Packet radio is a particular digital mode of Amateur Radio ("Ham"
Radio) communications which corresponds to computer telecommunications.
The telephone modem is replaced by a "magic" box called a terminal node
controller (TNC); the telephone is replaced
by an amateur radio transceiver, and the phone system is replaced by the
"free" amateur radio waves. Packet radio takes any data stream sent from
a computer and sends that via radio to another amateur radio station similarly
equipped. Packet radio is so named because it sends the data in small bursts,
What is the history of packet radio ?
Data packet technology was developed in the mid-1960's and was put
into practical application in the ARPANET which was established in 1969.
Initiated in 1970, the ALOHANET, based at the University of Hawaii, was
the first large-scale packet radio project. Amateur packet radio began
in Montreal, Canada in 1978, the first transmission occurring on May 31st.
This was followed by the Vancouver Amateur Digital Communication Group
(VADCG) development of a Terminal Node Controller (TNC) in 1980.
The current TNC standard grew from a discussion in October of 1981 at
a meeting of the Tucson Chapter of the IEEE Computer Society. A week later,
six of the attendees gathered and discussed the feasibility of developing
a TNC that would be available to amateurs at a modest cost. The Tucson
Amateur Packet Radio Corporation (TAPR) formed from this project. On June
26th 1982, Lyle Johnson, WA7GXD, and
Den Connors, KD2S, initiated a packet contact with the first TAPR unit.
The project progressed from these first prototype units to the TNC-1 and
then finally to the TNC-2 which is now the basis for most packet operations
Why packet over other modes?
Packet has three great advantages over other digital modes: transparency,
error correction, and automatic control.
The operation of a packet station is transparent to the end user; connect
to the other station, type in your message, and it is sent automatically.
The terminal Node Controller (TNC) automatically divides the message into
packets, keys the transmitter, and then sends the packets. While receiving
packets, the TNC automatically decodes, checks for errors, and displays
the received messages. Packet radio provides error free communications
because of built-in error detection schemes. If a packet is received, it
is checked for errors and will be displayed only if it is correct. In addition,
any packet TNC can be used as a packet relay station, sometimes called
a digipeater. This allows for greater range by stringing several packet
Users can connect to their friends' TNCs at any time they wish, to see
if they are at home. Some TNCs even have Personal BBSs (sometimes called
mailboxes) so other amateurs can leave messages for them when they are
not at home. Another advantage of packet over other modes is the ability
for many users to be able to use the same frequency channel simultaneously.
What elements make up a packet station?
Figure 1 shows an illustration of a typical station setup with a schematic
diagram of a station wiring.
What is the distance limitation for packet radio?
TNC (terminal Node Controller)
A TNC contains a modem, a computer processor (CPU), and the associated
circuitry required to convert communications between your computer (RS-232)
and the packet radio protocol in use. A TNC assembles a packet from data
received from the computer, computes an error check (CRC) for the packet,
modulates it into audio frequencies, and puts out appropriate signals to
transmit the packet over the connected radio. It also reverses the process,
translating the audio that the connected radio receives into a byte stream
that is then sent to the computer.
Most amateurs currently use 1200 bps (bits per second) for local VHF
and UHF packet, and 300 bps for longer distance, lower bandwidth HF communication.
Higher speeds are available for use in the VHF, UHF, and especially microwave
region, but they often require special (not plug-and-play) hardware and
Computer or Terminal
This is the user interface. A computer running a terminal emulator program,
a packet-specific program, or just a dumb terminal can be used. For computers,
almost any phone modem communications program (i.e. Procomm+, Bitcom, X-Talk)
can be adapted for packet use, but there are also customized packet radio
programs available. A dumb terminal, while possibly the cheapest option,
does have several limitations. Most dumb terminals do not allow you to
scroll backwards, store information, upload, or download files.
For 1200/2400 bps UHF/VHF packet, commonly available narrow band FM voice
radios are used. For HF packet, 300 BPS data is used over single side band
(SSB) modulation. For high speed packet (starting at 9600 bps), special
radios or modified FM radios must be used. 1200 bps AFSK TNCs used on 2-meters
(144-148Mhz) is the most commonly found packet radio.
