FAQ: Packet Radio
This document attempts to describe packetized data, packet-radio, and the several things hams think of when they hear the phrase "packet radio".
This document is under construction.
What is Packet-Radio?
Packets are groups of symbols (letters?) that are sent in all at once with addressing and error-recognition or correction included.
The usually scheme for delivering packets is to generate a sequence of electrical signals that are of an obvious pattern, to the recipient device, followed by a start symbol, that the recipient will recognize, then the body of the message, and finally an end that the recipient will recognize.
The body of the message will contain addressing, number of bytes (characters) and some information that allows the recipient to verify that all of the information is intact.
After the packet is finished, and before the next packet is started, there is a period of no-traffic.
The station that would receive can stand by for the start of another packet and will be looking for some sort of start sequence or external trigger to indicate start-of-packet-frame.
In our hobby, ham radio, we use packets in all sorts of places including Ethernet, WiFi, HDMI video signalling, USB cables, BlueTooth, and between processors in our complex radio equipment.
Packet radio is a loaded term.
Until the late 1970s, there was no such thing as packet radio in the ham radio space.
It was in use military and commercial space, espially by long distance microwave signalling equipment.
One major problem that prevented packets over radio was that the differnet radio agencies, DOC, FCC, etc. had rules preventing this.
I make no claims of being an expert in the legal definitions of things, but I was involved in both microprocessor programming and ham radio when packet radio suddenly became legal in the United States.
The specific detail that changed was that ASCII signalling was now allowed.
Another detail that existed after that was that automated operation of data communications was now permitted.
I'm not sure those details were related.
In 1984, I think, the Tucson Amateur Packet Radio club and corporation created a Terminal Node Controller that utilized a protocol called AX.25 which gained an enormous amount of attention and became the defacto definition of packet radio in the United States.
This wasn't the first on ham radio, that happened in Montreal a bunch of years before, and it wasn't the first TNC, that also was from Canada, but the TAPR TNC took the world by storm, possibly because they made it easy for corporations to manufacture and sell the TNC for profit.
Heathkit, and AEA sold a very close clone of the TAPR TNC in the year or two after TAPR started selling the unit.
AX.25 protocol defines a message that is sent over the radio between devices.
It's much like I described above but with specific details that I didn't mention inluding that there are several predefined messages that establish relationships between stations, break those relationships, send information, acknowledgte information, and several progress control messages used to verify that the two parties involved in the exchange are still on-line.
Each and every message includes a 16 bit CRC word which plays the part of the message verification element to the packet.
Each and every message includes the two callsigns involved in the exchange, using about 14 bytes of the message.
Packet-radio is a modification of RTTY or Radio Teletype. RTTY transmits characters with or without spacing, with no attempt at verification or correction.
Bytes can be sent slowly, or with close spacing. Data is sent incrementally as soon as any part of the data is available at the transmitter.
The decision of when to start and stop transmission is under operator control.
Packet-radio is a scheme for bundling addressing info, control info, data, and error checking into a single transmission which is sent all at once.
Because packet radio messages include addressing info, many stations could be on the same channel or frequency and a packet message could be directed by one station to one specific other station.
This permits multiple conversations to take place on the same channel where pairs of stations could each have what looks like a private conversation.
See Why do we packet later on this page.
In the case of a packet-radio message, a computer takes control of the push-to-talk on your radio.
That computer then decides when it is time to transmit.
When it is time, the computer keys the transmitter (causes it to transmit), sends a packet (of address, control, information, and error checking), and then unkeys, automatically.
The computer is responsible for generating the transmit audio to the transmitter, and it listens to the receiver as well.
The computer at the receive station takes the combined block of bits heard in the transmission and can make some decisions on what to do with that block.
The computer can know whether the entire contents was received intact, who it was sent by and who it was sent to.
Based on what has come before, the computer can also ignore the incoming data, or it can use the data and pass it on to the operating program.
Packetized transmission is used in most digital communications, wired or otherwise, although that has only been true since the 1970s.
The Internet, telephones, alarm systems, and many other common electronic systems use packets for data delivery.
Most systems where there are multiple computers (even very small computers) communicating in a network are probably using packets for communications.
In ham radio, packet-radio is used for sending text and data.
Text is used on HF and VHF, for short one-way messages, and for email operations.
Data is used for location plotting of hams and objects, and also for band condition monitoring, weather monitoring, and other interesting applications.
Sometimes a small computer is used to do most of the radio operating, tone generating, and receive tone decoding for a packet station.
That small computer could be a Terminal Node Controller or TNC. In other stations, the generating and decoding operation is performed in a desktop PC using the audio inputs and outputs provided by the desktop computer's sound card peripheral.
Amateur Radio packet is used on our entire spectrum, from very low frequencies to microwaves.
Packet messaging can be used with acknowledgments or without. In some applications, packet-radio is mostly used for sending text messages between stations.
On HF radio, packet-radio exists in dozens of different implementations which are categorized under the name "Digital Modes".
The protocols used for packet-radio are many. One option in packet-radio performs Guaranteed Delivery.
This is also called Connected Mode. When a packet is used in Connected mode, your message is sent and verified for unique error-free content by the recipient before the message is displayed, then the recipient acknowledges your message.
If they don't acknowledge, your end will automatically resend the message.
Redundant copies of the message are ignored. You are guaranteed either a perfect copy, or you give up on the QSO.
Packet-radio can be used to send blocks of text with high reliability and this capability is very handy in traffic handling or emergency communications.
