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What is Packet Radio?

PACKET RADIO

There are many different modes available to radio amateurs these days. Packet Radio is one of those modes and like the other modes 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.

 

WHAT IS PACKET RADIO?

Packet radio is communications for 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.

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.
  •  

HOW DOES PACKET WORK?

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.

 

PACKET RADIO REPEATING

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 their 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.

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. Most packet operators leave the digipeat function on.

Although you are not allowed to be a control operator of a voice repeater until you upgrade from the Novice class, you may leave your TNC's digipeater function enabled. The FCC recognizes the distinction between digipeaters and voice repeaters in this case.

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.

 

VHF/UHF VS. HF PACKET OPERATIONS

Today, most amateur radio packet activity occurs at VHF, on 2 meters, but activity on 222Mhz continues to grow as well as UHF 440 packet activities.

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 and tuning in your favorite packet radio frequency. On 2 meters, common packet frequencies are 144.91, 144.93, 144.95, 144.97, 145.01, 145.03, 145.05, 145.07, and 145.09. On 222Mhz, packet activities center around 223.400. 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.

 

PACKET BULLETIN BOARD SYSTEMS (PBBS)

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. Ask around in your area and you'll probably find that just about anybody on packet would be able to tell you what PBBS is local to you. In addition, there are several HF PBBS stations, although many of the HF stations are set up for long haul message traffic and not individual user connects. The ARRL operating manual contains more detailed information about using a PBBS.

 

THE  BEACON

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.  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 is a solution to the problem. It lets people know that a new packet station was on the air.

Today, (in some cases) beacons may be unnecessary.  On HF, 2 meters and 222Mhz, there is activity in most areas. If you are getting on the air for the first time, monitor 145.070 or 144.930 for a few minutes and you will quickly have a list of other stations that are on the air.

Sending beacons or leaving a message announcing your existence on the local PBBS, This is also as effective as sending beacons because your message will be read even when your station is off the air.

 

WHAT YOU NEED TO GET ON PACKET RADIO

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.

 

PACKET IN YOUR AREA ??? NEED HELP ?????

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, The best way to do this is to either contact a local club or ask on your local repeater, in many cases they will be more than happy to assist you. Below are some Packet Radio related Links.

Here are all the remaining links I could find at this time, Packet radio is on a downward spiral and is not used much in many areas

 

TAPR

Amateur Wireless Packet Internet Gateways

Introduction to Packet Radio