PACKET TECHNICAL INFO
Newcomers to the digital mode of AX25 or "Packet Radio" in the PE Area may wonder as to what and where their packet signals are connecting to. This page aims to enlighten packeteers and other interested parties on their local packet network configuration.
"Packet Radio" is the term given to this digital mode of communication, used mainly to send text messages to and from a communal bulletin board system (BBS). The name stems from the fact that the digital information is broken up into "packets" of data, which are then serially transmitted over the radio channel by means of audio frequency shift keying (AFSK) at 1200bd. Each packet is verified, any errored packets are detected and retransmitted. For a full write-up of the AX25 protocol, click here.
The Port Elizabeth Amateur Radio Society runs a communal Bulletin board system (BBS) with the callsign of ZS0NTP, open to all users, whether members or not. This is usually the "entry point" of most packet operators, since the BBS stores and forwards packet messages to and from local users to any destination in the world. ZS0NTP comprises a software package "FBB BBS", running on a standard ‘486 PC, resident in the repeater cubicle on Lady’s Slipper mountain. The PC has two serial ports, to which two Kantronics packet Terminal Node Controllers (TNC’s) are connected. These two TNC’s each have two radio ports, and all in all four transceivers are connected to them.
The task of the BBS is to allow users to connect to it via HF, VHF or UHF, at a speed of either 300 baud (normally HF operation), 1200 baud (VHF/UHF operation) or 9600 baud (UHF operation). These speeds are determined by the radio port used on the TNC’s. ZS0NTP runs the following ports:
300 baud 10.151 MHz HF APRS port 1200 baud UHF backbone channel (for forwarding) 1200 baud 144.625MHz User Port 9600 baud 434.875MHz User Port 1200 baud 433.800MHz APRS port 1200 baud EC Link Network Monitor (for APRS traffic)
Thus as a user in the immediate Lady’s Slipper coverage area, you have these frequencies available. Rural users have HF and possibly VHF if within the VHF coverage area of the Cradock digipeater or Grahamstown Node. The BBS allows users to check for any mail messages sent to them, or to send messages to other amateurs elsewhere. For a list of the commands to use on the BBS, connect to it and type H for "help". It is useful to save this listing to file for future reference. Or read them here.
Messages are forwarded direct from our BBS using the PEARS WiFi network, and a Telnet session to ZS6RO.
To connect to ZS0NTP in the PE Area, simply run up your Packet program and connect to "ZS0NTP", using the VHF port on 144.625MHz SIMPLEX, at 1200 baud (the standard soundcard interface and AGW PE). But if one is outside of the immediate PE Area, extra complications come into play. This is where the Grahamstown Node or Cradock Digi is used, along with the UHF backbone channel mentioned above, or else 300 baud HF packet must be used, on 10 151kHz.
Originally, to extend packet coverage both to users and the BBS, a digipeater was installed at Grahamstown (ZS0GHT), which most users and the BBS could access on 144.675MHz. The digipeater was invoked by including its callsign in the connect command, such as "C ZS2PP V ZS0GHT". But this form of repeating the digital message again on the same channel leads to a lot of clashing transmissions, and the end stations having to perform a number of retries, especially when the channel becomes busy. It was then decided to upgrade the digipeater to an improved system of message exchange – the Node. Nowadays, traffic from ZS0GHT to ZS0NTP travels on the backbone UHF channel, thus does not clash with user traffic on 144.675 or 144.625MHz
In the Eastern Cape, Cradock hams have erected a simple digipeater at Cradock mountain, which can see the Grahamstown Node, thus forming a packet path to the PE BBS. In future, it will make good sense to form another UHF backbone channel to this site from Grahamstown, and create a Cradock Node, thus improving on speed and coverage. A similar backbone link can be set up towards East London from Grahamstown, allowing high speed access between the two cities. The whole network can easily be expanded in this manner. If UHF backbone links are run at 9600 baud, remote users will not know that they are actually connected through so many hops!
See the Packet Network diagram on the previous page...
Remember that the PE BBS, ZS0NTP, has a "node" callsign of ZS0NTP-2. A Node allows users to connect from one port (say 144.625MHz) onto another port (say the UHF backbone). In this manner, one could connect from your QTH in say Jeffreys bay, via the ZS0NTP-2 node, via the UHF backbone to Grahamstown, then via the Cradock digipeater to hams in Cradock, or via some of the accessible sites in East London, to hams in EL……
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