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Ham Radio Pages
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.
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
Amateur Wireless Packet Internet Gateways
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