes
Yes
N/A
N/A
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Application Requirements
Bandwidth
• Peak Cell Rate (PCR)
• Sustained Cell Rate (SCR)
• Minimum Cell Rate (MCR)
Delay
• Cell Transfer Delay (CTD)
• Cell Delay Variation (CDV)
Reliability
Cost ($ or Admin)
• Cell Loss Ratio (CLR)
• Link Weighting
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