The NATO Communications Systems Network Interoperability (CSNI) programme was established in 1991 by memorandum of understanding (MoU) between the organisations concerned. It consisted of two phases, the first (CSNI-1) ran from 1991 until autumn 1995, the second (CSNI-2) from late 1995 until the expiry of the MoU in December 1996. The main focus of CSNI was to show that open systems protocols (open systems interconnection (OSI) and Internet Protocol (IP)) can be used to support information transfer over typical military type bearers (SHF satcom, UHF satcom, UHF LOS, long haul HF and HF ELOS). Commercial off the shelf (COTS) products were used wherever possible. The bearer to be used was selected on the basis of location of recipient and current link utilisation. OSI/IP routers were used to provide the dynamic switching of data between bearers. The overall network topology for CSNI-1 is illustrated. (11 pages)