Topics:
| 10.1 | HF,VHF and UHF Links |
| 10.2 | Satellite Links |
Packet radio is a form of data communication which has several forms e.g. X25 is in professional use, AX25 is used by amateurs
and there are other formats. The text message to be transmitted is broken up into packets which may contain up to 256 characters. AX25 packets take the following form:
They start and end with synchronising signal thus making the data within the packet a synchronous format.
The second part of the packet is an address area which contains the destination, origin and any station through which the originator may wish the packet to be relayed.
The relay stations are called digipeaters. They simply receive a packet and automatically transmit it to the next address on the packet. In addition to the text, each packet also contains control codes and error checking information. Until the receiving station confirms to the transmitting station that it has received the packet correctly, the same packet will be transmitted again. This procedure of re-tries will continue until the maximum number of re-tries e.g. 1O, is reached after which a failure will be indicated.
The packet radio operating system, packet format and other information is stored either in the Terminal Node Controller (TNC) or a computer program which interfaces between the keyboardand the radio. The TNC switches the transmitter on and off automatically. Initially the operator has to establish contact with the distant station using the keyboard. A connection will be indicated either by text on the VDU or printer, or by illumination of the appropriate LED, after which the operator sends the message using the keyboard. A further command is necessary from the operator to disconnect the link otherwise automatic disconnection may occur if no messages have been passed for a given time e.g. 1 minute.
In addition to a direct contact between two stations, packet radio is extensively used with mailboxes and bulletin boards. These facilities are similar to those which are available using modems over telephone lines. Mailboxes, as the name suggests, provide the facility for leaving mail at a distant station when the operator is not present. The mailbox may be at the distant station or some other station from where the mail can be collected, over the radio link. Bulletin Boards are again self explanatory. Messages are displayed on a bulletin board (BBS) at a station which has the necessary computer and software. These stations can be connected to and the list of bulletins observed. The full text of the required bulletin can then be down-loaded as indeed can any files or computer programs listed as being available at that particular BBS.
In addition to PACKET there are other systems for passing data such as AMTOR and PACTOR, the latter being a combination of PACket and amTOR.
Problems can be encountered when transmitting data over HF links. Signal fading can be more serious when data is transmitted than with voice transmissions. Whereas a human operator can decide whether a missed word needs repeating, a data system cannot so decide and is required to produce an error free message. On the other hand data transmissions require a lower signal to noise ratio than speech. A voice radio link can also be used to carry packet radio. However the highest baud rate that can be used will be dependant upon the quality of the radio link. This is the case when data is passed over the telephone network where the quality of various lines vary. A baud is the number of pulses transmitted per second.
On VHF and UHF circuits baud rates of 1200 to 9600 bauds are common. Because of effects such as multipath reception on HF the baud rate is lower, 300 bauds being common. An example of incorporating packet radio into a radio network can be seen in a unit developed by Dr S.A.G. Chandler of Warwick University, U.K. The unit can be used in developing counties which have limited telecommunications infrastructure, one network was established in the Bonthe area of Sierra Leone. The AFRICOM RADIO NETWORK CONTROLLER can be used to provide many very useful facilities on a radio network which would otherwise only have conventional voice capability. In its packet mode the unit enables the exchange of text messages between any two similarly equipped stations. If the stations are too far apart for direct contact then the messages can be automatically relayed via several intermediate stations. If the distant station is switched on, even though no operator is present, the text message can be received and stored.
Amongst other parameters the unit measures the stations battery voltage and solar charging current every 15 minutes and stores this data for the previous and current day. This data can be requested by a distant station and will be automatically transmitted to that station. The unit can measure the transmitter power output and VSWR of the aerial system. Again this information can be requested by a distant station.It appears that this unit is most useful, not only in the management of the station itself, but also for remote measurements of all the stations in a network from the maintenance headquarters.
An extension of the facilities envisaged by the author is that one station will collect daily, over the radio network, the 24 hour record of each stations parameters including transmitter power, VSWR etc. This data can then be transmitted via a store and forward packet satellite (see para 10.2) to another country, which may have the overall maintenance responsibility.
