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
- the address field has been expanded to include
radio Ham calls (every Ham has an international callsign, and Hams must always
identify themselves in their conversations by means of their callsign).
- it has been added the possibility to use UI
frames (Unnumbered Information), that is unnumbered packets; usually packets
are numbered to restore the sending sequence.
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:
- FLAG: is an identifier that marks the
start and the end of each packet.
- ADDRESS: contains informations needed
to route the packet, and it can contain 2 to 10 Ham calls. A secondary identification,
or SSID, can be added to each call.
- CONTROL: here are contained some control
informations, as the kind of packet, the number of the packet, and much more.
- PROTOCOL ID (PID): this field is included
only in type I (information) or UI (unnumbered information)
packets. It represents the kind of net protocol used.
- INFORMATION (I): this field contains
data to be sent (up to 256 bytes). OSI system superior levels can use
part of this bytes as service information of their own.
- FRAME CONTROL SEQUENCE (FCS): is a number
calculated by the receiver to control the integrity of the packet; it uses
algorythm HDLC. Protocols used in packet radio are normally mastered
by TNC. It is also possible to implement these protocols using the right PC
software; in any case all procedures are authomatic and do not require the
operator's intervention. What can be seen on the screen of a PC (which, connected
to a TNC, becomes a normal terminal) is a good approximation of what is passing
through the connection level. The screen becomes in this case the presentation
- DISCONNECTED. When TNC is disconnected
and in monitor mode, it is possible to see all traffic passing through the
radio channel (this because of laws that oblige Hams to send in clear or using
codes internationally admitted). The amount of informations that can be seen
depends on the parameter MONITOR (ON/OFF) of the TNC, which represents the
filter of passing informations. In DISCONNECTED state TNC is waiting for a
connection request, either from user to the outside, or the contrary.
- CONNECTED. When a station wants to connect
another one, it sends a request packet to the other station; if this one is
hearing and can establish a connection, it answers with an ACKNOWLEDGE packet.
If called station doesn't answer, after a certain interval more requests are
sent for a set number of times. The command to establish the connection is
CONNECT followed by the call of the desired station.
- INFORMATION EXCHANGE. It happens when
two stations have established a connection.
- DISCONNECT. While two stations are connected,
each one can send the request to disconnect. Disconnection however can take
place only if the other station answers affirmatively, or, without any answer,
only after repeated attempts to disconnect.
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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