What is packet Radio?
Packet radio is a Radio Networking scheme actually designed by Radio Amateurs and derived from a scheme for networking data on the telephone network called X.25. Hence the protocol is called AX.25. As I write it must be getting close to 30 years old! Unlike other pre-DSP age modes for data , such as CW, RTTY, Fax and Hellscriber it is designed to give an error free connection end to end.
The errors are correct by resending the "damaged" part of the message. To this end a message is broken up and sent as chunks or packets that must be acknowledged as correct. This is quite unlike CW, RTTY and Hellscriber which has no acknowledgement. Even today, modern phone fax is not error free, only confirmation of a link is made. HF and RF Fax systems do not acknowledge reception at all. Traditional SSTV is actually a variation of Fax and nothing really to do with "TV" or data transmission at all.
This is a basic feature of Packet. It means a basic Packet Repeater (Digipeater) only needs a single frequency and can alternately listen (Receive) and transmit. Every packet has an address. Every Packet Station (called a network Node or Node) has a unique address (usually the call sign of the operator). To operate as a "Digipeater" a node simply stores each packet, and if not addressed to the node simply retransmits them.
Imagine four radio operators on the same frequency. You first listen to see if the channel is "clear". If it is you can put out a call. But if someone else does the same, you might do it at the same time! This is called a Collision. The Packet Radio protocol has a mechanism to detect if this might have occured and then retries a nearly random time later. Additionally, each node (radio station) will transmit a maximum number of packets and then listen for not just packet acknowlegments but other stations. (Lets give a pause to see is anyone else on frequency!). This is all automatic.
Real Life is bit more complicated. Later we will consider the problem of a node in a group which can only be "heard" by one neighbour. This is called the Hidden Transmitter Syndrome. It can cause the data rate of the "network" to drop dramatically as unexpected "collisions" and packet resending occurs. Later versions of AX.25 dedicated Digipeater software have special mechanisims to reduce this effect by creating a the concept of a master node (Digirepeater) and slave nodes (packet nodes with repeating turned off). To avoid degrading the network speed you should have "Digipeating" or "Repeater mode" turned off on a normal user station. Only dedicated "repeater" sites should "digirepeat".
You can "lurk" or "listen only" to packet traffic without connecting. This will work for picking up "node list" broadcasts, APRS broadcast data or DX Cluster information. Anything "interactive" or involving transmission with a user node needs a "Connection". There are three kinds of connection: