Abstract: TDM (time division multiplexed) traffic is transported together with a TDM performance monitoring channel over a ATM (asynchronous transfer mode) network. The TDM traffic is segmented at ingress to the ATM network and is packed as payload data into ATM cells. Each cell payload is provided with an AAL1 segmentation and reassembly protocol data unit (SAR-PDU) containing a channel state information (CIS) field, a sequence number field, and a sequence number protection field. A performance monitoring channel is provided by utilising spare bits in the CIS field in alternate cells. On egress from the ATM network, the sequence number protection and performance monitoring data provide a means of identifying both missing and corrupted cells so as to determine a measure of the quality of transport of the TDM traffic over the ATM network. The measured transmission quality may be compared with service levels defined in a service level agreement.