ATM

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ATM
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Design Goals

• Single global voice/video/data network
• Goals
• Universal service all users worldwide connect
  any two (any time)
• All uses voice, video, data
• Single infrastructure for LANs and WANs
• Service guarantees on each connection, can
  commit to delay etc./ levels
• Low-cost devices can accommodate evolution of
  low cost network entities

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Design Goals Continued

• Key packet communication concepts
• Bandwidth (speed/capacity)
• Delay
• Jitter variance in delay
• Voice needs low delay and jitter and modest
  bandwidth
• Video needs low jitter and high data rate
  throughput

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Design Goals Continued

• Data less concerned about delay and jitter
• Resilient to changes in delay and jitter
• CSMA/CD has widely varying delays
• Even if data generated with zero jitter may not
  arrive with zero jitter

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General Purpose

• Data best sent in large chunks
• Lower overhead
• 4KB or greater
• Voice best sent in small chunks
• Need 8 bits every 125µs
• Dont want to buffer
• Very large packets cause intolerable delay (while
  waiting for big packets to move along) and delay
  defeats echo cancellation schemes

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Asynchronous Transfer Mode

• Developed by phone companies
• Seeking efficient universal high speed
  communication meeting all the above goals
• A switched connection oriented technology
  (star topologies glued together with paths sets
  up through it)

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ATM Cells

• For high speeds and low delay and jitter ATM uses
  small, fixed sized packets called cells
• Compromise between large versus small frames
• Each contains 53 octets
• 5 header and 48 data octets

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Cell Layout

• VPI/VCI identify cells destination
• Cyclic Redundancy Check
• Priority can cell be dropped?
• Has 10 overhead
• High for data

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ATM

• Is connection-oriented
• ATM connection term virtual channel, most call it
  a virtual circuit (VC)
• Each VC has 24-bit identifier
• 8-bit Virtual Path Identifier specifies path VC
  follows through network
• 16-bit Virtual Channel Identifier specifies
  given VC on this path
• Hierarchical
• Computer treat VPI/VCI as just one number

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ATM Continued

• Computers dont worry about VPI/VCI
• Each switch changes VPI/VCI
• Has forwarding table
• Indexed by incoming VPI/VCI number
• Contains port, incoming address, and outgoing
  address information
• Rewrites address
• Cell does not have single address while transiting

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ATM Continued

• Changing the VPI/VCI values in frames is termed
  label rewriting or label switching
• Eases creating and tracking of connections

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Permanent Virtual Circuits

• PVC set up for very long periods months!
• Survives crashes, power losses, failures
• Once established, PVC is provisioned
• provisioning is manually figuring out route and
  manually setting up all forwarding tables

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Switched Virtual Circuits

• Virtual circuit created dynamically in response
  to request
• Computer requests circuit from switch
• All switches along the way create circuit
• Message passed back to computer
• A number of VPI/VCIs reserved for requests and
  network control
• Denial can happen at any point in setup if ATM
  network cannot provide what is requested (BW,
  QoS, etc.)

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QoS

• Subscriber can specify Quality of Service
• Can specify Bandwidth needed, max delay, max
  jitter, etc.
• 64kbps, short delay, no jitter
• 10Mbps, long delay, lots of jitter
• ATM has several standard offerings for QoS levels
• Constant Bit Rate (CBR) for audio and video

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QoS Continued

• Variable Bit Rate (VBR) for compressed data
• Available Bit Rate (ABR) for bursty applications
• Whatever is available at the moment

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Cells vs. Packets

• Fastest if always same size and format
• No memory fragmentation in switches
• No end of packet detection
• Easier to guarantee QoS as transmission time
  fixed
• ATM often used in fast links
• OC-3 and better (155 Mbps or better)

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Cells vs. Packets Continued

• VPI/VCI extraction and rewriting simple
• Basically, doesnt need CPU
• Straightforward work can be done by hardware no
  software cycles taken up

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ATM and Data

• In order to interface better with data
  communication networks, ATM defines adaptation
  protocols
• ATM Adaptation Layer 5 (AAL5) used to send data
• Data ? AAL5 ? ATM ? AAL5 ? Data
• Accepts blocks of data up to 64K octets
• Divides into cells, transmits, reassembles
• Segmentation and reassembly

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ATM and Data Continued

• Sender doesnt know
• Blocks can be different sizes

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Critique

• Some drawbacks
• Expensive ATM switches expensive, ATM NICs
  expensive and not ubiquitous
• Setup latency
• Cell tax
• A priori service specification
• Client/server might not know
• Can have either under- or over-specification
• Inefficient broadcast

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Critique Continued

• QoS is complex to implement, monitor, support
• ATM assumes pretty homogeneous network entities
• Still, widely used by phone companies
• Not used very much for LANs
• Gigabit Ether being used for this
• IP over SONET
• No cell tax