IS 3413

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IS 3413

• Chapter 15 Asynchronous Transfer Mode (ATM)
• Dr. Jan Clark
• SPRING, 2003

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ATM

• Multi-speed network environment that provides a
  variety of complex network services
• Can carry voice, data, video separately or
  simultaneously
• Can be used in LANs, MANs, or WANs
• Connection-oriented virtual channel, pt-to-pt,
  full-duplex, and cell switched

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Cell Switched ATM

• Similar to frame relay
• Difference?
• Frame relay switches variable length frames
  within frame relay cloud from source to
  destination
• ATM switches fixed-length cells (48 byte
  information field, 5 byte header)
• Based on packet switching (connection-oriented)
• Cell sequence integrity preserved via virtual
  channel
• Information field is carried transparently
  through the network, with no error control

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ATM Layers
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ATM Physical Layer

• Transports cells via a communications channel
  (either optical or electrical)
• LAN support 25-155 Mbps copper or fiber
• WAN support SONET rates over fiber
• Physical Medium Sublayer bit transfer, bit
  alignment, and copper/fiber conversions
• Transmission Convergence Sublayer bit/cell
  conversion at sending and receiving nodes

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ATM Layer

• Handles functions of the network layer
• Connection-oriented without acknowledgements
• Two possible interfaces
• UNI User-Network Interface Boundary between an
  ATM network and host
• NNI Network-Network Interface Between two ATM
  switches

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ATM Adaptation Layer (AAL)

• Two sublayers
• Convergence accepts data messages from higher
  layer application protocols, interprets type and
  format of messages (based on CoS), prepares for
  processing by SAR
• Segmentation and Reassembly (SAR) processes
  data consistent with type and format, packetizes

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VBR vs. CBR

• VBR Variable Bit Rate - appropriate for data
• CBR - Constant Bit Rate - appropriate for voice
• Full motion video more demanding than voice or
  data, so need CBR PLUS reserve bandwidth in case
  it is needed.

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Isochronous Communication

• Intended for constant, complete delivery of
  communication (e.g. 30 frames per second for
  full-motion video)
• Provides constant bit rate must reserve
  bandwidth for this to occur
• Used in ATM by intermediary locations (ATM
  switching devices)

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ATM a Network within a Network

• ATM has its own internal hand-shaking and
  management protocols, QOS, flow control, etc.
• Cells are transmitted from source to destination
  via an ATM switched subnet
• End nodes communicate with an ATM device via a
  user-to-network interface (UNI)
• ATM switches communicate with each other via a
  network-to-network interface (NNI) (Fig. 15.3)

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NNI and UNI communications (Fig. 15.3)
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Voice, Video, or Data Communications
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ATM NIC

• Devices connected to an ATM network require an
  ATM NIC
• NIC is then connected to a local switch
• Can connect NIC to switch via copper
• UNI establishes hand shaking between NIC and
  switch

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PNNI Private Network-to-Network Interface

• Switch-to-switch communication in a private
  network

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ATM Cells

• 53 bytes (octets) 48 for data, 5 for overhead
• Fixed length
• How can we fit 6 byte source and destination
  addresses into 5 bytes?????? (the cliffhanger)

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Virtual Connections

• Virtual Channel Connection (VCC) Virtual
  circuit with logical connection between source
  and destination can be PVC or SVC
• Virtual Path Connection (VPC) Semi-permanent
  connection that provides a logical collection of
  virtual channels that have the same endpoint
• A single virtual path supports multiple virtual
  channels (analogy highway VPC, lane VCC)

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VCI vs VPI

• VPI Virtual Path Identifier identified in
  cells header. Cannot establish a virtual
  channel before virtual path
• VCI Virtual Channel Identifier only have
  local significance different virtual paths
  reuse VCIs (but VCIs on same path must be unique)

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What is so special about a virtual path?

• Switches can interconnect via a VPC without a
  direct physical link between them
• ATM is connection-oriented, so circuit must be
  established before transmission
• As route established, VPIs and VCIs are assigned
• VPI and VCI info suffices for addressing info

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ATM Header Bytes

• Byte 1 - bits 1-4 - VPI bit 5-8 - if NNI
  (network to network interface), VPI - could also
  be flow control (rare)
• Byte 2 - bits 1-4 VCI, bits 5-8 VPI
• Byte 3 VCI
• Byte 4 bit 1 CLP - cell loss priority bits 2-4
  PTI - payload type indicator, bit 5-8 VCI
• Byte 5 - error checksum

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ATM versus Fast and Gigabit Ethernet

• ATM originally designed as WAN technology for
  B-ISDN
• Ethernet designed for LAN technology
• ATM can work at LAN level, but gigabit Ethernet
  is faster and cheaper
• BUT, Ethernet is VBR, so video is not great.
• ATM working on 2.5 Gbps at LAN level

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ATM versus Frame Relay

• Frame relay uses variable length frames
• ATM fixed length cells
• ATM has higher overhead, but faster speed and
  traffic management (better suited for video and
  voice)

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ATM versus SONET

• SONET is a transport mechanism, transporting data
  over fiber.
• Can act as a transport carrier for ATM (or FDDI,
  or ISDN, etc.)
• ATM is a technology and protocol designed to use
  SONET as its carrier service