Asynchronous Transfer Mode (ATM) and QoS

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Asynchronous Transfer Mode (ATM)and QoS
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ATM Era Multiservice NetworksDeparture 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 Commercial Factors
Single network optimized for everything (Data, phone, TV) Same technology for WAN, MAN, LAN (Seamless integration) QoS oriented and high-speed oriented Fast Hardware Tremendous amount of research has been done Large overhead for packets QoS is a bit complicated from the applications point of view and network management point of view Not that great from web browsing (which is one killer application) Millions of networks already installed Lack of applications Expensive at the LAN (where it really matters) No strong business incentive for QoS (even up to now) Can achieve similar 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)

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ATM Cell RelayThe 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)
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Without Short Cells
A voice packet waits behind a large data packet
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With Short Cells

• Voice packet can go immediately after data packet
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• 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

• 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
ATM
Switch
Switch
ATM
Switch
ATM
ATM
Switch
Switch
Virtual
Circuits
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ATM Switches
Input
Output
45
VPI/VCI
Port
VPI/VCI
Port
29
1
45
2
64
29
45
2
29
1
64
1
29
3
29
3
64
1
29

• ATM switches translate VPI/VCI values
• VPI/VCI value unique only per interfaceeg
  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
Data Link
ATM Layer
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 1
VBR Traffic
Priority 2
Classifier
Output
ABR Traffic
Priority 3
Priority 4
UBR Traffic
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ATM Adaptation Layer Summary
ServiceCategories
Class
Bit Rate
ConnectionMode
TimingConcern
ApplicationExamples
ATM Adaptation Layer (AAL)

• Bandwidth andthroughput guaranteed
• Good for voice and video

AAL1
A
CBR(Constant)
Connection-Oriented
Yes

• Best effort bandwidth and throughput
• Good for live video,multimedia, LAN-to-LAN

AAL2
B
VBR(Variable)VBR-RTandVBR-NRT
Connection-Oriented
Yes
ATM Layer

• Best effort withcongestion feedback
• Reliable delivery of bursty traffic iflatency
  okay

AAL5
C
ABR(Available)
Connection-Oriented
No
Physical Layer

• No guarantee
• For SMDS/LAN

AAL3/4
D
UBR(Un-specified)
Connection-less
No
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QUALITY OF SERVICE

• Max CDT, Mean CTD, CDV, CLR, CER, SECBR, CMR

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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

• Cell Loss Ratio (CLR)

Cost ( or Admin)

• Link Weighting