Asynchronous Transfer Mode (ATM)

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Asynchronous Transfer Mode (ATM)

• Dr. E.C. Kulasekere

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Section Objectives

• To understand what ATM is.
• The architecture of ATM
• Key issues in ATM
• How ATM differs from frame relay
• Advantages and disadvantages of using ATM.

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Protocol Architecture

• Transfer of data in discrete chunks
• Multiple logical connections over single physical
  interface.
• Flow on each logical connection is in fixed sized
  packets called cells.
• Minimal error and flow control.
• Data rates (physical layer) 25.6Mbps to 622.08Mbps

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Asynchronous Transfer ModeProtocol Architecture
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Reference Model Planes

• User plane provides for user information
  transfer
• Control plane call and connection control
• Management plane
• Plane management whole system functions and
  provides coordination between planes.
• Layer management resources and parameters in
  protocol entities

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ATM Logical Connections

• Logical connections in ATM are referred to as
  Virtual channel connections (VCC)
• Analogous to virtual circuit in X.25
• Basic unit of switching between two end users
• Full duplex
• Fixed size cells
• Data, user-network exchange (control) and
  network-network exchange (network management and
  routing)
• Virtual path connection (VPC)
• Bundle of VCC with same end points

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Asynchronous Transfer ModeATM Logical Connections
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Call establishment using virtual paths
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Asynchronous Transfer ModeVP / VC Characteristics

• Quality of service
• Switched and semi-permanent channel connections
• Call sequence integrity
• Traffic parameter negotiation and usage
  monitoring
• VPC only
• Virtual channel identifier restriction within VPC

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Asynchronous Transfer ModeATM Cells

• Fixed size( 5 octet header and 48 octet
  information field)
• Small cells reduce queuing delay for high
  priority cells
• Small cells can be switched more efficiently
• Easier to implement switching of small cells in
  hardware

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Problems

• How does ATM Differ from Frame Relay?
• The most obvious feature of ATM compared to
  frame relay is that ATM makes use of a 53 byte
  fixed length cell while the frame in frame relay
  is much longer, and may vary in length, both in
  its header and its data fields. Additionally,
  error checking is only done on the header in ATM
  rather than on the whole cell or frame. Virtual
  channels of ATM that follow the same route
  through the network are bundled into paths. A
  similar mechanism is not used in frame relay.

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Problems

• What are the relative advantages and
  disadvantages of ATM compared to frame relay?
• ATM is even more streamlined than frame relay in
  its functionality, and can support data rates
  several orders of magnitude greater than frame
  relay.

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Problems

• What is the difference between a virtual channel
  and a virtual path?
• A virtual channel is a logical connection similar
  to virtual circuit in X.25 or a logical channel
  in frame relay. In ATM, virtual channels which
  have the same endpoints can be grouped into
  virtual paths. All the circuits in virtual paths
  are switched together this offers increased
  efficiency, architectural simplicity, and the
  ability to offer enhanced network services.