This file was written by Ed Harland, G3VPF some years ago but is still a good introduction to Packet Radio. Ed was the first station to conduct an AX25 QSO in the UK here in South Dorset.
AN INTRODUCTION TO PACKET RADIO - (C) by Ed Harland G3VPF - g3vpf@g3sds.org.uk
Packet radio is a new form of digital communication which allows100% error-free communication between amateur stations whilst aiding efficient use of the spectrum by allowing multiple QSO's to share the same channel. The system was devised by a group of American amateurs in 1982 and version 2, finalised in 1984, has now been adopted by the IARU as the international standard. In the UK, the RSGB, through its Packet Radio Working Group, recommended that AX.25 be used as the UK standard. AX.25 is a sub-set of the CCITT X.25 protocol optimised for use on radio links.
The first form of digital communication used by radio amateurs was RTTY. This used two frequencies to represent the digital levels '0' and '1' (usually referred to as space and mark). Each character to be transmitted was represented by a five bit code and the set of possible characters used was called the Murray code. Each bit of the character was transmitted sequentially at a rate of 50 bits/sec. In order to identify the start and finish of each character, three additional bits were added to the 5 data bits. A bit which is always set to space was added at the start and called the start bit. Another two bits always set to mark were added at the end and called stop bits. The purpose of the start bit was to engage the clutch in a mechanical teleprinter and allow the print mechanism to get ready to decode the following character. The stop bits allow the mechanism to return to the starting condition before the next character starts.
RTTY was used for many years but had the disadvantages that any QRM caused erroneous copy and that only one station could use a channel at any one time. Manual slow typing occupied channels for long periods. To try and overcome the problem of errors a new system was introduced in the mid-70's called AMTOR. This overcame the QRM problem by breaking the text into three character groups and waiting for an acknowledgement from the receiving station before sending the next three characters. If no acknowledgement was received, the same three character group was re-transmitted until the acknowledgement was received. This is known as Automatic Repeat reQuest (ARQ). In order to improve channel usage, the start and stop bits were discarded, and only character information sent. This is known as synchronous operation because the transmitter and receiver are synchronised together. The receiver achieved synchronisation with the transmitter at start of QSO by looking for a special character sequence. In order that the receiving station could know that the received characters were good, a special form of coding was used that allowed errors to be detected. This system gave much better performance in the sort of conditions experienced on the HF bands but still required the channel 100% of the time during the QSO.
AX.25 is a different approach to digital communication. As in AMTOR the message is broken up into character groups but these can be up to 256 characters long. Attached to each character group is a 16 bit error checking code, a control byte and callsigns.The whole assembly is called a packet. The start and end of a packet are marked by using special bytes called flags which are not allowed to appear anywhere within the packet. The transmitting station expects to get an acknowledgement of each packet from the receiving station before moving on and sending the next packet but, unlike AMTOR, because each packet contains callsigns it is possible for many stations to time-share a channel. The rules that govern the way the transmitting and receiving stations respond to the various possible packets are called the protocol.
The protocol used by most radio amateurs is AX.25.
STRUCTURE OF A PACKET
The basic unit of information in a packet is eight data bits called a byte or octet. Each character in text to be transmitted is represented by an eight bit binary number and the system used in AX.25 is the ASCII character set. The bytes which make up the packet are grouped in sub-sections called fields. The four fields are address, control, information and frame check sequence (FCS). These are arranged as follows:-
--------------------------------------------------------------------------------------------
I I I I I
FLAGS I ADDRESS I CONTROL I INFORMATION I FCS I FLAGS
I I I I I
--------------------------------------------------------------------------------------------
FLAGS:-
The flags are a special eight bit sequence (01111110) which denote start and end of packet. They may be (and usually are) repeated many times but must be sent at least once at start and end of packet.
ADDRESS:-
The address field is made up of several sub-fields and contains the callsigns of the sending station, receiving station and optionally the list of stations who will act as store-and-forward repeaters in order to get the packet to its destination. The order of sending is:-
<-----------------ADDRESS FIELD----------------------->
-------------------------------------------------------------------
I Receiving I Transmitting I Up to eight repeater I
I callsign I callsign I callsigns I
-------------------------------------------------------------------
Each sub-field of the address field consists of seven bytes.
