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Traffic Specification and Service Models

{\bf ATM}

\begin{description}
\item[Constant Bit Rate (CBR)] --- Intended for real-time
applications with tightly constrained delay variation (voice, video,
circuit emulation).  
Connection specifies a Peak Cell Rate.  Cells delayed beyond the
maximum Cell Transfer Delay are considered useless.

\item[Real-Time Variable Bit Rate (rt-VBR)] --- Intended for 
real-time applications with bursty traffic characteristics and tightly
constrained delay variation (voice, video, circuit emulation).
May support statistical multiplexing of real-time sources.
Connection specifies a Peak Cell Rate, Sustainable Cell Rate
and Maximum Burst Size.  Cells delayed beyond the
maximum Cell Transfer Celay are considered useless.

\item[Non-Real-Time Variable Bit Rate (nrt-VBR)] --- Intended for
non-real-time applications with bursty traffic characteristics.   May 
support statistical multiplexing of real-time sources.
Connection specifies a Peak Cell Rate, Sustainable Cell Rate
and Maximum Burst Size.  Connects expects a low Cell Loss
Ratio, but Cell Transfer Delay is irrelevant.

\item[Unspecified Bit Rate (UBR)] --- Intended for
non-real-time applications such as file transfer or email.
No service related guarantees on Cell Lose Ratio or Cell
Transfer Delay.  

\item[Available Bit Rate (ABR)] --- Uses a flow control mechanism to
allow the source to control its rate in response to changing ATM layer
characteristcs. 
Connection specifies a Peak Cell Rate and a Minimum Cell Rate.
Network guarantees to always provide at least the Minimum Cell Rate.
Connection expects a low Cell Loss Rate so long as it is complying
with flow control.
\end{description}

Open qustions: 
\begin{itemize}
\item How do you deal with multicast in ABR?
\item Who can make head or tail of ABR?
\end{itemize}

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