Introduction to Packet Radio

Radio amateurs in Canada, Sweden, and the United States have been experimenting with packet radio, a system of computer-based communications. This new mode can provide high-speed communication with efficient use of the spectrum, and is resistant to interference due to other stations and to signal degradation due to adverse band conditions. Not only can packet radio be used for informal amateur QSO's and traffic handling, but it has additional possibilities for exchange of data between hams with computers, "bulletin boards" and message systems, and remote computer programming.

WHAT IS PACKET RADIO?

Packet radio is a communication system in which information is digitally encoded. In this respect it is similar to RTTY or ASCII, but with important differences. These differences are the key to insuring error-free reception and at the same time allowing max- imum use of the spectrum through shared frequency use.

Data integrity is provided by packet radio through a "handshaking" technique and error detection. Along with each transmission, a computed value called a "frame check sequence" (FCS) is sent, which allows the receiving station to check for errors.

The receiving station acknowledges an error-free packet with a special acknowledgement (ACK) signal. If the sending station does not receive such a signal within a certain period of time, it automatically retransmits the packet.

A packet also contains identification of the destination station, permitting several QSO's to take place on the same frequency. A packet radio station can automatically ignore any packets which are not addressed to it. Due to the fact that the duration of most packet transmissions is very short, a user does not need the channel most of the time. The time between transmissions is available to other users on frequency. This system is called time-domain multiplexing. On a very busy channel, the user will notice an increased delay time before getting replies to transmissions, but the packet radio equipment will take care of automatic retransmissions and sorting out the replies meant for the station. The user never "hears" the QRM.

WHAT IS A PACKET RADIO STATION?

Packet radio requires the use of a microprocessor-based controller at each station, and it will obviously appeal to the ham who already has a computer in his shack. However, it does not require that the operator be a programmer, or even that the station have a personal computer. All that is really necessary is a terminal, a terminal node controller (TNC), and an amateur radio transceiver.

The terminal can be a simple display (CRT) or typewriter terminal that produces ASCII characters, a personal computer, or even a commercial mainframe computer. What you need is a terminal with a keyboad to allow you to talk and a screen or a printer to allow you to read incoming information. You can even get an inexpensive terminal that uses a TV set for the display.

The way in which most terminals encode ASCII characters is in "asynchronous" format. SInce characters are encoded as they are typed, there is a flag consisting of one or more "mark" (binary 1) values to mark the beginning and end of each character. The device decoding the characters expects a specific "baud rate", or number of transitions from "mark" to "space" (binary 0) per second during the character, but no particular time interval between characters themselves.

The terminal node controller is the heart of the packet radio system. It has one port that is connected to the terminal or computer, and communicates through it by asynchronous ASCII format at the baud rate required by the terminal. The TNC converts the data stream from the terminal to a packet by attaching a "header" showing the destination of the packet and control information for the network, a "tail" containing the result of the FCS calculation for error detection, and flags to mark the beginning and end of the packet.

The second port of the TNC connects it to the transceiver microphone and speaker audio lines, and the PTT line. Ordinarily, the TNC will produce AFSK modulation by putting one of two tones into the microphone input, corresponding to a "mark" or "space". In this fashion, the packet is sent out on the air at the packet channel baud rate, which is unrelated to the terminal baud rate at the other port of the TNC.

The receiving TNC reverses this procedure, decoding the audio tones from the speaker audio line of the radio, removing and reading the header and tail information, and passing a successfully received packet to the terminal at the terminal baud rate.

The part of the TNC that does the translation between the sequence of tone levels and the characters is called a "modem", short for MOdulator-DEModulator. This device may or may not be built into the TNC board. Most packet radio modems operate at 1200 baud, which corresponds to about 1200 wpm, although the FCC now authorizes much higher baud rates on some amateur bands. The audio tones used are 1200 hz and 2200 hz. This choice of frequencies is that of the Bell 202 modem, which is available as surplus.

The final component of a packet radio station is an amateur radio transceiver. Most packet radio activity so far has been in the 2-meter band. The only important requirement of the radio is that its audio frequency response at 2200 hz be adequate. In other words, the 2-meter FM rig you already have is probably just fine.

