Bulletins from the Pacific Packet Radio Society - page 177

program". The operating system provides a set of services to the application, mainly in the areas of memory management and 10 control. The operating system includes the software necessary to control the flow of data between PACMMS and ground stations, PACMMS and the control computer, PACMMS and the large memory.

The major advantage of this split is that the application software, the "bulletin board" system, can be designed to interface with a set of virtual devices. Checkout and testing can be done on a variety of ground based computer systems with floppy disk simulating the large onboard memory. As the inevitable hardware changes occur only the low level operating system routines will have to be changed.

PACMMS must also be a multiuser system. Several users can be "Logged on" to PACMMS at any time. Each must have a separate context area maintained, both at the protocol level and at the application level. Here again the operating system concept will be useful. As multiple users access multiple messages and files, standard OPEN/CLOSE/ READ/ WRITE operating system services will help solve multiple access and concurrency problems.

Mailbox.

PACMMS will offer an electronic mail service. Minimum attributes of mail messages would be from-address, to-address, creation date, title, and n lines of text. A minimal security mechanism should be provided to protect against accidental/malicious deletion of messages. Addresses should be callsign based. A mechanism for purging old messages should be provided.

Files.

Files are sent to PACSAT and stored for later retrieval. Files, unlike messages, have no internal structure forced on them by PACMMS. Files have names, creation dates, and the same level of security as is provided to messages. Files can also be created by PACMMS for its own use,e.g., for the storage of telemetry. Other uses for files include storage of PACMMS application software, raw data to be transferred among ground stations, new software for ground station control ready to be burned into ROM.

Store and forward.

PACMMS stores packets received from ground stations and sends them' thru the downlink when requested by the addressed ground station. This differs from messages and files in the PACMMS stores the packets without attempting to use the data in the packets. PACMMS acts as a delayed repeater, except that packets are acknowledged by PACMMS when sent and received. This allows ground stations to communicate with each other in a higher level protocol than PACMMS is aware of without forcing the constraints of messages and files.

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