t stays within the contracted service. If there are non-conforming cells, then the network takes appropriate action on them, such as setting the CLP bit of the non-conforming cells, thus making the cells eligible for discarding. This discarding is done to prevent any non-conforming cells affecting the QoS of conforming cells. The traffic shaping function modifies the traffic flow and changes the characteristics of the user cell streams to achieve improved network efficiency and to get the lowest cell loss. Queuing, Buffering, Cell Servicing and Congestion Control To maintain the network's optimum performance, the ATM performs a series of cell treat- ment mechanisms; queuing, buffering, cell servicing and congestion control Queuing will occur when two cells arrive at the same time and are going to the same destina- tion. When cells of higher bit rate pass through a virtual connection with a lower band- width, congestion takes place. Buffering of cells occurs when two or more conforming cells are destined to the same output at the same time. Cells servicing, such as dropping of cells, occurs when a non-conforming cell with the CLP bit set to 1 arrives which causes congestion. 8.6.3 Network Element and Interface Configuration Parameters Configuration of the network elements requires specification of a lot of parameters, which are related to the interfaces, ATM, IP, etc. Since ATM is used as a switching and multi- plexing technique in 3G networks, every base station requires a unit that can handle ATM connections. This unit can be integrated with the base station (as shown in Figure 8.2) or can be a standalone unit. Parameter planning/setting is required for the interface units/hardware (PDH/SDH), ATM terminations/cross-connections, and those related to the IP addressing and synchronisation of the ATM cross-connect (AXC) unit. Interface Unit Parameters The interface unit parameters are related to the hardware and the PDH/SDH interface units that are used for transmission of ATM on the PDH or SDH. Hardware unit parame