The main purpose of UPC (Usage Parameter Control) and NPC (Network Parameter Control) is to protect the ATM (see Asynchronous Transfer Mode) network resources from malicious connections and to enforce the compliance of every connection to its negotiated traffic contract . UPC and NPC are also known as traffic policing in general.
Usage parameter control (UPC) and network parameter control (NPC) perform similar functions at different interfaces. The UPC function is performed at the user-network interface (UNI), whereas the NPC function is performed at the network-node interface (NNI) .
UPC/NPC is defined as:
‘The set of actions taken by the network to monitor and control traffic in terms of traffic offered and validity of the ATM (see Asynchronous Transfer Mode) connection, at the user access and the network access respectively. Their main purpose is to protect network resources from malicious as well as unintentional misbehaviour which can affect the quality of service of other already established connections by detecting violations of negotiated parameters (in the traffic contract) and taking appropriate actions. Connection monitoring encompasses all connections crossing the user-network/network-node interface. UPC/NPC apply to both user VCC’s/VPC’s and signalling virtual channels (see Asynchronous Transfer Mode)’. 
Usage parameter monitoring includes the following functions:
What is actually carried out will depend on the access network configuration .
The parameters subject to monitoring and control may be the same as those used for source traffic characterisation to support connection admission control, namely average and peak bit rate, burstiness and peak duration (see traffic contract) .
UPC can simply discard cells that violate the negotiated traffic parameters (in the traffic contract). In addition, a ‘guilt’ connection may be released. Another, less rigorous option would be tagging of violating cells (see Priority Control) . These cells can be transferred as long as they do not cause any serious harm to the network. Thus the overall throughput of ATM cells might possibly be raised .
When, optionally, UPC and traffic shaping are combined, cells that comply with the negotiated connection parameters (in the traffic contract) may be rescheduled in order to improve network performance .
 Hiroshi Saito, Teletraffic Technologies in ATM Networks, Artech House, 1993. ISBN 0-89006-622-1.
 Handel R., Manfred H., Schroder S., “ATM Networks: Concepts, Protocols, Applications“, Addison-Wesley Publishing company Inc., 1994. ISBN 0-201-42274-3.
 Sexton M., Reid A., “Broadband Networking: ATM, SDH and SONET”, Artech House Inc., Boston, London, 1997. ISBN 0-89006-578-0.
 ITU-T: Recommendation I.371. ‘Traffic Control and Congestion Control in B-ISDN’. Geneva, 1993.