WHAT THE TNC DOES

The TNC consists of a special purpose microcomputer, containing all the necessary software and hardware to communicate with your terminal, assemble a packet, operate your transmitter and receiver to send and receive a packet, and decode a packet. The special functions of the TNC which would be difficult to implement with an ordinary personal computer are the use of protocol to communicate with other TNC's and real-time control.

The encoding and decoding of packets involves a carefully standardized set of procedures called "protocol". The protocol basically determines the exact form of the header and tail parts of the packet. The header allows receiving TNCs to automatically determine the purpose of the packet, e.g., net check-in, part of a QSO, or ACK to a previous transmission. The tail contains the FCS which allows the TNC to automatically determine whether the packet was received correctly, and if so, to automatically acknowledge it. Since the protocol is programmed into the TNC, the operator does not need to know exactly what his packet looks like. In particular, he does not need to know how the destination of his packet is indicated. The operator communicates with other amateurs by call sign, and the TNC translates the call sign into the identification required by the protocol.

The TNC is required to perform a number of tasks simultaneously, including responding to events such as the receipt of a packet or instructions from the operator in "real time", in other words, as they happen. This makes programming in BASIC, the common language of personal computers, undesirable. This is because BASIC use an "interpreter" which reads each line of the program and translates it into machine-type instructions every time the line is executed. The time required for the translation would prevent a program from responding rapidly enough in a packet radio environment. In order to meet the speed requirement, an assembly-language program or equivalent is required. While BASIC looks pretty much the same on any computer, assembly language is different for every machine. If the TNC were replaced by personal computers, program development would have to be redone for each variety of computer. In addition to maintaining the right pace, the TNC also must be constantly "listening" at both ports simultaneously while putting packets together or taking them apart. The hardware of personal computers may not even be capable of this sort of multi-task application.

Programming of individual TNC's must be as easy as possible, since there will inevitably be unforseen problems in the initial software. In addition, hardware changes may necessitate software changes. For this reason, TNCs are designed around erasable programmable read-only memories (EPROM's), which normally function like the ROM of a personal computer, where the vital software is storaed in an indestructible form. However, if the need arises, they can be reprogrammed by "burning in" the new program using special equipment.

WHAT IS A PACKET?

A packet is the basic message unit in packet radio. It ordinarily consists of a text message typed in by the operator, sandwiched between the header and tail information required by the protocol. In a typical QSO, a packet would be encoded and sent out by the TNC when the operator ends a line of typing by hitting the RETURN or ENTER key. In any event, the length of a packet is limited, usually to 128 characters. This helps to prevent a single user from "hogging" the channel, as well as making sure that the sending and receiving TNC's don't get swapmed with information.

A packet need not consist of ASCII or Baudot character strings, however. It could contain information in other coding systems, such as BCD or EBCDIC, or even binary data such as a compiled computer program. The TNC, which uses a "bit oriented protocol" based on a standard called High Level Data Link Control (HDLC), can encode any of these equally easily. An advantage to this choice of protocol is that the functions it requires are available on a single large-scale integration (LSI) chip, which simplifies the TNC hardware and software. A second advantage of HDLC protocol is that the beginning and end of the entire message are flagged, making the "start" and "stop" bits for each character unnecessary when the packet is transmitted in "synchronous" format.

The "frame" of an HDLC packet is represented below. Each field of the packet is encoded as a sequence of 1's and 0's (bits) to be transmitted as "mark" and "space" tones. With the exception of the DATA field, all these fields are generated by the TNC as it assembles the packet for transmission. The operator is concerned only with the contents of the DATA field.

| Flag | Address | Control | Data | FCS | Flag |

The FLAG is a unique bit sequence which identifies the beginning of a packet to the HDLC controller. This pattern corresponds to no sequence which would be encountered in any of the other fields, except possibly in the transmission of binary data. Even in this case, there are provisions for distinguishing data from the flag sequence.

The ADDRESS field contains routing information for the packet. This information may include the destination station, the originating station, and possibly intermediate routing information if the packet will be relayed to the destination. The destination and originating stations mights be identified by a network address number of by amateur call sign, depending on the exact form of the protocol being used.