When communication facilities were first provided by satellites in the 1960's it was anticipated that this would result in the demise of short wave radio communication systems. Despite the vast number of satellites in orbit, providing many thousands of telephone and data channels, short wave radio is still in common use and is the only means of communication that many organisations can afford.
Much of the international telecommunications traffic is carried by geostationary satellites in orbit at some 33,700 kilometres above the earth. Using one or two of these satellites gives virtually instantaneous communications between any two locations on the earth, except in the polar regions. Such satellites can cost $400,000,000. When instantaneous communications is not vital then a less costly satellite system is available using satellites in low earth orbit (LEO). These satellites can be in orbit some 800 kilometres above the earth and cost some $1,000,000. Such satellites are only visible to an earth station for up to 15 minutes at a time and take about one and a half hours to go around the earth. Hence messages are transmitted up to the satellite, stored and then transmitted down when the required ground station is in view. The satellite is a flying mailbox and the technique used is called STORE and FORWARD.
LEO's are a suitable and affordable solution to the problem of transferring information between two places on the earth without dependence upon the telephone systems of the countries concerned. Telecommunications to, from and within some developing countries can be difficult and in some cases impossible. By 1992 two groups, Volunteers in Technical Assistance (VITA) and SatelLife, had provided a satellite. This had a store and forward facility and enabled text messages to be sent by various groups involved in helping in developing countries. LEO's were described by some people as "the poor mans satellite", but in this application they can be described more correctly as the "appropriate satellite".
The opportunities offered by LEOs are limited, e.g. a satellite can only be contacted from a particular point on earth whilst it is passing over that particular region i.e. within a range of some 3,000 km from the ground station. Then the satellite is usable for 1 to 15 minutes during each pass and there are 2 or 3 consecutive usable passes, occurring at 100 minute intervals, every 12 hours. The method of handling the text messages is by packet radio. It may seem that a usable time of 10 minutes is rather short, but with a baud rate of 9,600 and a duplex system, the packet radio system used can in theory send and receive 700,000 characters in 10 minutes. In practice some 500,000 characters have been down-loaded in one pass.
A ground station for the VITA system consisted of a computer e.g. laptop, a packet radio terminal node controller (TNC), a transmitter for 148 to 149.9 MHz with a power output of some 30 to 40 watts, a receiver for the 420 to 430 MHz band and a modest aerial array made of thin metal tubing. The whole system could be solar powered with the aid of a storage battery. The cost of a ground station was some $4,500 to $6,000 in 1997. Such a VITA ground station, provided the the UK based Baptist Missionary Society, was rapidly deployed at Vanga Hospital, Zaire at the time of the ebola outbreak in May 1995. This facility provided a data link from this remote area to the international community, via satellite UO_14. Similar additional equipment was later provided from USA for a link to the SatelLife satellite HS_2.
At the beginning of 1997 the addresses of the organisations were:
| Volunteers in Technical Assistance | Tel: 703 276 1800 |
| Director of Informatice | Fax: 703 243 1865 |
| 1600 Wilson Boulevard. Suite 500 | E-mail: vita@vit.org |
| ARLINGTON | |
| Virginia 22209 | |
| USA |
| SatelLife | Tel: 617 789 5455 |
| Director of Operations | Fax: 617 789 4771 |
| 1360 Soldiers Field Road | E-mail: hnet@usa.healthnet.org |
| BOSTON | URL: http://www.healthnet.org/ |
| Mass. 02135 | |
| USA |
The World Administrative Radio Conference (WARC92) allocated 148 to 149.9 MHz on a primary basis, for use with small LEO satellites e.g. the VITA satellite. In the UK the radio regulatory body is the Radiocommunications Agency of the Department of Trade and Industry. This agency will not permit the use of 148 to 149.9 MHz by ground stations to communicate with LEOs. This is because they allocate the frequencies to the private mobile radio systems of the gas and electricity industries, for public safety and emergency situation communications. This was the situation in September 1992 but it may change with time. Current regulations can be obtained from:
Radio Communications Agency
Waterloo Bridge House
Waterloo Road
LONDON SE1 8UA
UK users are overcoming this restriction by sending their messages by e-mail to other countries e.g. Eire and USA, where there are ground stations in operation.