Since packet radio is most commonly used at the higher radio frequencies
(VHF), the range of the transmission is somewhat limited. Generally, transmission
range is limited to "unobstructed line-of-sight" plus approximately 10-15%.
The transmission range is influenced by the transmitter power and the type
and location of the antenna, as well as the actual frequency used and the
length of the antenna feed line (the cable connecting the radio to the
antenna). Another factor influencing the transmission range is the existence
of obstructions (hills, groups of buildings ,etc.). Thus, for two-meter
packet (144 - 148Mhz), the range could be 10 to 100 miles, depending on
the specific combination of the variables mentioned above.
What do you mean we can all use the same channel?
Packet radio, unlike voice communications, can support multiple conversations
on the same frequency at the same time. This does not mean that interference
does not occur when two stations transmit at the same time, known as a
collision. What 'same time' means in this sense is that multiple conversations
are possible in a managed, time shared fashion. Conversations occur during
the times when the other conversations are not using the channel. Packet
radio uses a protocol called AX.25 to accomplish this
AX.25 specifies channel access (ability to transmit on the channel)
to be handled by CSMA (Carrier Sense Multiple Access). If you need to transmit,
your TNC monitors the channel to see if someone else is transmitting. If
no one else is transmitting, then the TNC keys up the radio, and sends
its packet. All the other stations hear the packet and do not transmit
until you are done. Unfortunately, two stations could accidentally transmit
at the same time. This is called a collision. If a collision occurs, neither
TNC will receive a reply back from the last packet it sent. Each TNC will
wait a random amount of time and then retransmit the packet. In actuality,
a more complex scheme is used to determine when the TNC transmits. See
the "AX.25 Protocol Specification" for more information (ARRL, 1988).
What is AX.25?
AX.25 (Amateur X.25) is the communications protocol used for packet
radio. A protocol is a standard for two computer systems to communicate
with each other, somewhat analogous to using a business format when writing
a business letter. AX.25 was developed in the 1970's and based on the wired
network protocol X.25. Because of the difference in the transport medium
(radios vs wires) and because of different addressing schemes, X.25 was
modified to suit amateur radio's needs. AX.25 includes a digipeater field
to allow other stations to automatically repeat packets to extend the range
of transmitters. One advantage of AX.25 is that every packet sent contains
the sender's and recipient's amateur radio callsign, thus providing station
identification with every transmission.
Networking and special packet protocols
This is a sample of some of the more popular networking schemes available
today. By far, there are more customized networking schemes used than listed.
Consult your local packet network guru for specific network information.
Are there any other protocols in use other
AX.25 is considered the defacto standard protocol for amateur radio
use and is even recognized by many countries as a legal operation mode.
However, there are other standards. TCP/IP is used in some areas for amateur
radio. Also, some networking protocols use packet formats other than AX.25.
Often, special packet radio protocols are encapsulated within AX.25 packet
frames. This is done to insure compliance with regulations requiring packet
radio transmissions to be in the form of AX.25. However, details of AX.25
encapsulation rules vary from country to country.
What are some of those other networking schemes?
During the early days of amateur packet radio, it became apparent that
a packet network was needed. To this end, the following packet network
schemes were created.
BBS Message Transfer: Many of the BBS programs
used in packet radio allow for mail and bulletins to be transferred over
the packet radio networks. The BBSs use a special forwarding protocol developed
originally by Hank Oredsen, W0RLI. Besides full service BBSs, many TNC
makers have developed Personal BBS software to allow full service BBSs
to forward mail directly to the amateur's TNC. This allows operators to
receive packet mail at night and avoid tying up the network during busy
The first networking scheme with packet radio was Digipeaters. Digipeaters
would simply look at a packet, and if its call was in the digipeater field,
would resend the packet. Digipeaters allow the extension of range of a
transmitter by retransmitting any packets addressed to the digipeater.
This scheme worked well when only a few people were on the radio channel.
However, as packet became more popular, digipeaters soon were clogging
up the airwaves with traffic being repeated over long distances. Also,
if a packet got lost by one of the digipeaters, the originator station
would have to retransmit the entire packet again, forcing even more congestion.
