History
of Packet
Why
Packet Over Other Modes?
Elements
of a Packet Station
Distance
Limitations
Channel
Sharing
What
is AX.25?
Networks
and Special Protocols
Other
Protocols
Network
Schemes
What is the history
of packet radio ?
Data packet technology was
developed in the mid-1960's and was put into practical application in the
ARPANET which was established in 1969. Initiated in 1970, the ALOHANET, based at
the University of Hawaii, was the first large-scale packet radio project.
Amateur packet radio began in Montreal, Canada in 1978, the first transmission
occurring on May 31st. This was followed by the Vancouver Amateur Digital
Communication Group (VADCG) development of a Terminal Node Controller (TNC) in
1980.
The current TNC standard grew from a discussion in October of 1981 at a meeting of the Tucson Chapter of the IEEE Computer Society. A week later, six of the attendees gathered and discussed the feasibility of developing a TNC that would be available to amateurs at a modest cost. The Tucson Amateur Packet Radio Corporation (TAPR) formed from this project. On June 26th 1982, Lyle Johnson, WA7GXD, and Den Connors, KD2S, initiated a packet contact with the first TAPR unit. The project progressed from these first prototype units to the TNC-1 and then finally to the TNC-2 which is now the basis for most packet operations worldwide.
Why packet over
other modes?
Packet has three great advantages
over other digital modes: transparency, error correction, and automatic
control.
The operation of a packet station is transparent to the end user; connect to the other station, type in your message, and it is sent automatically. The terminal Node Controller (TNC) automatically divides the message into packets, keys the transmitter, and then sends the packets. While receiving packets, the TNC automatically decodes, checks for errors, and displays the received messages. Packet radio provides error free communications because of built-in error detection schemes. If a packet is received, it is checked for errors and will be displayed only if it is correct. In addition, any packet TNC can be used as a packet relay station, sometimes called a digipeater. This allows for greater range by stringing several packet stations together.
Users can connect to their friends' TNCs at any time they wish, to see if they are at home. Some TNCs even have Personal BBSs (sometimes called mailboxes) so other amateurs can leave messages for them when they are not at home. Another advantage of packet over other modes is the ability for many users to be able to use the same frequency channel simultaneously.
What elements make
up a packet station?
Figure 1 shows an
illustration of a typical station setup with a schematic diagram of a station
wiring.
Most amateurs currently use 1200 bps (bits per second) for local VHF and UHF packet, and 300 bps for longer distance, lower bandwidth HF communication. Higher speeds are available for use in the VHF, UHF, and especially microwave region, but they often require special (not plug-and-play) hardware and drivers.
What is the distance
limitation for packet radio?
Since packet radio
is most commonly used at the higher radio frequencies (VHF), the range of the
transmission is somewhat limited. Generally, transmission range is limited to
"unobstructed line-of-sight" plus approximately 10-15%. The transmission range
is influenced by the transmitter power and the type and location of the antenna,
as well as the actual frequency used and the length of the antenna feed line
(the cable connecting the radio to the antenna). Another factor influencing the
transmission range is the existence of obstructions (hills, groups of buildings
,etc). Thus, for two-meter packet (144 - 148Mhz), the range could be 10 to 100
miles, depending on the specific combination of the variables mentioned
above.
What do you mean
we can all use the same channel?
Packet radio,
unlike voice communications, can support multiple conversations on the same
frequency at the same time. This does not mean that interference does not occur
when two stations transmit at the same time, known as a collision. What 'same
time' means in this sense is that multiple conversations are possible in a
managed, time shared fashion. Conversations occur during the times when the
other conversations are not using the channel. Packet radio uses a protocol
called AX.25 to accomplish this shared channel.
AX.25 specifies channel access (ability to transmit on the channel) to be handled by CSMA (Carrier Sense Multiple Access). If you need to transmit, your TNC monitors the channel to see if someone else is transmitting. If no one else is transmitting, then the TNC keys up the radio, and sends its packet. All the other stations hear the packet and do not transmit until you are done. Unfortunately, two stations could accidentally transmit at the same time. This is called a collision. If a collision occurs, neither TNC will receive a reply back from the last packet it sent. Each TNC will wait a random amount of time and then retransmit the packet. In actuality, a more complex scheme is used to determine when the TNC transmits. See the "AX.25 Protocol Specification" for more information (ARRL, 1988).
