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   3.605 MHz
	1200 baud UHF backbone channel
	1200 baud 144.625MHz
	9600 baud 434.875MHz

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 ROSE switch. 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 kindly forwarded by our forwarding station, Jim, ZS2LR, who connects to the BBS on the UHF backbone channel, and also to Div 6 via HF, on 7, 10 and 14 MHz, all automatically switched by the same BBS software package.

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 Baycom-type modem). But if one is outside of the immediate PE Area, extra complications come into play. This is where the packet ROSE switch network is used, along with the UHF backbone channel mentioned above, or else 300 baud HF packet must be used, on 3605kHz.

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 "ROSE switch".

A basic "ROSE (RATS Open Systems Environment, where rats are the Radio Amateur Telecommunications Society, not the vermin type…!) switch" station consists of at least two TNC’s connected back to back, each connected to its own transceiver (each on a different frequency, obviously..!) This allows a user to come in on one frequency, such as the old user channel of 144.675MHz, and shoot off towards another destination such as the BBS in PE, on a completely different channel, in this case the UHF backbone channel. The name "switch" comes from the action of switching the message packets from one port to another. The user has the speed advantage of the repeated packets going off on the backbone channel at almost the same time, rather than on his user channel, and this can happen in "full duplex", so while the user is sending to the ROSE switch, it is busy sending the same message on towards the BBS on UHF.

This is essence is how the ROSE switch at Grahamstown works. The Grahamstown ROSE switch has a few callsigns, as a basic digipeater, it will still respond to just ZS0GHT. To make use of the ROSE switch features, one must use the callsign ZS0GHT-3. To connect via the ROSE system, this callsign must be included in the connect command. The exit-point of the network must also be included, and this is formed by the local telephone dialing code where the switch resides (such as 0461 for Grahamstown), plus an identifier 01, 02 etc. (These dialling codes have since changed, so we will update them on the Rose switch in due course). Because the Grahamstown Rose switch is the only working site at the moment, the exit point is always at Grahamstown itself, so the number is 046101 for VHF, or 046102 for UHF… If further ROSE switches are added to the network, then their codes could be used instead, to cause the network to route via the backbone channels to them, and then out to a station in their area.

The design of the ROSE switch network allows for a far more complex arrangement of backbone channels, but to the user, these are transparent, as all that is required is the callsign of the local entry point ROSE switch (ZS0GHT-3), and the area code for the destination station (046101 or 2).

In the Eastern Cape, Cradock hams have erected a simple digipeater at Cradock mountain, which can see the Grahamstown ROSE switch, 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 ROSE switch, 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……

Some of the more interesting addresses to connect to are:

In the PE Area, 0n 144.625 MHz:

C ZS0NTP-2, this will connect you to the Lady's Slipper Node, then issue the command:

	"MH 1" to get a listing of hams heard on 3605 kHz HF channel
	"MH 2" to get a listing of hams heard on the UHF backbone
	"MH 3" to get a listing of hams heard locally on 144.625 MHz
	"MH 4" to get a listing of hams heard on 434.875 MHz 9600 baud channel
	"C 2 heard V ZS0GHT-4,046101" to connect to the Grahamstown Rose switch and get a listing of hams heard by the Grahamstown Rose switch on its 144.675MHz port.
	Disconnect by sending B
	"C 2 ZS0CDK-2" to connect to the Cradock Node, then issue the command "MH 1" to get a list of hams heard by the Cradock Digipeater – you will note some Div 4 callsigns here…

In the  Grahamstown Rose switch area on 144.675 MHz:

	"C HEARD V ZS0GHT-3,046101" to get a listing of hams heard by the ROSE switch on 144.675 MHz
	"C ZS0CDK-2 V ZS0GHT-3,046101" then issue the command:

"MH 1" to get a listing of hams heard by the Cradock Digipeater

Enjoy & 73 

Chris
ZS2AAW