ATM or Asynchronous Transfer Mode is an ITU-T (International Telecommunication Union-Telecommunications Standard Section) standard for cell relay wherein information for multiple service types, such as voice, video or data is conveyed in small, fixed-size cells. ATM is an evolution of packet switching. ATM integrates multiplexing and switching functions. The small constant cell used with ATM is relatively small compared with units used with older technologies. The small, constant cell size allows ATM equipment to transmit video, audio, data over the same network. ATM creates fixed routes between two points before data transfer begins.
Generally, ATM provides four different types of bit rates :
- Constant Bit-Rate (CBR): CBR specifies a fixed bit rate so that data is sent in a steady stream.
- Available Bit-Rate (ABR): ABR provides a guaranteed minimum capacity but data can be burst to higher capacity when network traffic is minimum or network is free.
- Unspecified Bit-Rate (UBR): UBR doesn't guarantee any throughput level. It is used for applications, like file transfer that can tolerate delays.
- Variable Bit-Rate (VBR): VBR provides a specified throughput capacity but data is not sent in evenly. This makes it a popular choice for voice and video-conferencing.