In another mode, called UNPROTO in older TNCs, packets are sent without verification of delivery.
In UNPROTO mode the sending station repetitively (once a minute or less) sends a message containing the current status of something (like weather or location).
Then if the receive station misses, or declares corrupted (with errors), a message, only that point of data on a graph is lost.
The remainder of the points of data is enough to serve whatever the purpose is.
Thus no retries or acknowledgments are used or required.
Packet-radio can be used from ham to computer, or from ham to ham, or from computer to computer.
You can also use the computers to relay messages to the next computer along a path, allowing us to connect from ham to computer to computer to computer to ham.
This kind of packet-radio is called networking.
Why do we packet?
Of all of the modes of communications used in ham radio, packet-radio is the only mode that inherently allows several conversations to occur in the same piece of spectrum over the same path.
This means that on a network link between two sites, there can be several conversations at once without hindering each other.
We can take advantage of a linked network of packet stations without worrying about keeping other stations from doing the same.
We can connect our station or computer to other stations to run operations that might last for hours or even years!
This could be useful to tie together stations that would be used for a long coordinated operation or to receive telemetry from a remote station.
Packet messages can be routed. A packet radio network allows that you could have multiple routes for packets to travel.
So you could go
from Ham1 to ComputerA to ComputerB to ComputerC to Ham2
while another message could go
from Ham1 to ComputerA to ComputerP to computerQ to Ham3.
Packet-radio can be used on HF and VHF with or without a network, and it can be used to send text messages or for transmitting data encoded from some application.
On VHF and UHF the primary use of packet-radio is the Automatic Packet Reporting System, or APRS, which is used to send GPS coordinates, weather data, and other telemetry from an automated station, through (relatively) dumb relays, and eventually through an Internet Gateway to a server which processes the received signals.
APRS is infrequently used between mobile stations with a local non-internet display.
What is packet-radio over VHF Ham radio like today?
There are several kinds of VHF packet-radio activities today.
These are the most popular:
- APRS Automatic Position Reporting System -- transmit GPS coordinates of portable stations to Internet servers
- Winlink -- Enables a disaster site to be connected back to the Internet via packet radio -- also can be used to connect two sites together entirely via packet radio.
- BBS & CHAT -- G8BPQ node stations connected to both packet radio and Internet allows packet-radio-style emails to be sent via the Internet to other cities.
Also has a worldwide chat feature.
- BroadBand HamNet (BBHN) AREDN -- microwave and high UHF links using commercial-off-the-shelf WiFi equipment to create wide area high-speed networks.
Many regional networks exist covering multiple cities using exclusively Amateur-owned equipment.
There are also many BBHN systems that are mixed commercial (Internet) and Amateur-owned hardware.
Broadband-hamnet.org and Amateur Radio Emergency Digital Network
- ROSE, TheNET, G8BPQ, KANODE-- several Amateur-Radio-only systems use VHF/UHF radio hardware to do wide-area networks.
The Central North Carolina TARPN (NCPACKET) is one such.
- There is a network of 20 or so KAnodes in eastern PA, linked to a G8BPQ Internet/packet switch.
Also in the same area, KC3IBN has 3 or 5 TARPN nodes in a separate network.
- Charlotte NC's MARS club runs a dozen or so stations, several of which are G8BPQ linked to the Internet, and several of which are TheNET z80-TNC systems.
They also have at least one Raspberry PI with TNC-PI stacks.
What is TARPN and why re-invent the wheel?
Ham radio networks using data over VHF and UHF can do things that have never worked well before TARPN.
The biggest feature TARPN is trying to push is comradery in a big project.
We're trying to work together to make something bigger than the collection of individual efforts would produce.
We're also trying to do it cheaper, as a group, than the individual efforts would realize.
By building our project as a group, we, as individuals, get to do the tasks we have the most fun doing, and share the effort, maximizing our productivity, fun, and efficiency.
The most fun I've ever had on a packet network is real-time live chat.
Live chat across an area bigger than a repeater can cover, for instance, is even more fun, but creates a design challenge for packet messaging on VHF/UHF voice radios.
So why not use other kinds of radios?
The VHF/UHF voice solution is cheap for the range it can give us.
All we have to do to make it work is design the channel utilization effectively.
We can do a purposeful network architecture that produces the desired throughput result.
Designing a network with a well-understood and provable performance characteristic, and considering all participating stations, is not something that has been done on ham radio packet.
It takes an understanding of what happens with multiple stations sending and receiving data.
[homefully] Most of these issues are discussed in the TARPN FAQ page
I believe that our system is unique in that it results in individual hams learning how to build, expand and operate network switch hardware in their own ham-shack, where all traffic is moved using Amateur Radio.
This results in an expanding ham-radio network and infrastructure.
By working on a project with a large group of Hams, with regular communcations via a ham-controlled packet network, we're creating a user's group eager to play with new software and hardware.
This makes it fun for the participants to be creative and share with the group.
TARPN suggests a networking technology that is easy for all hams to participate in, and which promotes Amateur Radio.
TARPN is a design philosophy and operational philosophy. I think if we abide by this set of rules and use this design, the network can be built and promoted.
My (KA2DEW's) particular part of this is to document the methods we're using so other people may duplicate them.
I'm also working on developing software and procedures to make this work better and be easier+cheaper to implement and operate.
See also FAQ - What Do We Need In Order to Build and Operate a TARPN Station?