ATM Connections 7-3
Traffic Management Overview
Traffic Management Overview
The ATM Forum Traffic Management 4.0 Specification defines five basic traffic classes:
• CBR (Constant Bit Rate)
• rt-VBR (Real-Time Variable Bit Rate)
• nrt-VBR (Non-Real Time Variable Bit Rate)
• UBR (Unspecified Bit Rate)
• ABR (Available Bit Rate)
Table 7-1 summarizes the major attributes of each of the traffic management classes:
Table 7-1 Standard ATM Traffic Classes
Traffic parameters are defined as:
• PCR (Peak Cell Rate in cells/sec): the maximum rate at which a connection can transmit
• CDVT (Cell Delay Variation Tolerance in usec): establishes the time scale over which the PCR
is policed. This is set to allow for jitter (CDV) that is introduced for example, by upstream nodes.
• MBS (Maximum Burst Size in cells): is the maximum number of cells that may burst at the PCR
but still be compliant. This is used to determine the BT (Burst Tolerance) which controls the time
scale over which the SCR (Sustained Cell Rate) is policed.
• MCR (Minimum Cell Rate in cells per second): is the minimum cell rated contracted for delivery
by the network.
QoS (Quality of Service) parameters are defined as:
• CDV (Cell Delay Variation): a measure of the cell jitter introduced by network elements
• Max CTD (Cell Transfer Delay): is the maximum delay incurred by a cell (including propagation
and buffering delays.
• CLR (Cell Loss Ratio): is the percentage of transmitted cells that are lost.
Congestion Control Feedback:
• With ABR, provides a means to control flow based on congestion measurement.
Attribute CBR rt-VBR nrt-VBR UBR ABR
Traffic Parameters
PCR & CDVT x x x x x
SCR & MBS x x
MCR x
QoS Parameters
Pk-to-Pk CDV x x
Max CTD x x
CLR xxx nw specific
Other Attributes
Congestion Control
Feedback
x
