CBR traffic needs a single leaky bucket because it uses a sustained rate parameter in its network contract. If either value exceeds the contract parameters, the ATM switch polices the VBR traffic by manipulating the CLP bit. Traffic Shaping Similar to traffic policing, traffic shaping is performed at the user-network interface. Congestion Control Congestion can occur in any network, whether it uses shared media or ATM, whenever an application sends more data than the network can transmit with the available bandwidth. Also, as more applications send data over the same network, the bandwidth available to any one application changes over time.
Most networks fail to te ll applications how much bandwidth is available at any given instant.
As a result, applications have no basis on which to control the amount of data they send. When applications send more data than the network can handle, the network buffers fill up and can overflow. The application must then retransmit the data, which adds more traffic and further congests the network. An ATM network performs congestion control so that ABR traffic can efficiently use the bandwidth that has not been guaranteed to CBR and VBR traffic. Effective congestion control reduces the need to retransmit data due to congestion.