THE AX25 PROTOCOL


 

The name AX25 originates from the recommendation X.25 of CCITT, adding letter A that stands for Amateur; AX25 is therefore Amateur packet radio link layer protocol. These are the main differences between the two protocols:

The purpose of this protocol is to define the frame structure and to set the requirements of the station that sends or receives that frame or packet. Every packet, besides the data, contains other auxiliary and control informations, so that every packet includes all needed informations to reach its destination. This addressing technique allows packet radio stations to share the same frequency without interferring with each other; every station can monitor all the traffic in the frequency channel, or filter only activity related to one or more stations, ignoring the rest.


Without entering in the details, we here mention the main features of the protocol.

Each packet is composed of the following fields:

The use of protocol AX25 allows to use the same physical channel, the operating frequency, by many pairs of correspondents who see each other but completely ignore the traffic of others: we therefore have more logic channels on the same physical channel. The protocol allows to be virtually error-proof, speed being elevated for a radio channel (1200 baud normally, 9600 baud with particular hardware and software solutions). It is possible to transmit texts or programs: files in general.

It is also possible to send and receive E-Mail through a BBS, either private or directed to all users or groups. To optimize resources and frequencies, authomatic stations operating around the clock have been activated to manage, send and sort out bulletins, programs, private mail and more. These stations are interlinked on frequencies different from those used by normal traffic, so that with a system of store and forward it is possible to send mail or whatever to every italian or international Ham. Every node sorts out mail to the node geographically nearest to the receiver, depending on his call. Every message has a hyerarchical address, with a Ham call and a referring BBS. The address IK1QLD@I1YLM.IPIE.ITA.EU is the Ham E-Mail hyerarchical address of the writer, which tells that IK1QLD refers to the BBS I1YLM, included in the field IPIE (Italy-Piedmont region), included in the field Italy, included in the field Europe. A Ham that from the United States wants to send to me a digital radio message, has only to call the radio BBS nearest to him and send a message to the address above.

 

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