THE PROTOCOL AX25
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 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 level.
-
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 [email protected] 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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