What is
AX.25?
AX.25 (Amateur X.25) is the
communications protocol used for packet radio. A protocol is a standard for two
computer systems to communicate with each other, somewhat analogous to using a
business format when writing a business letter. AX.25 was developed in the
1970's and based on the wired network protocol X.25. Because of the difference
in the transport medium (radios vs wires) and because of different addressing
schemes, X.25 was modified to suit amateur radio's needs. AX.25 includes a
digipeater field to allow other stations to automatically repeat packets to
extend the range of transmitters. One advantage of AX.25 is that every packet
sent contains the sender's and recipient's amateur radio callsign, thus
providing station identification with every transmission.
Networking and
special packet protocols
This is a sample of
some of the more popular networking schemes available today. By far, there are
more customized networking schemes used than listed. Consult your local packet
network guru for specific network information.
Are there any
other protocols in use other than AX.25?
AX.25
is considered the defacto standard protocol for amateur radio use and is even
recognized by many countries as a legal operation mode. However, there are other
standards. TCP/IP is used in some areas for amateur radio. Also, some networking
protocols use packet formats other than AX.25. Often, special packet radio
protocols are encapsulated within AX.25 packet frames. This is done to insure
compliance with regulations requiring packet radio transmissions to be in the
form of AX.25. However, details of AX.25 encapsulation rules vary from country
to country.
Networking
Schemes
What are some of those other networking
schemes?
During the early days of amateur packet radio, it became apparent that a packet network was needed. To this end, the following packet network schemes were created.
NET/ROM doesn't use all of the AX.25 protocol. Instead, it uses special AX.25 packets called Unnumbered Information (UI) packets and then puts its own special protocol on top of AX.25. This is again used to increase efficiency of its transmissions. NET/ROM nodes, at regular intervals, transmit to other nodes their current list of known nodes. This is good because as new nodes come on-line, they are automatically integrated in the network. However, if band conditions such as ducting occur, ordinarily unreachable nodes can be entered into node lists. This causes the NET/ROM routing software to choose routes to distant nodes that are impossible. This problem requires users to develop a route to a distant node manually defining each hop instead of using the automatic routing feature.
NET/ROM is a commercial firmware (software put on a chip) program that is used as a replacement ROM in TAPR type TNCs. Other programs are available to emulate NET/ROM. Among them are TheNet, G8BPQ node switch, MSYS, and some versions of NET.
ROSE's use of static routing tables ensures that ROSE nodes don't attempt to route packets through links that aren't reliably reachable, as NET/ROM nodes often do. However, ROSE suffers from the inability to automatically update its routing tables as new nodes come on-line. The operators must manually update the routing tables, which is why ROSE networks require more maintenance.
BBS Message Transfer: Many of the BBS programs used in packet radio allow for mail and bulletins to be transferred over the packet radio networks. The BBSs use a special forwarding protocol developed originally by Hank Oredsen, W0RLI. Besides full service BBSs, many TNC makers have developed Personal BBS software to allow full service BBSs to forward mail directly to the amateur's TNC. This allows operators to receive packet mail at night and avoid tying up the network during busy hours.
Finke, C. R. (Ed.) (1992, February 15). TPRS Quarterly Report. Texas Packet Radio Society, Inc.
Jones, G., G. Knezek, M. Hata. (1992). Packet Radio Prospects for Educational Data Communications. Proceedings of the Ninth International Conference on Technology in Education, 1, 218-219. Paris, France.
Lucas, Larry, Greg Jones, David Moore. (1992) An Educator's Alternative to Costly Telecommunications. Texas Center for Educational Technology, Univ. of North Texas.
Steve Watt, KD6GGD, [email protected]. (1993). Frequently Asked Questions from the listing in the rec.radio.amateur.packet newsgroup for the USENET network. Version 1.11.
Tucson Amateur Packet Radio Corporation. terminal Node Controller Manual, Firmware Release 1.1.8, Tucson, AZ: Author.