The first six contain the callsigns in ASCII whilst the seventh contains additional information including a secondary station identifier (SSID) which is a number from 0 to 7 and allows each callsign to have up to eight TNC's active at a time. The SSID forms an extension to the callsign i.e. my main TNC would use G3VPF whilst a second would be G3VPF-1 and a third G3VPF-2 etc. The other bits in the seventh byte are used in conjunction with the control byte.
CONTROL:-
The control field is a single byte which is used to control the flow of information being sent.
For details of the control field make-up and use see ref. 1.
INFORMATION:-
The information field contains the actual data to be transmitted.
The maximum size within the AX.25 specification is 256 bytes.
The minimum size is zero.
FCS:-
The FCS field is a special 16 bit sequence which is calculated from all the other bytes in the packet.
At the receiving station this calculation is repeated as the incoming packet arrives and when the packet is complete the locally calculated FCS is compared with the received FCS and if equal then the packet contains no errors. If they are not equal an error has occurred and the packet is ignored by the receiving station.
THE PACKET STATION
The equipment required to operate packet consists of a radio transceiver, a terminal node controller (TNC), and a data terminal.The transceiver can be any of the standard commercial black boxes appropriate to the band used.
For VHF working a two metre FM rig is required whilst for HF any of the standard SSB rigs will suffice.
Frequencies used are 3.59MHz and 14.102MHz using 300 bits/sec data rate and 144.950 MHz using 1200 bits/sec.
There is no change-over timing problem experienced as in AMTOR and no duty-cycle problem as in RTTY.
Connections required to the rig are:-
Audio output - loudspeaker or headphone o/p.
Audio input - Microphone socket.
TX on/off - PTT.
The intelligence required to encode and decode packets and handle the protocol is provided by a microprocessor in the TNC. The TNC is a separate unit which sits between the radio and the users terminal.
The interface to the users terminal is usually RS232.
The TNC provides all the facilities of the AX.25 protocol as well as additional facilities like channel monitoring, tone calibration and answer-back messages.
The user terminal can be a dumb VDU or one of the many popular home computers running a suitable software package.
The only requirement is that it must have a suitable interface to match the TNC. Use of a computer rather than a dumb terminal allows additional facilities such as extended monitoring and file transfers to be used.
HISTORY
TAPR TNC1.
The original TNC produced by the Tucson Amateur Packet Radio
Corporation in America as a kit. Also available from Heathkit (the HD4040) and AEA (PKT1). Used a 6809 cpu and WD1935 HDLC controller.
The original software was adequate but an alternative set of software from WA8DED was far better.
TAPR TNC2
This was the second TNC to be produced by TAPR and used a Z80 cpu and Z80 SIO HDLC controller.
TAPR have now withdrawn from the market and handed over the design to commercial suppliers.
PACKET ORGANISATIONS
Tucson Amateur Packet Radio Corporation (TAPR), Tucson, Arizona, USA.
The original packet group who produced the TNC1 and TNC2 designs from which most of the current TNC's originate.
They produce a quarterly newsletter called 'Packet Status Register'.
Radio society of Great Britain (RSGB), Potters Bar, Herts.
The RSGB is negotiating packet facilities with the DTI and are responsible for planning the growing packet network.
Southwest AX.25 Group (SWAX25). A loose grouping of amateurs set up to promote AX.25 packet in the Southwest of England.
Please bear in mind that the above article was written during the mid 80’s and some of the information may be a little out of date. It is if anything a lot easier to start up in packet these days, when so many homes have computers. Just think what may be happening in say 15 years from now!
REFERENCES
'AX.25 Amateur Packet Radio Link-Layer Protocol. Version 2.0'
Terry Fox WB4JFI. Oct. 1984. Published by ARRL, available from RSGB.
'Your First Packet Station' – 1st Ed. - RSGB
'Packet Radio Primer' – 2nd Ed. - RSGB
'Packet, Speed & More Speed' - RSGB
The above article 'AN INTRODUCTION TO PACKET RADIO' by Ed Harland G3VPF was printed with his kind permission and must not be reproduced in any form without prior written consent.
(The information contained in this supplement whether Editorial or Advertisement is printed in good faith. We cannot be held responsible for any incorrect information. The opinions voiced in articles are not necessarily those of the Editor or Publisher.)
South Dorset Radio Society - www.g3sds.org.uk sdrs@g3sds.org.uk
Edited for SDRS web site by Geoff, G0EVW - May 2002 - g0evw@g3sds.org.uk