- What is Packet Radio

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There are many different modes available to radio amateurs these days and that is truely the beauty of our hobby. Packet Radio is one of those modes and like the other modes in amateur it will appeal to a specific group of people, especially those with an interest in computers and networking.

For emergency uses, a wireless network of laptop computers and TNC's with radios running on batteries can pass large volumes of traffic and also provide other tactical uses such as access to a database with information such as location of people and or emergency supplies.

Packet radio is communications fo the computer age. A computer in a ham shack is as common as a 2 meter handheld transceiver was 20 years ago. Computer programs allowed computers to send and receive CW and RTTY. Some farsighted hams, however, developed a new amateur mode of communications that unleashes the power of the computer. The mode is packet radio.

Being a child of the computer age, packet radio has the computer-age features that you would expect.

* It is data communications; high speed and error-free packet radio communications lends itself to the transfer of large amounts of data.

* It is fast, much faster than the highest speed CW or RTTY.

* It is error free, no "hits" or "misses" caused by propagation variations or electrical interference.

* It is spectrum efficient; several stations can share one frequency at the same time.

* It is networking; packet stations can be linked together to send messages over long distances.

* It is message storage; packet radio bulletin boards (PBBS) provide storage of messages for later retrieval.

Packet radio uses a terminal node controller (TNC) as the interface between the computer and the transceiver. A TNC is an enhanced modem. A modem is a device which converts the computers data into variable audio tones and on the other end converts the variable audio tones back to computer data. The TNC accepts information from your computer or ASCII terminal and breaks data into small pieces called packets. In addition to the information from your computer, each packet contains addressing, error-checking and control information. The addressing information includes the call sign of the station that sent the packet, and the call sign of the station the packet is being sent to. The address may also include call signs of stations that are being used to relay the packet. The error-checking information allows the receiving station to determine whether the received packet contains any errors. If the received packet contains errors, the receiving station asks for a repeat transmission until the packet is received error free.

Breaking up the data into small parts allows several users to share the frequency. Packets from one user are transmitted in the spaces between packets from other users. The address section allows each user's TNC to seperate packets intended for him from the packets intended for other users. The addresses also allow packets to be relayed through several stations before they reach their ultimate destination. Having information in the packet that tells the receiving station if the packet has been received correctly and assures perfect copy.

Sometimes terrain or propagation prevents your signal from being received by the other station. Packet radio gets around this problem by using other packet radio stations to relay your signal to ther intended station. All you need to know is which on-the-air packet radio stations can relay signals between your station and the station you want to contact. Once you know of a station that can relay your signals you can use it for this purpose.

Digital and voice repeaters repeat, but that is where the similarity ends. Notice that digital repeaters differ from typical voice repeaters in a number of ways. A digital repeater (digipeater) usually receives and transmits on the same frequency (whereas a voice repeater receives and transmits on different frequencies). A digipeater does not receive and transmit at the same time (as compared to a voice repeater, which immediately transmits whatever it receives). Rather, a digipeater receives a packet, stores it temporarily until the frequency is clear, and then retransmits the packet. Also, a digipeater only repeats packets that are specifically sent to be repeated by that station (the address in the packet contains the call sign of the digipeater). A voice repeater repeats everything that it receives on its input frequency.

If one digipeater is insufficient to establish a connection, you can specify as many as eight stations in your connect request.

Don't use more than one or two digipeaters at any one time, especially during prime time operating hours (evenings and weekends). Each time you use a digipeater, you are competing with other stations attempting to use the same digipeater. Each station that you compete with has the potential of generating a packet that may collide with your packet (which causes your TNC to resend the packet). The more digipeaters you use, the more stations you compete with, greatly increasing the chance of a packet collision. As a result, it may be difficult to get one packet through multiple digipeaters, and your TNC will quickly reach its retry limit and disconnect the link.

Any packet radio station can act as a digipeater. Most TNC's are setup to digipeat automatically without any intervention by the operator being used as a digipeater. You do not need permission, only his cooperation, because he can disable his stations digipeater function. In the spirit of Amateur Radio, most packet operators leave the digipeat function on, disabling it only under special circumstances.

