ATM QoS Classes
The ATM Forum has defined four ATM Layer service classes, each with
scalable Quality of Service (QoS) levels:
Class A
Class A, or Constant Bit Rate (CBR), traffic is characterized by a
continuous stream of bits at a steady rate, such as TDM multiplexer
traffic. Class A traffic is low-bandwidth traffic that is highly
sensitive to delay and intolerant to cell loss.
Class B and C
Class B and C traffic, defined as Variable Bit Rate (VBR), has a
bursty nature and can be characterized by voice or video applications
that use compression. Class B traffic is real-time VBR, (RT-VBR),
where end-to-end delay is critical, such as interactive video
conferencing. Class C is non-real time (VBR-NRT) traffic, where delay
is not so critical, such as video playback, training tapes and video
mail messages.
Class D
Class Dtraffic is split into two classes: Available Bit Rate (ABR) and
Unspecified Bit Rate (UBR). These 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 quality of service
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.
Excerpted from the June 19, 1995 Network World story "Portrait of an
ATM Switch," by David Axner.