A scheme called Priority Oriented Demand Assignment (PODA) was developed. PODA divided time on the satellite channel into frames and used one part of the frame to transmit data and another part of the frame to send reservation requests. Short reservation-request messages were sent in slots in the reservation part of the frame. These requests were received by all nodes listening on the satellite channel. All nodes performed a distributed scheduling algorithm and assigned a portion of the data part of a subsequent frame to the site that had been given the reservation. At the reserved time, that site would send its data and the other sites would be silent. On the downlink from the satellite, the data would be delivered to all sites on the channel and if it contained data destined for a host computer attached to that node it would be delivered; otherwise it would be discarded. Two variations of PODA were developed; Fixed PODA (FPODA) had a reservation slot for each site on the satellite channel and Contention PODA (CPODA) had a smaller number of reservation slots and the sites used the Slotted ALOHA method to contend for reservation slots. CPODA provided more efficient utilization of the satellite channel resources, though it was harder to implement and get to work.
In 1974, ARPA initiated a project to build a working PODA network named The Atlantic Packet Satellite Network, which was later shortened to SATNET. The purpose of SATNET was to extend the Internet to Europe and support experiments in the use of broadcast satellite channels for packet switching, as well as joint NATO experiments in distributed command and control. BBN was selected to implement the Satellite IMP as a modification to the standard ARPANET IMP. The SATNET Satellite IMP (SIMP), originally implemented on a Honeywell 316 minicomputer, contained more packet buffer memory to account for the long 250 msec. transmission delay between the earth and the satellite. It also had a special hardware interface to the satellite channel burst modem/codec which could precisely time radio transmissions and recover satellite channel clock times. The software implemented the PODA channel access and sharing algorithms. Comsat Labs had responsibility for the earth terminal equipment and Linkabit Corp. had responsibility for the burst modem/codec equipment. In addition to implementing the SATNET SIMP software and special hardware, BBN had overall responsibility for SATNET operations. The network was monitored and controlled by the ARPANET NOC at BBN using some of the same tools that were used for the ARPANET, or tools that had been modified for the SATNET application.
The original SATNET used a 64Kb/s channel on Intelsat’s Atlantic Ocean Region and included three main sites located at Intelsat country earth terminals in the US, Sweden and the UK. SATNET was operated as a separate network with early IP routers connecting to ARPANET in the US and local in-country networks at research institutions in Europe. In the UK, local area networks at the Computer Science Dept. of the University College London and the Royal Signals and Radar Establishment connected via Internet Gateways to the SATNET SIMP at Goonhilly Downs. In Norway, the Norwegian Defense Research Establishment and the Norwegian Telecommunications Research Establishment connected to the SIMP at Tanum, Sweden. The point-to-point transatlantic link of the ARPANET was disconnected and SATNET provide the primary network connection of the emerging Internet to Europe. In the early 1980s two new SIMP sites were established at Raisting, Germany and Fucino, Italy connecting the German Aerospace Research Agency (DFVLR) located outside Munich and the Italian Computer Research Center at CNUCE in Pisa.