The CONTROL field describes the purpose of the packet to the network. It identifies packets with such functions as network check-in or check-out request, packet acknowledgements, or request for information from net control. It may also contain a sequence number for a multi-packet message which must be received in the correct order.

The DATA field contains the message being sent, which will ordinarily be the text typed in by the user, converted into an ASCII data string. In the case of a packet identified in the control field as performing a control function, the DATA field may be absent.

The FCS allows the receiving station to verify that the packet has been received correctly. If the FCS calculated by the receiving TNC matches the FCS of the packet, an acknowledgement is sent; otherwise the packet is ignored.

WHAT IS A PACKET NETWORK?

A local area packet radio network (LAN) consists of a net control station and a number of individual operators. The net control station is sometimes referred to as the "station node" and the individual stations as "terminal nodes". The net may also contain a digital repeater or "digipeater", which may be the net control station or a separate repeater station. The repeater station may be a single-frequency simplex repeater which retransmits any correctly received packets, or it can be "normal" split frequency repeater.

As operators sign on to the net, they are recognized by the net control and given net address codes. An operator desiring to start a QSO with another net station will subsequently have his transmissions addressed to that station. Any operator may choose to have his TNC receive all transmissions, rather than just those addressed to his station. Of course, the TNC will only acknowledge those transmissions intended for that station. The operator whose station is functioning as net control participates in exactly the same way as other operators. The net control functions are taked care of automatically by his TNC.

As more packet radio LAN's become active, there will be the possibility of link stations with access to two distinct LAN's. These stations can be members of both nets and serve as communications links thorugh which packets originating in one net can be funneled to an addressee in the other net.

A more sophisticated possibility is that of a "gateway" station, which will be a specialized station having access to some long-distance mode of communications. The gateway station will reformat packets with another layer of protocol containing internetwork linking information and transmit it to another gateway station in a distant LAN. Three possibilities are being explored for long-distance links.

TERRACON will be a high-speed ground-based linking system utilizing UHF and/or microwave relays. It could potentially handle most long-distance packet radio communications in the United States and Canada. It will probably be a few years before TERRACON is implemented as a useful system, and somewhat longer before the continent is linked.

AMICON will be a satellite-based network utilizing one of the special-services channels on the AMSAT Phase III-B satellite. AMICON will allow intercontinental linking and contact with isolated areas not accessible to TERRACON. High data rate experiments are being planned for the 23cm uplink/70cm downlink (mode L) translator. There are also plans for a packet radio digital repeater aboard the AMSAT Phase III-C satellite.

SKIPCON is AMRAD's projected HF network of LAN gateway stations. The nature of HF propagation will require slower data rates (75 to 600 baud) and error correction as well as error detection protocol. SKIPCON experiments have been conducted since the end of 1981.

HOW TO GET IN ON PACKET RADIO

There are currently two TNC designs available. The first packet radio TNC was designed by the Vancouver Amateur Digital Communications Group (VADCG). The Vancouver TNC is available as a bare board, and requires a power supply, and external modem, and parts. It comes with instructions and notes on the power supply. A modem kit is also available from VADCG. The TNC design is based on the Intel 8085 CPU and 8273 HDLC controller and includes 4K bytes of 2114 RAM and 4 K bytes of 2708 EPROM. The TNC requires an 8250 (serial ports) or an 8255 (parallel ports) for interface to the terminal, as well as an interface to the radio.

The Tucson Amateur Packet Ra…io group (TAPR) is currently testing a second TNC design. This TNC has the modem, radio interface, serial and parallet terminal interfaces, and power supply circuitry (exclusive of the transformer) on a single board. It is based on the 6502 microprocessor (Editor's note: the board will now use the Motorola 6809E microprocessor), and can hold a total of 48K bytes of RAM and ROM on the board. the 1933 HDLC chip it uses is compatible with the 8273 chip used on the VADCG board, and the TAPR TNC will be capable of VADCG-compatible protocol.

Additional information on TAPR activites is available from Tucson Amateur Packet Radio, PO Box 22888, Tucson, AZ 85734.