Packet Radio Explained

By Jonathan Imberi


cmd:c PACKET Via THE SHACK 

*** CONNECTED to PACKET Via THE SHACK 

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Packet radio is communications for the computer age. More and more hams are adding computers to their ham shacks. Early ham-oriented computer programs allowed computers to send and receive CW and RTTY. Some farsighted hams, however, developed a new mode of communications that unleashes the power of the computer. This mode is Packet Radio. 

Packet radio has the computer age features 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 a network -- packet stations can be linked together to send data over large distances. 

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

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Packet radio uses a terminal node controller (TNC) as the interface between a computer and a transceiver. A TNC is nothing more than an enhanced modem. A TNC accepts information from a computer or ASCII terminal and breaks the data into small pieces called packets. In addition to the information from the 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 detection of packets containing errors. If the received packet contains errors, the receiving station waits until it is transmitted again. 

By breaking up the data into small parts, packet radio allows several users to share a frequency. Packets from one person are transmitted in the spaces between packets from other people. The address section allows each person's TNC to separate packets intended for him/her from packets intended for others. The addresses also allow packets to be relayed through several stations before they reach their destination. Having information in the packet that tells the receiving station if the packet has been received correctly is much like a read receipt and tells the sender it was received correctly. 


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All an amateur needs to set up a packet radio station is a VHF or UHF FM transceiver (with an antenna), a computer or ASCII terminal, and a TNC or multimode controller, which allows operation on several modes. 

The TNC connects to the transceiver microphone input. It also connects to a computer or terminal.



For operation on 10 meters you'll 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 local radio club will probably be able to help you if you have problems, or ask around on the local repeater or a simplex frequency. 

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To get started on packet radio, you must tell the TNC your call sign. Most TNCs allow the user to change their call sign at any time, so several family members could use one TNC with each of their call signs. Most TNCs also have a way to remember the call sign when the power is switched off. The TNC must be in command mode before commands may be entered. When the TNC is in command mode, you will see a prompt: 

cmd: 

This indicates that the TNC is waiting for input. To tell the TNC my call sign, KC0DMH types: 

MYCALL KC0DMH <enter> 

<enter> means "carriage return". On most computers this key is labeled "ENTER". 

As in all modes of Amateur Radio, packet allows you to "read the mail" or monitor channel activity. This is called the monitor mode, and looks like this: 

KC0DMH>KC0JKX: HELLO, WHEN IS THE NEXT NET? K 

KC0JKX>KC0DMH: HANG ON, I'LL CHECK. 

To enable monitor mode, simply type: MON ON at the cmd: prompt. You may also need to type MFROM ALL. Consult the operating manual for your TNC to be sure. The call signs of the stations involved appear as "FROM>TO:" with the contents of the packet following. When it is in monitor mode, your computer will display everything that is transmitted on the packet frequency, even if it is not addressed to you. In addition, a packet station that is monitoring is not replying to any messages, unless the message is directed to that station. 

You can send a CQ by entering the converse mode of the TNC. You go into converse mode by typing: 

CONV <enter> 

(Some TNCs allow you to type "K" instead of "CONV") 

You can then type your CQ: 

BOB IN MINNEAPOLIS LOOKING FOR ANYONE IN MANKATO 

Your TNC adds your call sign as the FROM address and CQ as the TO address. The receiving stations TNC adds these addresses to the front of the text when it is displayed. 

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You answer a CQ or establish a contact by using the CONNECT command. When two packet stations are connected, each station sends data packets specifically addressed to the other station. When a station receives an error-free packet, it transmits a reply packet to let the sender know the packet has been received correctly. The two stations take turns sending data or messages typed on the keyboard. 

To connect to another station, you type: 

Connect KC0JKX <enter> 

where KC0JKX is the call sign of the station you wish to contact. (Most TNCs let you use "C" as an abbreviation for "connect".) 

If KC0JKX's packet-radio station is on the air and receives your connect request, your station and her's will exchange packets to set up a connection. When the connection is completed, your terminal displays: 

*** CONNECTED to KC0JKX 

and your TNC automatically switches to the converse mode. 

Now, everything you type into the terminal keyboard is sent to the other station. A packet is sent whenever you press ENTER. It is a good idea to use K, BK, O, or > at the end of a thought to say "okay, I'm done. It is your turn to transmit." 

