QEX

amateur programs whenever possible.

Amateur Packet Radio

Efforts in the amateur packet-radio community are probably best described in terms of the International Standards Organization (ISO) Open Systems Interconnect (OSI) model.

In general, we look at the communication task and relate it to the OSI model. A representation can be made between the functions of the radio controller and the OSI Reference Model. For the functions associated with the data link layer, almost all amateur packet-radio work is being done with what is commonly known as the AX. 25 Data Link Layer Protocol. This protocol has been described in numerous amateur publications over the last few years.

Also described in great detail in numerous amateur publications are the efforts of amateurs to standardize the OSI network and transport layers. An article in a QST feature column

Dennis Bodson, W4PWF 233 N Columbus St Arlington, VA 22203

"Packet Perspective" recently brought us up to date on what is happening in packet networking within the amateur community.9 This article does an excellent job of summarizing many of the ideas presently being considered in amateur packet radio. The article describes the five most common ideas/ attempts at an HF ALE radio solution. Each of these schemes has a set of fol lowers, each attempting to develop the ideal networking solution

Federal Packet Radio

For many years, the federal community has also been exploring the application of networking technology using packet radio networks. Since 1983, US Government communications groups have been using a packet radio controller to provide data transmission to multiple users on a single radio channel on HF, VHF, and UHF frequencies. This network uses a terminal node controller (TNC), any two-way radio, and the AX.25 protocol.10

Not unlike those of the amateur committee, the federal network system requirements are:

1. uncomplicated operator interface and ease of use,

2. rapid access to the network, even during jamming or poor propagation,

3. continuous operation for all network configurations and propagation conditions,

4. automated routing so that the message gets through even during conditions of very poor connectivity,

5. minimum networking overhead so maximum throughput is obtained,

6. message transmission at all common data rates,

7. connectivity to a wide range of media.

To answer at least the first three of the above requirements, FED-STD-1045 HF ALE radios were developed.11 FED-STD-1045 radios use a very robust and redundant waveform and have traded data throughput for error control. Items 4 and 5 (ie, advanced networking) are being developed in the continuation standards, proposed FED-STD-1046 and proposed FED-STD-1047. Item 7 is being addressed in proposed FED-STD-1Q48, which will address multiple-media issues.

Federal HF ALE Radio

To bring you up to speed on the federal effort in HF ALE radio, a recap is necessary. With the recent integration of microprocessor control with state-of-the-art HF radio technology, radios are now being produced that are capable of providing automatic routine and emergency communications while maintaining interoperability between various vendors. Recent federal (see note 11) and military standards are encouraging vendors to produce interoperable (ie, between manufacturers) HF ALE radio systems.12-13 These radios have features such as selective calling, automatic handshaking, frequency scanning, link quality analysis, and common address group and net calls.

AIjE Controller Function Description

A typical ALE radio consists of a HF single-sideband (SSB) transceiver that is connected to a controller to function as an automated HF station. The controller can contain microprocessors or can be an addition to a conventional microcomputer. The present FED-STD-1045 type controller is considered to be a data link-layer controller in terms of the OSI model. The data link controller is roughly equivalent to the amateur TNC. Presently under development are network-layer and transport-layer controllers which will add features of advanced networking to the basic FED-STD-1045 radio. These networking controllers will be standardized by proposed FED-STD-1046 and proposed FED-STD-1047.

ALE radios scan over several channels (frequencies), at two to ten channels per second, looking for traffic. Upon detecting traffic, an ALE radio stops scanning to interrogate for traffic addressed to it; if none is found, it continues scanning. For transmitting, the system has previously determined by Link Quality Analysis (LQA) techniques which channel(s) will do the best job of propagating the message to the distant station. This system assumes that the other members of the network with which you wish to communicate can choose among several frequencies, and the choice may be based on the at mospheric conditions. Through a system of monitoring traffic, sounding, and LQA reporting it has been predetermined which channel(s) in the group will best support a transmission. The transmission is in the form of a link request and contains the address of the originator and the receptor, and can also contain an orderwire message. The actual message can be contained within the orderwire, or it can be indicated to the receptor that a link is requested for voice or modem traffic. After the link-layer controller establishes a link with another station, communication begins. If the message is transferred as part of the link request (as an order-wire mes-

Was this article helpful?

0 0

Post a comment