Asynchronous Transfer Modes By: Megan Cwiklinski Adam Nasset Brad Samples Will Vanlue

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Asynchronous Transfer ModesBy Megan
Cwiklinski Adam Nasset Brad Samples Will Vanlue
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Outline

• Introduction
• Why Cells and Cells in Practice
• Why Virtual Circuits
• Types of Virtual Circuits and Paths
• Cell Structure
• Quality and Classes of Service
• Benefits

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Introduction

• ATM is a data link layer protocol
• Uses fixed sized cells
• Utilizes Synchronous Optical Network
• Widely used by multiplex services and DSL

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Why Cells?

• Use small data cells to reduce jitter
• Important when carrying voice traffic
• Good job includes
• Evenly spaced stream of data items
• Measure in time variables

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Why Cells?

• Designed to run at 155 Mbit/s
• Typical 1500-byte data packet sent in 77.42
  microseconds
• Disadvantage
• Queuing delays can be larger than transmittal
  time
• Packets expensive to implement at high speeds

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Why Cells?

• Cells developed to carry large datagrams and
  provide short queuing delays
• Breaks up packets into 48-byte chunk with 5-byte
  header to be reassembled later

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Cells in Practice

• Reduced need for small packet cells
• However, Asymmetric Digital Subscribers Lines use
  ATM
• On slow links, makes sense to use ATM
• Provides compelling business advantage for small
  companies

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Why virtual circuits?

• Data is passed from source to destination over
  multiple real circuits
• ATM cell header contains
• 8- or 12-bit Virtual Path Identifier (VPI)
• 16-bit Virtual Channel Identifier (VCI)

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Why virtual circuits?

• VPI VCI make up a unique path for a virtual
  circuit
• Switching is done by altering VPI VCI
• VPI VCI may not stay constant throughout a
  circuit
• The circuit is consistent for the entire message
• Can be used as multiplexing layer
• Hierarchically identified first by VPI, then by
  VCI
• Simpler switching by only looking at one
  identifier

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Types of virtual circuits and paths

• Static
• Dynamic
• Switched

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Static

• Permanent Virtual Circuits (PVCs) and Permanent
  Virtual Paths (PVPs)
• Circuits are made of explicitly defined segments
  between endpoints
• Simple to understand not very scalable
• No dynamic rerouting in case of failure

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Dynamic

• Soft Permanent Virtual Circuits (SPVCs) and Soft
  Permanent Virtual Paths (SPVPs)
• Dynamically built from specified service contract
  and endpoints
• Types of service contracts
• ABR (Available Bit Rate)
• CBR (Constant Bit Rate)
• UBR (Unspecified Bit Rate)
• VBR (Variable Bit Rate)

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Switched

• Switched Virtual Circuits (SVCs)
• Created on demand one time use
• Defined by service contract and endpoints
• Example Individual telephone calls

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ATM Cell Structures
UNI ATM Cell
NNI ATM Cell
7 4 3 0
VPI VPI VPI VPI VPI VPI VPI VPI
VPI VPI VPI VPI VCI VCI VCI VCI
VCI VCI VCI VCI VCI VCI VCI VCI
VCI VCI VCI VCI PT PT PT CLP
HEC HEC HEC HEC HEC HEC HEC HEC
Payload (48 bytes) Payload (48 bytes) Payload (48 bytes) Payload (48 bytes) Payload (48 bytes) Payload (48 bytes) Payload (48 bytes) Payload (48 bytes)
7 4 3 0
GFC GFC GFC GFC VPI VPI VPI VPI
VPI VPI VPI VPI VCI VCI VCI VCI
VCI VCI VCI VCI VCI VCI VCI VCI
VCI VCI VCI VCI PT PT PT CLP
HEC HEC HEC HEC HEC HEC HEC HEC
Payload (48 bytes) Payload (48 bytes) Payload (48 bytes) Payload (48 bytes) Payload (48 bytes) Payload (48 bytes) Payload (48 bytes) Payload (48 bytes)
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Generic Flow Control

• Permits transmissions of several terminals to be
  multiplexed on the same user interface
• Contains 4 bits

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Virtual Path/Circuit Identifier

• Virtual paths are semi-permanent connections
  between endpoints
• Connections are identified by the virtual circuit

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Payload Type

• The first bit indicates user data or control data
• If the cell contains user data
• Second bit indicates congestion
• Third bit indicates whether the cell is the last
  in a series of cells that represent a single AAL5
  frame

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Cell Loss Priority

• Used for buffer management
• Only 1 bit
• 0 means no congestion problems
• 1 means congestion and should be discarded

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Header Error Correction

• Used to detect and correct errors in the header
• Correction attempted on single bit errors
• Calculated using the polynomial x8x2x1

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Classes of Service

• Constant Bit Rate (CBR)
• Variable Bit Rate-Non Real Time (VBR-NRT)
• Variable Bit Rate-Real Time (VBR-RT)
• Available Bit Rate (ABR)
• Unspecified Bit Rate (UBR)

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Technical Parameters

• Cell Loss Ratio (CLR)
• Cell Transfer Delay (CTD)
• Cell Delay Variation (CDV)
• Peak Cell Rate (PCR)
• Sustained Cell Rate (SCR)
• Burst Tolerance (BT)

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Class of Service CBR VBRNRT VBRRT ABR UBR
CLR yes yes yes yes no
CTD yes no yes no no
CDV yes yes yes no no
PCR yes yes yes no yes
SCR no yes yes no no
BT _at_ PCR no yes yes no no
Flow Control no no no yes no
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Benefits

• Scalability
• Hardware
• Speed
• Size
• Versatility
• Voice and Video data
• LAN and WAN

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Benefits contd.

• Consistency
• Efficient switching
• Predictable traffic flow
• Guaranteed delivery
• International standards

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Questions?