When you are finished conversing with the other station, return to the command mode by typing <CTRL-C> when the command prompt (cmd:) is displayed, type: 

Disconnect <enter> 

and your station will exchange packets with the other station to break the connection. (Most TNCs let you use "D" as an abbreviation.) When the connection is broken, your terminal displays: 

*** DISCONNECTED 

If, for some reason, the other station does not respond to your initial connect request, your TNC will send the request again until the number of attempts equals the internal retry counter. When the number of attempts exceeds the retry counter, your TNC will stop sending connect requests and your terminal displays: 

*** retry count exceeded 

*** DISCONNECTED 

A TNC can reject a connect request if it is busy or if the operator has set CONOK (short for CONnect OK) off. If this happens when you try to connect, your TNC displays: 

*** KC0JKX busy 

*** DISCONNECTED 

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Sometimes terrain or propagation can prevent 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 your 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, type: 

Connect KC0JKX Via W1AW-5 <enter> 

where KC0JKX is the call sign of the station you want to connect to and W1AW-5 is the call sign of the station that will relay your packets. The "-5" following W1AW is a secondary station identifier (SSID). The SSID permits up to 16 packet stations to operate with one call sign. For example, W1AW-5 is a 2 meter packet repeater and W1AW-6 is a 222 MHz packet repeater. 

When W1AW-5 receives your connect request, it stores your request in memory until the frequency is silent. It then retransmits your request to KC0JKX on the same frequency. This action is called digipeating, a contraction of "digital repeating". If KC0JKX's packet radio station is on the air and receives the relayed connect request, your station and her's will exchange packets through W1AW-5 to set up a connection. Once the connection is established, your terminal will display: 

*** CONNECTED to KC0JKX Via W1AW-5 

W1AW-5 will continue to relay your packets until the connection is broken. 

If one digipeater is insufficient to establish a connection, you can specify as many as 8 stations in your connect request. Additional digipeaters are added to the connect command separated by commas. For example typing: 

Connect KC0JKX Via W1AW-5, WA2FTC-1 <enter> 

after the command prompt (cmd:), causes your TNC to send the KC0JKX connect request to W1AW-5 which relays it to WA2FTC-1. Then, WA2FTC-1 relays it to KC0JKX. 

Do not use more than one or two digipeaters at a time, especially during the prime time operating hours of 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. 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 TNCs are set up to digipeat automatically without any intervention by the operator of the station being used as a digipeater. You do not need his/her permission, only his/her cooperation, because he/she can disable his/her station's digipeater function. 

The most common form of digipeater is the 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. 

Amateurs have set up nodes to connect with other nodes, so packet messages can be sent over longer distances. Such a system of interconnected nodes forms a network of packet stations. 

Amateurs have also established another type of network that operates in a manner very similar to the internet. This system is known as the TCP/IP network. TCP/IP is short for transmission control protocol/internet protocol, which describes the set of rules or commands that control the network operation. Here your station is assigned a network address, and the stations automatically know how to route your messages and data to other stations on the network. 

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Today, most amateur packet radio occurs at VHF, on 2 meters, but activity on 222 MHz continues to grow. The most commonly used data rate on VHF is 1200 bauds. 

Getting on the air is usually a simple matter of turning on your radio and tuning to your favorite packet radio frequency. On 2 meters, common packet channels are 145.01, 145.03, 145.05, 145.07, and 145.09 MHz. On 222 MHz, packet activity centers on 223.4 MHz. 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. 

If you are conducting a direct connect, move your contact to an unused simplex frequency. It is very inefficient to use a frequency where other stations, especially digipeater stations, are operating. The competition slows down your packets and, in return, you are also slowing down the other stations. You should use a frequency occupied by a digital repeater only if you are using that digital repeater. Check with other hams in your area for more information on local packet operating frequencies. Although you are operating your packet station on simplex, it is best not to use the designated FM simplex frequencies because packet and voice are not compatible modes. 

No discussion of modern packet radio would be complete without at least a mention of the automatic position reporting system (APRS). This system is a software application that uses a system of maps to display the location of stations. Combined with a global positioning system (GPS) receiver, many TNCs can be set to automatically decode your latitude and longitude information and transmit it for display on the maps of other stations in the network. 

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*** CONNECTED to PACKET Via THE SHACK 

cmd: D 

*** DISCONNECTED