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ATM Technical Overview

Why ATM?

• ATM a merging of two well-known techniques
  • Packet Switching which produces most flexible network in terms of
    • bandwidth requirements

    • resource allocation

  • Time Division Multiplexing which permits Continuous Bit-stream Oriented (CBO) services such as voice

ATM Features

• Error & flow control is moved to the network boundary
  • No error control on data field within the network

  • No flow control on links within the network

• Connection oriented at the lowest level
  • All information is transferred in a virtual circuit assigned for the duration of the connection

• Fixed cell (packet) size
  • Permits high speed switching nodes

  • No constraint on data services (segmentation)

• Efficient cell structure
  • 48 bytes of data

  • 5 byte header

ATM Cell Structure

• Cell
  
  

• Cell Header
  
  

ATM Cell Header Fields

• GFC (Generic Flow Control) permits transmissions of several terminals to be multiplexed on the same user interface - user to network traffic

• VPI/VCI (Virtual Path/Circuit Identifiers) field contains 24/28 bits UNI/NNI. VPI field of 8 to 12 bits allows 256 to 4096 virtual paths each of 64,000 VCIs

• PT (Payload Type) identifies payload type for OAM (Operations, Administration and Maintenance). Also reserved bits for explicit congestion control and an ATM Adaptation Layer 5 bit

• CLP (Cell Loss Priority) is used for buffer management
  • If set (1) then the cell may be discarded according to network conditions - congestion control

  • If not set (0) cell can not be discarded

• HEC (Header Error Control) is used to detect and correct errors in the header
  • Single bit error - correction is attempted

  • Multiple bit errors - cell discarded

  • Calculated using the polynomial x8+x2+x+1

VPI/VCI

• ATM is connection oriented

• Connections are identified by the Virtual Channel Identifier

• A virtual channel is established at connection time and torn down at termination time

• Multiple VCIs may be used for a multi-component service e.g. sound and video over separate VCIs in video-telephony

• ATM switches perform switching on a per VCI basis

Virtual Paths

• Networks will have semi-permanent connections between endpoints

• Such connections may contain many VCs and are known as Virtual Paths

• Virtual Paths are identified by a Virtual Path Identifier

• VCs may not be switched within a Virtual Path

• Switching is normally performed on a per VPI basis