Bonnie, Here is my experience with mobile messaging with Pskmail. This was done on HF NVIS but I believe the concepts are independent of the radio link method (HF or VHF). My background is four wheel driving in the Australian "Outback" and keeping a track log of my position a well as exchanging email/sms messages where there is no cellular coverage (we have a very low population density and therefore patchy coverage outside the coast and main roads).
The way Pskmail addresses the push messages is by using two concepts: 1. A notion of Linked-to-a-base status and 2. a centrally accessible (over the internet) database so that servers can be coordinated and avoid duplication). More details: The clients (mobile units) have to "check in" by sending a "link to base". From there on, the server will push any new APRS messages (without ack) to the client. Of course other message sources could be pushed as well. Note that this is different to "connecting to a base" like for email retrieval as there is no maintenance of the link through continuous exchanges and therefore very little time occupancy of the "channel". It therefore allows for several stations to share the same frequency. How many has not been tested in practice. At the moment the operator has to determine which base to link-to, but a more automated feature coud be used there: with Pskmail the clients can send an automatic beacon every x minutes (to all servers) with the position of the mobile unit. Coordinated over the APRS network only ONE server will reply with a QSL message. The improvement on the current concept could be that the replying server could automatically become the "linked-to" server pushing messages from there on. So if propagation/conditions change and another server becomes the best link, then it should take over the "message pushing" task until further notice. I imagine that sending the position from the mobile as a beacon is also valuable in emergency situation as an information to the central command/other mobiles. Another improvement on the current method would be to loosely verify the delivery of the messages. If the server, through the central database" were to keep track of the number of messages which should have been delivered, and the client the number of messages received (or the sequence number of the last contiguous messages received), and both client and server exchange this information during the beacon calls, then is is possible to know if the client has all of the messages up to date or not. Messages are packaged with a data integrity information (CRC) and therefore can be determined to be received correctly or not which of course you already have in the HFN system. Great system by the way. A separate point, but relevant in my opinion and was touched on in a previous post: the "terminal" unit needs to be frugal on power. Even my small eee laptop consumes over 2 AMPs of my 12V supply. If stationary for an extended time, and if not equipped with a separate dual battery system I would be reluctant to leave it on all the time. Maybe something like the a TNC as mentioned before or a software adaptation of the NUEPSK could provide a power budget more in line with a standard battery installation. Rein could give you more details (or correct my understanding) on Pskmail and the new developments. Hope this helps. All the best, 73, John (VK2ETA)
