Distributed Data Transfers in CMS

The multi-tiered computing infrastructure of the CMS experiment at the LHC depends on the reliable and fast transfer of data between the different CMS computing sites. Data have to be transferred from the Tier-0 to the Tier-1 sites for archival in a timely manner to avoid overflowing disk buffers at CERN. Data have to be transferred in bursts to all Tier- 2 level sites for analysis as well as synchronized between the different Tier-1 sites. The data transfer system is the key ingredient which enables the optimal usage of all distributed resources. The operation of the transfer system consists of monitoring and debugging of transfer issues to guarantee a timely delivery of data to all corners of the CMS computing infrastructure. Further task of transfer operation is to guarantee the consistency of the data at all sites, both on disk and on tape. Procedures to verify the consistency and to debug and repair problems will be discussed