Kantronics improved on the digipeater slightly and created KA-Nodes. As
with digipeaters, KA-Nodes simply repeat AX.25 frames. However, a KA-Node
acknowledges every transmission at each link (node) instead of over the
entire route. Therefore, instead of an end-to-end acknowledgment, KA-Nodes
allow for more reliable connections with fewer timeouts, because acknowledgments
are only carried on one link. KA-Nodes therefore are more reliable than
digipeaters, but are not a true network. It is similar to having to wire
your own telephone network to make a phone call.
NET/ROM was one of the first networking schemes to try to address the problems
with digipeaters. A user connects to a NET/ROM station as if connecting
to any other packet station. From there, he can issue commands to instruct
the station to connect to another user locally or connect to another NET/ROM
station. This connect, then connect again, means that to a user's TNC,
you are connected to a local station only and its transmissions do not
have to be digipeated over the entire network and risk losing packets.
This local connection proved to be more reliable.
NET/ROM doesn't use all of the AX.25 protocol. Instead, it uses special
AX.25 packets called Unnumbered Information (UI) packets and then puts
its own special protocol on top of AX.25. This is again used to increase
efficiency of its transmissions. NET/ROM nodes, at regular intervals, transmit
to other nodes their current list of known nodes. This is good because
as new nodes come on-line, they are automatically integrated in the network.
However, if band conditions such as ducting occur, ordinarily unreachable
nodes can be entered into node lists. This causes the NET/ROM routing software
to choose routes to distant nodes that are impossible. This problem requires
users to develop a route to a distant node manually defining each hop instead
of using the automatic routing feature.
NET/ROM is a commercial firmware (software put on a chip) program that
is used as a replacement ROM in TAPR type TNCs. Other programs are available
to emulate NET/ROM. Among them are TheNet, G8BPQ node switch, MSYS, and
some versions of NET.
ROSE is another networking protocol derived from X.25. Each ROSE node has
a static list of the nodes it can reach. For a user to use a ROSE switch,
he issues a connect with the destination station and in the digipeater
field places the call of the local ROSE switch and the distant ROSE switch
the destination station can hear. Other than that, the network is completely
transparent to the user.
ROSE's use of static routing tables ensures that ROSE nodes don't attempt
to route packets through links that aren't reliably reachable, as NET/ROM
nodes often do. However, ROSE suffers from the inability to automatically
update its routing tables as new nodes come on-line. The operators must
manually update the routing tables, which is why ROSE networks require
TCP/IP stands for Transmission Control Protocol/Internet Protocol. TCP/IP
is commonly used over the Internet wired computer network. The TCP/IP suite
contains different transmission facilities such as FTP (File Transfer Protocol),
SMTP (Simple Mail Transport Protocol), Telnet (Remote terminal protocol),
and NNTP (Net News Transfer Protocol) The KA9Q NOS program (also called
NET) is the most commonly used version of TCP/IP in packet radio. NOS originally
was written for the PC compatible. However, NOS has been ported to many
different computers such as the Amiga, Macintosh, Unix, and others. Smaller
computers like the Commodore 64 and the Timex-Sinclar do not currently
have versions of NOS available. TCP/IP based amateur networks are becoming
more common each day.
TexNet is a 3-port switch designed to create a 9600 baud backbone with
2 local access channels. The TexNet network provides transparent network
access to the user. The user simply accesses his/her local TexNet node
and then either connects to a user at another node or accesses various
system services. TexNet provides the stability of fixed routing, while
allowing new nodes to be automatically brought into the network.
Finke, C. R. (Ed.) (1992, February 15). TPRS Quarterly Report. Texas
Packet Radio Society, Inc.
Jones, G., G. Knezek, M. Hata. (1992). Packet Radio Prospects for
Educational Data Communications. Proceedings of the Ninth International
Conference on Technology in Education, 1, 218-219. Paris, France.
Lucas, Larry, Greg Jones, David Moore. (1992) An Educator's Alternative
to Costly Telecommunications. Texas Center for Educational Technology,
Univ. of North Texas.
Steve Watt, KD6GGD, steve@wattres.SJ.CA.US. (1993). Frequently Asked
Questions from the listing in the rec.radio.amateur.packet newsgroup for
the USENET network. Version 1.11.
Tucson Amateur Packet Radio Corporation. terminal Node Controller
Manual, Firmware Release 1.1.8, Tucson, AZ: Author.