Network Sales and Services Handbook (Cisco Press Networking Technology)

One of the biggest advantages of ATM over other LAN/WAN technologies, such as Frame Relay or Fast Ethernet, is that ATM supports QoS levels, enabling ATM service providers to guarantee to their customers some sort of end-to-end latency. The ATM Forum has defined four ATM Layer service classes, each with scalable QoS levels:

• Class A, defined as constant bit rate (CBR) Traffic is characterized by a continuous stream of bits at a steady rate, such as Time Division Multiplexer (TDM) traffic. Class A traffic is low-bandwidth traffic that is sensitive to delay and intolerant to cell loss.

• Class B and C, defined as variable bit rate (VBR) Traffic has a bursty nature and can be characterized by voice or video applications using compression.

  - Class B traffic is real-time VBR (RT-VBR), in which end-to-end delay is critical (for example, interactive video conferencing).

  - Class C is not real time (VBR-NRT) traffic and delay is not as critical (video playback, training tapes, and video mail messages are examples).

• Class D, defined as available bit rate (ABR) and unspecified bit rate (UBR) Classes are for bursty LAN traffic and data that is more tolerant of delays and cell loss.

  - UBR is a "best-effort" service that does not specify bit rate or traffic parameters and has no QoS guarantees. Originally devised as a way to make use of excess bandwidth, UBR is subject to increased cell loss and the discard of whole packets.

  - ABR, like UBR, is also a best-effort service, but differs in that it is a managed service based on minimum cell rate (MCR) and with a low cell loss. No delay variation guarantee is currently envisioned for either UBR or ABR service classes, making this service unsuitable for voice or video applications.