Another form of a digipeater is a NODE. To reach a distant station, first connect to the node. Then, instruct the node to connect you to the distant station. The node acknowledges packets sent from either station, then relays them to the other station. This has a number of advantages over a simple digipeater.

Today, most amateur radio packet activity occurs at VHF, on 2 meters, but activity on 430Mhz continues to grow as well.

The most common used data rate on VHF is 1200 baud with frequency modulated AFSK tones of 1200 and 2200 Hz. This is referred to as the "Bell 202" telephone modem standard.

Getting on the air is usually a simple matter of turning on your radio amd tuning in your favorite packet radio frequency. On 2 meters, common packet frequencies are 144.625, 144.675, 144.700. If there is a voice repeater on that frequency in your area, ask around at a club meeting or on the repeater. Someone is bound to know where the packet activity is.

HF packet radio is very different from VHF/UHF packet. An SSB transceiver is used to generate a 200Hz shift FSK signal, and 300 bauds is used rather than 1200 bauds. However, there is some 1200 baud packet activity on the 10 meter band.

Tuning is much more critical than it is on VHF. Tune your receiver very slowly, in as small an increment as possible until your terminal begins displaying packets. Do not change frequency until a whole packet is received. If you shift frequencies mid packet, that packet will not be received properly and will not be displayed on your terminal even if you were on the correct frequency before or after the frequency shift.

Some TNCs and external modems have tuning indicators on them that make tuning alot easier. Kits are also available to allow you to add a tuning indicator to a TNC without one.

A Packet Bulletin Board System (PBBS) is a computer that allows packet stations to store messages for other amateurs, upload and download computer files, and even link one packet station through a "gateway" to another band.

Some PBBS computers can automatically forward messages from one computer to another, so you can store a message at one PBBS that is ultimately meant for an amateur thousands of miles away. The message will be forwarded from one PBBS to another until it reaches its destination. A network is a system of packet stations that can interconnect to transmit data over long distances.

To use a PBBS, you must locate one. If you are a member of R.A.A.G. and if the sysop likes you and allows you to use his system, try the SV1SV PBBS.

All TNC's have a beacon function. This function allows a station to send an unconnected packet at regular intervals. These unconnected packets usually contain a message to the effect that the originating station is on the air and willing and able to carry on a packet radio contact.

The purpose of the beacon function is to generate activity when there is none. This purpose was legitimate when there was little packet radio activity. Back in the early 1980's, it was a rare occurence when a new packet station appeared on the air. Without beacons, that new radio operator might believe that his packet radio station was the only one active in the area. Similarly, packet radio stations already on the air would not be aware of the new stations existence. It would be very discouraging to build a TNC (they were all kits in the early days), get on the air and find no one to contact. The beacon function was a solution to the problem. It let people know that a new packet station was on the air.

Today, beacons are usually unnecessary. There is absoloutely no need to resore to beaconing in order to make your stations existence known. On HF, 2 meters and 430Mhz, there is plenty of activity in most areas. After a few connections, your existence on the air will be known.

Instead of sending beacons, leave a message announcing your existence on the local PBBS. This is more effective than sending beacons because your message will be read even when your station is off the air.

Beacons only add congestion to already crowded packet radio frequencies, so do the packet radio community a favor and disable your TNC's beacon function using the following command: BEACON EVERY 0

All you need to set up a VHF/UHF packet radio station is a VHF/UHF transceiver (with an antenna), a computer or ASCII terminal and a TNC. The TNC connects between the computer and the radio. For operation on 10 meters you will need a 10-meter SSB transceiver in addition to the TNC and computer. Your TNC manual should contain detailed instructions for wiring the TNC, radio and computer together. So many hams are on packet now that someone in the area will probably be able to help you if you have problems, or ask around on the local voice repeater.

There are many people on packet who would be willing to assist you in your packet operations. Should you have any questions please contact a local ham who is involved with packet radio, I am sure they will be more than happy to assist you.